WO1988004962A1

WO1988004962A1 - Apparatus and method for determining surface ionic contamination levels of electronic assemblies such as printed circuit assemblies

Info

Publication number: WO1988004962A1
Application number: PCT/US1987/003490
Authority: WO
Inventors: Jack Brous
Original assignee: Fry Metals Inc.
Priority date: 1986-12-24
Filing date: 1987-12-22
Publication date: 1988-07-14

Abstract

A method and apparatus for determining the surface ionic contamination level on an electronic assembly utilizes deionized, heated solvent mixtures consisting essentially of water and a water miscible solvent which, when heated well above room temperature, has a vapor pressure substantially less than that of water at the same temperature. A solvent mixture is pumped from a solvent reservoir (12) by the centrifugal pump (26), heated in a heater (30) and sprayed through conduits (18), (29), (24) and (20) on said electronic assembly (14). The water miscible solvent acts quickly on the electronic assembly (14) to dissolve and to remove the non-ionic residues, thereby exposing the ionic contaminants such that they may be dissolved and removed from said assembly (14). The aqueous solution (12) flows from the enclosure (10) to a dynamic contamination measurement apparatus (34) for the determination of the ionic contamination level in the assembly (14), then returns to the solvent reservoir (12). A change in an electrical property of the mixture, such as conductivity or resistivity, due to the presence of the dissolved ionic contaminants is measured to obtain an indication of the ionic contamination level originally present in said electronic assembly (14).

Description

TITLE: APPARATUS AND METHOD FOR DETERMINING SURFACE IONIC CONTAMINATION LEVELS OF ELECTRONIC ASSEMBLIES SUCH AS PRINTED CIRCUIT ASSEMBLIES

SPECIFICATION

Field of the Invention

This invention relates generally to the manufacture of electronic assemblies and is more particularly concerned with a method and apparatus for determining levels of residual surface ionic contamination of electronic assemblies such as printed circuit assemblies. The invention is especially useful for determining ionic contamination levels of printed circuit assemblies having surface mounted electronic components.

Background of the Invention

Printed circuit assemblies coming off the production line ordinarily carry surface residues of a variety of chemical agents used in their manufacture. These residues usually include deposits of highly ionic agents such as fluxing activators, and solutions employed in plating, etching, and other metal treatment operations. They also include deposits of non-ionic (or perhaps more precisely, weakly ionic) agents, such as rosin (a solder fluxing agent) and certain soldering oils. The ionic residues are of special concern since, if present in sufficient concentration, they can adversely affect both service life and operation of a circuit assembly. For example, these residues may corrode metal portions of the assembly, resulting in reduced service life. Ionic residues may also set up electrical leakage currents between insulated points of the circuit and thereby cause operational degredation or malfunction.

For applications requiring long service life and high operational reliability (e.g., military applications), printed circuit assemblies must be cleaned to bring concentrations of ionic residues within acceptable safety limits (see, e.g., Military Specification MIL-P-28809). This is commonly accomplished by washing the assemblies with cleaning solvents which dissolve the potentially harmful residues and thereby remove them from board and electronic component surfaces. However, cleaning operations do not achieve complete residue removal and may in some cases add ionic residues to the surfaces. In order to make certain that finished printed circuit assemblies are sufficiently free of ionic contaminants to meet applicable specifications, electronics manufacturers must often measure residual ionic contamination levels after cleaning has been completed.

Description of the Prior Art

Measurement of surface ionic contamination on a finished circuit assembly requires that the contaminant residues be extracted from the circuit board and component surfaces and then quantified. Due to the small traces of contaminants involved, the measurement process must be highly sensitive. Accuracy also demands that the extraction of contaminants be as thorough as possible. Perhaps the most widely used technique for determining ionic contamination levels involves electrical conductivity measurement of a deionized solvent liquid which is used to extract the ionic contaminants from the circuit assembly. The solvent washes the openly exposed surfaces of the assembly and penetrates spaces between electronic components and the circuit board surface to dissolve and thereby remove ionic contaminant residues. The conductivity of the solvent is highly sensitive to the presence of the dissolved ionic contaminants, and the ionic contamination level of the assembly may thus be determined by measuring changes in the solvent conductivity due to the dissolved ions. Resistivity, the reciprocal of conductivity, may similarly be used for quantification of ionic contaminants.

The generally preferred medium for extraction and measurement of ionic residues is water. However, because ionic contaminants may be entrapped in non- ionic, non-water soluble residues (such as rosin), conventional measurement systems utilize a mixture of isopropanol and water as the extraction/measurement medium. The mixture is applied to the circuit assembly usually by immersion of the assembly at room temperature. The isopropanol, which is of relatively low polarity, dissolves the non-ionic residues (which do not appreciably affect conductivity) so that the water component may reach and dissolve entrapped ionic contaminants. For additional background on ionic contamination measurement systems the reader is referred to Applicant's paper entitled "Extraction Methods for Measurements of Ionic Surface Contamination," Surface Contamination, Vol. 2, 1979; Applicant's prior U.S. Patent 3,973,572 issued August 10, 1986; and U.S. Patent 4,023,931 issued May 17, 1977 to Wolfgram (all of which are incorporated herein by reference).

A major factor affecting the accuracy of ionic contamination measurement of printed circuit assemblies is the ability of the extraction/measurement solvent mixture to penetrate the spaces between the electronic components and the surface of the circuit board. Until recently, electronic components were most commonly mounted on a circuit board by inserting leads of the components through corresponding metallized holes in the board and then soldering the leads to the metallization. Component-to-board clearances were generally about .25 mm (.01 in.) or more, which was sufficient to permit free penetration of ionic extraction/measurement solvents between the components and the underlying board surface.

Lately, there has been a rapidly growing trend toward the use of surface mounted electronic components which have very small component-to-board clearances in comparison with conventional leaded components. Surface mounted components may, for example, have very short, flattened leads which are soldered directly to underlying metallized pads on a board surface, with the leads supporting the component body almost flush with the board. Alternatively, they may have metallized contact surfaces which are soldered directly to corresponding pads on the board with minimal clearances being maintained, for example, by spacing elements (called "standoffs") incorporated into solder pastes used to mount and solder the components to the board. Typically, surface mounted component-to-board clearances are only about .05 mm (.002 in.). It has been found that the presence of surface mounted components in electronic circuits severely impairs the performance of conventional isopropanol/water systems for ionic contamination measurement. In particular, the small component-to- board clearances make it much more difficult for solvents to penetrate beneath the surface mounted components, thereby greatly reducing the efficiency of extraction of ionic contaminants from such spaces. For some circuits, especially those with surface mounted components of large surface area, complete extraction and thus accurate measurement within a reasonable period of time can actually be impossible.

Although it is known to use agitation (such as stirring) in isopropanol/water systems in order to enhance penetration of the solvent mixture, only minimal agitation may safely be used due to the need to avoid excessive error from atmospheric CO₂ interference. CO₂ interference results from the absorption of atmospheric CO₂ by the solvent mixture. In particular, absorbed CO₂ combines with the water component to form carbonic acid which dissociates into constituent ions in the solvent mixture. These ions change the conductivity of the solvent mixture and obscure the actual change in conductivity due to dissolved ionic contaminants. Whereas mild agitation does not improve the performance of conventional measurement systems sufficiently to make them truly practical for circuit assemblies with surface mounted components, vigorous agitation of the solvent mixture causes CO₂ absorption rates to increase and become erratic, thus making it impossible to account for CO₂ interference. Some currently available measurement systems do not even take CO₂ interference into account. Hence, there is a need for an improved technique for ionic contamination measurement which is effective with printed circuit assemblies having surface mounted components.

Summary of the Invention

In contrast to the prior art, the present invention offers a technique for ionic contamination measurement which provides prompt and accurate results even when applied to electronic circuits having surface mounted components and which notably avoids the problem of CO₂ interference described above. In accordance with the discovery of the invention, the foregoing advantages are obtained through a novel heated-solvent ionic extraction/measurement process, which will be described hereinafter in detail.

Briefly stated, in one of its broad aspects, the invention provides a method for determining the surface ionic contamination level of an electronic assembly, such as a circuit board having surface mounted electronic components soldered thereon, the assembly carrying residues of ionic and non-ionic contaminants. The method utilizes a solvent mixture consisting essentially of water and a water miscible solvent which, when heated to a temperature well above room temperature, is a solvent for the non-ionic contaminants and has a vapor pressure substantially less than that of water at the same temperature. The solvent mixture is heated (well above room temperature) and contacted in a deionized state with the electronic assembly to dissolve the ionic and non-ionic contaminants, until the assembly is rendered substantially free of the contaminant residues. The change in an electrical property of the solvent mixture due to the presence of the dissolved ionic contaminants is measured to determine the ionic contamination level of the assembly.

In another of its broad aspects, the invention provides a system for implementing the foregoing method. The system comprises a supply of solvent mixture constituted as outlined above, means for heating the solvent well above room temperature, means for contacting the heated solvent mixture in a deionized state with the electronic assembly, whereby the contaminant residues are dissolved in the heated solvent mixture, means for measuring a change in an electrical property of the contacted mixture due to the presence of the dissolved ionic contaminants therein, and means for supplying the contacted solvent mixture to the measuring means.

Further details and advantages of the invention will become apparent in the following description of the preferred embodiments taken in conjunction with the accompanying drawing.

Brief Description of the Drawing

The accompanying figure is a schematic diagram of a system for determining ionic contamination levels of electronic assemblies in accordance with the present invention.

Detailed Description of the Preferred Embodiments

The prior art has sought to improve the effectiveness of conventional isopropanol/water ionic contamination measurement systems by solvent agitation. The present invention is based on an entirely different approach — specifically, the use of high temperature solvent mixtures. Basically, the extraction/measure- ment solvents of the invention are mixtures which consist essentially of water and a water-miscible solvent which has a vapor pressure substantially less than that of water at temperatures well above room temperature (i.e., well above 25°C, 77°F) and which is a solvent for non-ionic contaminants (especially rosin) at such temperatures. By resorting to such mixtures at high temperature, the invention achieves dramatically improved results in comparison with prior ionic contamination measurement techniques. The ensuing description will first address the rationale behind the high-temperature extraction/measurement technique of the invention and will then address considerations pertinent to actual implementation of the invention (solvents, temperature, etc.). Thereafter, an ionic contamination measurement system according to the invention will be described.

The use of heated solvents in accordance with the invention is directed toward enhancing the extraction capabilities of the extraction/measurement solvent mixture per se, rather than to increasing the physical contact between the solvent mixture and contaminant residues, as is the case with agitation (although the use of agitation is consistent with the invention and preferred in the practice thereof) . High temperature enhances solvent properties in a number of important respects. For example, increased temperature reduces surface tension to provide a higher penetration capability for tight spaces, such as the small clearances between surface mounted electronic components and circuit boards. High temperature also lowers solvent viscosity, further improving penetration capability. Additionally, solvency for both ionic and non-ionic residues is substantially greater at high temperatures, as is thermal diffusion of contaminated solvent (i.e., the tendency of contaminants to diffuse within the solvent from higher to lower concentration levels). High solvent temperatures also provide certain ancillary benefits which further improve the extraction/measurement process. Specifically, the viscosities of residues such as rosin are substantially reduced at elevated temperatures, thus allowing for significant increases in solution rates. Furthermore, because carbonic acid is unstable at high temperatures, vigorous solvent agitation such as high velocity spraying may be used without adverse effects on measurement accuracy from CO₂ interference. In order to maximize the foregoing advantages, the ionic extraction/measurement solvent mixture should be heated as much as possible without danger to the electronic assembly being evaluated or to ionic contamination measurement equipment. Generally speaking, material-safety considerations will limit solvent temperatures to about 77°C (170°F). At higher temperatures semiconductor devices and certain plastics used in electronic circuits may be damaged. Also, the efficiency of ionic exchange resins used for deionizing the extraction/measurement solvents may be reduced, and water losses through evaporation may become excessive. Preferably, the solvent temperature should be limited to about 160 °F or less in order to avoid such problems. Insofar as minimum temperature is concerned, a temperature of at least about 49°C (120°F) should be used in most applications. This temperature will usually assure good extraction capability of the solvent mixture and prevent appreciable CO₂ interference. In practice, a minimum temperature of about 54°C (130°F) is preferred in order to provide greatly enhanced residue extraction capability of the solvent while also avoiding CO₂ interference. The most highly preferred solvent temperature range is from about 60°C (140°F) to about 66°C (150°F), based on considerations of equipment safety, residue extraction capability, and measurement accuracy.

Conventional isopropanol/water solvent mixtures are not practical at the above-described temperatures due to the high volatility of isopropanol. Although water is readily usable at such temperatures, evaporative losses of isopropanol would be excessive. Furthermore, isopropanol vapors might pose a danger to measurement technicians. The invention therefore employs a solvent component other than isopropanol for dissolving the non-ionic, non-water soluble residues, such as rosin, which may entrap ionic contaminants to be measured In practice of the invention, the selection of a solvent for the foregoing purpose should be consistent with four basic criteria:

1. The solvent should be capable, in the heated state, of dissolving the non-ionic contaminant residues present on an electronic circuit board most especially rosin.

2. The solvent should be fully miscible with water.

3. The solvent should exhibit a volatility substantially lower than that of water at the operating temperature in order to minimize evaporative losses.

4. The solvent should have dielectric characteristics which are sufficient to allow significant registration of ions in a measurement of conductivity or other electrical property (e.g., resistivity) of the solvent/water mixture.

Additional desirable characteristics for the solvent include the following: l. The solvent should not attack or otherwise adversely affect the materials of electronic assemblies or ionic contamination measurement equipment.

2. Solvent odor should be minimal at the operating temperature. 3. The solvent should present minimal toxicity risk.

4. The solvent should have a flash point above the operating temperature.

5. The solvent should have low viscosity and surface tension at the operating temperature to facilitate pumping and vigorous agitation such as high velocity spraying.

Based on consideration of all of the above factors, glycol ethers are the preferred solvents for practice of the invention. Especially preferred among the glycol ethers are ethylene glycol butyl ether, diethylene glycol butyl ether, diethylene glycol ethyl ether, and dipropylene glycol methyl ether, with the last-mentioned solvent being the most preferred. Table I lists important properties of the preferred glycol ethers, as well as corresponding properties of water for purposes of comparison.

All of the glycol ethers listed in Table I have excellent solvent power for rosin and other non-ionic contaminants typically found on printed circuit assemblies and are also fully miscible with water. As will be appreciated from Table I, these solvents also have very low vapor pressures in comparison with that of water. Note in addition the high flash points ranging from 74°C to 100°C (165°F to 212°F), the low viscosities ranging from 3.15cs to 5.17cs at 25°C (77°F), and the low surface tensions ranging from 27.4 dynes/cm to 31.8 dynes/cm. Glycol ethers also offer an additional advantage in that they generally dry without leaving a residue.

The choice of dipropylene glycol methyl ether as the most preferred solvent is based largely on its suitability for the workplace. In particular, this material has little odor in the temperature range contemplated by the invention and presents little or no toxicity hazard according to present knowledge. The current TLV (threshold limit value) for this material is 100 ppm, which may quite easily be satisfied in practice considering the material's low vapor pressure. In addition, dipropylene glycol methyl ether is believed to have the best overall balance of the various characteristics enumerated earlier.

The relative proportions of components in the solvent mixtures used in the invention can vary depending upon the characteristics of the residues on the particular circuit assembly being evaluated. The selection of proportions for any given situation can be determined empirically with minimal testing. As a general guideline, the solvent mixtures should usually contain water from about 20% to about 60% by volume, with the remainder of the mixture being constituted by the water miscible solvent component. Less than about 20% water may not provide sufficient sensitivity to dissolved ionic contaminants for accurate conductivity measurement. More than about 60% water may reduce the penetration capability for tight spaces (water itself being a relatively poor penetrant with a high surface tension above 70 dynes/cm) and may reduce the solubility of non-ionic residues such as rosin, so that extraction efficiency becomes less than desired. Mixtures having a solvency for rosin of at least about 1% by weight (i. e., .01 gm rosin per gm solvent mixture) will generally be effective to achieve thorough extraction of ionic contaminants, including those entrapped in residual rosin deposits. In the case of glycol ethers, 50/50 mixtures by volume will usually provide good results, but it may often be advantageous to reduce the mixture ratio to about 25% water/75% water miscible solvent in order to increase solvency for rosin. Table II shows the change in conductivity of 50/50 mixtures (by volume) of water and the preferred glycol ethers at 66°C (150°F) with sodium chloride additions of 10 μg per ml of mixture, thus demonstrating the sensitivity of the mixtures to dissolved ionic contaminants. For comparison, the corresponding information for deionized water is given.

Table III illustrates the greatly improved solvency for non-ionic residues attained by the present invention. In particular, Table III shows the results of comparative rosin extraction time tests for a heated, deionized mixture containing, by volume, 75% dipropylene glycol methyl ether (specifically, DOWANOL DPM available from Dow Chemical Company) and 25% water versus a 75/25 isopropanol/water mixture. For the purposes of each test, a glass microscope slide was prepared by applying seven drops of Alpha Metals Alpha- 100 flux (rosin - 40% in isopropyl alcohol) near one end of the slide, with the slide being heated on a hot plate. Evaporation of the alcohol left a fused rosin mass which was cooled to room temperature to yield a hardened lump of rosin. The slide was immersed in a beaker of extraction solvent which was connected to an Alpha Metals lONOGRAPH ionic contamination measurement system. A system of this type is described in Applicant's earlier mentioned patent and is sufficiently sensitive to detect accurately the very weak ionization of rosin and any trace ionic impurities. The solvent mixture was circulated between the beaker and the lONOGRAPH by an internal pump of the lONOGRAPH, with the lONOGRAPH monitoring the instantaneous conductivity of the solvent mixture. When the conduc- tivity returned to a baseline level, established for the mixture only, extraction was complete. Each slide was inspected to confirm the results.

Having explained the principles underlying the invention and the basic considerations pertinent to its implementation, it is now appropriate to consider an exemplary system for carrying out the invention. The accompanying drawing illustrates a system for this purpose. The illustrative system includes a tank 10 and a supply of extraction/measurement solvent 12 in the bottom portion of the tank. Solvent 12 is a solvent mixture of the type described earlier (e.g., 75% DOWANOL DPM/25% water by volume). A printed circuit assembly 14 to be tested is supported above the solvent supply 12 by a suitable rack or the like (not shown) with opposite sides of the circuit facing two high velocity sprayers 16, respectively. Each sprayer 16 is constructed to direct a two-dimensional array of high velocity sprays at the facing surface of circuit 14, so that contaminants may be extracted from both sides of the circuit. The high velocity sprays are indicated generally by arrows directed toward the opposite sides of circuit 14. in the form shown, sprayers 16, which rest on the bottom of tank 10, are each formed by a pair of plates 20, 22 spaced by a peripheral spacer 24 to which the plates are sealed. Each plate 22 is perforated to provide a two-dimensional array of spray outlets such that solvent supplied under pressure to the sealed chamber between plates 20, 22 will be discharged through the perforations as an array of high velocity sprays. In a practical embodiment, sprayers 16 may be spaced about 13 cm (5 in.) apart. Plates 22 may be about 30 cm high by 15 cm long by 1.6 mm thick (12 x 6 x 1/16 in.), with a 5 mm (3/16 in.) spacing, and made of aluminum. The perforations in plates 22 may be about 1 mm (.04 in.) in diameter and spaced 2.5 cm (1 in.) on center. A centrifugal pump 26 delivers solvent to sprayers 16 at a high flow rate such as 84 liters per minute (22 gpm) by way of a fluid line 28 connected between the bottom of tank 10 and a fluid line 29 which feeds inlets to sprayers 16, as shown. Pump 26 draws solvent from the bottom of tank 10 and pumps the solvent through a thermostatically controlled in-line heater 30, connected in line 28 at the pump outlet side, which heats the solvent to the desired temperature (e.g., 60°C, 140°F) for spraying onto circuit 14, after which the solvent collects in the bottom of tank 10. The high velocity sprays issuing from sprayers 16 are effective to wash the openly exposed surfaces of circuit 14 and to achieve thorough solvent penetration between the board and associated electronic components of the circuit, including any surface mounted devices.

To measure the ionic contamination removed from circuit 14, a dynamic ionic contamination measurement system 34 of the type described in Applicant's earlier mentioned patent is connected to tank 10 by fluid lines 30, 32. The measurement system may, for example, be an Alpha Metals lONOGRAPH Model 500. Basically, this system includes a pump, a conductivity measurement/integration stage, and a deionization stage, as indicated diagrammatically by dashed lines in the drawing. The lONOGRAPH pump draws fluid from the tank through the measurement/integration stage, which measures instantaneous conductivity of the solvent and integrates the conductivity over time. The solvent then passes through the deionization stage for return to tank 10 and reapplication to circuit 14. The integrated conductivity change is proportional to the total residual ionic contamination extracted from the circuit, and this total can readily be converted to contamination per unit surface area given the area of the circuit board.

It will be appreciated that although the illustrative apparatus employs the so-called dynamic extraction/measurement approach, in that solvent mixture from tank 10 is deionized, contacted with the circuit board, and measured for conductivity change in a continuous cycle, the use of high temperature solvents and high solvent agitation in accordance with the invention is equally applicable to the so-called static approach. In the latter approach, a fixed volume of solvent is used to extract the ionic residues, and the conductivity change of the entire volume is measured to determine the ionic contamination level (see, e.g., the Wolfgram patent). The reader is referred to Applicant's earlier mentioned paper for a discussion and a comparison of these approaches.

A system of the type described above but having only one sprayer was tested for comparative extraction efficiency against a Fry Metals ICOM-3000 system which utilizes a pressurized spray in air of a 75/25 isopropanol/water mixture (by volume) at room temperature. For purposes of the experiment, a test board arrangement was specially designed for evaluating the effectiveness of the systems in removing solder-flux residues from capillary spaces beneath surface mounted components. The test board arrangement was composed of a 127 x 254 x 1.6 mm (5 x 10 x 1/16 in.) FR-4 epoxyfiberglass sanel having an array of thirty-two 25.4 x 25.4 x 1.6 mm (1 x 1 x 1/16 in.) chips of the same material soldered to one of its surfaces. One face of each chip had a 2.5 mm (1/10 in.) square of copper metalization at each of its four corners, and the panel had metalized pads for four rows of eight chips to be soldered thereto at 5 mm (2/10 in.) spacings.

In preparation for soldering, the chips and panel were thoroughly cleaned of ionic contamination and then dried. Specifically, the chips and panel were cleaned for 30 seconds in a vapor degreaser using a boiling mixture of methylene chloride and fluorocarbon 113.

The components were then cleaned in the system of the invention for 30 minutes (75/25 DOWANOL DPM/water mixture at 60°C). The components were then dried in an air oven at 49°C (120°F) for 30 minutes. Next, three drops of Alpha 811 RA flux (available from Alpha

Metals, Inc.) were deposited in the center of the aforementioned face of each chip, and the chips were air dried and then heat dried in an air oven at 49°C (120°F) for 30 minutes. The corner pads of the chips were then soldered to corresponding pads of the panel using a clean 63/37 tin/lead solid solder wire and a clean soldering iron, without additional flux. The previously applied flux deposits were thus sandwiched between the chips and the panel. The chip-to-panel clearance was set at .076 mm (.003 in.) using a stainless steel shim.

For each test run, the measurement system of the invention was operated to bring the solvent temperature to 60°C (140°F), at which point a board was placed about 6.3 cm (2-1/2 in.) from the sprayer with the chip-bearing side facing the spray outlets, which discharged 42 liters of solvent per minute (11 gpm).

The Fry system was operated in the normal manner, except that each run was allowed to proceed for 10 minutes, which was well past the system's indication of complete extraction. After testing, each board was disassembled and all of its components were placed in the inventive system for a second measurement run which determined the remaining contamination on the board.

The extraction efficiency of the first test run (in the inventive system or the Fry system) was then determined as follows: extraction = first reading x 100 efficiency sum of first and second readings

Table IV demonstrates the remarkable improvement in extraction efficiency (and consequently measurement accuracy) achieved by the invention over conventional isopropanol/water measurement systems. Also shown in Table IV is the result of an additional test in which a room-temperature isopropanol/water mixture was used with submerged jets and a chip spacing of .25 mm (.01 in.). Even at this clearance, extraction efficiency was poor in comparison to the invention.

Finally, although the glycol ethers represent the most preferred class of solvents for non-ionic residues of electronic assemblies in the context of the invention, it will be apparent to those skilled in the art that a variety of solvent categories offer candidates which meet the four basic criteria discussed earlier, but these are less preferred on the basis of other considerations such as odor, possible effects on personnel, flash point, viscosity, etc. These categories include but are not limited to glycols, alcohols, polyglycols, amides, heterocyclics, organic sulfur compounds, and carbonates. Table V lists exemplary solvents representative of the foregoing categories and suitable for use in the invention. Table V indicates important properties of the solvents as well as conductivities of 50/50 mixtures (by volume) of the solvents with water for sodium chloride additions of 10 μg/ml of mixture at a mixture temperature of 66°C (150°F). While the preferred embodiments of the invention and some possible variations have been described, it will be apparent to those skilled in the art that other modifications are possible within the scope of the invention as set forth in the appended claims.

Claims

What is Claimed is: 1. A method of determining the surface ionic contamination level on an electronic assembly such as a circuit board having surface mounted electronic components soldered thereon, the assembly carrying surface residues of ionic and non-ionic contaminants, said method comprising: providing a solvent mixture consisting essentially of water and a water miscible solvent which when heated to a temperature well above room temperature is a solvent for the non-ionic contaminant residues and has a vapor pressure substantially less than that of water at the same temperature, heating the solvent mixture to said temperature well above room temperature, contacting the heated solvent mixture in a deionized state with the electronic assembly, such that the heated solvent dissolves the ionic and non-ionic contaminant residues, until the assembly rendered is substantially free of the ionic contaminant residues, and measuring change in an electrical property of the solvent mixture due to the presence of the dissolved ionic contaminants in the mixture.

2. A method according to Claim 1, wherein the electrical property is one of conductivity and resistivity.

3. A method according to Claim 1, wherein the solvent mixture is heated to a temperature of at least about 49°C (120°F).

4. A method according to Claim 3, wherein the solvent mixture is heated to a temperature no more than about 77°C (170°F).

5. A method according to Claim 4, wherein the solvent mixture is heated to a temperature from about 66°C (150°F) to about 71°C (160°F).

6. A method according to Claim 1, wherein the solvent mixture contains at least about 20% water by volume.

7. A method according to Claim 6, wherein the solvent mixture contains no more than about 60% water by volume.

8. A method according to Claim 1, wherein the water miscible solvent is selected from the group consisting of glycol ethers, glycols, polyglycols, alcohols, amides, heterocyclics, organic sulfur compounds, and carbonates.

9. A method according to Claim 1, wherein the water miscible solvent is selected from the group consisting of ethylene glycol butyl ether, diethylene glycol butyl ether, diethylene glycol ethyl ether, and dipropylene glycol methyl ether.

10. A method according to Claim 1, wherein said contacting includes agitation of the solvent mixture.

11. A method according to Claim 10, wherein said agitation includes directing a high pressure spray of the solvent mixture onto the electronic assembly.

12. A method according to Claim 1, including deionizing the solvent mixture after said measuring.

13. A method according to Claim 12, wherein the solvent mixture is heated, contacted with the electronic assembly, measured for change of the electrical property, and deionized, repeatedly, until the electronic assembly is substantially free of the contaminant residues.

14. A method according to Claim 1, wherein the solvent consists essentially of water and glycol ether, the ratio by volume of water to glycol ether being from about 20:80 to about 60:40, and wherein the solvent mixture is heated to a temperature from about 49°C (120°F) to about 77°C (170°F).

15. A method according to Claim 14, wherein the glycol ether is dipropylene glycol methyl ether.

16. Apparatus for determining the ionic contamination level on an electronic assembly such as a circuit board having surface mounted electronic components soldered thereon, the assembly carrying surface residues of ionic and non-ionic contaminants, said apparatus comprising: a supply of solvent mixture, said solvent mixture consisting essentially of water and a water-miscible solvent which when heated to a temperature well above room temperature is a solvent for said non-ionic contaminants and has a vapor pressure substantially less than that of water at the same temperature, means for heating said solvent mixture to a temperature v/ell above room temperature, means for contacting the heated solvent mixture in a deionized state with said electronic assembly, whereby said ionic and non-ionic contaminant residues are dissolved in the heated solvent mixture, means for measuring change of an electrical property of the contacted solvent mixture due to the presence of the dissolved ionic contaminants in the contacted solvent mixture, and means for supplying the contacted solvent mixture to said measuring means.

17. Apparatus according to Claim 16, wherein said electrical property is one of resistivity and conductivity.

18. Apparatus according to Claim 16, wherein said solvent mixture contains from about 20% to about 60% water by volume.

19. Apparatus according to Claim 16, wherein the water miscible solvent is selected from the group consisting of glycol ethers, glycols, polyglycols, alcohols, amides, heterocyclics, organic sulfur compounds, and carbonates.

20. Apparatus according to Claim 16, the water miscible solvent is selected from the group consisting of ethylene glycol butyl ether, diethylene glycol butyl ether, diethylene glycol ethyl ether, and dipropylene glycol methyl ether.

21. Apparatus according to Claim 16, wherein said contacting means includes means for agitating said solvent mixture.

22. Apparatus according to Claim 21, wherein said agitating means includes high pressure sprayer means for directing a high pressure spray of said solvent mixture onto said electronic assembly.

23. Apparatus according to Claim 16, wherein said heating means is adjusted to heat said solvent mixture to a temperature from about 49°C (120°F) to about 77°C (170°F).