The Effect of Flux Chemistry, Applied Voltage, Conductor...

PURPOSE

The purpose of this research was to investigatethe factors that enhance conductive anodic filament(CAF) formation. The variables studied were (1)water-soluble flux (WSF) formulation, (2) conductorspacing, (3) operating voltage, and (4) temperature.Quantification of the effect of each variable was de-termined through a series of accelerated life tests(ALTs), with each ALT consisting of 32 printedwiring boards (PWBs). An additional goal of this re-search was to acquire a fundamental understandingof the morphology and chemical composition of CAF.

BACKGROUND

Beginning in the mid-1970s and continuingthrough today, the wiring density of electronics hasincreased rapidly. Electronic equipment is also re-quired to operate in outside areas where elevatedtemperature and humidity are uncontrolled. Whileinvestigating the reliability concerns that might re-sult from these conditions, researchers at Bell Labsdiscovered a new failure mode.1–7 This failure modeis characterized by an abrupt, unpredictable loss ofinsulation resistance (IR) between conductors that

are held at a potential difference. The filament, nowtermed CAF, is a result of an electrochemical migra-tion (ECM) process that initiates at the anode andproceeds along separated fiber/epoxy interfaces.8–11

Just prior to this 1976 “definition” of CAF, Der-Marderosian12,13 had noted a similar failure mode.They observed that a filament penetrated betweentwo different layers in an MLB from anode to cath-ode. They termed this failure mode “punch-thru.”Punch-thru is similar to CAF, except the filamentgrows between circuit layers rather than along afiber.

The CAF should not be confused with dendritegrowth. In dendritic growth, metal ions go into solu-tion at the anode but plate out at the cathode, grow-ing in needle- or treelike formations across the sur-face of the PWB. In contrast, CAF growth emanatesfrom the anode. Furthermore, CAF contains copperand a halide ion (typically chloride) rather thanpure metal as in the case of dendrites. Additionally,dendrite growth is a surface phenomenon, whileCAF is a subsurface phenomenon.

In a typical PWB, there are surface tracks of cop-per metallization and copper plated through holes(PTH) that provide component insertion points aswell as electrical continuity between the PWBs topand bottom surfaces. A CAF may bridge two surface

Journal of ELECTRONIC MATERIALS, Vol. 31, No. 11, 2002 Special Issue Paper

The Effect of Flux Chemistry, Applied Voltage, Conductor Spacing, and Temperature on Conductive Anodic Filament Formation

W.J. READY1 and L.J. TURBINI2

1.—MicroCoating Technologies, Atlanta, GA 30341. E-mail: [email protected] 2.—Centre forMicroelectronics Assembly and Packaging, University of Toronto, Toronto, ON M5S 3E4

Conductive anodic filament (CAF) formation, a failure mode in printed wiringboards (PWBs) exposed to high humidity and high voltage gradient, hascaused catastrophic field failures. This study quantified the effect of flux chem-istry, applied voltage (V), spacing (L), and temperature on the failure rate. Testvehicles, which had hole-to-hole spacing of 0.5 mm or 0.75 mm, were processedwith one of three water-soluble fluxes (WSFs), and a heated control was alsoevaluated. The samples were placed in a temperature humidity chamber at85%RH, at one of three temperatures: 75°C, 85°C, or 95°C. A voltage of 150 Vor 200 V was applied to the test vehicle and periodically removed so that ameasurement could be taken. A specially designed linear circuit was used todetermine when the insulation resistance dropped significantly, indicating afailure. Activation energies were determined. The mean time to failure was afunction of L4/V2.

Key words: Flux chemistry, printed wiring boards, filament formation

(Received February 13, 2002; accepted May 21, 2002)

1208

The Effect of Flux Chemistry, Applied Voltage, Conductor Spacing,and Temperature on Conductive Anodic Filament Formation 1209

tracks, two PTHs, or a PTH and a surface track. Fig-ure 1 details these pathways graphically. The sub-surface deposits of corrosion byproducts emanatefrom the anode and eventually progress toward thecathode via a separated fiber/epoxy interface.

The initial Bell Labs researchers1–6 detailed amodel for the mechanism by which CAF formationand growth occurs. The first step is a physical degra-dation of the fiber/epoxy interface. When moisture ab-sorption occurs, it creates an aqueous medium alongthe separated fiber/epoxy interface that provides anelectrolytic pathway and facilitates the transport ofcorrosion products. The second step, electrochemicalcorrosion, results because the absorbed water acts asthe electrolyte, the copper circuitry becomes the elec-trodes, and the operating voltage serves as the drivingpotential. The second step occurs sequentially ratherthan in parallel with the first step.

Lando et al.2 determined that the filaments ap-peared most often in the PTH-PTH test pattern con-figuration (Fig. 1a), apparently as a result of the directcontact of copper and fiber/epoxy interface resultingfrom the hole drilling and plating process. The track-track configuration (Fig. 1d) was the least susceptibleto filament growth, and increasing the spacing be-tween conductors lowered susceptibility further.5 Theaddition of a “buttercoat” (a layer of epoxy resin with-out fiber reinforcement between the conductors andthe PWB surface) further decreased CAF formationfrequency emanating from surface tracks. This reduc-tion is apparently a result of the lack of fiber/copper orfiber/surface interfaces that could initiate debondingand lead to filament growth. Finally, the frequency offilament growth in the track-PTH (Fig. 1b) and PTH-track (Fig. 1c) configurations had intermediate life-times compared to the other conductor configurations.

Numeric Models of CAF Formation

Welsher et al.5 determined that, when the PWB isexposed to thermal transients, such as during multi-

ple soldering steps, the time to failure associatedwith CAF decreased by nearly a factor of 2. The ap-plication of an intense thermal transient served tospeed the debonding of the fiber/epoxy interface as aresult of a mismatch between the coefficients of ther-mal expansion (CTE) of the fiber and epoxy. Theyalso reported that the mean-time-to-failure (MTTF)obeys an Arrhenius relationship in the 50–100°Crange. Furthermore, the humidity dependence seemsto be material lot dependent. Time to failure (tf) re-sults show a strong dependence on relative humiditythat roughly obeys Eq. 1 and is inversely propor-tional to applied voltage according to Eq. 2.

tf 5 a(H)2b (1)

(2)

where H is the relative humidity; V is applied volt-age; and a, b, c, and d are positive material-dependentconstants. [Note: In equations, to prevent confusionwith the universal gas constant, “R,” (8.314 J/Kzmol),the more common “RH” designation for relative hu-midity will be designated simply as “H.” However,“RH” will be used in the text to represent relativehumidity.]

Welsher et al.4 performed experiments on PTH-PTH patterns with a 75-mil spacing biased at 200 V toascertain the effect of temperature, humidity, appliedvoltage, conductor spacing, and substrate materialchoice. They identified a composite of triazine/glass asa resistant material. Triazine is a thermosetting resinmade by reacting diglycedyl ether of bisphenol A(DGEBA) with a cyanogen halide rather than reactingit with dicyandiamide (DICY) as in FR-4. They foundthat lifetimes were 20 to 30 times greater for triazineas compared to typical FR-4.

Welsher et al.4 performed delayed bias applicationtests that confirmed the two-step sequential (ratherthan parallel) process proposed for CAF formation. Inparticular, the rate-limiting step involved fiber/epoxyinterfacial degradation and moisture absorption.This degradation is a function of temperature andrelative humidity. The interfacial degradation is fol-lowed by a very rapid electrochemical corrosion stepthat is an inverse function of the electric field (V/L) aswell as a function of temperature and humidity. Theyalso understood that the spacing between conductors(L) must be bridged by the filament so that the timefor the second sequential step will be proportional toL2/V. Their data confirmed that CAF in PWBs, partic-ularly those with closely spaced PTHs, is a poten-tially serious reliability problem. Combining Eqs. 1and 2, they developed the model shown in Eq. 3 forthe MTTF due to CAF:

(3)

where Ea is the activation energy for the process, Tis temperature (in Kelvin), V is the applied voltage,L is the spacing between conductors (in mils), kb is

MTTF 5 a(H)b z expa Ea

kbTb 1 daL2

Vb

tf 5 c 1 a dVb

Fig. 1. Schematic representation of CAF pathways (dimensions ex-aggerated for clarity): (a) PTH to PTH, (b) PTH to track, (c) track toPTH, and (d) track to track.

1210 Ready and Turbini

Boltzman’s constant (8.62 3 1025 eV/K), a and b arematerial dependent constants, and d describes thetemperature and humidity dependence.

Mitchell and Welsher6 further developed themodel to accommodate different conductor orienta-tions (Fig. 1). If the temperature and humidity de-pendencies for each step in the CAF process areequal, then they can both be approximated by asingle constant a(H)b?exp (Ea/kbT). They produceda revised MTTF due to the filament formationequation that is given in Eq. 4.

(4)

where a and b are material-dependent constants, gis a humidity-dependent factor, and n correlateswith the orientation of the conductors and has avalue of four for the PTH-PTH orientation.

Gandhi et al.9 stated that the voltage dependenceis closer to an inverse voltage squared or inversevoltage cubed relationship, rather than the inverselinear relationship suggested by Eq. 2, Eq. 3, andEq. 4.

Lahti et al.3 showed that for unprocessed PWBsbelow 60°C, the failure mechanism was not ther-mally activated and the Ea for CAF formation wasbetween 0.0 eV and 0.2 eV. Above 65°C, the Ea wasbetween 1.0 eV and 2.5 eV, which is a much strongertemperature dependence than was observed belowthis temperature. Based on this change in Ea, theyconcluded that PWB “lifetimes exceeding twenty tothirty years should be achievable” under normal(25–30°C and 40–50% RH) conditions. However, theyalso discovered that failure rates greatly increasedat a critical conductor spacing of 5 mils under their85°C/85%RH/45 V test conditions.

Research14–20 at the CALCE Electronic PackagingResearch Center used a “physics-of-failure” approachand found that several factors determined the forma-tion of CAF (CALCE research typically uses the term“conductive filament formation” [CFF]): the operatingconditions (humidity, temperature, and voltage), thelaminate choice, and the spacing and geometry of theconductors. They developed a model (Eq. 5) to recon-cile these various factors into a single equation:

(5)

where M is the percentage moisture content, Mt isthe percentage moisture content threshold, a is thefilament formation acceleration factor, f is a multi-layer correction factor, n is a geometry acceleratingfactor, m is the voltage accelerating factor, and k isthe conductor shape factor. A best-fit approach wasused to obtain the values for a, f, k, n, and m fromtheir experimental data. They concluded that theformation of CAF is most highly dependent on theoperating voltage and moisture content of the lami-nate. Though it may appear that the Arrhenius tem-perature dependence was excluded from their study,

tf 5 aa z f z (1000 z k z L)n

Vm z (M 2 Mt)b

MTTF 5 a z a1 1 b z Ln

Vb z Hg z expa Ea

kbTb

the temperature dependence has been incorporatedinto the Mt parameter.

The CALCE work also noted that hollow e-glassfibers may pose a preferential pathway for the metalmigration.18 A substantial amount of their resultsfocuses on the susceptibility of MLBs to filament for-mation, particularly under temperature and humid-ity cycling. The cycling serves to accelerate thedebonding of the fiber/epoxy interface. They notedthis was particularly deleterious when a PTH wasadjacent to the debonding.19

Augis et al.7 concluded that the linear acceleratingfactor extrapolations for the “very high stress testing”used in earlier work by Bell Labs researchers werenot valid. This is because the models based on the lin-ear acceleration factors would have predicted a 6%failure rate of a particular product after 5 years.They observed that this catastrophic failure ratewas not even remotely evident and thus the modelwas wrong if applied under the wrong circumstancesand environmental conditions. They used demarca-tion maps21–23 to understand the degradation modeand proved that a humidity threshold exists. Theyalso noted a wide variability between different lots ofmaterial. They suggest that there is a relative humid-ity threshold, below which CAF formation and growthwill not occur. They base this suggestion on the manyfactors identified by the early Bell Labs researchersand additional information such as the reversibilityof the hydrolysis of the silane-coupling agent.24

They found that, for a 50 V circuit operating at 25°C,the critical relative humidity for CAF formation wasnear 80%. Additionally, they developed a quantita-tive model shown in Eq. 10 that predicted this criti-cal threshold value:

(6)

The constant c (a measure of the rate of CAF forma-tion) was found to be 6.9 3 1024 for a failure proba-bility of 0.5%.

Voltage-Spacing Influence on CAF Formation

A critical factor used in determining CAF suscep-tibility is the “voltage gradient” that the assemblyexperiences. By normalizing the applied voltageover a standard distance, a comparison between dif-ferent assemblies with different pitch and operatingvoltages can be established. This normalization pro-cedure is most graphically demonstrated by exam-ple. An assembly containing a 120 V power planewith an adjacent PTH biased at 220 V has a poten-tial difference of 40 V. This is not considered a highpotential difference for an electronic assembly. How-ever, the 5-mil separation between the power planeand the PTH creates a voltage gradient of 8 V/mil.Figure 2 reveals the circuit failure that occurredwhen a CAF bridged the 5-mil spacing.10 Minimiz-ing voltage gradients is essential to reducing CAF

H 5 £2.3975 1 ln(c) 1

0.9kBT

2 1.52 z ln(V)

5.47§


formation. Using the physics-of-failure approach,CALCE researchers showed20 that the velocity ofthe migrating ions is proportional to the ionic mobil-ity and the voltage gradient.

Polyglycol and WSF Influences on PWBs andCAF Formation

It has been shown that polyglycols diffuse into theepoxy during soldering and degrade insulation re-sistance.25–27 This absorption occurs when the PWBis above its Tg, where elevated temperatures and amore “open” structure facilitate diffusion. This polyg-lycol absorption has been shown to reduce perform-ance by increasing moisture uptake by the sub-strate.28 Jachim et al.29 were the first to link the useof polyglycols in soldering fluxes and fusing fluids toincreased susceptibility to CAF formation. Further-more, Ready et al.10 detail a field failure, which oc-curred on only certain production lots. They showthat this failure resulted from the use of a polyglycolcontaining hot air solder leveling (HASL) fluid dur-ing production. This fluid also contained hydro-bromic acid that diffused into the brominated epoxysubstrate resulting in an increased bromide concen-tration in the PWB. Both of these constituents in-creased the assembly’s propensity for CAF formationby enhancing moisture absorption and providing anappropriate anion for the electrochemical reaction.

Diffusion of polyglycols into the PWB occurs dur-ing soldering. Since the diffusion process follows anArrhenius behavior, the length of time the PWB isabove the Tg will have an effect on the amount ofpolyglycol absorbed into the epoxy and that, in turn,will affect its electrical properties. Brous26 linkedthe level of absorbed polyglycol in a PWB to surfaceinsulation resistance (SIR) measurements. Jachimet al.29 reported on WSF treated PWBs that wereprepared using two different thermal profiles. Thosewhich experienced the thermal profile with highertemperatures exhibited a SIR level that was anorder of magnitude lower than those processedunder less aggressive thermal conditions. It is clearthat the higher the soldering temperature, thegreater the polyglycol absorption. Similarly, for eachthermal excursion that occurs, the fiber/epoxy inter-faces within the PWB are weakened due to differentthermal expansion characteristics of these two ma-terials. Likewise, the PTH and the PWB laminate

material will experience stresses due to a CTE mis-match, which may induce delamination.

Zado25,27 was the first to identify the deleteriousnature of unremoved WSF residues. In 1983, he re-ported on the effect of various constituents of WSFson SIR. He noted that for WSFs, PEG was the mostcommon flux vehicle at the time. Additionally, he de-termined that polyglycol esters and ethers used toprocess the PWBs produced a lowered SIR value.Furthermore, he proposed that PEG became hydro-gen bonded to the epoxy when the PWB was heatedto soldering temperatures. Upon cooling, the PEGbecame “locked” within the polymeric structure. Heshowed that the hydrophylicity of the epoxy in-creases when PEG remained in the PWB. He alsoshowed that a change in relative humidity from 55%to 75% resulted in a slight degradation of SIR, butan increase from 75% to 85% decreased SIR by twoorders of magnitude. He did not see a similar effectwhen polypropylene glycol (PPG) was used. He con-cluded that the steric hindrances between PEG andthe epoxy were the primary reason that PEG exhib-ited these SIR decreases while PPG did not.

Work done by Brous28 at Alpha Metals (Jersey City,NJ) extended the base of understanding of the delete-rious effect that WSF residues posed to PWB SIR. Hisresearch showed that SIR is affected by the (1) degreeof absorption of flux constituent, (2) hygroscopicity ofabsorbed flux constituent, (3) volatility (vapor pres-sure) of absorbed flux constituent, (4) effectiveness ofcleaning process, (5) temperature and humidity, and(6) presence of water soluble ions on the PWB sur-face. He tested several materials frequently used inWSF formulations: glycols, polyglycols, glycol ethers,polyglycol surfactants, polyols, and glycerine. Hefound that polyglycols (particularly those with lowermolecular weights) are strongly hygrosopic and thatsignificant amounts of moisture absorption can occurwithin 24 h if a critical humidity level is exceeded.

Brous28 confirmed that polyglycols diffused intothe PWB during the elevated temperature of solder-ing operations. Furthermore, he confirmed that (ingeneral) lower molecular weight polyglycols aremore highly absorbed into the substrate. His inves-tigations of various polyglycols showed that PWBstreated with them gave SIR values that were ini-tially low (108 2 109 V at 35°C/90%RH). For manypolyglycols, these SIR values rose appreciably (by anorder of magnitude) over short time periods as thepolyglycols diffused out of the epoxy. However, poly-ethylene glycol (PEG) SIR values did not increaseappreciably. He believed this was indicative of atrapping of the PEG within the polymeric backbone,which prevented its diffusion out.

Brous28 further stated that if hygroscopic contami-nants are present on the surface, a conductive con-densed film of water will deposit across the surface,possibly initiating electrolytic oxidation at the anodeand reduction of the metal ions at the cathode. Hefound that when the epoxy/glass laminate absorbedpolyglycols, the laminate became hygroscopic. He also

Fig. 2. Catastrophic failure due to a CAF blowout of layers 6 and 7 ofa 14-layer MLB.


found that under exposure to high temperatures inthe soldering process, many of the polyglycols appearto be capable of penetrating the surface sufficiently tobe retained by the PWB through the cleaning step.

Zado25,27 studied the effect of nonionic WSFresidues, such as PEG, on SIR. Test PWBs treatedwith PEG and cleaned in an aqueous mediumshowed reduced SIR values, while PWBs treated withthe other flux constituents gave SIR levels equiva-lent to unprocessed PWBs. He concluded that thecleaning step was ineffective in removing the polygly-col. Since the polyglycol rapidly diffuses into theepoxy at temperatures above Tg, diffusion out of theepoxy at temperatures below the Tg is expected totake much longer.

Brous26,28 studied several solvent combinationsfor removing PEG and other polyglycols from PWBsand determined that soaking the PWB in acetoni-trile for 24 h provides the best removal process.Ionox FCR (Kyzen) has also been successfully usedto remove polyglycol residues from PWBs processedwith HASL fluids. Some laboratory tests30,31 haveused 75% isopropyl alcohol and 25% deionized waterat 80°C for 1 h to remove polyglycols and other ioniccontaminants from processed PWBs. Bent et al.32,33

studied PWBs processed with PEG and several otherpolyglycols and noted that no CAF was observedwith PEG treated PWBs.

Chemical Composition of Conductive AnodicFilament Formation

Augis et al.7 noted that the filaments always con-tain copper and sometimes contain chlorine or sul-fur. Research by Ready et al.10,34 indicated thatCAFs generally contain copper and chloride but mayalso contain bromide when a bromine containingWSF is used. Meeker and LuValle35 also identifiedthat other anions could be present in the CAF. Mostnotably, they identified bromine and sulfur ions asbeing present in some specimens. CALCE research17

mistakenly identified an x-ray peak (Figure 6 oftheir work) as a CAF containing aluminum, when infact the CAF most likely contained bromine. Thismisidentification has occurred in other research36

and arises because the energy of the bromine La1 x-ray line is at 1.480 kV, while the aluminum Ka1 x-ray line is at 1.487 kV. This difference is so smallthat it is virtually impossible to distinguish the twoelements. In order to differentiate between thesetwo elements, it is necessary to look for the bromineKa1 line at 11.924 kV.

Improved Measurement System to Detect CAF

The SIR measurement method presently specifiedin J-STD-00437 is an ECM test, which should be ableto detect both surface SIR degradation and subsur-face CAF formation. The test method has two majorlimitations.38 (1) The test method takes SIR meas-urements at discrete and widely separated intervals(i.e., 24, 96, and 128 h). (2) It does not provide for aprompt termination of bias when ECM causes an

electrical short circuit. This allows dendrite or CAF“blow-outs” (similar to Fig. 2) to occur destroying theshort and preventing characterization.

A “linear circuit” (LC) has been developed39 toovercome the deficiencies of J-STD-004. Figure 3 isan electrical schematic of the LC. The IR of the testpattern (i.e., interdigitated comb, PTH-PTH, etc.) isdepicted as a variable resistor (RECM). The LC is es-sentially an inverting operational amplifier or op-amp (Fig. 4).40 In the LC, the feedback resistor (Rf) is1.0 MV and the input resistor (R1) is equal to 4.7 kVplus RECM and the additional resistors and capaci-tors are filters used to reduce high and low fre-quency noise. The theoretical gain (Vout/Vin) of theLC is plotted in Fig. 5.

A test pattern with good resistance characteristicswill have a high initial IR (RECM). As ECM formsand bridges between the anode and cathode, thevalue of RECM decreases. The value of RECM will dropto near 0 V at the instant of shorting. This drop (andresulting decrease in op-amp gain) causes the LC toreduce the current flowing through the ECM so thatthe dendrite or filament does not blow out.

EXPERIMENTAL PROCEDURE

Samples

A specialized PWB (Fig. 6) was created for this re-search. The nominal distance between the points ofclosest approach between the two PTH barrels was0.5 mm or 0.75 mm. The PWB is manufactured with1 oz. of copper and the thickness of the copper sur-face features is approximately 70 mm.

Fig. 3. Electrical schematic of linear circuit.

Fig. 4. Inverting op-amp.


A Zero Ion™ System containing a mixture of 75%isopropyl alcohol and 25% deionized water at 25°Cwas used to pre-clean the PWBs to a resistivity of150 MV/cm2. The PWBs were subsequently handledwith latex gloves to minimize any new contamination.

Three WSF formulations were tested. The WSFswere selected based on their propensity to form CAFduring SIR testing.32,33

Table I outlines the composition of each WSF. EightPWBs were prepared with each WSF and eight con-trol PWBs were included in each ALT (i.e., 32 PWBsper ALT). The four cathodic test sites on each PWB(PTHs on right in Fig. 6) were “ganged” together with

a connecting wire so that a failure at any single sitewould register as a failure for the entire PWB.

For each ALT, a pipette was used to dispense 100 mLof flux per PTH-PTH test point (i.e., 400 mL per PWB).The PWBs were reflowed in a bench-top OK Industries(Mt. Vernon, NY) model JEM-310N convection reflowoven with a profile that reached a maximum temper-ature of 205°C.

The post-reflow PWB cleaning procedure consistedof a vigorous tap water rinse at room temperaturefor 1 min. This rinsing was followed by a 5-min ul-trasonic cleaning with deionized water at 65°C in aBranson (Danbury, CT) 5210 ultrasonic bath. A finaldeionized water rinse completed the cleaningprocess. This manual method was used due to theunavailability of the common, industrial scale“aqueous in-line cleaner.” Consistency of the manualcleaning process from sample to sample and frombatch to batch will most likely be lower than thatfound in the automated system.

Accelerated Testing

Teflon-coated connecting wires were attached toeach PWB using rosin-cored solder. The solder jointsfor the wires were placed well away from the WSFtest points so that the rosin-cored solder would notimpact the results. The 32 PWBs were then placedin a Thermotron model SM-4S-SH temperature/humidity chamber. The connecting wires were passedthrough a side access port of the chamber and con-nected to the data acquisition and switching appara-tus described below. The relative humidity in thechamber was maintained at 85% 6 2% throughoutthe ALT. The temperature was also maintained at aconstant level (60.5°C) throughout the ALT, but itwas varied between each ALT. Temperature settingsof 75°C, 85°C, and 95°C were used. Table II detailsthe temperature settings used for each ALT.

To prevent condensation, the chamber was “ramped”from room temperature to the desired ALT tempera-ture. Once the temperature had stabilized at the de-sired level, the relative humidity was slowly rampedover about 2 h to 85%. In addition, the PWBs are al-lowed to equilibrate in the stabilized chamber for 24 hprior to commencing the electrification of the PWBs.

Automated Measuring System

Two Hewlett Packard (Palo Alto, CA) modelE3612A power supplies were connected in series toapply the high voltage necessary to cause CAFgrowth. Each power supply can generate a maximum

Fig. 5. Theoretical gain of linear circuit (Fig. 3) as a function of RECM.

Fig. 6. PWB with variable PTH-PTH spacing.

Table I. Flux Formulation Matrix*

Code Polyglycol Type (always 20 wt.%) HCl, 37.4% HBr, 48% IPA

PG3-2 Poly(ethylene/propylene) Glycol (Avg. mol. wt. 5 1,800) 0 0 80PG3-4 Poly(ethylene/propylene) Glycol (Avg. mol. wt. 5 1800) 0 9.4 70.6PG7-3 Modified linear aliphatic polyether 5.5 0 74.5

*All components are listed in weight percent. HCl and HBr are calculated to provide halides as chlorides equal to 2 wt.%


of 120 V at 0.25 A. Voltage noise was less than 200 mVand current noise was less than 200 mA.

In order that multiple test points could be meas-ured with a single linear circuit (LC), a switching ap-paratus consisting of 1 National Instruments (Austin,TX) SCXI-1001 chassis, 8 SCXI-1160 relay modules(each with 16 double-pole, single-throw electrome-chanical relays), and 8 SCXI-1324 wiring blocks wasconstructed. This chassis also contains one SCXI-1120 data acquisition (DAQ) module that makesvoltage measurements of the LC output. Figure 7schematically illustrates the switching arrange-ment that was applied to 32 test points (four perSCXI-1160 relay module). The 1 MV resistor isknown as a “limiting resistor,” because it limits the

amount of current flowing through the CAF at theinstant of bridging.

The control software, especially developed withLabView (National Instruments, Austin, TX), can dis-play the data as a web page for remote monitoring ofsystem and test status. In addition, the system ob-tains and automatically records the failure times ofthe various test points as tab delimited data in anExcel™ (Microsoft, Redmond, WA) spreadsheet. Thisautomation facilitates the analysis of data and allowsfor an accurate determination of the failure distribu-tion and MTTF. Also, since the system will automati-cally remove the high voltage test bias once a failureoccurs, the filament is preserved for future analysis.

Design of Experiment

Four factors were varied in this study: (1) WSFcomposition, (2) voltage, (3) PTH-PTH spacing, and(4) ALT temperature. The software program CARDPro® (S-Matrix, Eureka, CA) v5.1.2 by S-Matrix wasused to design the experiment and analyze the MTTFresults. It also randomized (1) the PWB preparationorder, (2) placement within the environmental cham-ber, and (3) data connection “port” on the DAQ equip-ment. If all combinations of variables were run, then36 separate experiments would have been required(not including control PWBs). Through the use of theDOE and a quarter-fraction factorial design, thisnumber was reduced to nine (not including controlPWBs). The fractional design matrix used is pre-sented in Table III. Eight PWBs were run at each testcondition.

Electrical Characterization

Electrical observations consisted of failure timedata obtained by the LC. During each of the ALTs,eight PWBs for each WSF chemistry and eight con-trol PWBs (32 PWBs per ALT) were simultaneouslytested to failure. The failure times were then fit to aWeibull distribution. The scale parameter was usedto estimate the MTTF for each PWB set.

Microscopic Characterization

An Olympus SZ-40 optical microscope (magnifica-tion range of 6.73 to 403) was used for transmission

Table II. Temperatures for Each ALT

Test ALT Maximum ReflowNumber Temperature Temperature

1 75°C 205°C2 85°C 205°C3 95°C 205°C

Fig. 7. Switching arrangement for automated LC measurement sys-tem. The figure on the left details the switch arrangement for the highbias phase of the test, while the figure on the right illustrates theswitch settings for the LC test phase.

Table III. Fractional DOE Matrix

Flux Spacing (mm) Voltage (Volts) Temperature (°C) Test Number

Control 0.50 200 — —PG3-2 0.75 200 75 1PG3-4 0.50 150 — —PG7-3 0.75 150 — —Control 0.50 200 — —PG3-2 0.75 200 85 2PG3-4 0.50 150 — —PG7-3 0.75 150 — —Control 0.50 200 — —PG3-2 0.75 200 95 3PG3-4 0.50 150 — —PG7-3 0.75 150 — —


optical observations of the “shadows” of the CAF.Images were recorded with a charge coupled device(CCD) camera attached to a video capture card insidea computer. The colors of the CAF were observedusing reflected light.

A Hitachi (Tokyo, Japan) S-800 scanning electronmicroscope (SEM) with a cold field emission sourcewas used to observe CAF morphology on polishedsections of the samples. Backscattered electronswere used because compositional contrast is supe-rior to that of secondary electrons. Energy-disper-sive x-ray spectroscopy (EDS) was used to analyzethe composition of the CAF. An SEM acceleratingvoltage of 20 kV was selected to obtain the most pre-cise EDS data possible.

A Hitachi HF-2000 transmission electron micro-scope (TEM) with a 200 kV field emission source wasused for TEM observations. The HF-2000 is equippedwith a thin window EDS for compositional measure-ments. The sample was prepared as an acetate replicaby dipping cellulose tape into acetone and laying thesoftened tape across a cross-sectioned CA. Once thetape dries, it is carefully peeled from the surfaces andpulls material from the CAF. The removed material istransparent enough for an electron diffraction pat-tern to be obtained using standard methods. Themeasured radius (r) of the polycrystalline diffractionrings is related to the d spacing of the crystalline com-pound through

l ? L 5 r ? d (7)

where l is the wavelength of the incident radiation(2.51 3 1026 m) and L is the camera length (1.2 m).The experimental values of d are compared to apowder diffraction database to determine the phase.

RESULTS

Electrical Observations

In order to determine the MTTF for the PWBs dueto CAF formation, an analytical technique that in-corporates the entire distribution of failure times isrequired. The Weibull distribution contains a value,

the scale factor (h), which details the point at which63.2% of the distribution has failed for any valuethat the shape factor of the distribution may take.Therefore, it accommodates all the failure points on aweighted basis according to the distribution and al-lows a much more accurate gauge of the failure ratethan would be achieved using the mean, the median,or the mode. The scale factor is also known as the“characteristic life” of the PWB population. Table IVdetails the characteristic life (h), while Table V detailsthe shape factor for the specimens at 85% relative hu-midity and at the various WSF, voltage, spacing, andtemperature conditions for the DOE ALTs. The uncer-tainties are given as the “6” values and correspond tothe standard error based on the Weibull distribution.

Activation Energy

To quantify the degree to which polyglycols, poly-ethers, or halides enhance CAF formation, a deter-mination of the Ea for this failure mode was made.Augis et al.7 has shown that there is a minimum RHrequired for CAF formation. Thus, 85%RH (which isabove the minimum RH value) was the humiditychosen for use throughout this study. By maintain-ing a constant relative humidity throughout thetests and using the same lot of material for all ALTs,the pre-exponential humidity-dependent factor inEq. 3 becomes a constant, C, as shown in Eq. 8:

(8)

Mitchell and Welsher6 showed that the L2 depend-ence above, for track-track CAF, was actually betterrepresented by L4 for the PTH-PTH conductor con-figuration. Additional researchers9 found that the1/V term may also be modeled as 1/V2 or 1/V3.

Therefore, the d?(L2/V) term should be better repre-sented as d?(L4/Vm), where m is a constant betweenone6 and three.9 Equation 9 depicts this modificationto Eq. 8:

(9)MTTF 5 C z expa Ea

kbTb 1 da L4

Vmb

MTTF 5 C z expa Ea

kbTb 1 daL2

Vb

Table IV. Characteristic Life Comparison

Flux Voltage (V) Spacing (mm) 75°C (h) 85°C (h) 95°C (h)

Control 200 0.50 515 6 103 434 6 99 300 6 93PG3-2 200 0.75 566 6 96 529 6 77 443 6 69PG3-4 150 0.50 514 6 76 477 6 76 337 6 63PG7-3 150 0.75 888 6 167 842 6 99 823 6 102

Table V. Shape Factor Comparison

Flux Voltage (V) Spacing (mm) 75°C 85°C 95°C

Control 200 0.50 1.9 6 0.5 1.6 6 0.5 1.2 6 0.3PG3-2 200 0.75 2.1 6 0.6 2.5 6 0.7 2.4 6 0.7PG3-4 150 0.50 2.5 6 0.7 2.3 6 0.7 2.0 6 0.5PG7-3 150 0.75 2.0 6 0.6 3.1 6 1.0 2.9 6 0.9


Since all PWBs of a given WSF were run at the samevoltage and spacing, an iterative methodology (Eqs.10-12) was used to determine optimum values of Eaand C based on the MTTF values at the DOE temper-atures (converted to degrees Kelvin). These valueswere in turn used to determine optimum values of dand m. Table VI presents the summary of these itera-tive results. Based on the derived values in Table VI,Eq. 9 can be simplified further to Eq. 13:

(10)

(11)

(12)

(13)

A plot of the MTTF results for each WSF (normal-ized to each other with respect to the different volt-ages and spacings via Eq. 13) on a logarithmic scaleversus inverse temperature is shown in Fig. 8. Theerror bars indicate the uncertainty based on theWeibull assessment of the MTTF. The equation ofthe “best-fit” line for each data series is given. Also,the “goodness of fit” (R2) value for each best-fit line isgiven.

MTTF 5 C z expa Ea

kbTb 3 3.84 3 107 z aL4

V2b 2 expa Ea

kbT2b d

MTTF1 2 MTTF2 5 C z cexpa Ea

kbT1b

MTTF2 5 C z expa Ea

kbT2b 1 da L4

VmbMTTF1 5 C z expa Ea

kbT1b 1 da L4

Vmb

Microscopic Characterization of Control PWBs

An SEM image of a two sequential cross sectionsof a CAF that formed on a control PWB is shown inFig. 9. The micrograph shows the filament to beconfined to a thin region at the separated fiber/epoxy interface. The EDS spectrum in Fig. 10shows that the CAF is copper and chlorine con-taining. The calcium, aluminum, and silicon peaksare artifacts from the interaction volume of the

Fig. 8. Activation energy determination for all DOE PWBs. The dot-ted line in each plot indicates the relative position of the control line.

Fig. 9. SEM image of two sequential cross sections of CAF on con-trol PWB. (a) approximately 100 mm from the anodic PTH. (b) ap-proximately 50 mm from the anodic PTH. The individual E-glassfibers are labeled to aid in visualization.

Fig. 10. EDS spectrum of CAF on control PWB shown in Fig. 9.

Table VI. C and Ea Values

ActivationC Energy D

(h) (eV) (V2/mm4) m

Control 2.9 3 1023 0.36PG3-2 1.2 3 1023 0.37 3.84 3 107 2PG3-4 7.4 3 1023 0.33PG7-3 3.3 3 100 0.14


electron beam and the e-glass fiber. The bromine,carbon, and oxygen peaks are due to the FR-4epoxy. The gold peak is due to the conductive coat-ing placed on the sample to facilitate SEM analy-sis.

Microscopic Characterization of PG3-2 PWBs

Figure 11 is an SEM image of a CAF that formedon a PWB processed with PG3-2. Note the stratifiedmorphology and the crack associated with the up-permost fiber. The accompanying EDS elementalmap reveals that the CAF contains copper and chlo-rine. The occurrence of the stratified copper andchlorine containing morphology was universallypresent in all PWBs processed with PG3-2 that wereobserved with the SEM.


Figure 12 is an SEM image showing the initia-tion of a CAF at the PTH barrel on a PWBprocessed with PG3-4. Figure 13 is an SEM imageshowing the initiation of yet another CAF at thePTH barrel. Note the corrosion of the copper anodeadjacent to the CAF initiation point. The accompa-nying EDS data reveal that the CAF does not con-tain bromine as might be expected due to the bro-mide containing WSF, but rather is predominantly

Fig. 11. SEM image (a) and EDS elemental map (b) of CAF on aPG3-2 PWB.

Fig. 12. SEM image of CAF initiation point at PTH on the PG3-4PWB. The portion labeled “Copper Land” is actually the PTH “Nail-head” on PWB surface.

Fig. 13. SEM image (top) and EDS elemental map (bottom) ofCAF initiation point at PTH on PWB processed with PG3-4. Theportion labeled copper land is actually the PTH “Nailhead” onPWB surface.


copper and chlorine as in earlier cases. Figure 14shows another example of the stratified CAF mor-phology that was universally present on PWBsprocessed with PG3-4 that were observed with theSEM.


Figure 15 is an SEM image of a CAF that formedon a PWB processed with PG7-3. Note the striatedmorphology. Other PWBs showed CAFs that hadspatially varying chlorine concentrations (Fig. 16).Another CAF was seen at the initiation point withthe PTH (Fig. 17). Again, note the striated mor-phology.

Chemical Nature of Conductive Anodic Filament

The SEM/EDS data show that the CAF observed inthis study is copper and chlorine containing. In an at-tempt to determine the exact crystalline composition ofcopper-bearing CAF, TEM was used to obtain electrondiffraction data41 on a CAF studied earlier.36 When in-dexed (Table VII), the micrographs and electron dif-fraction patterns (Fig. 18) showed good correspondenceto that of synthetic atacamite (2CuCl2 ? 5Cu(OH)2 ?H2O). Indexing of d spacings above ,2.7 Å is not possi-ble due to overexposure of the film by the transmittedelectron beam. When present in nature, atacamite istransparent to translucent and deep green in color. Itdisplays a slender, striated, and acicular to fibrousrhombic or orthorhombic crystal structure.42–44 Syn-thetic atacamite, or copper chloride hydroxide hydrate,is detailed in card number 23–948 from the Interna-tional Centre for Diffraction Data (ICDD).

DISCUSSION

Shape Factor Analysis

In the first detailed study of CAF by Boddy et al.,1 itwas found that the failure mechanism followed “a log-

Fig. 14. SEM image of stratified CAF on PWB processed with PG3-4.

Fig. 15. SEM image of CAF on PWB processed with PG7-3.

Fig. 16. SEM image and EDS elemental map of CAF on PWBprocessed with PG7-3.

Fig. 17. SEM image of CAF on PWB processed with PG7-3. Theportion labeled “Copper Land” is actually the PTH “Nailhead” onPWB surface.


approximates the lognormal distribution. Table V de-tails the shape factor data that results when the DOEALT data are fit to a Weibull distribution. With the ex-ception of the control, all the data values contain 2.5within the standard error limits. Thus, the presentdata agree with the earlier lognormal distributionfound by Boddy et al. A likely reason that the controlPWBs do not follow a similar distribution as that ob-served by Boddy et al. is that the control PWBs wereheated in the reflow oven for these data, while theywere unheated in the Bell Labs research.

Activation Energy

The majority of previous CAF research was per-formed on unprocessed PWBs. The results from BellLabs researchers showed an Ea for CAF formation tobe between 0.0 eV and 0.2 eV below 60°C and be-tween 1.0 eV and 2.5 eV above 65°C.3

Calculating the Ea for unprocessed PWBs mayyield interesting data. However, since all “real”PWBs must undergo a soldering step, it is more use-ful to determine the Ea for CAF formation that re-sults when various WSFs are used during PWB pro-cessing. This is particularly true since it has beenshown in previous research10,29 that the occurrenceof CAF can be enhanced by using WSFs that containcertain ingredients such as halides and polyglycols.

Table VI details the Ea results. The effect of usingPG3-4 is virtually indistinguishable from the con-trol and does not appear to be appreciably deleteri-ous to MTTF. The graphical depiction of these “nor-malized” MTTF results (Fig. 8) suggests that theaddition of bromide in this formulation (PG3-4) ac-tually improves CAF resistance as compared to thenonbromine containing version of the same polygly-col (PG3-2). Rubin et al.45 similarly observed thatbromine additions to no-clean fluxes improved SIRperformance. It is unclear why this behavior occurs,though the bromide anion may have some competi-tive effect with the chlorine reaction kinetics,thereby slowing CAF formation. Also, note that allEa values are less than or equal to 0.5 eV, which isindicative of a relative humidity driven corrosion ordegradation mechanism.46 Also of interest is thevery low Ea for the PG7-3 PWBs. This low value im-plies that there is a very low energy barrier to CAFformation with PG7-3 and, hence, a much faster rateof formation. A plausible reason for this dramaticdifference from the other WSFs is that PG7-3 con-tains chlorine, whereas other fluxes do not. Chlorineis a key component of CAF. For the cases of the otherWSFs, the chlorine must be scavenged from residueswithin the PWB. However, when using PG7-3, therewill be an abundance of excess chlorine available forionization and incorporation into the CAF. The ex-cessive CAF formation with PG7-3 may also be dueto the interaction between the modified linearaliphatic polyether and the polymeric epoxy.

To verify that the iterative values were accurate,substitutions into Eq. 13 were made using the given T,V and L values in coordination with the calculated Ea

Table VII. Electron Diffraction Pattern Indexing

Tabulatedd Spacing

for Measured Calculated Synthetic

Ring Radius d Spacing Atacamite(mm) (Å) (Å)

11.5 2.62 2.6513.5 2.23 2.2715.5 1.94 1.9716.5 1.82 1.8217.5 1.72 1.7222.0 1.37 1.37

Fig. 18. TEM micrograph (left) and electron diffraction pattern (right)of CAF.

normal distribution for the central portions of the dis-tribution.” When the shape factor for the Weibull dis-tribution is 2.5, the Weibull distribution very closely


and C values (Table VI). The calculated MTTF resultshave a very good fit (Table VIII) to well within the ex-perimental error of the measured MTTF for each WSF.

Data from another set of experiments processedat a different reflow temperature were used to verifythat the model was applicable to other data sets.Table IX shows that the calculated MTTF resultsare also comparable to the experimental error. Thisindicates that the activation process is consistenteven for a slightly lower reflow temperature.

It was noted that the calculated Ea value for thecontrol PWBs was less than that observed by theBell Labs researchers.3 The most likely cause of thedifference is that the control PWBs experienced thethermal reflow excursion in this study but did not inthe Bell Labs work. Thus, the fiber/epoxy interfacewas weakened due to the CTE mismatch and ther-mal excursion. In addition, the PTH-PTH spacingused for Bell Labs research was 75 mils (2 mm), ver-sus 20 mils (0.5 mm) or 30 mils (0.75 mm) here.

Drilling PTHs or reflowing the PWBs can weakenor separate the fiber/epoxy interface. If the PTHsare spaced closely together, fiber/epoxy separationsat one PTH may intersect the separations from an-other PTH. For the wider spacing in the Bell Labswork, the separations may not intersect.

WSF Formulation Effects

Control PWBs

Identifying CAF on control PWBs is often difficult dueto the very fine filament size (i.e., ,10–50 mm in breadth).

The SEM cross sections of the CAF taken within ,100 mmof the PTH barrel (Fig. 9) reveal that the CAF is confinedto a very small “halo” region immediately adjacent to theseparated fiber/epoxy interface. The size of the CAF doesappear to increase slightly; however, this apparent thick-ening is actually due to a larger crack opening closer to theanodic PTH.

The EDS spectrum presented in Fig. 10 shows nu-merous peaks for a CAF on a control PWB. One rea-son for the numerous peaks is the fine filament size.Despite being able to precisely select the spot on theCAF where a spectrum is obtained, as the electronbeam decelerates in the analyzed material, it formsa teardrop-shaped “interaction volume” below thespot selected for analysis where x-rays may be ex-cited and detected. Since the acceleration voltage isfairly large, a relatively large interaction volume isgenerated. The calcium, aluminum, and siliconpeaks result from the e-glass fiber adjacent to the“spot” on the CAF selected for analysis. The carbon,oxygen, and bromine peaks are due to the epoxy ma-trix that is also adjacent to the CAF. The gold peaksare due to the conductive coating. The peaks due tothe actual CAF are copper and chlorine. The sourcefor the copper is obviously the PTH. Chlorine con-tamination can come from a variety of sources. How-ever, the most likely source for the chloride is fromthe epichlorohydrin (C3H5OCl) used in the epoxymanufacturing process. Generally, 1000–2000 ppmof total chloride are present in the finished epoxy.However, the amount of available chloride is typi-cally 100 ppm.47

Table IX. Comparison of Measured and Calculated MTTF for Other Experimental Data

ALT Meas. Std. Calc. MTTFTemp. V L C Ea MTTF Error MTTF Difference

Flux (°C) (V) (mm) (h) (eV) (h) (h) (h) (h)

Control 85 200 0.50 2.9 3 1023 0.36 356 50 398 42PG32 85 200 0.50 1.2 3 1023 0.37 255 53 259 4PG34 85 200 0.50 7.4 3 1023 0.33 332 43 386 54PG73 85 200 0.50 3.3 3 100 0.14 307 34 368 61

Table VIII. Comparison between Measured and Calculated MTTF

Meas. Std. Calc. MTTFV L C Ea Temp. MTTF Error MTTF Difference

Flux (V) (mm) (h) (eV) (°C) (h) (h) (h) (h)

Control 200 0.50 2.9 3 1023 0.36 75 515 103 534 1785 434 99 399 3595 300 93 307 7

PG3-2 200 0.75 1.2 3 1023 0.37 75 566 96 585 1985 529 77 503 2695 443 69 448 5

PG3-4 150 0.50 7.4 3 1023 0.33 75 514 76 553 3985 477 76 435 4295 337 63 352 15

PG7-3 150 0.75 3.3 3 100 0.14 75 888 167 895 785 842 99 851 995 823 102 815 8


PG3-2 and PG3-4

The chemical composition of CAF that forms withPG3-2 includes copper and chlorine (Fig. 11); the sameis true for PG3-4 (Fig. 13). Since PG3-2 does not con-tain chlorine as an intentional additive, the epichloro-hydrin remaining from the epoxy manufacturingprocess is again a likely source for this anion. Testingof PG3-4 was based on the hypothesis that a bromine-containing WSF would induce bromide-containingCAF as had been observed previously.10,34 Figure 13and all other SEM/EDS analyses showed that therewas a normal concentration of bromine in the EDSspectra for the PG3-4 CAFs. Since the bromine con-centration in PG3-4 (2 wt.%) is lower than that fromearlier work10,34 (approximately 15%), the results sug-gest that a critical threshold must exist for bromidecontaining CAF to form.

A CAF that bridged the PTHs on a PG3-4 PWBwas sectioned at different locations as it approachedthe anodic PTH (Fig. 19). The SEM micrographs ofthe two sections (Fig. 20) show that the CAF islarger close to the PTH. The individual e-glass fibersare labeled to aid in visualization.

Since the CAF is growing from the anodic PTH(PTH on left in Fig. 19), the copper ions must travelthe length of the filament if it is to be extended to-ward the cathodic PTH (right PTH in Fig. 19). Mi-gration along the exterior surface of the filamentprovides the most favorable pathway, since migra-tion along the interior of the already formed CAFwould be impeded by the significantly less mobilecopper-chloride compound, which is bound withinthe epoxy structure. Thus, it is reasonable to assumethat the filament would be larger near its base thanat its tip, since the copper compound must continu-ally advance radially outward if it is to advance to-ward the cathodic PTH. This type of behavior hasalso been observed previously.36

Figure 11 is an SEM micrograph of a CAF thatformed on a PWB processed with PG3-2. The sepa-rated fiber/epoxy interface is visible, with a crack ex-tending through the epoxy to the upper left. Notethat this CAF extends into the epoxy unlike CAF ob-served on the control PWB. Furthermore, there is astriated appearance of the deposits. The polymericstructure of the epoxy (Fig. 21) has both distinct“layers” and functional groups where a copper ioncan form a complex or chelated structure along thepolymeric backbone. Since a high voltage gradient isone of the driving forces for filament growth, thestratified morphology may be due to the alignmentof the electric field.

PG7-3

Figure 22 shows the locations of the cross sectionsshown in Fig. 23. It is obvious that the filament islarger closer to the anodic PTH (right of Fig. 23).There is a distinct striated appearance to the CAF.Whereas a stratified appearance is associated withPG3-2 and PG3-4 processed PWBs, the striated (i.e.,spotted) appearance for CAF is associated withPG7-3 PWBs (Figs. 15-17). PG7-3 contains a modi-fied linear aliphatic polyether with a structure R-(O-CH2-CH2)x-R9 (where R is an alkyl or alkylarylgroup, R9 is a modifying “cap,” and x is the number ofmoles of ethylene oxide) and the other fluxes con-tain a poly(ethylene/propylene) glycol. Zado25,27 has

Fig. 19. Optical transmission micrograph showing several CAFs andapproximate location of cross section 1 (left of Fig. 20) and crosssection 2 (right of Fig. 20). Cross sectioning proceeded from right toleft on this PG3-4 PWB. Anode is at left.

Fig. 20. CAF cross section 1 (left) approximately 100 mm from PTHand cross section 2 (right) adjacent to PTH on PG3-4 PWB. The por-tion labeled “Copper Land” is actually the PTH “Nailhead” on PWBsurface. Fig. 21. Epoxy structure.48


shown the effect of polyethylene oxide on epoxy, andthis striation behavior is likely attributable to theabsorption characteristics of the polyether into the“pockets” of the epoxy backbone (Fig. 21).

The CAFs formed on PWBs processed with PG7-3also were copper and chlorine containing. However,unlike previous filaments, the chlorine content var-ied with spatial location within the CAF (Fig. 16).This striation of chloride deposits was only associ-ated with PWBs processed by PG7-3. The additional2% chlorine in this WSF formulation provides signif-icantly more available anions for inclusion withinthe CAF than the residual chloride from theepichlorohydrin. The absorption characteristics ofthe modified linear aliphatic polyether may be in-strumental in the formation of these striations.

Chemical Nature of Conductive AnodicFilament

The qualitative EDS data from the SEM analysisclearly showed that CAF in this study is copper andchlorine containing. The electron diffraction results(Fig. 18 and Table VII) provide evidence that the CAFis synthetic atacamite. Using x-ray powder diffrac-tion, Raffalovich49 also identified green atacamite asa common corrosion product on PWBs. The formationof atacamite in moist air can be completed in a few

hours,50 and this lends credence to the Bell Labs datashowing that the electrochemical corrosion step inCAF formation is quite rapid.4 Equations 14-17 pro-vide the stoichiometric chemical equations that leadto synthetic atacamite formation. The term R desig-nates a “generic” cation.

7Cu → 7Cu21 1 14e2 (14)

4RCl → 4R1 1 4Cl2 (15)

11H2O → 10(H)1 1 10(OH)2 1 H2O (16)

7Cu 1 4RCl 1 11H2O → 2CuCl2 ? 5Cu(OH)2z H2O 1 14e2 1 4R1 1 10(H)1 (17)

In addition, the Pourbaix diagram51 for the cop-per-chlorine-water system (Fig. 24) reveals that acompound similar to synthetic atacamite known asparatacamite with the form CuCl2 ? 3Cu(OH)2 is sta-ble in the mid to upper left-hand portion of the dia-gram, a region which corresponds to anodic voltagesand acidic conditions. These are the precise condi-tions required for CAF formation. Equations 18-21provide chemical reactions that lead to the forma-tion of the stable product predicted by the Pourbaixdiagram. It is obvious that the only differences be-tween synthetic atacamite and the compound in thePourbaix diagram is the relative stoichiometric pro-portions of copper, chloride-salt, and water. Further-more, mineralogists sometimes refer to atacamite asparatacamite or botallacktite,43,44 so these differ-ences may be a matter of semantics between Profes-sor Pourbaix and mineralogists.

Fig. 24. Pourbaix51 diagram for copper-chlorine-water system at 25°Cwith 35 ppm Cl2. The chemical formulas indicate stable compoundsfor that particular range of pH and potential.

Fig. 22. Approximate cross section locations of CAF from a PWBprocessed with PG7-3. Cross sectioning proceeded from right to left.

Fig. 23. SEM image of PWB in Fig. 22 showing cross section 1 (left)located approximately 100 mm from the anodic PTH and cross sec-tion 2 (right) located adjacent to the PTH.


4Cu → 4Cu21 1 8e2 (18)

2RCl → 2R1 1 2Cl2 (19)

7H2O → 6(H)1 1 6(OH)2 1 H2O (20)

4Cu 1 2RCl 1 7H2O → CuCl2 z 3Cu(OH)2z H2O 1 8e2 1 2R1 1 6(H)1 (21)

Chloride Containing CAF versus BromideContaining CAF

Although bromine containing CAF has been ob-served in previous research,10,34 it was not observedin the present work. Nevertheless, it is important tounderstand why chloride CAF is more common thanbromide containing CAF. Table X includes the perti-nent atomic information for copper, chlorine, andbromine. It can be seen that when bound to a copperion, chloride has a much larger electronegativity dif-ference than bromide. This larger difference creates amore favorable attraction between copper ions andchloride than between copper ions and bromide. Fur-thermore, the ionic radius of chloride is smaller thanbromide, which would tend to favor chloride contain-ing CAF, since the steric hindrances to ion transportalong the fiber/epoxy interface will be lower. As dis-cussed earlier, it is believed than bromide containingCAF will form when there is a level of bromine in ex-cess of 2 wt.% (but less than 15 wt.%). When this oc-curs, it is possible for chloride containing CAF andbromide containing CAF to form simultaneously. Thiswas observed in previous research.34

CONCLUSIONS

The purpose of this research was to investigate thefactors that enhance CAF formation in printed wiringboards. The variables studied were (1) flux formula-tion, (2) conductor spacing, (3) operating voltage, and(4) temperature.

A Weibull distribution of failure times due to CAFwas observed. It was found that the flux formulationaffected the rate of CAF formation. A modified linearaliphatic polyether flux with a chloride activator hada significantly different activation energy than controlprinted wiring boards or those boards processed witha poly(ethylene/propylene) glycol flux or a poly(ethyl-ene/propylene) glycol flux with a bromide activator.

The addition of bromine to a poly(ethylene/propy-lene) glycol flux decreased the rate of CAF formationas compared to poly(ethylene/propylene) glycol with-out a halide activator. The inter-relation between

voltage and conductor spacing was quantified as aL4/V2 relationship for the plated through hole testpattern used in this study.

Microscopic analysis showed distinct differencesin CAF morphology between the various processedboards. Control boards had small halolike CAF forma-tions around a separated fiber/epoxy interface. TheCAF that formed on boards processed with poly(ethyl-ene/propylene) glycol or poly(ethylene/propylene) gly-col with a bromine activator had a stratified appear-ance that penetrated well into the epoxy. Boards thatwere processed with the modified linear aliphaticpolyether with chlorine activator had a striated mor-phology that also penetrated into the epoxy.

All CAFs were consistently copper and chlorinecontaining despite the use of a bromine containingflux. Electron diffraction revealed that a CAF ob-served in this study was synthetic atacamite.

ACKNOWLEDGEMENTS

This work was supported by United States ArmyMissile Command Contract Nos. DAAH01-92-D-R005-0013 and DAAH01-92-D-R005-0039, by the Of-fice of Naval Research Contract No. N0001149710057,and by an International Microelectronics and Packag-ing Society Educational Foundation grant.

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Table X. Pertinent Atomic Data

Ionic IonicElectro- Radius Mass

Name Z negativity Ion (nm) (g/mol)

Chlorine 17 3.16 Cl2 0.181 35.4

Copper 29 1.90 Cu1 0.096 79.9Cu21 0.072

Bromine 35 2.96 Br2 0.196 63.5


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