WO1990008405A1

WO1990008405A1 - Assembly of electrically interconnected articles

Info

Publication number: WO1990008405A1
Application number: PCT/US1990/000049
Authority: WO
Inventors: Stephen H. Diaz
Original assignee: Raychem Corporation
Priority date: 1989-01-13
Filing date: 1990-01-02
Publication date: 1990-07-26
Also published as: JPH04502682A; EP0453499A1; KR910700553A; AU4967890A

Abstract

An assembly of electrically interconnected articles which are substantially fixed spatially relative to one another, each article having an aperture extending through it, the assembly including a wire which is positioned so that it extends through the apertures in the articles, at least part of the wire being formed from a shape memory alloy which exhibits pseudoelasticity when deformed under an applied stress, and being in contact with, and deformed by, the interior surface of each aperture, so that it presses against the interior surface of each aperture, and forms a connection thereto.

Description

ASSEMBLY OF ELECTRICALLY INTERCONNECTED ARTICLES

The invention relates to an assembly of interconnected articles and to a method of interconnecting articles, the connections being electrical in nature.

It can be necessary to form electrical connections between articles which are fixed spatially relative to one another, for example to supply power to the articles, to connect the articles to ground, to supply electrical signals to the articles or to receive such signals from the articles. It is known to fasten spaced apart portions of an electrically conductive element such as a wire or a bus bar to respective articles, for example by soldering or brazing. However, this has the disadvantages that it requires a high level of skill of the person making the connection, and is somewhat laborious, particularly when the number or density of con¬ tacts on the articles to be interconnected is high. Such interconnection techniques also have the disadvantage that they require the use of a material other than the materials of the articles and the conductive element, which can be undesirable from the point of view of chemical stability, for example towards corrosive agents, and in some applica¬ tions electrical performance.

The present invention is concerned with forming electrical connections between articles, in particular between com¬ ponents of electronic apparatus, without the use of a material other than the materials of the articles and a con¬ ductive element which extends between them.

Components of electronic apparatus, such as capacitors, resistors, transistors, diodes, integrated circuits and memory modules, are frequently mounted on a substrate com- ponent such as a printed circuit board. It is necessary to supply electrical signals or power or both to components, and to receive electrical signals from components. This may be achieved by means of an array of contacts provided on the surface of the components; such contacts are frequently provided around the edge of a substrate component such as a printed circuit board.

In order to optimize the use of space in electronic appara¬ tus, it can be desirable to mount components in layers and to form connections between the facing surfaces of the com¬ ponents. This can be achieved conveniently by forming recesses, or more preferably apertures, in the components, and forming connections between the components and a conduc¬ tive member positioned in the recesses. For example, US-4737114 discloses an electrical contact pin which has a compliant part to be fitted into a plated aperture in a printed circuit board. The compliant part consists of a pair of elongate legs which are joined together at opposite ends and define a slot between them. The legs are deformed by the walls of the aperture in which the pin is positioned and, due to the springy material from which the pin is formed, press against the walls of the aperture.

DS-4746301 discloses an electrical contact which includes a transversely compliant portion having an S-shaped cross- section. The contact is made from an alloy based on copper, nickel or steel. The compliant portion of the contact is arranged so that it is deformed when inserted in an aper¬ ture. For insertion into a small aperture, the contact is formed from a material which can withstand severe plastic deformation. US-3217283 discloses a technique for making connections bet¬ ween contacts in layers of a printed circuit pinboard, in which a wire is threaded through a pinhole in the board, an dis deformed laterally by and from electrical connections to contacts on the walls of the pinhole. The pinholes may be straight and the wires may have laterally extending resi¬ lient bends offset from the axis of the wire, or they may consist of two straight line bores which are formed at an acute angle to a line substantially perpendicular to the surfaces of the board, and the wires may be substantially straight.

Compliant pins of the type disclosed in the above-mentioned patent specifications have the disadvantage that they are difficult to manufacture with the desired level of accuracy, particularly in view of the small sizes involved. Moreover, they generally cannot be reused after they have been deformed beyond their elastic limit as will often occur when they are inserted in relatively small apertures, and the range of sizes of apertures in which the pins can be used without risk of deformation beyond their elastic limit is limited by the low elastic limit of the materials from which compliant pins are conventionally formed.

US-3913444 discloses a fastening pin which may be used to make an electrical bus connection between an stack of printed circuit boards. The pin comprises two tubular mem¬ bers telescoped one inside the other, the inner member comprising a shape memory alloy. When the shape memory alloy is in its martensitic phase, the inner member is held in a radially inwardly deformed configuration by the outer member. When the shape memory alloy recovers to its auste- nitic phase, it overcomes the deforming force exerted by the outer member and expands outwardly, in use to grip the internal surface of a recess or aperture in which it is positioned. A fastening pin of this type has the disadvan¬ tage that it cannot conveniently be made for use in recesses or apertures having a diameter of 200 micrometers or less as are commonly found in printed circuit boards and other com¬ ponents.

According to the present invention, connections between articles are made by means of a wire which extends through apertures in the articles, the wire being formed from a shape memory alloy which exhibits pseudoelasticity, and being so configured that when it is positioned so that it extends through the apertures, it is deformed in a bending mode so that it is pressed against the interior surface of each aperture.

In a first aspect, the invention provides an assembly of electrically interconnected articles which are substantially fixed spatially relative to one another, each article having an aperture extending through it, the assembly including a wire which is positioned so that it extends through the apertures in the articles, at least part of the wire being formed from a shape memory alloy which exhibits pseudoelasticity when deformed under an applied stress, and being in contact with, and deformed by, the interior surface of each aperture, so that it presses against the interior surface of each aperture and forms a connection thereto.

In another aspect the invention provides a method of electrically interconnecting at least two articles which are substantially fixed spatially relative to one another, each article having an aperture extending through it, the method comprising positioning a wire so that it extends through the apertures in the articles, at least part of the wire being formed from a shape memory alloy which exhibits pseudoelasticity when deformed under an applied stress, and being in contact with, and deformed by, the interior surface of each aperture, so that it presses against the interior surface of each aperture and forms a connection thereto.

The invention takes advantage of the high elastic limit of shape memory alloys which exhibit pseudoelasticity. As used herein, the term "elastic limit" denotes the maximum strain that can be imparted to the wire and recovered substantially elastically. The use of a wire formed from such an alloy has the significant advantage that the wire can tolerate a range of sizes of apertures and, in particular, can accom¬ modate the tolerances to which the apertures are formed, which can be as much as 10% in small components. These ranges can be accommodated without exceeding the elastic limit of the wire while also ensuring that the wire presses against the interior surface of each aperture. Furthermore, the effective elastic moduli (as defined below with reference to Figure 1) of materials which exhibit pseudoelasticity are generally lower, even at their elastic limit, than those of conventional spring materials. In the context of the interconnection technique of the present invention, this has the advantage that a wire may be drawn through the apertures in the articles, being deformed as much as 4% or even more as it is so drawn, according to the configuration of the apertures, while exerting a relatively low force on the material lining the apertures. This mini- mizes the wear on that material as the wire is drawn through the apertures. Moreover, the high elastic limit of a pseudoelastic wire allows small diameter wire to be used, and ensures that the wire can be deformed to contact aper¬ tures with a wide range of transverse dimensions. The low elastic modulus of a small diameter wire reduces wear on the material lining the apertures yet further.

Moreover, the high elastic limit of a pseudoelastic wire ensures that the wire can be deformed elastically after it has been deformed during installation. Thus connections that are made according to the present technique can be suf¬ ficiently resilient to tolerate stresses encountered in use, for example due to differential thermal expansion.

The wire may be deformed in such a way that its elastic limit is exceeded. If it is desired that the wire should be reusable, it will then generally be necessary that the non- recovered strain imparted to the wire not be so high that the wire is incapable of making electrical connections to each aperture when so reused.

The configuration and dimensions of the wire, and the nature and characteristics of its pseudoelasticity, will be selected according to the arrangement of the apertures in the articles. As discussed in the paper presented by T.W. Duerig and G.R. Zadno at the International meeting of the Materials Research Society in Tokyo in June 1988, certain alloys are capable of exhibiting pseudoelasticity of two types. "Non-linear pseudoelasticity" arises in appropriately treated alloys while they are in their auste- ni ic phase at a temperature which is greater than M_s and less than M^ (M_s is the temperature at which, when a shape memory alloy in its austenitic phase is cooled, the trans¬ formation to the martensitic phase begins, and ^ is the maximum temperature at which the transformation to the mar¬ tensitic phase can be induced by the application of stress). It is generally required that the alloy be annealed at a temperature which is less than the temperature at which the alloy is fully recrystallized. An article formed from an alloy which exhibits non-linear pseudoelasticity can be deformed substantially reversibly by 8% or more. In contrast, "linear pseudoelasticity" is believed not to be accompanied by a phase change. It is exhibited by shape memory alloys which have been cold worked while in the mar¬ tensitic phase, but have not been annealed in the manner discussed above. An article formed from an alloy which exhibits linear pseudoelasticity can be deformed substan¬ tially reversibly by about 4%.

While the wire that is used to form connections using the present technique may exhibit either of the extreme types of pseudoelasticity, or pseudoelasticity of an intermediate type, it is generally preferred that it exhibit non-linear pseudoelasticity because of the higher elastic limit, and also because the effective elastic modulus which is charac¬ teristic of such deformation is lower, giving rise to the advantages discussed above. In addition, the relatively constant stress exerted by a wire that exhibits non-linear pseudoelasticity when deformed over a certain strain ensures that a relatively constant force is exerted by the wire on an aperture in which it is positioned, the force being rela¬ tively insensitive to the size of the aperture. This allows contact forces and pullout forces to be determined for the wire, these forces being relatively constant over a range of sizes of apertures. For certain applications, however, it will be preferred to use a wire which exhibits linear pseudoelasticity, for example, because of the insensitivity to temperature of the pseudoelastic characteristics of this type of wire.

"Non-linear" pseudoelastic properties may be conferred on the wire by annealing at a temperature below that at which the allow is fully recrystallised. The wire may be provided with a desired configuration, for example generally sinu¬ soidal, by holding the wire in that configuration during the annealing step.

The configuration of the wire may be selected by use of a suitable forming fixture. For example, the wire may be wound around suitably arranged pins, or it may be clamped between a pair of dies having an appropriate configuration.

The material of the wire will be selected according to the desired pseudoelastic characteristics. It will generally be a nickel-titanium based alloy, which may include additional elements which might affect the yield strength that is available from the alloy or the temperature at which par¬ ticular desired pseudoelastic characteristics are obtained. For example, the alloy may be a binary alloy consisting essentially of nickel and titanium, for example 50.8 atomic percent nickel and 49.2 atomic percent titanium, or it may include a quantity of a third element such as vanadium, chromium or iron. Alloys consisting essentially of nickel, titanium and vanadium, such as disclosed in US-4505767, are particularly preferred for some applications. Copper based alloys may also be used, for example alloys consisting essentially of copper, aluminum and nickel, copper, aluminum and zinc, and copper and zinc.

The wire may be provided with a coating. For example, it may be coated with a lubricant to facilitate drawing the wire through the apertures. Preferably it is plated with a metal which has a higher electrical conductivity than that of the pseudoelastic material. Gold is particularly pre¬ ferred.

An additional advantage arising from the high elastic limit of the pseudoelastic wire used in the present invention is that repair is facilitated. Disconnection of the articles is possible simply be removing the wire from the apertures; such removal is possible without damaging the apertures, which allows connections to the article to be remade without any need to repair the apertures. Because of its high elastic limit, the wire is generally able to withstand the deformations imposed on it both during manufacture of the assembly and when the wire is removed, thereby allowing the wire to be reused.

The configuration of the wire will depend on the arrangement of the apertures in that, when the wire is positioned so that it extends through the apertures, it should be deformed so that it presses against the interior of each aperture. The deformation of the wire will be such that the transverse dimension of the wire, measured perpendicular to the axis of the aperture, will be reduced as a result of the wire being constrained within the aperture. The deformation will generally appear to involve bending of the wire, although frequently, more detailed analysis will reveal that the deformation is in a bending mode or a torsion mode or a com¬ bination of the two. For example, when the wire is formed with a helical configuration, reduction of the transverse dimension of the helix will deform the wire in a torsion mode. When the wire is formed with a generally sinusoidal configuration, reduction of the transverse dimension will deform the wire in a bending mode.

The wire may be straight initially, and it may be bent as a result of the deformation. In another embodiment, the wire may be bent initially, and it may be straightened, at least partially, as a result of the deformation. In yet another embodiment, the deformation of the wire may involve a com¬ bination of bending and straightening.

The articles may be positioned one above the other, so that they are in the form of a stack. The articles may, however be positioned side by side so that a wire which extends through the apertures in the holes is ϋ- or S-shaped. It will be understood therefore that the present invention can be used to interconnect articles having a variety of spatial relationships by selection of a wire having an appropriate configuration.

In a preferred embodiment of the assembly of the invention, the articles are positioned one above the other in such a way that a line passing through each of the apertures through which the wire extends is substantially straight. In this event, it will generally be required that the wire is initially bent. Preferably the bend introduced to the wire is such that, when the wire is viewed in elevation, at least part of it appears to have an undulated configuration consisting of a number of peaks or waves on alternating sides of a notional center-line. More preferably, the distance between adjacent peaks in the wire is so arranged that the wire is in contact with the internal surface of the apertures at least at one peak. This may be achieved by arranging the spacing of the articles to b equal to a multiple of the distance between adjacent peaks in the wire, the spacing being the distance between adjacent articles measured from the center of one of the articles to the center of the other article. More preferably, the spacing of adjacent articles is approximately equal to the distance between adjacent peaks in the wire. For example, the wire may be formed as a helix. More preferably however the undu¬ lations can be fitted substantially in a single plane, so that the wire has a generally sinusoidal configuration. This has the advantage that sufficient contact points are established to make a connection, while the force required to draw the wire through the apertures is not so great that the wire wears the material lining the apertures.

The configuration, in terms of longitudinal and transverse dimensions, of the undulations imparted to the wire will be selected according to the number of articles and the spa- cings between them, the length and transverse dimensions of the apertures, and the accuracy with which these dimensions can be defined. When the configuration of the wire is generally undulating, but in a single plane, it is preferred that the wire is in contact with the interior surface of each of at least some of the apertures at the peak of at least one wave. It has been found however that it can be important, particularly when the number of articles to be interconnected is large, that the change in longitudinal dimension of the wire when it is deformed to contact the apertures be maintained as small as possible, consistent with the requirement that the contact be made with the aper¬ tures as desired. It has been found that this can be achieved by keeping the distance between undulations relati¬ vely long, compared with the transverse dimensions of the wire.

The cross-sectional dimensions of the wire and the apertures will affect both the extent to which it is necessary for the wire to be deformed to make contact with the apertures and the frictional forces encountered by the wire as it is drawn through the apertures. Generally, the wire will be circular in cross-section, and for many applications, the apertures will also be circular in cross-section; it has been found that the ratio of the diameter of the wire to the diameter of the apertures will be from about 0.3 to about 0.9; pre¬ ferably the ratio is at least about 0.55, or less than 0.75, or both, especially about 0.65.

The technique of the present invention is applicable to forming connections to articles in which the apertures have a diameter of 200 micrometers or less, for example 100 to 150 micrometers or less. Such articles might be, for example components of electronic apparatus, such as capaci¬ tors, resistors, transistors, diodes, integrated circuits, memory modules and circuit boards on which they are mounted. Preferably at least one of the articles is a printed circuit board, the wire making an electrical connection to electri¬ cally conductive material associated with (for example, lining) the aperture in the board. For these applications. it will generally be preferred, therefore, that the diameter of the wire be about 130 micrometers or less, for example 50 to 120 micrometers or less.

The technique of the invention may also be used to form con¬ nections between larger articles in which the apertures have a diameter of, for example 1 to 10 millimeters or more. Such articles might form parts of apparatus used in the transmission of electrical power or in electric motors etc.

In order to increase the number of points of contact with the articles, more than one wire may be positioned as to extend through the apertures. The wires will preferably be connected to one another at least at one end to facilitate positioning in the apertures.

The assembly may include articles in addition to those articles through which the wire or wires extend. Each article may have several apertures extending through it, of which only some have one or more wires extending through them.

The wire that is used to form connections in accordance with the present invention may be formed continuously in a desired configuration and provided on a reel. The wire con¬ sists of alternating sections which are straight and bent (for example undulating) respectively, which can be separated for use. The wire can be supplied to the assembly by means of apparatus of the type that is used in the inter¬ connection of electronic components by thermocompression wire bonding, in which wire is supplied through a bonding head. Preferably the bonding head includes means for severing the wire to a desired length, for example as disclosed in WO-A-88/04829. The use of wire in the connec¬ tion technique of the present invention has the significant advantage over discrete pins that continuous assembly tech¬ niques can be used in which wire is supplied from a reel, in contrast to techniques in which discrete connection pieces are supplied and positioned individually.

Preferably, the wire comprises a first portion which is straight and a second portion which is adjacent to the first portion and which appears to have an undulated configuration when viewed in side elevation. More preferably the length of the straight first portion is at least about equal to the distance between the articles at opposite ends of the assembly. This facilitates a preferred technique for posi¬ tioning the wire so that it extends through the apertures, which includes the steps of:

(a) threading the straight portion of the wire through the apertures;

(b) applying a longitudinal force to the wire so as to reduce the major width of the undulated portion;

(c) positioning the wire so that the undulated portion extends through the apertures; and

(d) removing the longitudinal force to allow the major width of the undulated portion of the wire to increase, so that it presses against the interior surface of the apertures. This technique for inserting the wire in the apertures has the advantage that contact between the wire and the internal surfaces of the apertures is minimized during insertion, thus reducing abrasion and wear of the materials of the wire and the internal surfaces of the apertures. It is also par¬ ticularly suitable for positioning the wire in apertures in spaced apart articles, whether or not the apertures are arranged so that a line passing through them is straight.

Preferably, the wire is deformed in a bending mode as a result of the application of a load for moving the wire through the apertures.

The invention will now be described further with reference to the accompanying drawings, in which:

Figure 1 shows the stress-strain behaviour on loading and subsequent unloading of wires made from conventional elastic material, linear pseudoelastic material and non¬ linear pseudoelastic material;

Figures 2a, 2b," 3a and 3b"show two forms of wire, each in side and end elevation, for forming connections in accor¬ dance with the present invention; and

Figure 4 shows a assembly of printed circuit boards that are interconnected by means of wires.

Referring to the drawings. Figure 1 is a graph which shows how the stress that is applied to each of three wires is related to the resulting strain during a single loading and unloading cycle. The stress is applied to a straight wire. and the imparted strain is in the stretching mode, rather then a bending mode.

The portion of Figure 1 marked (a) arises from a heat- treated wire that is formed from a beryllium-copper alloy on cycling to its elastic limit A.

The portion of Figure 1 marked (b l arises from a wire that is formed from a nickel-titatium alloy, containing 50.8 ato¬ mic percent nickel and 49.2 atomic percent titanium. The wire was formed by cold drawing at room temperature. The upper line shows the stress-strain behavior on loading to the- elastic limit B, and the lower line shows that behavior on unloading.

The portion of Figure 1 marked (c) arises from a wire which has been produced by the technique used for the wire used to produce Figure lb; with the addition of an annealing step after cold drawing. Annealing the wire at 500°C for two minutes in a salt bath enables the wire to transform from the austenitic phase to the martensitic phase when stress is subsequently applied. The upper line in Figure lc shows the stress-strain behavior on loading to the elastic limit C, and the lower line shows that behavior on unloading.

Figure 1 enables comparisons to be made between the elastic properties of the three materials, in particular between the imparted strain at the elastic limit, and the effective elastic modulus (defined as the ratio of the applied stress at the elastic limit to the imparted strain), as follows:

B

Imparted strain 0.8% 4% 8% Effective elastic modulus 1.28xl0⁵ 2.9xl0⁴ 1.03xl0⁴ (

It is to be understood that the data represented in Figure 1 is for stretching rather than bending of the wires, but the behavior in bending closely follows the behavior in stretching.

Figure 2 shows a wire 3 having a round cross-section of which a central portion 5 has a helical configuration. When viewed from one side, the configuration appears to be undu¬ lating. The wire is extended to make connections to aper¬ tures in articles, the internal diameter of the apertures being less than the external diameter of the helix so that, when portions of the wire are positioned in the apertures, those portions are deformed in such a way that their exter¬ nal diameter is reduced, and the wire contacts the internal surfaces of the apertures.

Figure 3 shows a wire 7 of which a central portion 9 has an undulating configuration, such that the undulations can be fitted in a single plane which, as shown in Figure 3a, is in the plane of the paper. The configuration of the wire can thus be considered as including a plurality of waves, or as a zig-zag. The wire is intended to make connections to apertures in articles, the centers of the apertures being substantially aligned on a common axis and the internal diameter of the apertures being less than the transverse distance between adjacent waves of the wire so that, when portions of the wire are positioned in the apertures, those portions are deformed such that the transverse distance is reduced, and the wire contacts the internal surfaces of the apertures.

Figure 4 shows an assembly of electronic components, which comprises components 11 such as resistors, capacitors, integrated circuits and the like, mounted on substrate com¬ ponents such as printed circuit boards 13. The boards 13 are fixed spatially relative to one another by means of a frame (not shown). Each board has apertures 17 extending through it. Wires 19 extend through the aligned apertures in adjacent boards. The wires have an undulating con¬ figuration which is such that they are deformed inwardly and constrained by the apertures, when positioned such that they extend through the apertures. At least some of the aper¬ tures 17 are lined with conductive material through which an electrical connection is made between the wire and a com¬ ponent 11 on a board 13, or a contact 21 through which an electrical signal or power can be supplied to, or received from, the assembly.

EXAMPLE 1

An alloy consisting of 50.8 atomic percent nickel and 49.2 atomic percent titanium was formed into a wire having a cir¬ cular cross-section with a diameter of 1.83 mm, and a degree of cold work of 31%. A length of the wire was fitted onto a forming fixture which imparted an undulating configuration to the wire, generally as shown in Figures 3a and 3b." More specifically, a central portion of the wire had a generally sinusoidal shape, consisting of six equal periods (so that there were twelve peaks), each period having a wavelength of 18.3 mm and an amplitude (measured from the axis) of 0.89 mm. The resulting major width of the wire was therefore 3.61 mm. Each end portion of the wire was substantially straight.

The wire was heat treated in a salt bath at 500°C for 120 seconds. The wire was removed from the bath and allowed to cool to room temperature. It was then removed from the forming fixture. The wire was found to exhibit non-linear pseudoelastic behavior.

The wire was stretched longitudinally until its major width was reduced to 2.54 mm. As a result of the stretching, the wavelength of the periods was increased to 19.4 mm. Further stretching resulted in a reduction of the major width to 2.29 mm and an increase in the wavelength to 19.9 mm. The wire was used to form connections between six rigid metal plates which were mounted in a stacked relationship by means of threaded fasteners and insulating spacers. The thickness of each of the plates was 5.08 mm. A circular aperture was formed in each plate, the apertures in five of the plates having a diameter of 2.29 mm, and the aperture in the sixth plate having a diameter of 2.54 mm. The plates were mounted so that the axes of the apertures were aligned, and the distance between the surfaces of adjacent plates was 4.75 mm.

An end section of the wire was threaded through the holes in the plates. The wire was then stretched longitudinally until the major width of the wire was about 2.16 mm and the wavelength was about 21.5 mm. The wire was moved through the apertures while so stretched until the central portion of the wire was positioned within the apertures in the plates.

Electrical continuity between the wire and each of the plates was monitored as the axial tension on the wire was slowly reduced, causing the wire to shorten and its major width to increase. Electrical contact was established bet¬ ween the wire and each of the five plates in which the diameter of the aperture was 2.29 mm. Further reduction of the tension of the wire resulted in electrical contact bet¬ ween the wire and the sixth plate. The remaining axial ten¬ sion was then removed. It was found that the wire was held firmly in the apertures in the plates as a result of the laterally extending peaks pressing against the apertures. Electrical continuity was maintained between the wire and each of the six plates.

This example demonstrates the ability of the connections technique of the present invention to tolerate apertures having a range of sizes. This can be important when the technique is used to form connections to small articles, in which the manufacturing tolerances can be at least 5%.

EXAMPLE 2

It is desired electrically to interconnect six printed cir¬ cuit boards arranged in a stacked relationship. The thickness of each board is 0.38 mm, and the boards are spaced 0.38 mm apart. Each board has a number of apertures extending through it, the apertures being plated internally to an internal diameter of 0.15 mm. The boards are posi¬ tioned as that the axes of the apertures are aligned. A wire having a diameter of 0.11 mm formed from the nickel titanium alloy used in Example 1, and being plated with gold, is fitted to a forming fixture which imparts to a central section of the wire an undulating configuration, in which approximately 50° arcs are formed on alternate sides of the longitudinal axis. The internal radius of each arc is about 0.68 mm, and the external radius is about 0.79 mm. The wavelength of the resulting generally sinusoidal shape is about 1.14 mm.

Once applied to the forming fixture, the wire is annealed so as to render it pseudoelastic. When such a wire is stretched such that its major width is 0.15 mm, its wave¬ length will be increased to about 1.52 mm; adjacent peaks will therefore be about 0.76 mm apart, which corresponds exactly to the distance between the centers of the apertures in adjacent boards. When the wire is positioned, under longitudinal tension, so that it extends through the aper¬ tures in the boards and the tension is removed, the wire will press against the center of each aperture at about a peak. The force which the wire exerts on the aperture will be from about 4 to about 20 grams.

Claims

CLAIMS:

1. An assembly of electrically interconnected articles which are substantially fixed spatially relative to one another, each article having an aperture extending through it, the assembly including a wire which is posi¬ tioned so that it extends through the apertures in the articles, at least part of the wire being formed from a shape memory alloy which exhibits pseudoelasticity when deformed under an applied stress, and being in contact with, and deformed by, the interior surface of each aperture, so that it presses against the interior sur¬ face of each aperture, and forms a connection thereto.

2. An assembly as claimed in claim 1, in which at least one of the articles is a printed circuit board, the wire making an electrical connection to electrically conduc¬ tive material associated with the aperture in the board.

3. An assembly as claimed in claim 1 or claim 2, in which the wire is plated with a metal which has an electrical conductivity which is higher than that of the pseudoelastic material of the wire.

4. An assembly as claimed in any one of claims 1 to 3, in which at least a portion of the wire appears to have an undulated configuration when viewed in side elevation.

5. An assembly as claimed in claim 4, in which the undula¬ tions can be fitted substantially in a single plane, so that the wire is formed with a generally sinusoidal con¬ figuration.

6. An assembly as claimed in claim 4 or claim 5, in which the wire is in contact with the interior surface of each of at least some of the apertures at the peak of at least one wave.

7. An assembly as claimed in claim 6, in which the distance between adjacent peaks in the wire is approximately equal to the spacing of adjacent articles.

8. An assembly as claimed in any one of claims 1 to 7, in which a line, passing through each of the apertures through which the wire extends, is approximately straight.

9. An assembly as claimed in any one of claims 1 to 8, in which the wire is circular in cross-section.

10. An assembly as claimed in claim 9, in which the aper¬ tures are circular in cross-section, the ratio of the diameter of the wire to the diameter of the apertures being from about 0.3 to about 0.9.

11. An assembly as claimed in any one of claims 1 to 10, in which the diameter of the wire is from about 50 to about 130 micrometers.

12. An assembly as claimed in any one of claims 1 to 11, in which at least two of the wires are positioned so that they extend through the apertures in the articles.

13. An assembly as claimed in any one of claims 1 to 12, in which a first portion of the wire is straight and an adjacent second portion of the wire appears to have an undulated configuration when viewed in side elevation.

14. An assembly as claimed in claim 13, in which the length of the straight first portion of the wire is at least about equal to the distance between the articles at opposite ends of the assembly.

15. An assembly as claimed in any one of claims 1 to 14, in which more than one wire extends through the apertures in the articles.

16. A method of electrically interconnecting at least two articles which are substantially fixed spatially rela¬ tive to one another, each article having an aperture extending through it, the method comprising positioning a wire so that it extends through the apertures in the articles, at least part of the wire being formed from a shape memory alloy which exhibits pseudoelasticity when deformed under an applied stress, and being in contact with, and deformed by, the interior surface of each aperture, so that that it presses against the interior surface of each aperture, and forms a connection thereto.

17. An method as claimed in claims 16, in which the articles are arranged so that a line, passing through each of the apertures through which the wire extends, is substan¬ tially straight.

18. A method as claimed in claim 16 or claim 17, in which the wire is positioned so that it extends through the apertures by application of a longitudinal force by which the wire is drawn through the apertures.

19. A method as claimed in any one of claims 16 to 18, in which at least a portion of the wire appears to have an undulated configuration when viewed in side elevation.

20. A method as claimed in claim 19, in which a first por¬ tion of the wire is straight and an adjacent second por¬ tion appears to have an undulated configuration when viewed in side elevation.

21. A method as claimed in claim 20, which includes the steps of:

(a) threading a straight portion of the wire through the apertures;

(d) removing the longitudinal force to allow the major width of the undulated portion of the wire to increase, so that it presses against the interior surface of the apertures.