WO2000048837A1

WO2000048837A1 - Apparatus and method for depositing a viscous material

Info

Publication number: WO2000048837A1
Application number: PCT/GB2000/000546
Authority: WO
Inventors: Ricky Bennett; Clément KAISER; Francis Bourrieres; Michael Bartholomew; Charles Moncavage
Original assignee: Dek Printing Machines Limited
Priority date: 1999-02-16
Filing date: 2000-02-16
Publication date: 2000-08-24

Abstract

An apparatus for and method of depositing a viscous material on a substrate, the apparatus comprising: a delivery head (27) for containing a viscous material and delivering the viscous material to a substrate (3), wherein the delivery head (27) includes first and second thermally-connected surfaces, the first surface being in thermal contact with the contained viscous material; a temperature sensor (83) for sensing the temperature of the viscous material; heat transfer means thermally connected to the second surface of the delivery head (27); and a control unit (121) coupled to the heat transfer means and the temperature sensor (83) and being operably configured to control the heat transfer means to transfer heat selectively to and from the second surface of the delivery head (27) in response to the sensed temperature such as to maintain the temperature of the viscous material within a predeterminable temperature range.

Description

APPARATUS AND METHOD FOR DEPOSITING A VISCOUS MATERIAL

The present invention relates to an apparatus for and a method of depositing a viscous material on a substrate through the apertures of a stencil. In particular, the present invention provides for setting and maintaining a solder paste in a screen printing apparatus at a predetermined temperature.

As is known, a substrate, such as a printed circuit board, on which electronic components are mounted, requires that the components be soldered to the substrate. A viscous material, such as a non-conductive or conductive adhesive, solder paste or silicon-type material, is often deposited on a substrate prior to the mounting of components. Screen printing apparatus are used to automatically deposit viscous material through a stencil, such as a mesh or metallic screen, onto a substrate.

Generally, the solder pastes used in the screen printing of substrates for electronic application are heterogeneous materials, the components of which are a metallic material and an organic material, often referred to as a flux. Such solder pastes comprise microspheres of a metallic alloy bound by an organic flux. This viscous flux comprises a solvent and rheologic agents, adhesive agents and cleaning agents, which together confer the necessary properties to allow for the assembly of components on substrates. The process, which is well known, involves screen printing solder paste contacts onto selected areas of a substrate, locating component leads on the deposited contacts, with the adhesive agent of the paste holding the components to the substrate, and reflowing the paste in a furnace or oven to cause the coalescence of the metallic microspheres, which reflowed solder when cooled fixes the components to the substrate and provides the required electrical interconnection between the component leads and the substrate.

Of particular importance in ensuring optimal printing of solder paste is maintaining the temperature of the paste within a predetermined temperature range, as otherwise the viscosity of the paste, which is dependent on temperature, becomes undesirably low or high and the paste exhibits a poor transfer efficiency.. Solder pastes are formulated for use within a predetermined temperature range. If the temperature of the solder paste is too low. the viscosity of the paste is increased such as to prevent the complete filling of the apertures in the stencil at high printing speeds. On the other hand, if the temperature of the solder paste is too high, the viscosity of the paste is reduced and the paste flows too easily, causing the paste to flow between the stencil and the transfer head, leaving paste on the surface of the stencil. The optimum temperature range for the deposition of solder paste is typically in the narrow range of about 22 to 24 °C.

The difficulty in maintaining the temperature of solder paste within a narrow temperature range is exacerbated by any screen printing apparatus including heat-generating devices, such as motors, pumps and control circuitry, located within the enclosure thereof. The heat generated by these devices, which in the main operate intermittently, causes the temperature of the delivery unit and hence the contained solder paste to fluctuate with time. Further, the enclosure of the screen printing apparatus has to be opened periodically to allow the operator to make adjustments and to perform maintenance, which action causes the temperature within the enclosure to be rapidly lowered, and in turn the temperature of the delivery unit and hence the contained solder paste. Such variations in the temperature of the solder paste alter the viscosity of the paste and undesirably lead to inconsistent printing characteristics.

In one known configuration, it has been proposed to cool the interior of the enclosure of a screen printing apparatus by providing a flow of cool air through the enclosure, as typically provided by an evaporative heat exchanger, such as a kelvinator refrigeration device. In this apparatus the heat exchanger is thermostatically controlled to maintain the air temperature within the enclosure within a predetermined range.

However, controlling the temperature of the environment within the enclosure of a screen printing apparatus by providing a flow of cool air presents a number of problems. Firstly, this air flow tends to dry the solder paste, which adversely effects the chemical and rheological properties of the paste. way of example, the loss of 1 vol% of the solvent from a solder paste can be sufficient to completely alter the rheology of the paste and make screen printing very difficult, if not impossible. Secondly, the thermal coupling between the contained solder paste and the air within the enclosure is poor, resulting in a long equilibration time. It will be understood that long equilibration times can lead to imprecise temperature control where temperature fluctuations occur, such as on opening the enclosure of an apparatus. In addition, some solder pastes have to be stored at a low temperature when not in use to prevent degradation, which cold storage requires the pastes to be warmed up to a printing temperature before commencing the printing operation. As a result of the poor thermal transfer characteristics between the paste and the surrounding air. a significant delay is experienced between the paste being removed from cold storage and reaching the operating temperature. Thirdly, mechanical conduction paths invariably exist between the solder paste and the heat-generating devices, causing the paste to remain at a temperature greater than the air in the enclosure. Furthermore, refrigeration systems are inefficient, bulky, and are prone to mechanical failure.

WO-A-96/20088 discloses an apparatus for and a method of compressing viscous material through the apertures of a stencil. The apparatus comprises a compression headcap which includes two generally parallel wiper blades in sliding communication with a stencil and a plurality of diffuser plates for guiding and equalizing the flow of a viscous material between the wiper blades. This apparatus does not. however, incorporate means for controlling the temperature and hence viscosity of the viscous material.

Accordingly, the present invention provides an apparatus for depositing a viscous material on a substrate, comprising: a delivery head for containing a viscous material and delivering the viscous material to a substrate, wherein the delivery head includes first and second thermally-connected surfaces, the first surface being in thermal contact with the contained viscous material: a temperature sensor for sensing the temperature of the viscous material; heat transfer means thermally connected to the second surface of the delivery head; and a control unit coupled to the heat transfer means and the temperature sensor and being operably configured to control the heat transfer means to transfer heat selectively to and from the second surface of the deliveiy head in response to the sensed temperature such as to maintain the temperature of the viscous material within a predeterminable temperature range.

Preferably, the first and second surfaces of the deliver.' head comprise the same surface.

Preferably, the delivery head comprises a transfer head and a reservoir.

In one embodiment the second surface of the deliver ' head is a surface of the transfer head.

In another embodiment the second surface of the delivery head is a surface of the reservoir.

Preferably, the delivery head is an elongate body and the second surface of the delivery head comprises at least one elongate surface extending along one side of thereof.

More preferably, the second surface of the delivery head comprises first and second surfaces extending along opposite sides of the delivery head.

In one embodiment the heat transfer means comprises a heat dissipator and at least one heat transfer unit thermally connecting the heat dissipator and the second surface of the delivery head, the at least one heat transfer unit being operably configured, under the control of the control unit, to transfer heat selectively to and from the second surface of the delivery head.

In one preferred embodiment the heat dissipator comprises at least one conductive body having an extended dissipative surface.

Preferably, the dissipative surface is configured to transfer heat to a surrounding gas.

In another preferred embodiment the heat dissipator comprises at least one heat pipe. Preferably, the at least one heat transfer unit is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head in response to the polarity of an applied electrical current.

More preferably, the at least one heat transfer unit is a Peltier effect device.

Preferably, the heat transfer means comprises a plurality of heat transfer units.

In another embodiment the heat transfer means comprises a thermal energy source and at least one heat conductor thermally connecting the thermal energy source and the second surface of the delivery head, the thermal energy source being operably configured, under the control of the control unit, to be set at a temperature such that heat is transferred selectively to and from the second surface of the delivery head through the at least one heat conductor.

In a preferred embodiment the thermal energy source comprises a remote energy exchanger.

Preferably, the remote energy exchanger comprises a conductive body having an extended dissipative surface.

More preferably, the dissipative surface is configured to transfer heat to a surrounding gas.

Preferably, the remote energy exchanger includes a mechanical heat pump.

In a preferred embodiment the at least one heat conductor comprises a circulatory line through which a fluid is in use circulated.

Preferably, the circulatory line is provided in part by a channel within the delivery head. Preferably, the fluid comprises a liquid.

In another preferred embodiment the at least one heat conductor comprises a heat pipe.

Preferably, the heat transfer means further comprises at least one heat transfer unit thermally connecting the at least one heat conductor and the second surface of the delivery head, the at least one heat transfer unit being operably configured, under the control of the control unit, to transfer heat selectively to and from the second surface of the delivery head.

More preferably, the at least one heat transfer unit is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head in response to the polarity of an applied electrical current.

Still more preferably, the at least one heat transfer unit is a Peltier effect device.

Preferably, the heat transfer means further comprises at least one heat transfer block thermally connecting the at least one heat conductor and the at least one heat transfer unit.

More preferably, the heat transfer means comprises a plurality of heat transfer units and a plurality of heat transfer blocks thermally connecting the at least one heat conductor to respective ones of the heat transfer units.

Preferably, the heat transfer means comprises a plurality of heat conductors.

More preferably, the plurality of heat transfer units are each thermally connected in series with the plurality of heat conductors such that the direction of fluid or heat flow in one of the heat conductors is opposite the direction of fluid or heat flow in another of the heat conductors.

Preferably, the control unit and the delivery head are detachable such as to allow for storage of the delivery head.

More preferably, the control unit is configured to cool the viscous material to a storage temperature.

The present invention also provides a method of controlling the temperature of a viscous material to be deposited on a substrate, comprising the steps of: providing a delivery head for delivering a viscous material to a substrate, wherein the deliver}' head contains a viscous material and includes first and second thermally-connected surfaces, the first surface being in thermal contact with the contained viscous material; sensing the temperature of the viscous material: and transferring heat selectively to and from the second surface of the delivery head by heat transfer means thermally connected to the second surface of the delivery head in response to the sensed temperature such as to maintain the temperature of the viscous material within a predeterminable temperature range.

Preferably, the first and second surfaces of the delivery head comprise the same surface.

Preferably, the delivery head comprises a transfer head and a reservoir.

In one embodiment the second surface of the delivery head is a surface of the transfer head.

Preferably, the delivery head is an elongate body and the second surface of the delivery head comprises at least one elongate surface extending along one side of thereof. More preferably, the second surface of the delivery head comprises first and second surfaces extending along opposite sides of the delivery head.

In one embodiment the heat transfer means comprises a heat dissipator and at least one heat transfer unit thermally connecting the heat dissipator and the second surface of the delivery head, and the heat transfer step comprises the step of actuating the at least one heat transfer unit to transfer heat selectively to and from the second surface of the delivery head.

In another preferred embodiment the heat dissipator comprises at least one heat pipe.

Preferably, the at least one heat transfer unit is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head in response to the polarity of an applied electrical current, and the step of actuating the at least one heat transfer unit comprises the step of selectively applying electrical current of one or other polarity to the at least one heat transfer unit.

In another embodiment the heat transfer means comprises a thermal energy source and at least one heat conductor thermally connecting the thermal energy source and the second surface of the delivery head, and the heat transfer step comprises the step of setting the thermal energy source at a temperature such that heat is transferred selectively to and from the second surface of the delivery head through the at least one heat conductor.

Preferably, the thermal energy source comprises a remote energy exchanger.

More preferably, the remote energy exchanger comprises a conductive body having an extended dissipative surface.

Still more preferably, the dissipative surface is configured to transfer heat to a surrounding gas.

Preferably, the remote energy exchanger includes a mechanical heat pump.

In one preferred embodiment the at least one heat conductor comprises a circulatory line through which a fluid is circulated.

More preferably, the circulatory line is provided in part by a channel within the delivery head.

Preferably, the fluid comprises a liquid.

Preferably, the heat transfer means further comprises at least one heat transfer unit thermally connecting the at least one heat conductor and the second surface of the delivery head, and the heat transfer step comprises the step of actuating the at least one heat transfer unit to transfer heat selectively to and from the second surface of the delivery head.

More preferably, the at least one heat transfer unit is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head in response to the polarity of an applied electrical current, and the step of actuating the at least one heat transfer unit comprises the step of selectively applying electrical current of one or other polarity to the at least one heat transfer unit.

Preferably, the heat transfer means comprises a plurality of heat conductors.

More preferably, the plurality of heat transfer units are thermally connected in series with the plurality of heat conductors such that the direction of fluid or heat flow in one of the heat conductors is opposite the direction of fluid or heat flow in another of the heat conductors.

Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates a screen printing apparatus in accordance with a first embodiment of the present invention:

Figure 2 illustrates a perspective view of the deliver}' unit of the screen printing apparatus of Figure 1. illustrated in the closed configuration: Figure 3 illustrates a perspective view of the delivery unit of the screen printing apparatus of Figure 1. illustrated in the open configuration:

Figure 4 illustrates a perspective view of the delivery unit of the screen printing apparatus of Figure 1. illustrated with the pressure mechanism in the open position;

Figure 5 illustrates a side view of the deliver}' . unit of the screen printing apparatus of Figure 1 , illustrated in the closed configuration:

Figure 6 illustrates an underneath view of the delivery unit of the screen printing apparatus of Figure 1, illustrated in the closed configuration;

Figure 7 illustrates a fragmentary vertical sectional view (along section I-I in Figure 6) of the delivery unit of the screen printing apparatus of Figure 1 ;

Figure 8 schematically illustrates the electrical interconnection of the heat transfer components of the screen printing apparatus of Figure 1 ;

Figure 9 schematically illustrates a fragmentary plan view of a screen printing apparatus in accordance with a second embodiment of the present invention:

Figure 10 schematically illustrates a fragmentary side view of the screen printing apparatus of Figure 9;

Figure 1 1 illustrates a vertical sectional view (along section II-II in Figure 9) of the screen printing apparatus of Figure 9;

Figure 12 schematically illustrates a fragmentary plan view of a screen printing apparatus in accordance with a third embodiment of the present invention; Figure 13 schematically illustrates a fragmentary side view of the screen printing apparatus of Figure 12:

Figure 14 illustrates a vertical sectional view (along section III-III in Figure 12) of the screen printing apparatus of Figure 12:

Figure 15 schematically illustrates a fragmentary plan view of a screen printing apparatus in accordance with a fourth embodiment of the present invention;

Figure 16 schematically illustrates a fragmentary side view of the screen printing apparatus of Figure 15;

Figure 17 illustrates a vertical sectional view (along section IV-IV in Figure 15) of the screen printing apparatus of Figure 15;

Figure 18 schematically illustrates a fragmentary plan view of a screen printing apparatus in accordance with a fifth embodiment of the present invention;

Figure 19 illustrates a vertical sectional view (along section V-V in Figure 18) of the screen printing apparatus of Figure 18;

Figure 20 illustrates a fragmentary side view of a screen printing apparatus in accordance with a sixth embodiment of the present invention:

Figure 21 illustrates a vertical sectional view (along section VI-VI in Figure 20) of the screen printing apparatus of Figure 20;

Figure 22 schematically illustrates a fragmentary plan view of a screen printing apparatus in accordance with a seventh embodiment of the present invention: Figure 23 illustrates a vertical sectional view (along section VII-VII in Figure 22) of the screen printing apparatus of Figure 22; and

Figure 24 schematically illustrates a temperature control unit for use with the delivery units of screen printing apparatus in accordance with embodiments of the present invention.

The screen printing apparatus comprises a support unit 1 for supporting a substrate 3, in this embodiment a printed circuit board, at a printing location, a stencil 5 disposed above the printing location and defining the pattern to be deposited onto the substrate 3, a delivery unit 7 for delivering a viscous material, in this embodiment a solder paste, through the stencil 5 to print the required pattern on the substrate 3, and a drive unit 9 for moving the delivery unit 7 during printing, all enclosed in an enclosure 10.

The support unit 1 is provided by first and second rails 11, 13 along which the substrate 3 is transferred to the printing location, in this embodiment by a belt and pulley system.

The drive unit 9 comprises a first, support member 15, a second, horizontal drive member 17 movable horizontally relative to the support member 15, a third, vertical drive member 19 movable vertically relative to the horizontal drive member 17 and to which the delivery unit 7 is mounted, a first, printhead motor 21 for moving the horizontal drive member 17 and hence the delivery unit 7 horizontally in a forward and reverse direction, and a second, squeegee motor 23 for moving the vertical drive member 19 and hence the delivery unit 7 vertically.

The delivery unit 7 comprises a printhead 27 which contains the viscous material to be deposited on the substrate 3. and a pressure mechanism 31 for applying a pressure to the viscous material which is coupled to the printhead 27 by a hinge 32. The printhead 27 comprises a transfer head 33. and a carrier 35 for receiving a material- containing cassette 37 which is coupled b} a hinge 38 to the transfer head 33 such as to be rotatable between a first, open position (as illustrated in Figure 3) in which the cassette 37 can be loaded into the carrier 35 and a second, closed position (as illustrated in Figure 4) in which the cassette 37 is in communication with the transfer head 33.

The transfer head 33 comprises an elongate main block 41 having substantially planar upper and lower surfaces 43, 45 and including an elongate cavity 47 extending along the length thereof between an upper opening 49 in the upper surface 43 thereof and a lower opening 51 in the lower surface 45 thereof, a first, upper plate 53 including a plurality of apertures 55, in this embodiment in a symmetrical grid-like arrangement, located in the upper opening 49. and a second, lower plate 57 including a plurality of apertures 59. in this embodiment in the same symmetrical grid-like arrangement as the apertures 55 in the upper plate 53, located in the lower opening 51.

The transfer head 33 further comprises first and second wipers 61, 63 attached to the longitudinal edges of the lower surface 45 of the main block 41, in this embodiment clamped to the main block 41 by retaining strips 65. 67, and end retainers 71, 73 attached to the ends of the lower surface 45 of the main block 41 so as to enclose the regions between the ends of the wipers 61. 63. in this embodiment clamped to the main block 41 by brackets 75. 77. The wipers 61. 63 enclose an acute angle α, preferably not more than 60°, with the lower surface 45 of the main block 41. In this embodiment the wipers 61, 63 are formed of polyester, but alternatively could be formed of metal or a composite of metal and polyester.

The transfer head 33 further comprises clamping brackets 78, 79 at each of the ends of the main block 41 which serve in part to hold the carrier 35 in the closed position as will be described in more detail hereinbelow. The transfer head 33 further comprises a plurality of thermal transfer units 80 thermally connected to the main block 41. in this embodiment along each of the longitudinal sides thereof, first and second heat sinks 81. 82 thermally connected to the respective ones of the thermal transfer units 80 on each side of the main block 41. and a temperature sensor 83 disposed within the cavity 47 in the main block 41. in this embodiment attached to the lower surface of the upper plate 53. for sensing the temperature of the viscous material. In a preferred embodiment thermal grease is provided between the thermal transfer units 80 and both the main block 41 and the heat sinks 81. 82 in order to enhance the thermal coupling.

The thermal transfer units 80 are thermoelectrical devices which utilize the Peltier effect to transfer heat to or from the viscous material in response to the polarity of the applied electrical current. In this embodiment the thermal transfer units 80 are CP series thermoelectric modules as manufactured by the Melcor Company.

In this embodiment the heat sinks 81 , 82 are formed of metal, preferably as aluminium extrusions, and include fins in order to enhance the heat transfer with the surrounding air.

In this embodiment the temperature sensor 83 comprises an immersible J-type thermocouple. The temperature sensor 83 may, however, be any sensor which is physically and chemically compatible with the viscous material. In other alternative embodiments the temperature sensor 83 can be attached to an inner peripheral surface of the cavity 47 in the main block 41. the upper surface of the lower plate 45. or indeed the upper surface of the upper plate 53. In another embodiment the transfer head 33 can include a plurality of temperature sensors 83.

The carrier 35 comprises an elongate housing 84 which is of substantially the same length as the main block 41 and includes an elongate cavity 85 which is of substantially the same dimension as the elongate cavity 47 in the main block 41 and into which the cassette 37 is loaded. In this embodiment the carrier 35 includes spring clips for retaining the cassette 37 in position. The elongate cavity 85 extends between a first opening 87 in one surface thereof, a lower surface thereof when in the closed position, and a second opening 89 in another surface thereof, an upper surface thereof when in the closed position. The carrier 35 further comprises thumbscrews 93. 95 at each of the ends thereof which serve in part to lock the carrier 35 in the closed position as will be described in more detail hereinbelow.

The cassette 37 comprises a base plate 97 which includes a plurality of apertures 99 having the same grid-like arrangement, that is. pitch and dimension, as the apertures 55 in the upper plate 53 of the transfer head 33. and a material reservoir 101 containing a viscous material connected in fluid communication to the base plate 97 such that in use, in this embodiment by the application of a force to the material reservoir 101, the viscous material is driven through the apertures 99 in the base plate 97 to the transfer head 33. In this embodiment the base plate 97 comprises a rigid plastics material and the material reservoir 101 comprises a flexible plastics material. In alternative embodiments the material reservoir 101 could comprise a rigid structure which incorporates either a movable element in the manner of a piston or a refillable container such as a bladder.

The pressure mechanism 31 comprises a piston crosshead 105 for applying a force to the material reservoir 101 of the cassette 37 so as to deliver viscous material to the transfer head 33, first and second piston actuators 107. 109 for moving the piston crosshead 105, and an enclosure 111 for enclosing the piston crosshead 105 and the piston actuators 107, 109. In this embodiment the piston actuators 107. 109 comprise double-acting pneumatic cylinders, which, by regulation of the air pressure supplied thereto, allow the pressure applied to the viscous material to be accurately controlled. The screen printing apparatus comprises a pneumatic supply 1 12 for actuating the piston actuators 107, 109.

In a cassette loading operation, a cassette 37 is clipped into the carrier 35 when in the open position, and. with the clamping brackets 78, 79 of the transfer head 33 rotated outwardly and the pressure mechanism 31 in the raised, open position (as illustrated in Figures 3 or 4), the carrier 35 is rotated to the closed position such that the base plate 97 of the cassette 37 abuts the upper plate 53 of the transfer head 33. The clamping brackets 78. 79 are then rotated inwardly so as to be located above the thumbscrews 93. 95 on the carrier 35 and the thumbscrews 93. 95 tightened against the clamping brackets 78. 79 such as to lock the carrier 35 in the closed position. The pressure mechanism 31 is then lowered to the closed position (as illustrated in Figure 2).

In operation of the screen printing apparatus, the delivery unit 7 is first raised by actuation of the squeegee motor 23 to a home position during initialization. The delivery unit 7 is then lowered by actuation of the squeegee motor 23 to the so-called contact height, at which height the wipers 61, 63 of the transfer head 33 of the delivery unit 7 are just in contact with the stencil 5. but with no downward force exerted (zero pressure) on the stencil 5. Next, a downward print pressure is applied to the delivery unit 7 by further actuation the squeegee motor 23. This downward print pressure causes gasketing to occur between the wipers 61, 63 and the end retainers 71, 73 of the transfer head 33 and the stencil 5. Typically, a downward print pressure force of about 8 to 10 kgf is suitable for a 300 mm sized delivery unit 7. Next, a downward paste pressure of about 2.5 to 3 bar is applied by the piston actuators 107, 109 of the pressure mechanism 31, resulting in the piston crosshead 105 of the pressure mechanism 31 applying a force to the viscous material in the cassette 37. Then, by actuation of the printhead motor 21, the delivery unit 7 is driven to perform a print stroke by moving horizontally along the stencil 5 and the substrate 3. Typical print stroke speeds are about 0.1 1 to 0.15 ms^"1. The delivery unit 7 is stopped when centered over the opposite rail 13, ensuring that the delivery unit 7 is well supported while under the print pressure. The paste pressure applied by the piston actuators 107. 109 is removed as soon as the delivery unit 7 has stopped moving. Then, by actuation of the squeegee motor 23 the print pressure is removed, and, as a result, the delivery unit 7 returns to the contact height. The delivery unit 7 is then ready to apply viscous material to the surface of the next substrate 3. which is moved into the printing location along the rails 1 1. 13, by repeating the above sequence while travelling in the opposite direction. The screen printing apparatus further comprises a control unit 121 for controlling the current flow in the thermal transfer units 80 and hence the temperature of the viscous material contained in the printhead 27. As illustrated schematically in Figure 8. the control unit 121 comprises a temperature controller 123 coupled to the temperature sensor 83. a power supply 125 for supplying electrical current, and a power controller 127 connected to the thermal transfer units 80. the temperature controller 123 and the power supply 125 and being configured such as to supply current to the thermal transfer units 80 under the control of the temperature controller 123. In this embodiment the control unit 121 is located within the enclosure 10 of the screen printing apparatus. In an alternative embodiment the control unit 121 can be located externally of the enclosure 10 and connected with the thermal transfer units 80 and the temperature sensor 83 via a cable.

In this embodiment the thermal transfer units 80 are arranged in four groups of two connected electrically in series, which groups are connected in parallel with the power controller 127. In an alternative embodiment the transfer head 33 can include fewer or greater numbers of thermal transfer units 80, and indeed the thermal transfer units 80 can be electrically connected in other configurations.

In this embodiment the temperature controller 123 is a model E5CK PID controller as manufactured by the Omron Corporation which includes a manual interface for allowing the operating temperature window to be set by an operator. In a preferred embodiment the temperature controller 123 includes an interface for connection to a computer, for example, an RS-232 interface, for allowing the operating temperature window to be set remotely by an operator.

The power controller 127 includes polarity reversing means for reversing the polarity of the current supplied to the thermal transfer units 80, in this embodiment a solid-state relay for selectively supplying current of reverse polarity to the thermal transfer units 80. In operation, the temperature controller 123. which senses the temperature of the viscous material through the temperature sensor 83. causes the thermal transfer units 80 to be energized when the temperature sensed by the temperature sensor 83 is outside a predetermined temperature range. When the sensed temperature exceeds a predetermined upper limit, the power controller 127. under the control of the temperature controller 123. causes current to flow through the thermal transfer units 80 in one direction such as to transfer heat from the main block 41 to the heat sinks 81. 82. thereby cooling the viscous material. When the sensed temperature is below a predetermined lower limit, the power controller 127, under the control of the temperature controller 123. causes current to flow through the thermal transfer units 80 in the other direction such as to transfer heat from the heat sinks 81, 82 to the main block 41. thereby heating the viscous material.

Figures 9 to 1 1 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a second embodiment of the present invention. This screen printing apparatus is substantially identical to that of the above-described first embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the first-described embodiment only in the construction of the heat exchanger. In this embodiment, instead of the heat sinks 81, 82, the transfer head 33 includes heat exchange blocks 130 thermally coupled to respective ones of the thermal transfer units 80. The screen printing apparatus further comprises a remote energy exchanger 131, and first and second circulatory lines 133, 135 through which fluid is circulated and which thermally couple the heat exchange blocks 130 and the remote energy exchanger 131 , with the fluid in the first, upper line 133 flowing in a first direction, an anti-clockwise direction when viewed from above, and the fluid in the second, lower line 135 flowing in a second opposite direction, a clockwise direction when viewed from above. In this embodiment the circulatory lines 133. 135 extend through each of the heat transfer blocks 130 about the periphery of the main block 41 of the transfer head 33. The fluid may be any suitable heat exchange fluid, such as water, glycol or fluorocarbon. In this embodiment the remote enerev exchanger 131 includes a conventional heat sink with a large surface area and a fan for maintaining an air flow over the heat sink, and is configured to dissipate heat outside the enclosure 10 of the screen printing apparatus. In an alternative embodiment the remote energy exchanger 131 can include a mechanical heat pump.

By arranging the separate fluid flows in each of the first and second lines 133. 135 to flow in opposite directions, the temperature differential between the heat transfer blocks 130 is minimized as the first heat transfer block 130 to be provided with heated or cooled fluid supplied from the remote energy exchanger 131 through one of the lines 133. 135 is the last heat transfer block 130 to be provided with heated or cooled fluid supplied from the remote energy exchanger 131 through the other of the lines 133. 135 and vice versa.

In operation, the temperature controller 123. which senses the temperature of the viscous material through the temperature sensor 83. causes the thermal transfer units 80 to be energized when the temperature sensed by the temperature sensor 83 is outside a predetermined temperature range. When the sensed temperature exceeds a predetermined upper limit, the power controller 127. under the control of the temperature controller 123, causes current to flow through the thermal transfer units 80 in one direction such as to transfer heat from the main block 41 to the heat transfer blocks 130, and thereby cool the viscous material, which heat is dissipated by the remote heat exchanger 131. When the sensed temperature is below a predetermined lower limit, the power controller 127. under the control of the temperature controller 123. causes current to flow through the thermal transfer units 80 in the other direction such as to transfer heat from the heat transfer blocks 130 to the main block 41. and thereby heat the viscous material, which heat is supplied by the remote heat exchanger 131.

Figures 12 to 14 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a third embodiment of the present invention. This screen printing apparatus is substantially identical to that of the above-described first embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the first-described embodiment only in the construction of the heat exchanger. In this embodiment, instead of the heat sinks 81, 82, the transfer head 33 includes heat exchange blocks 130 thermally coupled to respective ones of the thermal transfer units 80. This screen printing apparatus further comprises a remote energy exchanger 131. and first and second heat pipes 139. 141 which thermally couple the heat exchange blocks 130 and the remote energy exchanger 131. In this embodiment the remote energy exchanger 131 includes a conventional heat sink with a large surface area and a fan for maintaining an air flow over the heat sink, and is configured to dissipate heat outside the enclosure 10 of the screen printing apparatus. In an alternative embodiment the remote energy exchanger 131 can include a mechanical heat pump. The heat pipes 139. 141 are known elements for transferring heat energy using a sealed tube containing a wicking material and a fluid at low pressure, whereby heat is transferred without mechanical parts. In this embodiment the heat pipes 139, 141 comprise THERMAPIPE heat pipes manufactured by the Indek Corporation. The heat pipes 139, 141 extend through each of the heat transfer blocks 130 about the periphery of the main block 41 of the transfer head 33, with one of the heat pipes 139 extending through an upper part of the heat transfer blocks 130 in a first direction, a clockwise direction when viewed from above, and terminating at one of the heat transfer blocks 130 and the other of the heat pipes 139 extending through a lower part of the heat transfer blocks 130 in the opposite direction, an anti-clockwise direction when viewed from above, and terminating at that transfer block 130 adjacent the one heat transfer block 130 in the first, clockwise direction.

In operation, the temperature controller 123, which senses the temperature of the viscous material through the temperature sensor 83, causes the thermal transfer units 80 to be energized when the temperature sensed by the temperature sensor 83 is outside a predetermined temperature range. When the sensed temperature exceeds a predetermined upper limit, the power controller 127. under the control of the temperature controller 123, causes current to flow through the thermal transfer units 80 in one direction such as to transfer heat from the main block 41 to the heat transfer blocks 130. and therebv cool the viscous material, which heat is dissipated by the remote heat exchanger 131. When the sensed temperature is below a predetermined lower limit, the power controller 127. under the control of the temperature controller 123, causes current to flow through the thermal transfer units 80 in the other direction such as to transfer heat from the heat transfer blocks 130 to the main block 41, and thereby heat the viscous material, which heat is supplied by the remote heat exchanger 131.

By arranging the first and second heat pipes 139. 141 to extend around the periphery of the transfer head 33 in opposite directions, the temperature differential between the heat transfer blocks 130 is minimized as the first heat transfer block 130 to be provided with heated or cooled fluid from the remote energy exchanger 131 through one of the heat pipes 139, 141 is the last heat transfer block 130 to be provided with heated or cooled fluid from the remote energy exchanger 131 through the other of the heat pipes 139, 141 and vice versa.

Figures 15 to 17 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a fourth embodiment of the present invention. This screen printing apparatus is similar to that of the above-described second embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the second-described embodiment only in that the thermal transfer units 80 and the heat transfer blocks 130 are omitted and the first and second circulatory lines 133, 135 are thermally coupled directly to the periphery of the main block 41 of the transfer head 33.

In operation, the remote energy exchanger 131 is controlled by the temperature controller 123, which senses the temperature of the viscous material through the temperature sensor 83, to transfer heat to or from the fluid flows in the circulatory lines 133. 135, and thereby control the temperature of the viscous material within the transfer head 33. In this embodiment the remote energy exchanger 131 is a mechanical heat pump that can be reversibly cycled to either transfer heat to or from the circulating fluid flows, which heat pump utilizes a compressor to provide evaporative cooling, the compressor being cycled on and off under the control of the temperature controller 123.

Figures 18 and 19 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a fifth embodiment of the present invention. This screen printing apparatus is similar to that of the above-described second embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the second-described embodiment in that the thermal transfer units 80, the haet transfer blocks 130 and the second circulatory line 135 are omitted, and in that the first circulatory line 133 is thermalh' coupled directly to the inner periphery of the cavity 47 in the main block 41 of the transfer head 33. In an alternative embodiment, instead of providing the circulatory line 133 as a pipe within the cavity 47 in the main block 41, the circulatory line 133 could be provided by channels within the main block 41 of the transfer head 33. In another alternative embodiment first and second circulating lines 133, 135 having counter-directed flows could be used as in the above-described second embodiment.

In operation, the remote energy exchanger 131 is controlled by the temperature controller 123, which senses the temperature of the viscous material through the temperature sensor 83, to transfer heat to or from the fluid flow in the circulatory line 133. and thereby control the temperature of the viscous material within the transfer head 33. In this embodiment the remote energy exchanger 131 is a mechanical heat pump that can be reversibly cycled to either transfer heat to or from the circulating fluid flows, which heat pump utilizes a compressor to provide evaporative cooling, the compressor being cycled on and off under the control of the temperature controller 123.

Figures 20 and 21 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a sixth embodiment of the present invention. This screen printing apparatus is substantially identical to that of the above-described first embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the first-described embodiment in that the carrier 35 and the cartridge 37 of the delivery unit 7 are replaced by a permanent material reservoir 151 which includes ports 153 for re-filling, and in that the temperature sensor 83 is attached to the upper surface of the upper plate 53 of the transfer head 33. In an alternative embodiment the thermal transfer units 80 and the heat sinks 81, 82 could be thermally coupled to the periphery of the material reservoir 151. It will, of course, be understood that any of the other above-described embodiments could be modified to include a permanent reservoir as in this embodiment. Operation is the same as for the above-described first embodiment.

Figures 22 and 23 schematically illustrate fragmentary views of a screen printing apparatus in accordance with a seventh embodiment of the present invention. This screen printing apparatus is similar to that of the above-described second embodiment, and thus in order to avoid unnecessary duplication of description only the differences will be described in detail, with like parts being designated by like reference signs. This screen printing apparatus differs from that of the second-described embodiment, similarly to the fifth- described embodiment, in that the thermal transfer units 80, the heat transfer blocks 130 and the second circulatory line 135 are omitted, and in that the main block 41 includes a peripheral channel 155 which provides in part the first circulatory line 133. In an alternative embodiment the main block 41 could include first and second peripheral channels 155 provided as part of first and second circulating lines 133, 135 having counter- directed flows as in the above-described second embodiment. This screen printing apparatus further differs from that of the second-described embodiment, similarly to the sixth-described embodiment, in that the carrier 35 and the cartridge 37 of the delivery unit 7 are replaced by a permanent material reservoir 151 which includes a port 153 for refilling, and in that the temperature sensor 83 is attached to the upper surface of the upper plate 53 of the transfer head 33. Operation is the same as for the above-described first embodiment.

Figure 24 schematically illustrates a temperature control unit 157 for maintaining the viscous material contained in a delivery unit 7 at a predetermined temperature when stored remotely of the screen printing apparatus of any of the above-described embodiments

The temperature control unit 157 comprises a control unit 158 comprising the same elements as the above-described control unit 121 , namely, a temperature controller 123 coupled to the temperature sensor 83 in the transfer head 33 of the delivery unit 7, a power supply 125 for supplying electrical current, and a power controller 127 connected to the thermal transfer units 80 of the transfer head 33 of the delivery unit 7. the temperature controller 123 and the power supply 125 such as to supply current to the thermal transfer units 80 under the control of the temperature controller 123, and a fan 159 configured to direct a flow of air across the heat sinks 81. 82 in order to enhance the heating or cooling of the viscous material by the thermal transfer units 80. In this embodiment the temperature control unit 157 includes a connector 161 for connection with a complementary connector 163 on the delivery unit 7 which provides electrical connection to the thermal transfer units 80 and the temperature sensor 83.

In operation, where the viscous material contained in the delivery unit 7 has to be stored within a particular temperature range, for example, where solder paste must be maintained below a cold-storage temperature when not in use. the delivery unit 7 is removed from the screen printing apparatus and connected to the temperature control unit 157 by the connectors 161. 163. When so connected, the temperature controller 123. which senses the temperature of the viscous material through the temperature sensor 83. causes current to flow through the thermal transfer units 80 in a direction such as to transfer heat from the main block 41 to the heat sinks 81. 82 and the fan 159 to be operated to draw air away from the delivery unit 7, thereby cooling the contained viscous material to the required temperature. When at the required temperature, the temperature control unit 157 maintains the viscous material at the required temperature by selectively energizing the thermal transfer units 80 to transfer heat to or from the main block 41 as necessary. When the delivery unit 7 is to be used, the temperature of the solder paste must be raised to the required operating temperature, and. under the control of the temperature controller 123, the power controller 127 causes current to flow through the thermal transfer units 80 in the direction such as to transfer heat from the heat sinks 81. 82 to the main block 41 and the fan 159 to be operated to provide warm air to the delivery unit 7, thereby heating the viscous material to the printing temperature. Once at this delivery temperature, the delivery unit 7 can then be fitted to the screen printing apparatus.

Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

Claims

1. An apparatus for depositing a viscous material on a substrate, comprising: a delivery head (27) for containing a viscous material and delivering the viscous material to a substrate (3). wherein the deliver}' head (27) includes first and second thermally-connected surfaces, the first surface being in thermal contact with the contained viscous material; a temperature sensor (83) for sensing the temperature of the viscous material; heat transfer means thermalh' connected to the second surface of the delivery head

(27); and a control unit (121) coupled to the heat transfer means and the temperature sensor

(83) and being operably configured to control the heat transfer means to transfer heat selectively to and from the second surface of the delivery head (27) in response to the sensed temperature such as to maintain the temperature of the viscous material within a predeterminable temperature range.

2. The apparatus of claim 1, wherein the first and second surfaces of the delivery head (27) comprise the same surface.

3. The apparatus of claim 1 or 2, wherein the delivery head (27) comprises a transfer head (33) and a reservoir (35. 151).

4. The apparatus of claim 3, wherein the second surface of the delivery head (27) is a surface of the transfer head (33).

5. The apparatus of claim 3, wherein the second surface of the delivery head (27) is a surface of the reservoir (35. 151).

6. The apparatus of a ^r of claims 1 to 5. wherein the delivery head (27) is an elongate body and the second surface of the delivery head (27) comprises at least one elongate surface extending along one side of thereof.

7. The apparatus of claim 6, wherein the second surface of the deliver}' head (27) comprises first and second surfaces extending along opposite sides of the delivery head (27).

8. The apparatus of any of claims 1 to 7, wherein the heat transfer means comprises a heat dissipator and at least one heat transfer unit (80) thermally connecting the heat dissipator and the second surface of the delivery head (27). the at least one heat transfer unit (80) being operably configured, under the control of the control unit (121), to transfer heat selectively to and from the second surface of the delivery head (27).

9. The apparatus of claim 8, wherein the heat dissipator comprises at least one conductive body (81 , 82) having an extended dissipative surface.

10. The apparatus of claim 9, wherein the dissipative surface is configured to transfer heat to a surrounding gas.

11. The apparatus of claim 8, wherein the heat dissipator comprises at least one heat pipe (139, 141).

12. The apparatus of any of claims 8 to 1 1. wherein the at least one heat transfer unit (80) is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head (27) in response to the polarity of an applied electrical current.

13. The apparatus of claim 12. wherein the at least one heat transfer unit (80) is a Peltier effect device.

14. The apparatus of any of claims 8 to 13. wherein the heat transfer means comprises a plurality of heat transfer units (80).

15. The apparatus of any of claims 1 to 7, wherein the heat transfer means comprises a thermal energy source and at least one heat conductor thermally connecting the thermal energy source and the second surface of the delivery head (27), the thermal energy source being operably configured, under the control of the control unit (121), to be set at a temperature such that heat is transferred selectively to and from the second surface of the deliver}' head (27) through the at least one heat conductor.

16. The apparatus of claim 15. wherein the thermal energy source comprises a remote energy exchanger (131).

17. The apparatus of claim 16. wherein the remote energy exchanger (131) comprises a conductive body having an extended dissipative surface.

18. The apparatus of claim 17. wherein the dissipative surface is configured to transfer heat to a surrounding gas.

19. The apparatus of claim 16, wherein the remote energy exchanger (131) includes a mechanical heat pump.

20. The apparatus of any of claims 15 to 19. wherein the at least one heat conductor comprises a circulatory line (133. 135) through which a fluid is in use circulated.

21. The apparatus of claim 20. wherein the circulatory line (133, 135) is provided in part by a channel within the delivery head (27).

22. The apparatus of claim 20 or 21. wherein the fluid comprises a liquid.

23. The apparatus of any of claims 15 to 19. wherein the at least one heat conductor comprises a heat pipe (139. 141).

24. The apparatus of any of claims 15 to 23. wherein the heat transfer means further comprises at least one heat transfer unit (80) thermally connecting the at least one heat conductor and the second surface of the delivery head (27), the at least one heat transfer unit (80) being operably configured, under the control of the control unit (121). to transfer heat selectively to and from the second surface of the delivery head (27).

25. The apparatus of claim 24. wherein the at least one heat transfer unit (80) is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head (27) in response to the polarity of an applied electrical current.

26. The apparatus of claim 25. wherein the at least one heat transfer unit (80) is a Peltier effect device.

27. The apparatus of any of claims 24 to 26. wherein the heat transfer means further comprises at least one heat transfer block (130) thermally connecting the at least one heat conductor and the at least one heat transfer unit (80).

28. The apparatus of any of claims 24 to 27. wherein the heat transfer means comprises a plurality of heat transfer units (80).

29. The apparatus of claim 27. wherein the heat transfer means comprises a plurality of heat transfer units (80) and a plurality of heat transfer blocks (130) thermally connecting the at least one heat conductor to respective ones of the heat transfer units (80).

30. The apparatus of any of claims 15 to 29. wherein the heat transfer means comprises a plurality of heat conductors.

31. The apparatus of claim 30 when appendant upon claim 28, wherein the plurality of heat transfer units (80) are each thermally connected in series with the plurality of heat conductors such that the direction of fluid or heat flow in one of the heat conductors is opposite the direction of fluid or heat flow in another of the heat conductors.

32. The apparatus of any of claims 1 to 31. wherein the control unit (121, 157) and the delivery head (27) are detachable such as to allow for storage of the delivery head (27).

33. The apparatus of claim 32, wherein the control unit (121. 157) is configured to cool the viscous material to a storage temperature.

34. A method of controlling the temperature of a viscous material to be deposited on a substrate, comprising the steps of: providing a delivery head (27) for delivering a viscous material to a substrate (3), wherein the delivery head (27) contains a viscous material and includes first and second thermally-connected surfaces, the first surface being in thermal contact with the contained viscous material; sensing the temperature of the viscous material; and transferring heat selectively to and from the second surface of the delivery head

(27) by heat transfer means thermally connected to the second surface of the deliver}' head (27) in response to the sensed temperature such as to maintain the temperature of the viscous material within a predeterminable temperature range.

35. The method of claim 34. wherein the first and second surfaces of the delivery head (27) comprise the same surface.

36. The method of claim 34 or 35, wherein the delivery head (27) comprises a transfer head (33) and a reservoir (35, 151).

37. The method of claim 36, wherein the second surface of the deliveiy head (27) is a surface of the transfer head (33).

38. The method of claim 36, wherein the second surface of the deliver}' head (27) is a surface of the reservoir (35, 151).

39. The method of any of claims 34 to 38, wherein the delivery head (27) is an elongate body and the second surface of the delivery head (27) comprises at least one elongate surface extending along one side thereof.

40. The method of claim 39, wherein the second surface of the delivery head (27) comprises first and second surfaces extending along opposite sides of the delivery head (27).

41. The method of any of claims 34 to 40, wherein the heat transfer means comprises a heat dissipator and at least one heat transfer unit (80) thermally connecting the heat dissipator and the second surface of the delivery head (27), and the heat transfer step comprises the step of actuating the at least one heat transfer unit (80) to transfer heat selectively to and from the second surface of the deliver}' head (27).

42. The method of claim 41, wherein the heat dissipator comprises at least one conductive body (81 , 82) having an extended dissipative surface. s s

43. The method of claim 42, wherein the dissipative surface is configured to transfer heat to a surrounding gas.

44. The method of claim 41, wherein the heat dissipator comprises at least one heat pipe (139. 141).

45. The method of any of claims 41 to 44. wherein the at least one heat transfer unit (80) is a thermoelectric device configured to transfer heat selectively to and from the second surface of the deliver}' head (27) in response to the polarity of an applied electrical current, and the step of actuating the at least one heat transfer unit (80) comprises the step of selectively applying electrical current of one or other polarity to the at least one heat transfer unit (80).

46. The method of claim 45, wherein the at least one heat transfer unit (80) is a Peltier effect device.

47. The method of any of claims 41 to 46, wherein the heat transfer means comprises a plurality of heat transfer units (80).

48. The method of any of claims 34 to 40. wherein the heat transfer means comprises a thermal energy source and at least one heat conductor thermally connecting the thermal energy source and the second surface of the delivery head (27), and the heat transfer step comprises the step of setting the thermal energy source at a temperature such that heat is transferred selectively to and from the second surface of the delivery head (27) through the at least one heat conductor.

49. The method of claim 48, wherein the thermal energy source comprises a remote energy exchanger (131).

50. The method of claim 49. wherein the remote energy exchanger (131) comprises a conductive body having an extended dissipative surface.

51. The method of claim 50, wherein the dissipative surface is configured to transfer heat to a surrounding gas.

52. The method of claim 49, wherein the remote energy exchanger (131) includes a mechanical heat pump.

53. The method of any of claims 48 to 52. wherein the at least one heat conductor comprises a circulatory line (133. 135) through which a fluid is circulated.

54. The method of claim 53, wherein the circulatory line (133, 135) is provided in part by a channel within the delivery head (27).

55. The method of claim 53 or 54, wherein the fluid comprises a liquid.

56. The method of any of claims 48 to 52. wherein the at least one heat conductor comprises a heat pipe (139, 141).

57. The method of any of claims 48 to 56. wherein the heat transfer means further comprises at least one heat transfer unit (80) thermally connecting the at least one heat conductor and the second surface of the delivery head (27), and the heat transfer step comprises the step of actuating the at least one heat transfer unit (80) to transfer heat selectively to and from the second surface of the delivery head (27).

58. The method of claim 57. wherein the at least one heat transfer unit (80) is a thermoelectric device configured to transfer heat selectively to and from the second surface of the delivery head (27) in response to the polarity of an applied electrical current, and the step of actuating the at least one heat transfer unit (80) comprises the step of selectively applying electrical current of one or other polarity to the at least one heat transfer unit (80).

59. The method of claim 58. wherein the at least one heat transfer unit (80) is a Peltier effect device.

60. The method of any of claims 57 to 59. wherein the heat transfer means further comprises at least one heat transfer block (130) thermally connecting the at least one heat conductor and the at least one heat transfer unit (80).

61. The method of any of claims 57 to 60, wherein the heat transfer means comprises a plurality of heat transfer units (80).

62. The method of claim 61, wherein the heat transfer means comprises a plurality of heat transfer units (80) and a plurality of heat transfer blocks (130) thermally connecting the at least one heat conductor to respective ones of the heat transfer units (80).

63. The method of any of claims 41 to 62, wherein the heat transfer means comprises a plurality of heat conductors.

64. The method of claim 63 when appendant upon claim 61, wherein the plurality of heat transfer units (80) are thermally connected in series with the plurality of heat conductors such that the direction of fluid or heat flow in one of the heat conductors is opposite the direction of fluid or heat flow in another of the heat conductors.