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The Corrosive Effect of Soldering Fluxes and Handling on Some Electronic Material!

A variety of rosin-based liquid soldering fluxes are characterized as to halide content, and investigation indicates galvanic corrosion dmS stress corrosion cracking are mechanisms oi material degradation thy

can cause premature failure of inadequately cleaned electronic devicy

BY B. D. DUNN AND C. CHANDLER

ABSTRACT. A preliminary survey has been conducted to assess the variety of rosin-based liquid soldering fluxes utilized by European Space Agency contractors. These fluxes were charac­terized according to their halide con­tents, and a limited number of the more common commercially available types were further evaluated in order to assess their performance in terms of solderability and corrosiveness. The investigation included both standard and ad hoc corrosion tests and the determination of flux halide content and pH values. The corrosion tests were performed in a warm, damp atmosphere fol lowing the deliberate contamination by the fluxes of sam­ples of electronic materials.

The corrosive effects of residual flux on the surfaces of stressed Kovar com­ponent leads and silver-plated copper wires are correlated against the phys­iochemical properties of these fluxes. The results were compared with those obtained from similar control samples either in clean condit ion or after delib­erate contamination during handling.

Galvanic corrosion and stress corro­sion cracking are considered to be mechanisms of material degradation which can cause the premature failure of inadequately cleaned electronic devices. The acceptance of supposedly "non-corrosive" l iquid soldering fluxes on the basis of routine standard tests is unlikely to obviate all the potential corrosion problems associated with electronic hardware.

Introduction

Electronic packages intended for European Space Agency (ESA) space­

craft projects are generally assembled by hand-soldering methods. Compa­nies which have been contracted to manufacture such equipment wi l l fo l ­low the general soldering require-ments specified by ESA1 in order to obtain an adequate standard oi soider joint reliability. This is achieved mainly by the employment of trained and certified operators and inspectors, but also by the control of materials and soldering techniques. The mass assem­bly of components to printed circuit boards by wave soldering has b'jen agreed for the ESA Spacelab project in view of the large number of identical circuits util ized by this project and, fol lowing qualification programs, a l imited number of wave-soldering lines have been approved.

The successful outcome ot all sol­dering operations wil l depend on sev­eral material factors. The choice of a suitable soldering flux is very impor­tant, because it is the flux medii.m which wil l provide for the imlial trans­fer of heat from the hot-soldering iron, or liquid wave of solder, *o the sur­faces being joined together. The ESA soldering specification limits the choice of material finishes which may be interconnected to those having an excellent solderability, so that highly activated fluxes are not needed during the actual assembly process. r luxes of high activity, which are potentially more dangerous from a corrosion viewpoint, are permitted during the

B. D. DUNN and C CHANDLER are with the Materials Section, Product Assurance Group, European Space Research anc Tech* nology Centre, Noordwijk, Ti:e Nether­lands

initial pre-tinning of "diifie-.i**- ,.-".* ponent leads—particularly m r i s •• -of nickel-based alloys—to adW&we h•---•• ter solderability. After pre»l?«v . .H , »he flux residues must be thorotggjMy cleaned from the component \e-rn ,*, face to preclude time-depef***;-- J cor­rosive attack.

Notwithstanding the tight Sofefe process controls, several p associated wi th the corrosive -liquid-soldering fluxes arid If? dues have cost certain ESA < v much wasted time and «•-,-majority of these problem-! 4 • •' as non-conformances to :S inspection requirements, ••>"'; the formation of corrosic^'. ; r on the surfaces of both s; . copper wires and fus coated printed circuit L-cases, soldering had been wi th the additional applicat1 •:- ii uid fluxes, supplied by rr-;: -ufacturers to both U.S." an.' ; . '••••• specifications, purporting .••'••, v . dues of a non-hygroscopk', mm • sive and non-conducpv- tgtytf Met­allographic examine: gi . r strands and printed wi i • .•••im­ported both bright g • >-.y-brown corrosion p odi :i : reveal reduced co tional areas; it is as • ' short term such disc •.:<.•<< ,.-from surface corrosi; > cosmetic defect. v ;-spacecraft electron:.' . • - . •,: harnesses have long •:; : • to launch. Once it . to assess the long te r corrosion mechar sim conductors w i t h . - -: ; i which have beer (I •• ••:. •._•• for at least 10 year

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Problems of a more serious nature included the failure of various gold-plated Kovar leads on flat-packaged components. These leads had been formed and then prepared according to the ESA requirement1 for gold removal prior to soldering. De-golding and pre-tinning were performed by dipping the leads into solder baths fluxed wi th a mildly activated rosin. After a few months of storage, the leads from several batches of flat pack­ages were observed to fracture com­pletely when exposed to light han­dling operations. Metallographic in­vestigations strongly indicated that the failure mechanism of these leads was one of stress corrosion cracking of the Kovar alloy due to the combined effect of residual stresses in the lead material fo l lowing the forming opera­tion and the presence of a thin surface fi lm of supposedly "non-corrosive" flux residue. Other failures have involved the fracture of mounted tran­sistor leads fol lowing equipment level vibration testing; fractography re­vealed that stress corrosion had ini­tiated a crack in the lead material and this had later propagated by a fatigue mechanism.

Discussion of these sporadic corro­sion-related problems during Project Material Review Board meetings has led to the supposition that, although ESA contractors purchased both cored and liquid solder fluxes to recognized specifications, the complex chemical composition of activators contained in proprietary rosin-based fluxes may change from one batch to another with resultant modification of proper­ties such as corrosiveness and solder­ability.

Initially, a survey of soldering fluxes used by some of the major ESA con­tractors was made; the results are pre­sented in the Appendix. This survey established that all the liquid soldering fluxes employed for component as­sembly work and wire interconnec­tions would satisfy the corrosion requirements of recognized flux speci­fications.2"4 It indicated that even the strongly activated fluxes, normally used for the pre-tinning of "poor sol­derabil ity" component leads, might not cause extensive corrosion of stan­dard copper mirror test pieces; it was recommended that the true corrosive nature of any flux can be realistically assessed only if the corrosion test assembly reproduces the essential characteristics of the individual metals which make up a soldered connec­tion.

Additional test methods to the screening tests reported in the Appen­dix have now been selected in an attempt to assess the relative effective­ness of different liquid fluxes. Thirteen flux types have been chosen from the

71 previously examined in the flux survey; they represent typical commer-ical products ranging from non-acti­vated to fully activated rosin-based fluxes containing halogenated addi­tives. The copper mirror test proce­dure2-4 is now re-examined against a new flux test proposal.13 Also, certain special corrosion tests have been devised to reproduce those material compositions and environments thought most likely to have promoted both the previously mentioned space­craft corrosion problems and the few problems which have been reported in the literature.5-10

The ionic content of each flux was assessed together wi th the effect of excessive operator handling contami­nation on the ensuing corrosion of component leads. The various flux types which have been subjected to this test program are listed in Ta­ble 1.

Experimental and Test Procedures

Chemical Analysis

Chemical analysis of solder fluxes is extremely difficult to perform and is often found to be inaccurate. Rosin fluxes can include a vast range of additives known only to the flux man­ufacturers themselves. These may in­clude solvents and wett ing, foaming and viscosity agents which have been chosen to strengthen the fluxing prop­erties of the rosin.

Full chemical analyses have been attempted. Fluorine, chlorine, bro­mine, iodine, sulpher and phosphorus have been detected by emission spec-troscopy, X-ray fluorescence spectros-copy and activation analysis. Gas chro-matography and infrared absorption spectroscopy have been used to sepa­rate and determine the volatile organic components. Non-volatile compo­nents have been identified by both ionic and non-ionic chromatography on ion exchange columns. Those analyzed compositions contained ethanol, methanol, water, fatty acid monoethanolamide, alkylbenzene-sulphonate and many unidentif ied compounds derived from abietic acid.

A full chemical analysis of each flux under evaluation was soon discontin­ued, particularly as it was thought unlikely that such details could ever be related to the effectiveness or corro­sive properties of individual fluxes. The chemical analysis was, therefore, limited to the two simple checks used in the initial flux survey described in the Appendix—halide content deter­mination, and pH value determina­tion. Dynamic Conductivity Monitor (DCM)

system for the measurement of ionic conductivity of flux residues. This equipment, often referred to as the lonograph,* has been evaluated by Naval Avionics and pronounced as a method which provides for the semi-quantitative measurement of flux resi­dues on printed circuit boards.1112

The test is l imited to the detection of ionizable constituents in solder fluxes which are monitored by the DCM on a scale based on a known quantity of sodium chloride dissolved in either pure water or a 1:1 solvent mixture of isopropanol and water. Cal­ibration of this system was based on the conductivity of 1 /ig NaCI per 1 ^l water.

For the purpose of this test program, a constant amount of flux (5 /xl) was added to the equipment's solvent which was continuously pumped in a closed loop and then passed through two conductivity cells. Values of con­ductivity were measured for each of the 13 fluxes in their as-received con­dit ion. Each flux sample was then boiled for 1 minute (min) at -l-200oC (392°F) in an attempt to simulate flux composition modifications which may occur during a soldering operation; the DCM test was then repeated by introducing 5 /xl of the boiled flux concentrate into the solvent.

Effectiveness of Flux Based on Solderability Tests

The soldering efficiencies of the various fluxes were assessed by means of a standard solderability test method compliant wi th B.S. 2011, Part 2T (Phil­ips Globule Method). The solderability test was applied to both gold-plated copper wire and plain copper wire by the horizontal immersion of short lengths of each degreased wire in a globule of l iquid eutectic solder held at 4-235°C (455°F) on a heated steel block.

Immediately prior to wire immer­sion, a standard volume of the flux under investigation was dripped onto the molten solder surface. Once immersed, the wire split the solder globule into two halves and the time was taken for each half to wet, f low and coalesce around the wire sur­face.

This test was repeated 50 times for each combination of wire and flux type. New 200 mg pellets of solder were applied to the heated block at the commencement of each test. The mean soldering times were calculated for each flux when applied to the gold-plated wire (xU]) and the copper wire (xc„). An arbitrary unit, termed flux efficiency (FE), was generated to

The DCM utilizes a solvent pumping *Trademark ol Alphametal.

290-sl OCTOBER 1980

Table 1—Identification of Fluxes

Identification code

A1 A2 A3 A4 K1 K2 K3 M1 M2 M3 Z1 Z2 Z3

Act by

vity as described manufacturer"'

RA RA R RMA RA RA RA R RA RMA RMA RA R

(a 'R—pure or l ow act ivi ty rosin; RMA—mild ly act ivated rosin; RA—activated rosin.

enable a comparison between the ability of different fluxes to effect solder wett ing of these particular me­tallic finishes:

FE = x0l, + xAu .

Copper Sheet Corrosion Test

A new test which evaluates the cor-rosiveness of flux residues has recently been proposed13 and may become applicable in the assessment of rosin-based fluxes. The test method involves melting a piece of solder on a copper sheet in the presence of the flux under evaluation, and submitting this test piece to a humid environment. The test results are subjective and based on visual inspection for corrosion or chemical reaction between the copper sheet, solder alloy and constituents in the flux residue.

Square copper sheet test pieces (50 X 50 mm, i.e., 2 x 2 in.) were made from 0.5 mm (0.02 in.) thick material in the half-hard condit ion. An indentor and die were applied to each test piece to form a central 4 mm (0.16 in.) diameter depression.

Before this corrosion test could be initiated, it was necessary to calculate the non-volatile content of each flux so as to enable a constant weight of solid flux to be transferred into the test piece depression. The solid content of each flux was found from weight-loss calculations based on the evaporation of volatiles from flux samples situated in dried aluminum containers after three hours in an oven held at 110 ± 2°C (230 ± 3.6°F). Desiccators containing silica gel were employed for storage of the containers and solid fluxes. The copper test pieces were thoroughly cleaned and pretreated in line with the recommended proce­dures13 and then a sample of each flux was transferred to the indentations.

The solid flux samples were heat-treated at 60°C (140°F) for 10 min, and this was fol lowed by the addition of a

rerspex Container

S = l o c a t i o n oi' i tnndard scratch Fig. 1—Test configuration for possible corrosion of Kovar component leads under constant deformation

1 g pellet of 60:40 tin-lead solder to each depression. By means of tongs, the test pieces were in turn lowered onto the surface of a heating bath containing l iquid solder at 235°C (455°F). Contact between test piece and bath was maintained until 5 sec­onds (s) after the initial melting of the solder pellet. The test pieces were removed in the horizontal position and cooled for 15 min.

They were then transferred to a humidity chamber and held, in a verti­cal position, for 21 days at a tempera­ture of between 38 and 42°C (100.4 and 107.6°F) and at a relative humidity of between 91 and 95%. Assessment of any corrosion product was made after the test period wi th an eyeglass at x 7 magnification. The test pieces were considered to have failed the test should any blue-green corrosion prod­uct have formed on the copper sheet, or if discrete white or colored spots had appeared in the flux residue or on the surface of the solder alloy.

Copper Mirror Corrosion Test

This test was performed wi th flux in both the as-received condit ion and after it had been boiled for 1 min at approximately 200°C (392°F). Labora­tory production of the mirrors fo l ­lowed the method prescribed in MIL-F-14256C4 and the test accept/reject criteria are as described in the Appen­dix.

"Ad Hoc" Corrosion Tests

The tests were devised to reproduce and investigate the corrosion mecha­nisms which have occurred during the fabrication of several ESA spacecraft. The samples consisted of:

1. Insulated silver-plated copper wire, partly stripped to expose approx­imately 8 mm (0.31 in.) of stranded wire. The materials have been ap­proved for space use; however, the silver-plated strands were nicked and scraped wi th a stripping tool so as to reveal the underlying copper.

2. High reliability, space quality, 14-pin flat-packaged components were chosen to reproduce service failures as

best as possible. These components contained glass-to-Kovar seals wi th gold-plated Kovar leads and were of a type known to pass regularly electron­ic component environmental qualif i­cation tests.14 A jewellers' tooling jig was used to form a standard scratch on each component lead. The scratches were positioned mid-length across the upper surface of the leads; the jig was then set such that a diamond cutting edge passed through the gold-plated layer and was just deep enough to penetrate and expose a microscopical­ly thin band of the Kovar base materi­al.

The test configuration was chosen to represent both residual and applied stresses which may be expected of a component lead during service. Exam­ples of the causes of such stresses are lead bending prior to mounting the component on a printed circuit board, the soldering operation which may constrain the leads, particularly if plated-through holes are employed, and differences in coefficient of expansion of interconnected materials during thermal cycling.

In order to reproduce these stresses, small containers were accurately ma­chined from thick perspex sheets. The inside width of the containers was slightly less than the measured length of each component—as sketched in Fig. 1^so that all leads required to be deflected at their ends as they were slipped between the perspex walls. Once released, each component was thus suspended with in the container sides by the spring properties of its Kovar leads. The dimensional mis­matches were calculated to produce a constant tensile strain on the lead top surfaces approaching that of the Kovar material's yield strength.*

The silver-plated copper wire and flat-packaged component test pieces

* Metallography had revealed that the com­ponent leads were in the fully annealed condition. Mechanical testing of individual leads produced the following results: 29.2 kg/mm 2 yield strength; 43.4 kg/mm 2 ulti­mate tensile strength; 14 X 104 kg/mm 2

Young's modulus; 43% elongation at frac­ture and 150 VPN micro-hardness

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Fig. 4—Relationship between flux efficiency and the ionic content of boiled fluxes

100 000

split solder globule to f low and co­alesce around the wire. It should be noted that, before molten solders can wet and spread, the metallic surfaces must be free of any tenacious non-metallic films such as oxides or sul­phides. One purpose of the applied flux is to remove these barrier films either by reducing them to the metal­lic state or by chemically dissolving them.

Two common, easily reproducible metal surfaces were chosen to repre­sent typical material finishes that are frequently interconnected by solder­ing techniques in the electronics industry. Gold-plated (extremely thin to preclude the formation of britt le tin-gold intermetallics) copper wire represented a barrier-free surface of high solderability: degreased plain copper wire represented a surface which, due to long atmospheric expo­sure, supported a thin film of oxidation products.

The results of the solderability tests are presented in Fig. 3. It is seen from the solderability distribution curves for the gold-plated wires that most of the soldering times are very short, particu­larly when the fully activated fluxes were applied. In these instances when the fluxes are not expected to undergo any surface chemical reactions, it is only just possible to differentiate between the respective wett ing abil i­ties of the various fluxes.

Typical mean wett ing times for RA,

RMA and R-type fluxes on gold wires are observed to be 0.14, 0.25 and 0.35 s, respectively. Repetition of these tests on plain degreased copper wires proved to be more discriminatory owing to the additional t ime taken by the flux to remove an adherent surface film of tarnish. Typical mean wett ing times for RA, RMA and R-type fluxes are 0.5, 1.3 and 2.0 s, respectively.

The arbitrary unit of flux efficiency (FE) which is ascribed to each flux listed in Table 2 has been plotted against the ionic content, as indicated by the DCM test result for boiled flux, in Fig. 4. The fast acting fluxes are noted to be of the RA-type and gener­ally contain a high ionic concentra­tion. However, the remaining R and RMA fluxes show no particular rela­tionship between FE and ionic con­tent.

Results of Standard Corrosion Tests

The flux corrosiveness performance results show good agreement between the standard copper mirror test and the proposed copper sheet test, as shown in Table 2. However, the inspectors performing the visual exam­ination of both types of humidity-tested samples regarded the specified classes of acceptance and rejection criteria to be very subjective.

One inspector considered that the "marginal passes" attributed to the copper mirror test identified as Class 2

(see Table 2, footnote c) ought to have been recorded as "fai lures" according to his interpretation of the specifica­t ion. ' All inspectors agreed that the Class 3 failures, evident as corrosion and flux penetration of the copper, were immediately apparent and that quality checks of the "goods inward" type, for this classification of corrosive flux, would probably be both rapid and straightforward to control.

It is also noticed from the copper mirror results in Table 2 that identical results were obtained for both the as-received fluxes and the same sam­ples after boiling. This is somewhat surprising since the DCM results indi­cated that, once boiled, the ionic con­tent of these fluxes increased; this would be expected to create a more corrosive environment. It is probable that the copper mirrors have not been greatly affected by this increase in ionic content because stable basic chlorides, such as those green patinas which form on copper, wi l l reduce the Ck activity and may form barrier layers that retard corrosion. In fact, only fluxes which contain very high (greater than 4.45%) halide concentrations and extremely high ionic contents were seen actually to penetrate the thin copper f i lm. The copper mirror test is, however, l imited in that it does not indicate the danger of electrolyte cor­rosion accelerated by direct contact between different metals.

Under actual soldering conditions, a

296-sl OCTOBER 1980

l iquid flux wi l l f low to surround the molten front of solder and wil l be in contact wi th both hot metallic and non-metallic materials that may sub­stantially modify the chemical compo­sition and activity of the flux. Once

cooled, some fluxes are extremely hygroscopic and may promote gal­vanic corrosion between the various interconnected materials. The pro­posed copper sheet test method may go some u a \ to assess this problem of

AIR AiB A2R A2B

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bi-metallic corrosion resulting from the applied solder.

Cooper is more noble than solder. When these are in contact in the pres­ence of a chloride concentration, this factor wil l cause galvanic corrosion of

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W E L D I N G RESEARCH SUPPLEMENT I 297-s

Table 3—Detailed Results of the "Ad Hoc" Testing

Flux type""

A1a A1b A2a A2b A3a A3b A4a A4b K1a K1b K2a K2b K3a K3b M1a M1b M2a M2b M3a M3b 21a Z1b Z2a Z2b Z3a Z3b

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80

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1 = no corros ion, no discolorat ion or tarnish 3 = less than 50% of surface corrosion 5 = extensive surface corrosion w i t h severe d iscolorat ion.

" 'Classi f icat ion of depth of stress-corrosion cracking based on data appear ing in Table II. " "The 'Corrosivi ty Index' presupposes the presence of bo th unhealed and bo i led flux on these samples and represents the summat ion of all classif ications for a part icular flux type. ,e 'These samples consist of stressed go ld-p la ted Kovar leads ei ther del iberately con tamina ted w i t h hand perspirat ion or as-cleaned in I.P.A. and de- ion ized water; no flux had been appl ied.

the electronegative solder metal. The potential difference which wi l l arise is difficult to estimate accurately, but as a guide1"' is taken to be 0.3 volts (V). (Some fluxes can contain the same chloride concentration as seawater, 2.2 g / l , the basis of this guide.) As far as electronic circuit materials are con­cerned, copper and solder are relative­ly "compat ible," and this copper sheet test would probably be more selective if thin platings of a noble metal such as silver or gold were applied to the copper, these metals being separated from solder by a potential of 0.5 and 0.65 V, respectively.

Results of the Ad Hoc Corrosion Tests

It is believed that the standard tests for establishing the corrosiveness of liquid soldering fluxes bear little resemblance to those conditions in which electronic materials may be­

come contaminated by a flux and sub­sequently corrode. Assimilation of the conditions which are considered to have promoted ESA spacecraft elec­tronics corrosion problems are likely to have been achieved by the "ad hoc" corrosion tests of this program.

Visual Inspection Results. A general overall view of the component and stranded copper wire samples is seen in Fig. 5 which depict the corroded appearances of many items after 56 days exposure to 95% RH at 40°C (104°F). It is noted that the control specimens show no evidence of corro­sive attack at the end of the test period. The progressive inspection results and a description of the numer­ical classification system employed throughout this evaluation are listed in Table 3.

It is important to note that, after only 10 days subjection to the test environment, both the damaged sil­

ver-plated wires and the scratched flat-pack leads have invariably at­tained their worst visual appearance. Samples supporting a typical RA type flux and one RMA type flux have been detailed in Figs. 6 and 7 after 21 days of humidity exposure. These photo­graphs are captioned according to the inspectors' observations.

It is to be observed that the fluxes containing the higher ionic content and having higher activity (RA) pro­duce the most severe forms of surface corrosion. It is also observed that, des­pite the claims made by certain flux manufacturers, many products are hy­groscopic. Such fluxes are likely to have become electrically conductive gels during the test period and are able to initiate and sustain various forms of corrosive attack that are dependent on electrochemical reactions.

Effect of Fluxes on Silver-plated Wires. The deliberately damaged si I -

298-sl OCTOBER 1980

.4.1 . lu»

f/g. 6—Flux type Al (RA) after 27 days exposure. Corrosion products appear on all metallic surfaces, the wires support a green product, and the leads have several micro pits in their plating. A—as-received flux; B—boiled flux

i i j I MI in • r

Fig. 7— Flux type Z1 (RMA) after 27 days exposure. A non-hygroscopic flux showing absolutely no sign of corrosion on metallic parts A—as-received flux; B~boiled flux

ver-plated wires show varying degrees of surface corrosion products after exposure to flux and humidity; the results are listed in Table 3. Unfortu­nately, it was not possible to quantify the extent or depth of corrosion by means of either surface scanning elec­tron microscope or metallographic examination of individual samples.

The depth of the deliberate score varied from one wire strand to anoth­er. While the non-surface-corroded samples failed to reveal any metal wasting, it was extremely difficult to establish the internal corrosive depth of attack due to the presence of pref­erential sites of corrosion and the non-uniform cross-sectional areas of the damaged strands. The more reactive RA fluxes were observed to cause rath­er extensive corrosion in locations where the copper substrate had been exposed and only slight corrosion beneath the layer of supposedly intact and pore-free silver plating.

The worst case of wire strand corro­sion produced by one of the stronger RA fluxes is shown in Fig. 8. It is noted

that, once the flux and its residue had been cleaned from the strands during the preparation of samples for SEM and metallographic inspection, the remaining adherent corrosion product appeared to have a green-brown col­oration. It is believed that this debris is a mixture of so-called red and green plagues which have been previously reported in the literature.1'1 Red plague is the oxidation product, Cu.O, which forms because of galvanic action between silver and copper in the pres­ence of the electrically conductive flux

gel-Figure 8 illustrates how the more

noble silver does not enter into the corrosion reaction. This galvanic attack is believed to be intensified by the presence of active chloride ions, Cl , and abietic acid present in the flux which form a green product (green plague) which can be washed safely away during post-soldering cleaning. A potential danger arises, however, when strong RA-type fluxes and resi­dues are entrapped under the wire insulation material by capillary action

during soldering or by wicking of con­taminated cleaning fluid along the stranded wires.

On the basis of these results, it is not considered possible to predict the long-term reliability of spacecraft wires which turn green shortly after the introduction of flux during solder­ing operations. The greatest danger wil l occur when the flux is hygroscop­ic, contains a high ionic concentration and is present on wires which wil l be periodically subjected to a humid environment.

Stress Corrosion of Component Leads. Probably the most revealing results to have been produced by this evaluation program are those which indicated the low stress corrosion cracking resistance of gold-plated Kovar leads when stressed close to this material's yield point, then subjected to the soldering fluxes under evalua­tion. These results are listed in the last two columns of Table 2 and, for full details, the photomicrographs and captions presented in Figs. 9-21 must be reviewed.

WELDING RESEARCH SUPPLEMENT I 299-s

The extensive degree of general and stress corrosion tracking (SCC) shown by these longitudinal sections is quite surprising because close visual inspec­tion of the lead surfaces does not always indicate any sign of surface corrosion (e.g., see Fig. 7, flux Z1, type RMA). The photomicrographs (Figs. 9-21) depict longitudinal sections made through the lead mid-planes under the "standard scratch" and in any region away from the scratch which possesses a particularly severe corrosion site. The results of applying either as-received or boiled flux to the stressed Kovar leads may be compared and, although not consistent, most fluxes appear to have an enhanced corrosiveness once boiled.

It should be noted that several of the component samples suffered lead breakages as they were being carefully removed from their containers at the end of the 56-day test period. Many of these fractured items were viewed by scanning electron microscope. With one typical example, the lead was microsectioned, polished and—at a

later stage—lightly etched to reveal a transgranular mode of SCC propaga­tion.

As in the case of the example cited, several photographed SCC paths did not begin at the standard scratch. It is thought that the gold plating on the lead surfaces is porous and capable of initiating and sustaining SCC growth by diffusion of chloride ions from the residual flux to the crack tip. The elec­trochemical mechanism of SCC in alloy steels is accelerated by the appli­cation of anodic currents, and a similar situation is thought to exist in the case of these gold-plated Kovar leads, anodic currents being set up between the porous plating and the less noble Kovar alloy. It is to be noted that the corrosion products occupy a larger volume than the Kovar from which they are formed; these products wil l tend to increase the SCC propagation rate by creating a wedging action and additional stress concentrations at the crack tip.

The fracture of Kovar alloy as a result of exposure to SCC environmental

Fig. 8—One of the strongest and hygroscopic RA-type fluxes produced severe galvanic corrosion of the copper conductors from these stranded wires (A). The SEM photograph (B) of one strand shows a silver "shell" whin h, when microsectioned (Cand D) reveals extensive wasting of copper. (The original form of silver plate is marked on C, but this was crushed by the mounting media; the polished sections have been lightly etched in ammonium peroxide to highlight the copper grain structure.) A—optical photograph; B—SEM photograph, X350; C-microsection, X400; D—detailed photomicrograph, X900 (reduced 23% on reproduc­tion)

conditions have not been widely reported in the l i terature. ' I" i , s Effec­tive protection of Kovar component leads from SCC has been achieved1" by firstly chemically removing the work-damaged (from lead-stamping opera­tions) surface of the lead prior to thin gold plating. The thin plate is designed to protect the lead from oxidation during component manufacture, but is finally removed by dip-coating with a ductile pore-free finish of eutectic t in-lead solder, and any gold plate remain­ing adjacent to the component-to-lead glass-to-metal seal is additionally protected by a silicone varnish.

Another method" to minimize the SCC failure of leads is to plate the Kovar with 12.5 microns of nickel prior to gold-plating. However, even this process wil l not overcome chloride-assisted SCC if the nickel is mechani­cally cracked during either component manufacture or subsequent lead-form­ing operations.17

It has been suggested that it is impossible to avoid the SCC failure of Kovar leads by solely increasing the thickness of the gold finish. One report1'' states that many thick gold-plated transistor leads containing re­sidual stresses induced by 90 deg angle bends, in the presence of a commer­cial soldering flux and a humid envi­ronment, were observed to fracture after only 23 days.

The halide concentration necessary to promote SCC in Kovar has not been previously studied. In the chemical industry, where these corrodents are a serious problem, it has been ob-served111 that an extremely small con­centration of 0.02% aqueous NaCI wil l readily cause SCC of high alloy stain­less steels, including AISI 316 stainless steel.

Based on the flux evaluation results presented in Table 3, an attempt has been made to index the various fluxes according to their ability to cause cor­rosion. This "corrosivity index" ap­pears as the summation of all the numerically classified results for gen­eral surface corrosion and SCC.

A comparison between this index and the ionic and halide contents of the various fluxes is presented in Figs. 22 and 23. These show some correla­tion between a high corrosivity and either a high halide or high ionic con­tent. There is, however, no relation­ship between the SCC susceptibility of Kovar leads and either halide or ionic content of individual fluxes. In the case of one as-received R-type flux, designated A3, it is seen that a halide concentration as low as 0.0011% may initiate and propagate sufficient SCC to cause lead fracture (Fig. TIB).

Effect of Skin Secretions on Stressed Component Leads. Spacecraft elec-

300-sl OCTOBER 1980

Fig. 9-Flux A1. Pitting corrosion seen under scratch (A), with general stress corrosion at sites B, C and D

Fig. 70— Flux A2. Surprisingly little corrosion under scratch (A, Q, but severe lead embrit-tlement in one selected region (B)

Fig. 11—Flux A3. Only one site of stress corrosion (B) with cracks propagating to 75% of lead thickness

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A (As- rece ived f lux; reg ion under scratch)

C (Bo i led f lux; reg ion under scratch)

(As- rece ived f lux; se lec ted regions)

D (Bo i led f lux; se lec ted reg ions)

Fig. 12— Flux A4. Only very slight pitting corrosion adjacent to scratch on (A)

Fig. 13—Flux Kl. Extremely severe form of stress corrosion cracking through the lead thickness (A-D) with some exfoliation cor­rosion (D). In one region (D) the gold plate is blistering due to the buildup of corrosion products

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Fig. 15—Flux K3. Leads have fallen apart due to the presence of a fine network oi hairline stress corrosion crac ks

Fig. 76—Flux Ml. Pitting corrosion adjacent to scratches (A,C) with some surface corro­sion and blistering of gold plating (D)

W E L D I N G R E S E A R C H S U P P L E M E N T ; 301 -s

Fig. 17—Flux M2. Severe case of stress corro­sion cracking and associated blistering (B-O)

Fig 19— Flux Zl. Slight intergr'anular corro­sion in one region of stressed lead (D—boiled flux)

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Fig. 20—Flux Z2. Surface pitting corrosion (A,B,D) and severe stress corrosion (C)

Fig. 18-Flux M3. An extraordinary mixture of corrosive classes, general Kovar corrosion and blistering due to corrosion product (A), slight intergranular and pitting corrosion (B), zero corrosion (C) and severe stress corrosion (D)

tronic units are generally handled by operators under "clean room" condi­tions. The preferred codes of practice for component assembly by hand or wave soldering to printed circuit boards recommend that operators wear finger cots or lint-free gloves when they bend, straighten and insert component leads into pcb termination areas. These practices are certainly not universal since some operators feel restricted when wearing hand covers for deiicate soldering operations or tor reworking incorrect joints on high density boards. The use of bare hands in these instances may be justified provided the boards are thoroughly solvent-cleaned by approved meth­ods1 immediately after assembly.

The effect of severe handling, which

causes skin secretions to be deposited upon stressed component leads, was evaluated by means of two clean flat-packaged components supporting the "standard scratch." Once contami­nated by perspiration, these samples were subjected to the stress and humidity environment in parallel wi th cleaned control samples and the pre­viously' described fluxed samples. All devices were inspected at regular intervals and classified according to the degree of observed surface corro­sion. The results are listed in Table 3.

After 21 days, the contaminated sample had become surface-corroded to an extent less than 50"o of its lead surface area. The cleaned control sam­ple remained uncorroded. At the end of the 56-day period, the contami­nated and control samples were microsectioned; these as-polished photomicrographs appear in Figs. 24 and 25, respectively. Despite the somewhat innocuous surface appear­ance of the contaminated leads, the sections reveal highly branched stress corrosion cracks beneath the stress-raising scratch as well as in selected regions beneath the gold plate. Many of the fine cracks which penetrated more than 50% of the lead thickness occurred in regions well away from surface blisters and would not have been apparent from visual inspection alone.

The component control samples showed absolutely no signs of surface or stress corrosion cracking fo l lowing the 56-day exposure to temperature and pure water humidity.

Measurements have been made1'1 on the salt contributed by fingerprints on handled printed circuit boards. Indi­vidual prints were found to contribute as much as 30 micrograms of NaCI as measured on the DCM. It may be

Fig. 2'1-Flux Z3. Surface pitting corrosion (A,B,C) and severe stress corrosion (D)

difficult to remove completely such high concentrations of salt from the vicinity of similar fingerprints when they are present on component sur­faces due to the porous nature of the gold plating on Kovar leads.

An induction period prior to Kovar cracking may be dependent on the bui ldup and penetration of a local corrosive solution somewhere along the lead surface and SCC, in the worst case promoting lead fractures, may be observed only after a long period of time when the environmental condi­tions are conducive to crack propaga­tion.

These test results emphasize the need to preclude unnecessary han­dling of Kovar and probably other iron-nickel-cobalt alloy lead materials by bare hands. When circumstances permit handling, every precaution must be taken to ensure complete

302-sl OCTOBER 1980

B

CD

<

o tr u.

X UJ Q

1/1 O CC L X

o u

130

120

110

100

90

80

70

60

50

A0

30

20

10

0

© = R

• = RMA AS-RECEIVED TO BOILED CONDITION

• = RA

KEY MANUFACTURERS' Dl SCRIPT I ON OF FLUX ACTIVITY

A1 K3

A 2

K2 —<—•

K1

M3 Z2 >

M 2

© -Z3

A 3

- © © -

M l ~®

AA

Z1

10 100 1000

D.C.M. TEST RESULTS ( j j g N a d /j_tI FLUX )

10000 100 000

Fig. 22—Relationship between the ionic content of a flux and its corrosive index

® = R

• = RMA

• = RA

KEY MANUFUCTURERS DISCRETION OF FLUX ACTIVITY

00001 0 001 0 01 01

HALIOE CONTENT OF AS -RECEIVED FLUX, °l. BY WEIGHT

Fig. 23-Relationship between the halide content of the as-received fluxes and their corrosivity. based on ad hoc tests

removal of potential corrosive films by adequate post-assembly cleaning pro­cedures.

Conclusions

1. The flux survey and subsequent program of testing and evaluation of specific types of commercial l iquid soldering fluxes may serve as a rough guide to flux selection for application during the soldering of spacecraft

hardware. The surprisingly strong cor­rosive nature of many commercial fluxes precludes their use whenever there is the slightest chance that a trace of their residues may remain on delicate electronic materials which, for the new generation of ESA spacecraft, may require long storage times prior to launch and may have, in the case of communications satellites and Space-lab, operating lives of 10 years.

2. The mean solder wett ing times

for the various classes of fluxes exam­ined in the program are as follows: RA 0.14 s for gold-plated and 0.5 s for copper wires; RMA 0.25 s for gold-plated and 1.3 s for copper wires; R 0.35 s for gold-plated and 2.0 s for copper wires.

The fully activated RA-type fluxes generally contain a high ionic concen­tration and possess a high fluxing effi­ciency (FE) whereas no particular rela­tionship could be established between

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W E L D I N G RESEARCH SUPPLEMENT : 303-s

Fig. 24—Section across handled speci­men—severe stress corrosion cracking. A—under scratch; B-selected region

FE and ion ic c o n c e n t r a t i o n for the RMA and R-type f luxes. The app l i ca ­t i on of heat was f o u n d to increase the ion ic c o n t e n t of most t luxes.

3. A l t h o u g h s o m e w h a t sub jec t i ve , the standard c o p p e r mi r ro r test and the p roposed coppe r sheet test p ro ­v ided c o m p a r a b l e results. Bo i l i ng i n d i ­v idua l f luxes d i d not p r o d u c e d i f fe ren t results. Ne i the r test m e t h o d is c o n s i d ­ered to be par t icu la r ly se lect ive in assessing the corros iveness of f luxes under service c o n d i t i o n s .

4. O n l y the " a d h o c " co r ros ion tests are cons ide red l ike ly to shed l ight o n the c o m p o n e n t lead and w i r e p r o b ­lems e n c o u n t e r e d on ESA spacecraf t pro jects ; they rep roduce the mater ia l character is t ics and the in te r re la ted mechan isms of ga lvanic co r ros ion and stress co r ros ion c rack ing .

5. C o n c e r n i n g s i l ve r -p la ted c o p p e r w i r e , f luxes of the RA- type shou ld no t be used. If there is any l i k e l i h o o d that the p la t i ng is d a m a g e d or po rous , t hen cer ta in of the RMA and R-type f luxes l isted in Table 3 w o u l d not be r e c o m ­m e n d e d , par t i cu la r ly w h e n the ingress of f lux or c o n t a m i n a t e d c lean ing so lu ­t ions canno t be p reven ted . In case of d o u b t , the c l e a n i n g / v a c u u m bake remedy o u t l i n e d in the l i te ra ture" may e l im ina te co r ros ion .

Fig. 25—Section across clean control sam pie—no evidence oi any form of corrosion A—under scratch; B-selected region

6. C o n c e r n i n g g o l d - p l a t e d Kovar c o m p o n e n t leads, these may b e c o m e s l igh t ly s u r f a c e - c o r r o d e d , bu t degrada­t i on by stress co r ros ion c rack ing (SCC), as caused by the ma jo r i t y o f t h e tested f luxes, is no t a lways v is ib le un t i l such leads f rac tu re in t w o . To avo id SCC i n i t i a t i on and p r o p a g a t i o n , the f o l l o w i n g ESA so lde r ing r e q u i r e m e n t s ' must be e n f o r c e d :

• Flux and res idue are r e m o v e d i m m e ­d ia te ly after so lde r ing .

• Stress rel ief bends must be p r o v i d e d b e t w e e n c o m p o n e n t b o d y and part t e r m i n a t i o n . • Leads must not be sharp ly ben t ; use the m i n i m u m l e a d - b e n d i n g requ i re ­ments . • Leads must not be f o r ced to l ie f lat d u r i n g so lde r i ng , and they must be f o r m e d accura te ly to p reven t res idual stresses, e.g., c o m p o n e n t s must no t be he ld i n to PCB p l a t e d - t h r o u g h holes by the spr ing p roper t ies of the i r leads; this c o u l d p rov ide ideal c o n d i t i o n s for ca tas t roph ic SCC fa i lu re .

• N o n - a u t h o r i z e d f luxes, so lvents , etc., must be r e m o v e d f r o m the w o r k area. • A lso , m o u n t i n g pads, c o n f o r m a l coa t ings , etc. , shou ld be des igned to l im i t stresses d u e to d i f f e ren t i a l ther ­mal expans ion . - "

Fig. 2b—Photograph showing the deleter­ious eiiect oi Class 2 top-marginal pass) and Class 3 (bottom-tail) (luxes following the copper mirror test

7. The leads o f e l ec t r on i c c o m p o ­nents may b e c o m e severely deg raded by the t ransfer of c o n t a m i n a t i o n , such as pe rsp i ra t i on f r o m an opera to r ' s bare hands. Such pract ices must be a v o i d e d , and it is an ESA r e q u i r e m e n t ' that c lean w h i t e g loves or f inger cots be w o r n in o rder to avo id the f o r m of ca tas t roph ic SCC d e p i c t e d in Fig. 27.

Acknowledgment

The au thors w i sh to t hank Mr . D.S. Co l l i ns for his assistance w i t h the m e ­ta l l og raphy and Mr . H. Smi th of the Fulmer Research Ins t i t u te , Eng land , f o t the ha l i de c o n t e n t and pH d e t e r m i n a ­t ions .

References

1. ESA-PSS-14/QRM-08, "The Manual Soldering of High Reliability E lectrical Con­nections," 1918.

2. QQ-S-571, "Non-corrosive Rosin-cored Solder Wire," 1963.

3. DTD-599A, "Non-corrosive Flux for Soft Soldering," March 1961.

4. MIL-F-14256C, "Flux Solutions of Ros­in or Mi ld ly Activated Rosin," 1963.

5. Peters, S.T., and Wesling, N., "Corro­sion of Silver-plated Copper Conductors," SAMPE 13th National Symposium, May

-MARGINAL PASS

. . . . .

_|_ _l_ 0 001 0.01 01 1.0

HALIDE CONTENT OF AS RECEIVED FLUX , 7. BY WEIGHT •

27-Effect oi halide content on copper mirror test results

3 0 4 - s l O C T O B E R 1980

1968. 6. Peters, S.T., "Review and Status of Red

Plague Corrosion of Copper Conductors," Insulation/Circuits, May 1970, p. 55.

7. Reich, B., "Stress Corrosion Cracking of Cold-plated Kovar Transistor Leads," Sol­id State Technology, April 1969, pp. 36-38.

8. Studnick, W.R., and Foune, C.C, "Testing for Corrosivity in Activated Liquid Soldering Fluxes," The Western Electric Engineer, |an. 1973, Vol. XVII, No. I, pp. 3-8.

9. Weirick, L.J., "A Metallurgical Analysis of Stress Corrosion Cracking of Kovar Pack­age Leads," Solid State Technology, 1975, Vol. 18, No. 3, pp. 25-30.

10. Dunn, B.D., "Reliable loints for Spacecraft, Part 2," Electronic Production, 1978, Vol. 7, No. 4, p. 23.

VI. Brons, )., "Evaluation of Post-solder Flux Removal," Welding journal, 54(12), Research Suppl. Dec. 1975, pp. 444-s to 448-s.

12. Tautscher, C.)., "Printed Wir ing Board Cleanliness Testing," Circuit World, Vol. 4, No. 2, p. 30.

13. B.S. Draft Specification No. 77/78244 intended to replace B.S. No. 41 1, 1954.

14 MIL-S-19500D, "General Specifica­

tion tor Semiconductor Devices—Group B Tests."

15. Dunn, B.D., "Product Assurance and Choice of Materials for Satellite Construc­t ion, " Metall, August 1976, 30 (9), pp. 711-720.

16. Elkind, M.|., and Hughes, H.E., "Pre­vention of Stress Corrosion Failure in Fe-Ni-Co Alloy Semiconductor Leads," Bell Telephone Laboratory Report in Physics oi Failure in Electronics, 5 (1967), pp. 447-495.

17. Harboe, R., and Adams, L., "An Inves­tigation of the C M O S Lead Corrosion Prob­lem." ESTEC-Working Paper No. 1023 (con­fidential).

18. Reich, B., "SCC of Gold-plated Kovar Transistor Leads," Solid State Technology, 1969, Vol. 12. No. 4, pp. 36-38.

19. Spa'hn, H., "Performance Require­ments tor Stainless Steels in the Chemical Process Industry," Proceedings of the Stain­less Steel 1977 Conference, London, p. 161.

20. Dunn, B.D.. "The Resistance of Space-quality Solder Joints to Thermal Fa­tigue," Circuit World, Part I, Vol. 5, No. 4, 1979, pp. 11-17; Part 2, Vol. 6, No. 1, 1979, pp. 16-27.

A p p e n d i x : S u m m a r y of Survey C o n d u c t e d t o Establ ish S o l d e r i n g Fluxes

U t i l i z e d by E u r o p e a n M a n u f a c t u r e r s of E l e c t r o n i c E q u i p m e n t f o r Space

A p p l i c a t i o n

Thi r ty of the major European c o m ­panies engaged in t he m a n u f a c t u r e o f e lec t ron ic ha rdware for ESA spacecraf t app l i ca t ions we re a p p r o a c h e d . T w e n ­ty - f i ve c o m p a n i e s r e s p o n d e d by for ­w a r d i n g for analysis and tes t ing o n e or

m o r e samples of f resh u n c o n t a m i -na ted f lux as used at the i r p lants. The samples we re a c c o m p a n i e d by c o m ­p le te i n f o r m a t i o n a b o u t ba tch i d e n t i ­ty , manu fac tu re r ' s da te o f pu rchase and i n t e n d e d usage.

It was ev iden t f r o m the repl ies rece ived that t he ma jo r i t y o f c o m p a ­nies d o no t carry ou t any f o r m of i n c o m i n g i nspec t i on tests or c h e m i c a l con t ro l s o n any of the i r l i qu i d f lux purchases. In genera l , it was f o u n d that c o m p a n y purchase orders refer­enced on l y tha t p r o c u r e m e n t shou ld be against cer ta in na t i ona l spec i f ica­t ions and that accep tance test ing was assumed to have been p e r f o r m e d at t he supp l ie r ' s p lan t p r io r t o ba tch re­lease.

In to ta l , 71 f lux samples w o r e rece ived—Table 4. The most c o m m o n f lux p roduc t s o r i g i na ted f r o m Br i t ish , U.S. a n d G e r m a n manu fac tu re rs . A f e w we re rece ived w i t h D u t c h and Belg ian b rand names. A l l samples we re sub­jec ted to a halide content determina­tion a c c o r d i n g to the m e t h o d de­scr ibed in t he l i te ra tu re 1 , here, t he percen tage by w e i g h t of ha l i de is ca l ­cu l a ted as c h l o r i n e o n the w e i g h t of the n o n - v o l a t i l e p o r t i o n of f lux.

A pH value determination was a t t e m p t e d by p lac ing c h e m i c a l l y t reated pH papers in the as-received f lux for o n e m i n u t e ; c o l o r changes o f the papers w e r e d e p e n d e n t on the h y d r o g e n - i o n c o n t e n t of each f lux. Al l f luxes w e r e ac id ic , hav ing a p H of less than 7. H o w e v e r , the dye in t he papers d i d no t r espond s t rong ly to all f luxes and these results are s o m e w h a t sub­jec t ive .

Each f lux was sub jec ted to a copper mirror test. This test was p e r f o r m e d

Table 4—Summary of Various Flux Types Tested, User Companies and Laboratory Results

Laboratory test results

Flux code no.

1

2

3

4" "

Country and no. of comp

GB S NL GB

GB

D DK NL B NL DK

D GB DK B NL E

B DK NL S

user any

4 11 13 24

25

9 10 13 14 18 26

1 3

10 14 18 19

5 7

13 27

Batch identity of flux

a a

j a b c c

a a c b b b

a a b a a a

b b d a

Halide content of as-received

sample, %

0.069 0.037 0.022 0.016 0.054 0.028 0.053

0.0012 0.0003 0.0011 0.0064 0.0003

< 0.0004

0.03 0.051 0.038 0.044 0.032 0.024

0.01 0.0095 0.012 0.01

pH of original flux

5.1 4.5 4.8 4.8 4.5 4.8 4,8

4.0 4.8 4.8 4.5 3.9 4.8

5.1 5.0 5.1 4.5 5.0 5.1

4.5 4.8 4.8 4.8

Copper

24 h at 30

As-receiv

(1) (0) (1) (1) 0) (1) (1)

(1) (0) (0) (0) ( I ) (0)

(1) 0) (0) (0) (0) (1)

(0) (1) (0) (1)

mirror test

C ;d

and 50% RH

3oiled

(1) (0) CM (1) (1) (1) (I)

( I ) (0) (0) (0) ( I ) (0)

( I ) (1) (0) (0) (0) (I)

(0) (1) (0) (I)

(Continued on next page)

W E L D I N G R E S E A R C H S U P P L E M E N T 305-s

using flux in the as-received condit ion and after boiling a standard volume of flux for one minute at approximately 200°C (392°F) in order to simulate the soldering operation. The copper mir­rors were prepared in the laboratory by vacuum-depositing 5000 A of pure copper onto treated vapor-degreased

glass slides as required by MIL-F-14256C" Approximately 0.05 ml of flux was placed on the copper side of the mirrors which were then stored in the horizontal position inside a humidity chamber at 30 ± 2°C (86 ± 3.6°F) and 50% relative humidity for 24 h. Each slide was then visually examined for

corrosive attack on the copper. The pass/fail criteria of this test are

given in the footnote to Table 4. Any penetration of the copper thickness could be readily seen by holding each mirror against a light source. Such cor­rosive attack was classified as a class 3 flux failure. Attack on, or modification

Table 4—Summary of Various Flux Types Tested, User Companies and Laboratory Results (Continued)

Laboratory test results

Flux code no.

5

Country no. of

and jser

company

E NL B D

6 13 14 22

Batch identity of flux

a w c a

Halide content of as-received

sample, %

0.46 0.48 0.48 0.46

pH of original flux

4.5

-4.5 4.5

Copper mirror test, 24 h at 30°C and 50% RH

As-received

(0) -(0) (1)

Boiled

(0) -(0) (I)

6""

7""

8

9

10

11

12

DK S NL

DK NL F

NL CB CB

DK GB

\ l GB

DK GB

B

7 11 13

7 13 31

13 15 25

10 30

13 25

10 15

5

a b k

c e a

t b b

d a

q q

e a

q

0.83 0.28 0.59

0.067 0.011 0.0009

0.42 0.40 0.36

2.31 2.32

0.0002 0.0018

0.70 0.44

0.0035

4.5 3.9 4.8

3.9 4.5 4.8

-4.5 4.8

4.5 4.8

-4.5

3.9 3.9

4.8

d) (1) (2)

(I) (2) (2)

-(0) (0)

(2) (1)

-(2)

(0) (2)

(2)

(1) (1) (2)

0) (2) (2)

-(0) (0)

(2) (1)

-(2)

(0) (2)

(2)

13 14 15 16 17

18

19 20 21 22 23

D D D D D

DK

NL NL NL NL NL

8 8 8 8 9

10

13 17 17 17 18

< 0.0006 5.84

< 0.0006 0.0009 0.0019

4.8 3.9 4.5 4.2 4.5

(2) (2) (0) (0) (2)

(2) (2) (0) (0) (2)

0.88 3.9 (1) (1)

27 28 29

3 2 « b

33"" 34"" 35"" 36"" 37"" 38""

NL 13 NL 13 NL 13

\ l NL NL NL NL NL NL

13 13 13 13 13 13 13

0.0009 0.004 0.0028 0.68 0.012

0.006 0.0046 0.008

7.93 4.45 0.0011 0.0013 0.73 0.75

62.50

4.2 4.8 4.5 4.5 5.1

3.4 3.6 4.8 4.8 4.8 4.8 1.0

(0) (1) (0) (2) (2)

(0)

(3) (3) (0) (1) (D (2) (3)

(0) 0) (0) (2)

24

25 26

I F

D DK

32 21

22 26

a a

b q

0.078 0.048

0.019 12.95

5.0 4.5.

4.8 2.0

-(1)

(1) (3)

-(1)

d) (3)

(0)

30"

31

NL

D

13

8

f

e

0.0029

< 0.0004

4.5

4.2

(2)

(2)

(2)

(2)

(3) (3) (0) (I) (I) (2) (3)

39 I 32 < 0.0002 10.0

t a ,The copper mirror test was per formed on flux samples in the as-received cond i t i on , in the same concent ra t ion as used by the part ic ipat ing ESA contractors The test was repeated after each sample had been boi led for 1 mm at 200 ± 5°C to simulate the soldering operat ion. The pass/fai l criteria are classified as fo l lows: Class 0—Pass: no d iscolorat ion of flux or copper; class 1-Marg ina l pass: strong discolorat ion of flux, but no attack on copper, class 2 - M a r g i n a l pass: sl ight d iscolorat ion of flux w i t h salmon pink 'e tch ' on copper, class 3—Fail: excessive etch ing wi th penetrat ion of copper Ihickness 'h lHuxes chosen for fur ther testing

306-si OCTOBER 1980

of, the copper surface was not classi­fied as a failure, but as a marginal pass (either class 1 or 2). Examples of classes 2 and 3 are shown in Fig. 26. The results of the copper mirror test and the halide content analysis are compared in Fig. 27. The fol lowing observations are made fo l lowing the results of the tests:

1. Based on the copper mirror test results, the extremely corrosive fluxes have a halide content in excess of 4.0% and pH values of less than 3.5. Such fluxes are rarely employed by ESA con­

tractors and used only for the pre-tinning of "di f f icul t " metals.

2. Wi th the exception of the ex­tremely corrosive fluxes, there appears to be no relationship between the resistance to copper mirror corrosion and the individual flux halide concen­trations (from Fig. 27). However, this is unlikely to be true for bi-metallic corrosion or in the presence of alloys susceptible to stress corrosion. For a realistic assessment of the corrosive nature of any flux, it is therefore rec­ommended that the corrosion test assembly reproduce accurately the

physical makeup of each particular solder joint configuration.

3. Slight variations are observed in the halide content and pH values of different batches of the same flux type when purchased in different countries. This indicates that flux manufacturers may change the chemical formulat ion of their brand name products. Howev­er, this has no marked effect on the copper mirror test.

4. Flux types as-received and after boiling produce identical copper mir­ror test results.

z UJ Q-O _ i UJ >

WRC Bulletin 258 May 1980

International Benchmark Project on Simplified Methods for Elevated Temperature Design and Analysis: Problem I—The Oak Ridge Pipe Ratchetting Experiment; Problem II—The Saclay Fluctuating Sodium Level Experiment

by H. Kraus

Problem I—The Oak Ridge Pipe Ratchetting Experiment is analyzed by general purpose finite element computer programs and by approximate analytical techniques. The methods are described and the results are compared to the experimental data.

Problem II—The Saclay Fluctuating Sodium Level Experiment is analyzed by special purpose computer programs and by approximate analytical techniques. The Methods are described and the results are compared. Experimental data are not yet available.

Publication of these reports was sponsored by the Subcommittee on Elevated Temperature Design of the Pressure Vessel Research Committee of the Welding Research Council.

The price of WRC Bulletin 258 is $10.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St.. Room 801. New York, NY 10017.

WRC Bulletin 259 June 1980

Analysis of the Radiographic Evaluation of PVRC Weld Specimens 155, 202, 203, and 251J

by E. H. Ruescher and H. C. Graber

This report is one of a series of analyses of nondestructive examination data obtained from heavy-section steel weldments with intentionally introduced flaws prepared by the PVRC Subcommittee on Nondestructive Examination of Pressure Components.

The primary objective in this work area was to determine the radiographic detectability of deliberately induced flaws. A group of evaluation teams without prior knowledge of the number, type or location of the intentional discontinuities independently examined each specimen in accordance with identical instructions. The results of these examinations were used as the basis for decisions regarding the flaws.

This report describes the evaluation techniques used to reduce the data from the detectability of the deliberately induced and naturally occurring flaws in the weld specimen.

Publication of this report was sponsored by the Subcommittee on Nondestructive Examination of Pressure Components of the Pressure Vessel Research Committee of the Welding Research Council

The price of WRC Bulletin 259 is $11.00 per copy, plus $3.00 for postage and handling. Orders should be sent with payment to the Welding Research Council, 345 East 47th St., Room 801, New York, NY 10017.

W E L D I N G RESEARCH SUPPLEMENT , 307-s