Pergamon Corrosion Science, Vol. 39, No. 8, pp. 1415-1430, 1997

0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved

001~938X/97 $17.00+0.00

PII: s0010-938x(97)0003s-3

ELECTROCHEMICAL MIGRATION TESTS OF SOLDER ALLOYS IN PURE WATER

T. TAKEMOTO*, R. M. LATANISIONI_, T. W. EAGARt and A. MATSUNAWA*

*Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567, Japan

t Department of Materials Science and Technology, Massachusetts Institute ofTechnology, Cambridge MA 02139, U.S.A.

Abstract-Electrochemical migration (ECM) tests on solder alloys were conducted by applying constant voltage

with a power supply or sweeping the voltage at a constant rate with a potentiostat. Distilled water was used as the

test environment. Time to short and short voltage were the measure of ECM susceptibility. Similar results were

obtained in both test methods. In Sn-Pb alloy system, pure lead showed the highest susceptibility. Alloys with 5-

60%Sn showed similar high susceptibility. Further addition of tin lowered the susceptibility and pure tin had the

lowest susceptibility. Dendrites grew from cathode and reached to the anode. The composition of dendrites of Sn-

40Pb alloy was enriched in lead. Some tin base solder alloys without lead were more resistant to ECM than Sn4OPb

alloy. Pure indium, In-48Sn and In-5OPb alloys were found to be immune to ECM in pure water. Large anodic

dissolution rate seems to be responsible to high ECM susceptibility. 0 1997 Elsevier Science Ltd

INTRODUCTION

Soldering is one of the key technologies for assembling microelectronics components on printed circuit boards. Recently surface mount technology has become the primary process to produce high functional, high response compact and light weight electronic products.‘4 Also the size of the component and the conductor spacing has been reduced from 0.65 mm to 0.3 mm in Quad Flat Packages, and the tip components such as capacitors and condensers with 1 .O mm length and 0.5 mm width have been introduced.5

A board with a high density of components generates much more heat during operation resulting in more severe thermal fatigue of the soldered joints. Other problems arising from boards with greater mounting density include electrochemical reactions such as corrosion,6 metal migration ’ and reduction of surface resistance.8 Metal migration is often called electrochemical migration, electrolytic migration, or ion migration.97’0 The phenomenon is based on growth of metal dendrites in humid atmospheres. In most cases the metal migrates as ions from the anodic site to the cathodic site where it is deposited. In this paper, metal migration related to electrochemical reactions is referred to as electrochemical migration (ECM). In the future, it is felt that electrochemical migration will become one of the most severe problems in electronic soldering for the following two reasons. One arises from the narrow conductor spacing. At constant voltage, the electric field between the conductors rises inversely with the conductor spacing, and electrochemical migration is known to be enhanced under high electric fie1ds.“-‘3 The second reason is based upon changes in the

Manuscript received I.5 April 1995; in amended form 15 September 1995.

1415

1416 T. Takemoto ef al.

micro-soldering process. The cleaning process of the board after soldering has been changed due to regulation of chlorofluorocarbons such as Freon, CFC and other organic cleaning

solvents which damage the ozone layer.14 These solvents are the most reliable cleaning

agents to remove rosin flux residues2 but their use is now severely restricted. There are some replacing cleaning methods using substitutional organic detergents having low or no-ozone damaging factor for rosin based fluxes or water-based detergents for water soluble fluxes.

Under these circumstances, soldering in inert gas atmospheres using low residue fluxes and so-called no-clean fluxes’5m22 that require no cleaning step after soldering have become popular. But the reliability of assembled boards after long time use, especially chemical stability has not been fully established yet. This change in the cleaning and soldering processes provides a higher likelihood of corrosion and ECM.8

Consequently, selection of solder alloys resistant to ECM becomes important in maintaining high reliability of electronics joined by soldering technology. Most studies

have been conducted on silver and copper and their alloys.“,23m28 The purpose of the present experiments is to obtain basic data on the susceptibility of solder alloys to ECM and to clarify the effect of solder alloy composition on ECM behavior.

EXPERIMENTAL METHOD

In the present study, distilled water was adopted as the primary test environment. The resistivity of the distilled water was 0.18 MR m. The solder alloys which were studied consisted mainly of Sn-Pb binary system alloys, and other binary tin alloys and indium alloys. Silver and copper were also used for comparison. Solder alloys were cold rolled to 0.15 mm thickness and cut to the shape shown in Fig. 1. Two solder specimens of the same composition were placed opposed to each other and clipped together by glass slides as shown in Fig. 1. The spacing between the two specimens was adjusted under a microscope using a micro-scale. After clamping the assembly, pure distilled water was added using a thin capillary.

Electrochemical migration tests were carried out using a power supply and a potentiostat. Using a constant voltage a short circuit due to growth of dendrites was

Solder Gla;s !&ate Solder(O.15 t)

‘Glass plate

(mm)

Fig. 1. Schematic of setting of specimen for ECM test.

ECM tests of solder alloys in pure water 1417

clearly observed by a sudden IR drop between specimens. The time to short, tM was used as a measure of the tendency for ECM. In the potentiostat test, the voltage was scanned linearly toward the noble direction against the counter electrode with a scanning rate of 0.15 - 30 mV/s, usually 5 mV/s. A platinum counter electrode with the same size as solder specimen was also used as the cathode material to investigate the effect of cathode material on ECM. Short circuits due to growth of migration products were clearly observed by a drastic increase of current. The voltage and current at the time of shorting were measured as parameters for the susceptibility to electrochemical migration (ECM). The slope of current

increase, i.e. the increment of current per unit voltage was also measured for each specimen which did not produce a short circuit. The short voltage using a platinum counter electrode is expressed by the difference in potential at short and the initial potential of the solder vs the platinum, for example, the short voltage of 2 V means the solder short circuited at 2 V higher than its natural potential vs platinum.

EXPERIMENTAL RESULTS

Sn-Pb binary alloys Many studies using the constant voltage method have shown that the time to short is

reduced by increasing the applied voltage and reducing the gap between the electrodes.“-‘3 In the potentiostat test, it is expected that lower voltage sweep rates and reduction of the gap

would lower the short voltage, similar to the behavior of the time parameter in the constant voltage method. Figure 2 shows the effect of sweep rate (scanning rate) on short voltage of Sn-40Pb solder alloy specimens with 0.5 mm gap distance. The short voltage decreases with decreasing sweep rate, as does the current. Many dendrites were observed at high sweep rates, whereas only a limited number of small fine dendrites grew at slow sweep rates.

Figure 3 is an example of voltage-current curves. There seems to be no significant changes with increments of current up to 1 V, however as mentioned above, the short voltage and the current at short became higher with increases in the sweep rate. The

6

E

,.,.- 100 Sn-40Pb

Distance: 0.5mm 7 3

1

Swe:p rate (S/s) loo0

Fig. 2. Effect of sweep rate on short voltage and current at short of SnAOPb alloy under voltage

sweep method.

1418 T. Takemoto et al.

80

z 3 60

1 .SmV/s

5mV/s ! ; 15mVls

30mVls 1 &:-f-.N D,Sta”ce: 0,5mm Sn-40Pb/DW 0 1 2 3 4 5 6

Voltage (V)

Fig. 3. Examples of voltage

ECM tests of solder alloys in pure water 1419

250. . I . . I ’ I Sn-40Pb

5

Time to short Electric field: W/mm

3 200 - -4 0

__-- -0 z

5 150- -3 5 3 .

5

s loo- Sweep rate: 5mVls

-2 L g 5 ._ 6 c 50 - -1

t I...., . . . . . . . . . . . . . . lo

X.0 0.5 Distanck’ (pm)

1.5 2.0

Fig. 4. Effect of gap distance between two specimens on short voltage in voltage sweep test and time

to short in constant voltage test.

rich alloys with more than 8O%Sn showed higher short voltages with increasing tin. Higher short voltage means higher resistance to ECM, therefore, in tin rich alloys ECM is more difficult to initiate than in lead rich and Sn-Pb alloys with less than 60%Sn. Comparing the effect of the counter electrode, VM, in the solder/platinum electrode is slightly shifted toward higher voltages than in the solder/solder electrode. The reason for this will be discussed later.

Two of three pure tin specimens showed no ECM irrespective of the counter electrode, as indicated by upward arrows. The wide difference in the results of pure tin seems to be based on the surface condition of the tin.

1 Electric field: W/mm

z Distance: 0.5mm

0 A b A

A

a2 lo2 A ’

E A

.-

10” . ’ ’ . ’ . ’ . ’ 0 20

Tin co%nt %ass~~ 100

Fig. 5. Effect of Sn-Pb solder alloy composition on time to short in constant voltage test.

1420 T. Takemoto et al.

10 Distance: 0.5 mm 0 Solder/Solder Sweet rate: 5 mV/s + SolderR

0’ I 0 20

Tin cogent 80

?rhss%) 100

Fig. 6. Effect of Sn-Pb solder alloy composition on short voltage in voltage sweep test.

Figure 7 shows the ECM current at short when one dendrite grows from the cathode to the anode site. The dependence of solder alloy composition was almost the same with both solder/solder and solder/platinum electrodes. A maximum value was observed in pure lead, while the small addition of tin drastically decreased the current at short. The effect of further addition of tin seems to have no clear relation with tin content, but on the whole, the current at short gradually decreased with increasing tin content. All Sn-Pb alloys showed similar currents at short on the order of 20- 30 PA. Nearly the same current was observed irrespective of the counter electrodes.

Figure 8 shows optical micrographs of the ECM products observed in pure lead, Sn- 40Pb and Sn-5Pb alloys. Many dendrites grew with the pure lead, however, in the Sn-5Pb alloy the number of dendrites was small. The higher number of dendrites corresponded well with the increased current at short as shown in Fig. 8.

1404 . Distance: 0.5 mm l Sold&Solder

, 120 -

Sweep rate: 5 mVls x Solder/Pt

.

0’ I 0 20

Tin cofnknt 80

~rktss%l 100

Fig. 7. Relation between Sn-Pb solder alloy composition and current at short.

ECM tests of solder alloys in pure water 1421

c > .5 1’ b 9 5 S n 200~ m I I

Fig. 8. Optical micrographs of migration, left; cathode, right; anode, the gap between cathode and anode is 0.5 mm.

Figure 9 shows the secondary electron image of nodular dendrites formed after the migration test. In pure lead many bold and long dendrites with many dendrite arms are observed. In the Sn+lOPb alloy both the primary and the dendrite arms are finer and shorter than those in pure lead, while globular products were observed at both the top and the root

I422 T. Takemato et al

a) IOOPb

b) 4OPb-6OSn

c) SPb-OSSn

Fig. ‘9. SEM of migration products

of the arms. In Sn-SPb alloy both primary dendrites and secondary arms are short and fine, and the total number is small.

In practical use a short between the circuits due to migration of the solder alloy causes failure of the circuit, therefore, the length of the dendrites is more important than the

ECM tests of solder alloys in pure water 1423

Table 1. Examples of quantitative EDX point

analysis on dendrites, mass%

Solder alloys

1 OOPb

Sn40Pb

Sn-5Pb

Elements

Sn Pb

0 100

18.6 81.4

94.6 5.4

number, however, in the Sn-Pb system, both the length and the number are large at lead rich compositions.

Table 1 shows the examples of quantitative energy dispersive X-ray (EDX) point analysis on dendrites. The main constituent of the dendrites in Sn4OPb alloy was lead. This coincides with the ECM test results which show that tin rich alloy has superior ECM resistance to lead rich alloys.

As indicated in Figs 7 and 8, similar results were obtained for counter electrodes of the solder and pure platinum. However, the ECM initiation voltage was slightly higher in the test with the platinum counter electrode. In the case of Sn-Pb alloys, the ECM products mainly consisted of lead, therefore, the following reaction for migration should be considered at cathodic site.

Pb3+ + 3e = Pb

This suggests the above cathodic reactions might be suppressed or difficult to occur at platinum surface compared to the solder alloy surface. The reduction process produces the nodular migration products from the cathode. On the platinum cathode surface, the reduction of the hydrogen ion should be easier because of its high hydrogen exchange

current.29’30 Accordingly, the following reduction reaction,

2Hf+2e=H2

would occur easily at the platinum cathode, consuming the current and reducing the reduction rate of metallic ions. This results in the less reduction of lead at the cathode site because the observed current is almost the same irrespective of the counter electrode. Consequently, the initiation voltage moved toward higher values.

Similar effects were observed with palladium on silver migration.25 Palladium is known to reduce the hydrogen overpotential in an electrodeposition cell.3’ Thus the presence of palladium reduce the deposition rate of silver by consuming the current to produce hydrogen gas at the cathode. The results show that the cathodic reaction also influences for the ECM growth rate.

The ECM susceptibility of some solder alloys ECM tests were conducted using a potentiostat and pure platinum as the cathode. Table

2 summarizes the short voltage, V M, ECM current at short, Z,, and slope of current increase for alloys which do not exhibit ECM. All binary tin alloys except Sn-1.2Al and SnAOPb, showed ECM between 3.2 and 4 V. The current at short ranged between 9 and 15 PA. Sn-1.2A1, pure indium, In48Sn and In-5OPb showed no susceptibility to ECM in distilled water at applied electric fields up to 16 V/mm, and voltage sweep up to 8.5 V.

1424 T. Takemoto et ul.

Table 2. ECM test results on some solder alloys, mean values more than duplicate tests under

sweep rate of 5 mV/s

Solder alloys

(mass %)

Sn4OPb

Sn48In

Sn2.7Cu

Sn3.8Ag

Sn4.6Ag

SnS.OSb

Sn-I .2AI

In48Sn

In-5OPb

IOOln

Short voltage

(V)

2.58

3.20

3.31

3.78

3.91

4.00

No short

No short

No short

No short

Current at short

@A)

I8

I5

II

I2

I3

9

No short

No short

No short

No short

Slope of current increase*

@A/V)

(7.0)

(4.7)

(3.3)

(3.2)

(3.3)

(2.3) 0.5

I .3

2.0

2.7

* The values in parentheses are the current at short divided by the short voltage.

Addition of ECM susceptible elements such as copper or silver induced susceptibility to ECM in tin alloys, which shorted at about 3.3-3.9 V, as compared to pure tin specimens which showed no ECM, Fig. 7. The Sn-481n alloy did not exhibit ECM, as pure indium is immune to ECM and pure tin is highly resistant. It is likely that Sn48In will show higher resistance under special surface conditions as mentioned later. In-5OPb possessed resistance to ECM even though it contains lead. In-Pb binary alloys form a solid solution at all concentrations, therefore, lead as a solute element in indium seems to have no role in producing ECM. Within the range of the present experiments, all binary tin alloys had superior resistance to SnllOPb alloys from the point of view of the ECM initiation voltage, current and slope of current increase per unit voltage. The Snl.2Al alloy was especially resistant to ECM, however, the alloy has poor solderability. Indium and In-Pb alloys are candidates for ECM resistant alloys, however, it is important to check the ECM susceptibility as a function of environmental conditions and humidity in practical use.

To reduce the soldering temperature the use of bismuth bearing Sn-Pb alloys has become popular in the production of electronics for commercial use. Figure 10 represents the effect of bismuth addition to Sn4OPb alloys on short voltage. The short voltage becomes higher with increasing the bismuth content indicating the substitution of tin for bismuth relieved the ECM susceptibility. There seems to be a linear relationship between the short voltage and bismuth content. The mechanism of improvement is not clear, however, the use of bismuth bearing alloys might be effective in reducing the susceptibility of soldered

products to ECM.

DISCUSSION

Susceptibility of tin to ECM The ECM tests on pure tin given in earlier reports are not consistent. DerMarderosian32

reported pure tin migrated in pure water; on the other hand, Kawanobe et al. I2 classified tin as one of the most resistant metals. According to Murata’s constant current method in pure water, 33 tin is classified as relatively resistant but is not completely immune to ECM. Similar discrepancies are also found for gold and palladium.‘2*32533

In our experiments using a potentiostat, some pure tin specimens did not exhibit ECM

ECM tests of solder alloys in pure water 1425

101 Sweep rate: l.SmV/s 1

8-

6- 0

4-

,

-0 5 10 15 Bi content (mass%)

20

Fig. 10. Effect of bismuth addition to Sn40Pb alloy on short voltage.

growth, while some migrated and shorted at less than 5 V. In migrated pure tin, the current increased at relatively low voltage leading to initiation and growth of dendrites. It was postulated that the rough surface would increase the dissolution rate by enhancement of anodic and/or cathodic reaction(s) and might effect the formation of dendrites. Therefore, the effect of surface treatment of the specimen was investigated. Table 3 shows the effect of surface treatment on the shorted voltage of tin specimens. For emery polished (EP) specimens, only the edge of the specimen was polished using 600 grade emery paper and then corner burrs were removed. Other specimens were polished with filter paper(

Table 3. Effect of edge surface treatment of tin specimen on electrochemical migration in distilled water, numbers

in parentheses are the mean values, distance; 0.5 mm, sweep rate; 5 mV/s

Edge surface Short voltage, Short current,

treatment FM (V) iM (114

ih.f/V~ or slope of current increase

WV) Remarks

Filter paper polish,

FP

(Both anode and cathode)

Emery paper polish,

EP

(Both anode and

cathode)

Anode: FP Cathode:EP

Anode: EP

Cathode; FP

No short No short 0.28 *

No short No short 0.56 No short No short 1.20

(No short) (No short) (0.68)

2.75 5.0 1.82 3.60 7.5 2.08 4.62 3.8 0.82

(3.66) (5.4) (1.57)

8.10 7.50 0.93 No short No short 0.70

2.93 8.25 2.82 5.90 8.00 1.36

(4.42) (8.13) (2.09)

* Formation of small dendrite were observed, the dendrites did not grow to reach anode even after repeated

twice scans up to 10 V.

1426 T. Takemoto et al.

In specimens treated with FP on both the anode and cathode, shorting by ECM did not occur, while for specimens with both the anode an the cathode treated by EP, short

circuiting occurred at about 3.7 V. The combined treatment of EP and FP offered interesting results. ECM occurs easily with an EP anode, while specimens with EP edges on both sides are the most susceptible to ECM. In tests under constant voltage, all tin specimens short circuited within several hundred seconds, while in the potentiostat mode, tin with a FP edge migrated only if the voltage increased slowly, i.e. at slow sweep rate. Thus, the difference caused by the surface treatment of tin and the experimental conditions in the prior studies is likely to be responsible for the difference in the measured susceptibility of tin. In our work tin is not immune to ECM: under certain surface and experimental conditions it migrates. Soldered surfaces are usually smooth with high reflectivity, therefore, tin is expected to be relatively resistant to ECM as long as surface scratches or pollutants are not present.

Susceptibility of some pure metals Table 4 summarizes the ECM results of various metals. In the ECM test in pure water,

lead and Sn-Pb alloys are more susceptible to ECM than silver and copper which have previously been reported as the most susceptible metals.32 It is important to note that lead bearing solder alloys possess high susceptibility. Further tests in solution containing chemical reagents will be reported in a separate paper. In the present experiments, immune metals such as indium and tin have a passive region near neutral pH, whereas the more susceptible silver and lead alloys have corrosion regions throughout the entire pH region.34%35 The band for lead is wider and is located at lower potentials as compared to silver. Copper has passivation regions just under pH 7;3’ however, it still migrated, although the susceptibility is lower than solder alloys and silver.3’ The ECM susceptibility in pure water seems to be related to the pH-potential diagram. The metals having immunity or passive region at neutral pH range would be immune to ECM. In fact metals such as aluminum, zirconium have been classified as immune,12 however, tungsten and nickel have

been reported to be immune I2 in spite of the fact that they have corrosion domains near the neutral pH region. In addition gold and titanium have been also reported to be immune;12 they have corrosion regions at relatively high potential near the neutral pH. The immunity

Table 4. ECM characteristics of some pure metals under the sweep rate of 1.5 mV/s and 5 mV/s, the values are

mean values more than duplicate tests

Pure metals and

Sn4OPb solder

alloy

Sweep rate: 1.5 mV/s Short voltage Current at short

(V) @A)

Sweep rate: 5 mV/s Short voltage Current at short

(V) (CIA)

Pb

Sn4OPb

Ag* Cut S” I”

1.65 58.8 2.49 123 1.73 5.5 2.58 17.8 2.02 3.7 2.71 4.4 3.11 6.8 4.17 13.4 5.88 5.3 No short (0.6811

No short (0.25)f No short (1.5)$

*Thickness of specimen is 0.1 mm.

TThickness of specimen is 0.3 mm. 1 The value in parentheses are the slope of current increase, i.e. increment of current per unit voltage

ECM tests of solder alloys in pure water 1427

of these metals have not been strictly established, because Der Marderosian32 reported that gold and titanium migrates under special condition. Susceptibility of these metals depends on the test methods including purity of test water and surface preparation of specimen. It could be suggested from potential-pH diagrams that the metals having immunity or passive region at neutral pH range would be immune to ECM under pure water. If this relation applies to all metals, beryllium, gallium, hafnium, tantalum and niobium should be immune to ECM under pure water. Although the potential-pH diagram cannot predict the

susceptibility to ECM completely, it could offer valuable information for immunity to ECM.

Current and susceptibility Figure 11 shows examples of voltagecurrent relationships on pure lead, Sn4OPb and

pure tin. For pure tin both migrated and non-migrated specimens were observed. Pure lead showed the highest current using the low sweep voltage, while tin showed the minimum current. The figure suggests that the high current might be responsible for the higher ECM susceptibility.

Figure 12 shows the plots of short voltage and current at shorting for various Sn-Pb solder alloys as based on Figs 7 and 8. For pure tin which exhibits no ECM under some conditions, the slope of current increase is plotted. There is a large amount of scatter, however, on the whole, the short voltage becomes lower under higher currents. As indicated in Fig. 12, high current at shorting corresponds to a high slope of current increase, therefore, high current is the result of a high dissolution rate which would provide high sensitivity to ECM.

The following conditions seem to be responsible for high ECM susceptibility.

(1) High dissolution rate of metal as ionic species (2) Low hydrogen evolution at cathode

In addition to the above, low saturation concentration of metallic ions in pure water might also be responsible for high susceptibility.

lo3

2 lo2

3

E 10’ Q k =I o loo

10-I. 0 2

Voltage 4W) 6

Fig. Il. Changes of current during sweep of voltage for lead, Sn-Pb and tin, sweep rate of 1.5 mV/s.

1428 T. Takemoto et al.

Sweep rate: 5mVls 0 Solder/Solder -

E

+ Solder/Pt

8-

+ ++o 0

0..

0’ . ’ ” . I . n “. ’ ” 0 20 100 120 140

Cu%mt 'a'l st% @A)

Fig. 12. Plots of short voltage vs current at short ofvarious Sn4 - 1 OOPb alloys, data based on Figs I and 8.

Tin is found to have superior ECM resistance to lead and Sn-Pb alloys. This behavior corresponds well with the corrosion susceptibility of tin, lead and their alloys in pure water.36 Nevertheless in boric acid and in KCl04/H2Cl04 solutions, the dependence of corrosion resistance on tin-lead composition was similar in pure water.36 The corrosion rate follows the following order.

Pb = Pb2Sn > Pb35~80Sn > Sn

The tendency is very similar to Fig. 6. Randovici et al. 36 investigated the corrosion of Sn-Pb alloys in aqueous solutions with

different pH. Their results showed that the corrosion current measured by the Tafel slope was a minimum with Sn4OPb alloys, but the dependence of maximum current density in the anodic polarization curve on alloy composition is similar to the present results, Figs 7 and 8. The changes in passivation current with alloy composition also showed the same tendency37 as Fig. 6, with the exception of a minimum at the Sn+IOPb alloy. Based upon these results, it is believed that a large dissolution rate at the anode is necessary for the easy migration.

CONCLUSIONS

ECM tests using constant voltage and sweep voltage in pure water revealed that both methods showed similar results suggesting the voltage scanning method is effective in predicting ECM susceptibility of solder alloys. The ECM short voltage, current at short an degree of increase of current during scanning could be obtained using the voltage scanning method. These parameters seemed to be effective in comparing the ECM susceptibility of different solder alloys. In the Sn-Pb system, pure lead was the most susceptible to ECM and the most popularly used Sn40Pb alloy also showed high susceptibility to ECM, while tin exhibited relatively high resistance to ECM. This order of susceptibility corresponds well to the corrosion resistance of these alloys in aqueous environments. The surface treatment of tin greatly affected ECM. On a carefully polished surface, ECM did not occur; however, tin is not immune to ECM.

ECM tests of solder alloys in pure water 1429

The ECM products were composed mainly of lead in Sn-4OPb alloy. Among the solder alloys tested, pure indium, In48Sn, In-5OPb and Sn-1.2Al were found to be immune to

ECM in pure water. Thus these alloys are candidates for ECM resistant solder alloys for practical use. In tin base alloys, the addition of silver or copper, elements susceptible to ECM, decreased the short voltage and increased the current at short suggesting that the addition of silver or copper to the solder alloy causes a deterioration in the resistance to ECM. Sn-Pb solder alloys were found to possess higher susceptibility to ECM than silver and copper. These alloys are believed to have high sensitivity to ECM and, therefore, ECM of solder alloys can be an important problem on the printed circuit boards with high component density. It is necessary to clarify the ECM susceptibility of solder alloys under various conditions in addition to these studies in pure water.

Acknowledgements-The authors would express their hearty thanks to Prof. Y. Matsumura at Faculty of Eng, Kansai University for helpful discussions. Thanks are also to Nihongenma MFG Co., Ltd., Tanaka Precious Metal

Industries and Sanbo Copper and Brass Co., Ltd. for providing materials.

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