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Jack Brous's white paper

Karen Tellefsen < [log in to unmask]> Date: Wed, 16 Oct 1996 10:25:54 -0400 (EDT) Content-Type: text/plain CIRCUIT BOARD IONIC CLEANLINESS MEASUREMENT -WHAT DOES IT TELL US? by Dr. Jack Brous Alpha Metals, Inc. Jersey City, New Jersey 1.

Introduction In the early 1970's the only fluxes permissible for electronics In the early 1970's the only fluxes permissible for electronics manufacturing for military applications were types R (pure rosin only) and RMA (rosin based, mildly activated). Before any of these fluxes could be approved and established on the military "qualified products list", it was necessary to demonstrate that

B they were non-corrosive and did not contain ionic halides in excess of established critical levels. All fluxes were required

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to pass a silver chromate paper test which sensitively indicates the presence of halide ions. Additionally, the flux was subjected to a copper mirror corrosion test which is a sensitive indicator of any corrosive behavior of the flux toward copper. Because of the desirability for using stronger, more active fluxes in electronic assembly, the military agencies decided to include the category RA (fully activated rosin) on their approved flux list. RA fluxes do not, as a rule, pass the copper mirror or chromate paper tests and there was, therefore, increased concern about allowing the more highly active rosin flux into military electronics manufacture. Although circuit assemblies are ordinarily cleaned after assembly soldering, traces of RA fluxes, left after the final cleaning, would be considered a greater risk than those of RMA or R types. It was, therefore, imperative to thoroughly clean these assemblies to provide assurance that the potentially harmful active residues had been thoroughly removed. 2.

Cleanliness Measurement The RA fluxes most commonly use halide salts, such as amine hydrochlorides, as additives to enhance fluxing activity. In cleaning, these additives are removed along with the rosin residue. If, however, some of the residue is not removed in the cleaning process, some of the halide activators can remain behind, entrapped in the solid rosin residue. Such highly ionic material can be very sensitively detected and measured if brought into a solution containing water. Using electrochemical conductometric methods, ionic materials, which are easily detectable, can act as tracers for the presence of any RA flux residue remaining after the cleaning process. It is well known, to physical and electrochemists, that the addition of a strongly ionized salt to deionized water will enhance its electrical conductance to a degree that is nearly proportional to the concentration of the salt {1,2}. Conductance measurement can thus be used to indicate concentration of an ionic salt extracted into a solution. This was discussed, by T.F. Egan of Bell Laboratories, in a paper published in 1973{3}, who used this method to measure ionic plating

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salt residues on plated components. The method described by his was, however, unsuitable to measure the ionic content of rosin flux residues from an electronic assembly. Rosin is insoluble in pure water, therefore, any ions present in the residue would be trapped in a matrix of the solid, insoluble rosin and not brought into aqueous solution where they could be measured. In 1972, W. Hobson and R. DeNoon, at the Naval Avionics Center, Indianapolis, IN, showed that rosin flux residues could be solubilized in a solvent mixture containing 75% isopropanol and 25% water{4}. This mixture could be used as the solvent base for the conductometric measurements. The alcohol, at that level, is able to solubilize any rosin traces and the water is needed to sustain ionization for a conductivity measurement. 3.

Testing Methods Several testing procedures and instruments were established to monitor the levels of residual ions extracted in alcohol/water mixtures: a.

b.

c.

The original method of Egan was adapted by Hobson and DeNoon to include the use of an isopropyl alcohol/water mixture as the extracting solvent. In this method, the assembled board is flushed with a pre-determined quantity of the mixture and the resistivity of the "contaminated" solvent measured{4}. An instrument was developed in which an assembly was immersed in an agitated, fixed volume of a mixture of isopropanol/ water{5}. The resistivity of the solution is monitored until there is no further indication of resistance change. The effective level of ionic contamination can then be calculated from the change in solution resistivity. This instrument, utilizing extraction into a static volume of alcohol/ water mixture was named the Omega Meter. Another process was developed utilizing a continuously recirculating system in which the solvent mixture was deionized by passing it through an ion-exchange column before recirculating it back into the extraction tank{6,7}. During the course of ionic extraction from an assembly, the conductivity of the mixture falls continuously as the sample and solution are cleaned. The integrated ionic measurement, over time, is indicative of the total extracted ionic material.

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This process, known as the dynamic extraction method, will more sensitively indicate the completeness of the extraction. This process is used in the Ionograph. 4.

Cleanliness Standards - How Clean is Clean? In 1977-78, a program was run at the Naval Avionics Center, Indianapolis, to evaluate various ionic testing systems and to attempt to establish ionic residue levels for ionic fluxes which would be "safe" if left on circuit assemblies{8}. At a meeting held in Indianapolis, February 1978, all of the data of the program was reviewed and pass/fail limits were established for ionic cleanliness levels of electronic assemblies for military applications. These values were different for the different instruments which were available since ionic residues were extracted from the assemblies with various degrees of efficiency. These efficiency, or equivalence factors, were determined from the extensive data accumulated in the prior testing program and resulted in recommendations for pass/fail limits for general military requirements. These values, subsequently, became generally adopted as the control levels for the cleaning processes used to remove rosin flux residues{9} and were widely accepted for industrial non-military as well as the military applications for which there were developed. In time, they were also accepted by many as the cleanliness standards for assemblies made with non-rosin fluxes, such as water-soluble (OA type) and synthetic-activated (SA type) fluxes. In short, these tests and their associated values, which were derived for RA type rosin fluxes, were gradually and arbitrarily extended to cover a variety of other flux types. Other processing operations were included, as well, in this type of testing to these same limits. One example is the ionic testing of bare boards, either prior to or after application of solder masks. While it is true that the presence of contaminants, on a laminate surface, could affect subsequent coating adherence as well as electrical characteristics, specific critical levels could differ significantly from those established for activated rosin fluxes. Another example is in applying ionic testing to circuits which are assembled, entirely or in part, using solder pastes. In such precesses, the paste is applied in very localized areas. During

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the reflow process the flux runs out in these limited areas and is not distributed over the entire board as is the case for the wave soldering process. Ionic measurement of these assemblies, after cleaning, assumes that any residual ionic contamination is distributed over the entire surface of the board since the total extracted ionic material is averaged over the estimated surface area of the whole assembly. In using solder pastes, however, traces of flux residues, which are present, will be concentrated in the very much smaller areas of initial application and thus would represent a much higher locallized contamination level that is indicated by a general ionic extraction test. 5.

How Clean is Good? Perhaps a more pertinent question, that should be asked, is "How clean should an assembly be to be functionally good?" An ultimate test must be one of performance rather than the processing standard of a measured ionic level. Unfortunately, ionic residue levels have, in themselves, become confused in many minds as a criterion of "goodness" or "badness" of the product. In particular, attempts have been made to correlate ionic levels to more functional indicators such as Surface Insulation Resistance (SIR) testing. SIR is a measure of how various materials and processes affect the electrical insulating characteristics of the laminate surfaces between conductors. In the earlier history of electronic assembly, when discreet wiring was used to connect the various components, insulation between conductors was derived from the insulating coating on the wires. With the advent of printed wiring assemblies, the insulation became the bare or solder-masked laminate spaces between the printed wires. This exposed type of insulation is potentially more susceptible to the effects of residues from the soldering and cleaning processes, particularly if the assembly is to be subjected to various conditions of temperature and humidity. A measurement of the electrical effects of the processing and chemicals used in the assembly would, therefore, be most indicative of functional performance in the subsequent application. We know, today, that is not merely the presence or absence of ions, as such, that determines good or bad behavior of a material residue. Some of the poorest electrical behavior experienced has been seen with surfaces which had been contaminated with polyglycols and polyglycol surfactants{10,11}. These materials, although

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completely non-ionic, may, nevertheless, have SIR and Electromigration if they are residual face. Such materials have often been used in many water-soluble assembly fluxes and reflow levelling fluxes used in PCB fabrication.

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disastrous effects on on the laminate surthe formulation of and hot air solder-

To compound the problems caused by these materials, many of them are absorbed into the surface of the plastic laminate when exposed to the high temperature of the soldering or HASL or reflow processes{11,12. They are then not easily removable in normal aqueous or solvent cleaning processes. These retained materials can give rise to serious degradation of the subsequent measured values of SIR. The most important factor determining a material's ability to affect SIR is its ability to absorb water molecules from the ambient air. The hygroscopic nature of the molecule determines its ability to interact with atmospheric water molecules to form multilayer films of water on the surface. Such water films are, of themselves, slightly conductive and can result in degradation of the SIR. The presence of ionic materials on the surface, which could dissolve in this water film would serve only to exacerbate the surface conductance and further lower SIR. Although many polyglycols are examples of some of the worst offenders in degrading SIR, such non-ionic materials are not the only electrically hazardous materials. The key point of commonality of such materials is their ability to absorb moisture from the ambient atmosphere (hygroscopicity). This property can be associated with ionic or non-ionic materials, but is not universal for either type. 6.

"No Clean" Fluxes In recent years, under the impetus of environmental protection, a new breed of fluxes has evolved - "no clean" fluxes. These fluxes are composed largely of weakly-ionized organic acids at low solids levels which, at soldering temperatures, are sufficiently active to provide good cleaning of the metallic surfaces and wetting by the molten solder. In most instances, much of the organic acid is volatillized at the soldering temperatures or neutrallized by chemical reaction. Some traces of the organic acids, however, are always present after the

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soldering. Some of these have very low levels of hygroscopicity and exhibit good electrical performance if they are limited to controlled amounts. It has been shown, however, that excessive levels of these organic acid residues can significantly degrade electrical characteristics such as SIR and Electromigration{13,14}. Conventional ionic extract testing could be of great value as a quality control tool in a production environment. This test can be used as a periodic check of the ability of the "no clean" process to leave residue amounts in a consistent range below levels that can seriously affect electrical characteristics. Significant increases of ionic levels, in a periodic testing program, would then indicate changes in the process which result in heavier residue levels and their associated effects on the electrical characteristics of the board surface. It would be necessary, however, to initially develop the information to define the "normal" ionic operating range for a specific "no clean" process and flux. Once established, further periodic ionic tests could serve as a rapid monitoring check - as compared to lengthy and more difficult SIR tests - to indicate the degree of control maintained over the "no clean" process and flux. CONCLUSIONS -

Ionic extract testing is a valuable testing technique for monitoring and controlling the effectiveness of a cleaning process.

-

Original pass/fail limits were established for wave soldering processes using rosin fluxes. These limits are not necessarily applicable to other types of fluxes and other soldering processes.

-

Ionic extract values are not necessarily valid indicators of the "goodness" or "badness" of the final assemblies in terms of the electrical properties of their laminate surfaces.

-

Ionic measurement can be very valuable in controlling processes using "no-clean" flux technology. This can be used in monitoring amounts of flux solids applied as well as levels of residues remaining on assemblies after soldering.

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REFERENCES 1.

J.C.Bockris and A.K.N.Reddy, Modern Electrochemistry, Plenum Press, New York (1970)

2.

F. Daniels and R.A.Alberty, Physical Chemistry, 2nd Ed., John Wiley & Sons, New York, London (1961)

3.

T.F.Egan, Determination of Plating Salt Residues, Plating, 50(4), 350-354 (1973)

4.

W.T.Hobson and R.J.DeNoon, Printed Wiring Assemblies, Detection of Ionic Contaminants On, Materials Research Report 3-72 (1972) Naval Avionics Center, Indianapolis, IN

5.

E. Wolfgram, Means and Method for Measuring Levels of Ionic Contamination, U.S.Patent 4,023,931 (May 17, 1977)

6.

J. Brous, Self-Purging Apparatus for Determining the Quantitative Presence of Derived Ions, U.S.Patent 3,973,572 (Aug. 10, 1976)

7.

J.Brous, Evaluation of Post-Solder Flux Removal, Welding Journal Research Supplement, 444s-444s (Dec. 1975)

8.

W.Hobeen and R.J.Donoon, Review of Data Generated with Instruments Used to Detect and Measure Ionic Contaminants on Printed Wiring Assemblies, Materials Research Report 378 (1978) Naval Avionics Center, Indianapolis, IN

9.

MIL-P-28809, Military Specification, Printed Wiring Assemblies

10.

F.M.Zado, Proceedings of Technical Program-NEPCON, 1979 Philadelphia, PA, 3876-354

11.

J.Brous, Water Soluble Flux and Its Effect on PC Board Insulation Resistance, Electronic Packaging and Production, July 1981, 79-87

12.

J.Brous, Electrochemical Migration and Flux Residues Causes and Detection, Proceedings of the Technical Program, NEPCON West 1992, 387-394

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13.

L.A.Guth, To Clean or Not to Clean?, Circuits Manufacturing, Feb. 1989

14.

J.E.Sohn and U.Ray, Weak Organic Acids and Surface Insulation Resistance, IPC Technical Paper 1081, IPC, Lincolnwood, IL June 1994

-Karen Tellefsen [email protected] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [email protected] with and no text. * ***************************************************************************

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SIR and Electromigration if they are residual on the laminate sur- face. Such materials have often been used in the formulation of many water-soluble assembly ...

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