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The potentiodynamic polarization behavior of Pb-free XIn-9(5Al-Zn)-YSn solders
MATERIALS CHEM;$fW&lD ELSEVIER
Materials Chemistry and Physics53 ( 1998) 55-59
The potentiodynamic polarization behavior of Pb-free XIn-9 ( SAl-Zn) -YSn solders Kwang-Lung
Lin ‘:, Fu-Chih
Chung, Tzy-Pin Liu
Received 21 April 1997: received in revised form 30 October 1997; accepted 28 November 1997
Abstract In an investigation for Jcveloping Pb-free Al-Zn-Sn solders, indium was added to lower the melting point of the solders. The melting point of a 1OIn-9( SALZn)-Sn solder was found to be 192°C when measured with differential scanning calorimetry. Meanwhile, the potentiodynamic electrochemical behaviors of thus prepared In-Al-Zn-Sn solders in a 3.5% NaCl solution were explored. The investigated solders exhibit electrochemical passivation behaviors. The corrosion products formed during the polarization study were characterized with scanning electron microscopy and X-ray diffraction. The corrosion products formed, depending on potential, during polarization were found to be ZnO, SnO, 0 1998 Elsevier Science S.A. SnO, and Sn,O,. Kqxvrds:
Potentiodynamic polarization; Solders
The eutectic 9 ISn-9Zn
1. Introduction
has a melting
point of 198°C which
The health andenvironmental concernshave called for the development of Pb-free solders [ I-31. Quite a number of Pb-free soldershave beendisclosedin the literature. Among theseare the binary systems,e.g. Ag-Sn, Au-Sn, Bi-Sn, CdSn, Sb-Sn and Zn-Sn [ 4-1 I ] and certain ternary systems such as Sn-Ag-Zn [ 121, Sn-Zn-In [ 13,14], Sn-Ag-Sb [lS],BiSb-Sn [ 16],Sn-Bi-Ag [ 171andSn-Ag-Cu [18].
is very close to the eutectic temperature, 183”C, of the PbSn system. The incorporation of Zn in the Zn-Sn solders gives rise to a poor oxidation resistance,although it resulted in a better mechanicalproperty than the conventional Pb-Sn solders.The corrosion resistanceof Zn has been improved by the addition of Al [ 23.241 in the applications for atmospheric corrosion resistance,asin the caseof the conventional galvanizing coatingsfor steel.SAI-Zn and55Al-Zn coatings
Wetting
are the most commonly
property,
melting
point,
fatigue
life and corrosion
resistanceareamongthe mostimportant considerationswhen developing solderalternatives.The melting point of the solder governs the processing conditions. The wetting property determines the bonding strength of the solder joint. The fatigue life and corrosion r&stance determine the joint life and thus the component life. The metallurgy and mechanical properties of a number of soldershave been reviewed [ 193.
commercialized
Al-Zn
coatings.
It
[ 13,14,20] will give rise to a good mechanicalproperty and fatigue life. Indium wasapplied to lower the melting temper-
was tried in a seriesof study to incorporate Al with the ZnSn soldersin order to enhancethe corrosion resistanceof the solders. Al may form solid solutions with Zn and Sn. An equilibrium phasediagram has been reported [25] for this particular ternary systemwhich hasa eutectic point of 197°C. The diffusional pathsof various Al-Zn-Sn systemsat various isothermshave also been discussed[ 251. The Al dosageis kept low in order to keepthe melting point aslow aspossible. It was attempted in this work to lower the melting point of the solderby incorporating In andto investigate the corrosion
ature of the corresponding solder, yet with a loss in mechanical properties. Indium, as much as IO%, was added to lower
resistance of thus prepared solders. The wetting and mechanical properties are also undergoing investigation and are to
the melting temperature of Sn-Zn solder [ 211. The soft indium solder also served as a die attachment medium in a multichip module design [ 221,
be discussedin other work.
Among
all of the alloying
elements of the Pb-free solders, Zn
* Corresponding author. Tel: + 886-6-2757575-6290016-238-3866:
fax:
+ SS6-6-233-6290. 0254-0584/98/$19.00 PI180254-0584(
0 1998 Elsevier Science S.A. All rights reserved 97 )02062-2
2. Experimental The Al and Zn were added with a SAI-Zn masteralloy. The In, Sn and SAl-Zn were melted in a furnace to give Xln-
56
K.-L. Lin el cd. /Mnteriais
Chemistry
9(5AI-Zn)-Sn solders, The In contents are 5%’ and 10%. The metals were kept melting for a few hours in the furnace to assure a uniform composition. The potentiodynamicpolarization experiments were conducted from - 1500 mV ( SCE) to - 100 mV (SCE). The specimens were cathodically treated at - 1500 mV ( SCE) for 10 min in the 3.5% NaCl testing solution, which was deaerated with N2 for 1 h, prior to the polarization scanning. The specimens were investigated for their surface corrosion products formed after the polarization scanning up to the potentials of interest, refer to the polarization curve. The corrosion products were characterized for their morphology, compositions and microstructure, respectively, by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction ( XRD) The melting points of the solder alloys were measured by differential scanning calorimetry (DSC) under an Ar atmosphere at a 10°C min-’ of heating and cooling rates.
3. Results and discussion Figs. 1 and 2 are the DSC results of the 5In-9(5Al-Zn)Sn and lOIn-9( SAL-Zn)-Sn solders, respectively. The DSC curves of both solders exhibit a single peak upon heating from ambient temperature to 400°C. This behavior delineates that these two solders have a eutectic property. The eutectic temperatures are 191.9”C and 196/K, respectively, for 10 In- and 5 In-containing solders. The melting point of the lOIn-9( SAl-Zn)-Sn solder is around 9°C greater than that
and Physics 53 (1998)
55-59
of the conventional eutectic 63Sn-37Pb -solder. The DSC cooling curves show distinct sharp peaks at 172.4”C and 175.2”C for 10 In- and 5 In-containing solders, respectively. The deviations of the solidifcation temperatures from the melting temperatures are believed to be caused by supercooling 1261. A tiny peak occurs for both solders at a temperature somewhat greater than the detected solidification temperature on the DSC cooling curve. This is assumed to be due to a precipitation behavior as discussed in another work
v71. The polarization curves of the In-containing solders are presented in Fig. 3 along with that of the 9(5Al-Zn)-Sn solder. All three polarization curves possessverysiitnilarprofiles. The corrosion potentials are all in the range of - 1350 N -1250 mV (SCE) wherein the 5In-9( SAI-Zn)Sn solder is the most subjective to corrosion while the 9( 5AlZn)-Sn solder is the most noble one. These solders exhibit passivation behaviors in the potential range between - 1000 and - 500 mV (SCE). The passivation current densities are around the order of lo-’ A cm-? with the descending sequence of 5In-9(5Al-Zn)-Sn> lOIn-9(5Al-Zn)-Sn > 9 (5 Al-Zn)-Sn. The variations in the polarization curve reflect the different corrosion reactions. It is of interest and of importance to characterize the corrosion products formed at every particular stage of the polarization curve so as to provide a sketch of the corrosion behavior. In a series of XRD investigation, it was found that the corrosion products of the solders investigated in this present Cwe A : Temperature CweB : Dsc (mW)
Sl~-9(5Al-Zn>S~ loin-9(5Al-Zn)-Sn
Curves4 : Tempaature CweB : Dsc (mW)
(c) VS.Tii (ti) vs.Tii (mitt)
(‘C ) vs. Time ( min ) vs.Tiw (mill)
DSC
-===P
TCXXIp
DSC
00.00
,mzi--goo.oo
00.00
00.00
00.00
a,.,+,
0.00
100.00 Time[min]
200.00
Fig. I. The DSC curve of rhe lOIn-9(5
. 0.00
,
,
,
,
.
100.00
,
,
)
)
.
*
,
.oo
200.00300.0
TimcLmin]
Al-Zn)-Sn
solder.
Fig. 2. The DSC curve of the .51n-9(5
Al-Zn)-Sn
solder.
K-L.
-a 00
-a 00
4 00
Lin et al. / Maierials
-2 00
Chernisrry
53 (1998)
55-59
57
most of the In added forms InSn,. ZnO and SnO, were detected in the as-prepared solder. However, In and Al were not seen to form a corrosion product either in the as-prepared solder or during the potentiodynamic polarization. The surface morphology of the solder polarized to - 900 and - 700 mV (SCE) are shown in Figs. 5 and 6, respectively. The metal compositions (Table 1) analyzed with EDS on the surface of these two specimens show an enrichment in In. 13.5% and 16.2% compared to 10% of the as-prepared solder. It is evident from Figs. 5 and 6 that the corrosion products became visible when polarized up to -700 mV (SCE). These products consist of ZnO and SnOz as revealed by the XRD results of Fig. 4. The primary passivation behavior may be due to the formation of thick, compact corrosion products of SnO, and ZnO which slow down the diffusion of corrosives and metal ions. The XRD results show that the corrosion products formed at the potentials from - 500 to - 150 mV (SCE) consist of SnO and Sn,04 in addition to ZnO and SnO,. The surface composition of the specimen polarized up to -500 mV (SCE) has an even lower content of Zn compared to the Zn content achieved at -900 and -700 mV (SCE). It is believed that the ZnO tends to dissolve especially when the potentials move to a further anodic direction. The dissolution of the ZnO is further confirmed through the fact that only 2.0 - 2.4% (Table I ) of Zn was detected at the surface when polarization moves to - 300 and - 150 mV (SCE). In the meantime, the surface Sn content increases gradually from 84.6% to 9 1.8%. The dissolution of the ZnO at - 300 mV (SCE) from the surface corrosion product layer results in a porous product layer as revealed in Fig. 7. The porous product
0 00
log(l/Area)(Akm”Z) Fig. 3. The potentiodynamic polarization solution of the 9(5 Al-Zni-Sn, 5In-9(5 Zn)-Sn solders.
rind Pizysics
curves, in deaerared 3S%NaCl Al-Zn)-Sn and lOIn-9(5 Al-
work are of similar types. Fig. 4 shows the XRD curves of the lOIn-9(5Al-Zn)-Sn solder as a representative. after being potentiodynamically polarized to particular potentials of interest. The XRD of the as-prepared solder shows that the In and Sn can foml intermetallic compounds InSn, and In,Sn. However, In,Sn and pure In were not detected for all the samples other than the as-prepared solder. This means that
.
0
-15OOmV--150mV
Sn
-1500mV--300mV
In sno : Sn,O, v sno, v zilo * I&n, A In,Sn 0
zn
l
Ii
-1500mV--5OOmV
-1500mV--9M)mV 4
0.W
Fig. 4. The XRD analysis
20.W
I
40.w 28
60.00
6O.W
ot’thcpotentiodynamically polarized solders atdifferentpotentials, andas-prepared lOIn-9(
5 Al-Zn)-Sn
solders.
58
K.-L. Lin et cd. /Mnrehls
Table 1 The hut-face composition
In Al Zn Sn
of the IOIn-91.5
Al-Zn)-Sn
Chnnimy
solder at different
polarization
mri Physics 53 (IY98J 55-59
potentiais
-
IOin-9(5Al-Zn)-Sn
- 900 mV
- 700 mv
- 500 mV
- 300 mv
- IS0 mv
10 0.45 8.55 81
13.5 0.75 1.5 1 81
16.2 0.083 5.88 77.8
11.3 0.12 4.04 84.6
10.0 0.20 2.0 87.8
5.09 0.73 2.35 91.8
Fig. 5. The SEM surface morphology p01; irked to - 900 mV (SCE)
of the lOIn-9(5
Fig. 6. The SEM surface morphology polarized to - 700 mV f SCEj.
of the lOIn-915
Al-Zn)-Sn
Al-Zn)-Sn
solder, Fig. 7. The SEM surface morphology polarized to - 300 mV (SCE),
of the IOIn-915
Al-Znj-Sn
solder,
Fig. 8. The SEM surface morphology polarized to - 150 mV (SCE).
of the lOIn-9(5
AI-Zn)Sn
solder.
solder,
layer will not provide any protection to the underlying metal against corrosion. Accordingly, a peak current density is achieved at the potential of around - 300 mV CSCE). and similarly for the other two solders as seen in the polarization behaviors of Fig. 3. On the basis of the above discussion a reaction sequence of the potentiodynamic polarization behavior is proposed as follows. The as-prepared solder exhibits
in the passivation between - 900 and - 500 mV. At potential From - 500 to - 300 mV, the ZnO dissolves or falls and the current density increases, ZnO-+Zn+‘+O’At potentials above -300 mV, the reaction starts forming SnO and Sn,O,
Zn+ l/2 0,+&O
Sn+ l/2 02+Sn0
Sn + O2 3 SnO,
3 Snf20,+Sn,O,
The growth of these products to a thick, compact layer results
and thus the second passivation occurs.
The further dissolution of the ZnO product and the tremendous growth of the oxides of Sn up to - 150 mV (SCE) gave rise to the flake products as seen in Fig. 8. At this stage. the surface composition analysis shows a diminishing of In down to as few as 5. I ‘%. In other words. the dissolution of In may not occur significantly until the potential approaches as anodic as - 150 mV ( SCE) This is understandable as most of the added indium form intermetallic compound. mainly In,%,, which is relatively noble in comparison with the constituents of the investigated solders, as shown by the XRD results (Fig. 4).
4. Conclusions The 5In-9( SALZn )-Sn and lOIn-9( SAl-Zn)-Sn solders exhibit eutectic behavior. The melting points of the 5In9(5Al-Zn)-Sn and lOIn-9(SALZn)-Sn solders are, respectively, 19 I .9 and 196.3”C. The potentiodynamic polarization behaviors of these two solders are very similar to that of the 9( SALZn)-Sn solder. The 5 wt.% In-containing solder is slightly more susceptible to corrosion than the 10 wt.% In-containing solder. The corrosion products formed during the potentiodynamic polarization in a 3.5% NaCl solution are SnO, and ZnO in potentials from - 1500 to - 700 mV (SCE). SnO and SnxO, were formed when polarized to - 500 mV (SCE j and above. Iridium form In,Sn and InSn, compounds in the solders. The In and Al did not form detectable corrosion products during the polarization investigation; In did not dissolve until - 150 mV (SCE).
Acknowledgements The financial support of this work from the National Science Council of the Republic of China under NSC85-22 16E-006-03 1 is gratefully acknowledged.
References Environtnental Protection Agency, Strategy for Reducing Lead Exposure. 21 Feb. 1991. Environmental Protection Agency, Advanced Notice of Rulemaking, 131May 1991. Environmental Protection Agency, Comprehensive Review of Lead in the Environment under TSCA, 56FR 22096-98, 13 ~May, 199 I. P.T. Vianco. D.R. Frear, J. Met. ( 1993) 14-19. J.W. Morris, Jr., J.L. Freer Goldstein, 2. Mei. J. Met. ( 1993) 25-27. C.H. Rneder, L.E. Felton, V.A. Tanzi, D.B. Knorr, J. Electron. Mater. 23 (7) (1994)6ll-617. L.E. F&on, C.H. Raeder, D.B. Knorr, J. Met. t 1993) 28-32. W.L. Winterbottom. J. Met. ( 1993) 20-N. W.J. Tomlinson, I. Collier, J. Mater. Sci. 22 (1987) 1835-1839. M. Harada, R. Satoh, IEEE Trans. Components, Hybrids and Manufacturing Technol. 13 (4) ( 1990) 736-742. 2. Mei, J.W. Morris Jr., J. Electron. Mater. 2 I (6) t 1992) M. McCormack, S. Jin, G.W. Kammlott, H.S. Chen, Appl. Phys. Lett. 63 (I) (1993) 15-17. M. McCormack, S. Jin, H.S. Chen, D.A. Machusak,J. Electron. Mater. 23 (7) (1994) 687-690. M. McCormack, S. Jin, J. Electron. Mater. 23 (7) ( 1994) 635-640. D.B. Masson. B.K. Kirkpatrick. J. Electron. Mater. I5 (6) ( 1986) 339-353. G. Ghosh. M. Loomans. M.E. Fine, J. Electron. Mater. 23 (7) ( 1994) 619-623. U.R. Kattner. W.J. Boettinger, J. Electron. Mater. 23 (7) ( 1994) 603610. CM. Miller, I.E. Anderson, J.F. Smith. J. Electron. Mater. 23 (7) (1994) 595401. I. Glazer, Int. Mater. Rev. 40 (2) ( 1995) 65-93. H. IMaroori. S. Vaynman, J. Chin, B. Moran. L.M. Keer, M.E. Fine, Mater. Rea. Sot. Symp. Proc. 390 ( 1995) 161-175. Mark McCormack. Sungho Jin, J. Met. ( 1993) 36-40. R. Darveaux, I. Turlik, IEEE Trans. Components, Hybrids and Manufacturing Technol. I3 (4) ( 1990) 929-939. L. Allegra, J.C. Zoccola, Mater. Perform. 22 ( I) ( 1993) 5. H.E. Townsend, J.C. Zoccola, Mater. Perform. I8 ( 10) ( 1979) 13. A. Sebaoun, D. Vincant, D. Treheux. Mater. Sci. Technol. 3 ( 1987) 241. 2. Mei. J. Electron. Packaging. Trans. ASME I I7 (2) (1995) 105. K.L. Lin, L.H. Wen. T.P. Liu. The microstructures of the Al-Zn-Sn solder alloys. J. Electron. Mater.. accepted for publication.
Materials Chemistry and Physics53 ( 1998) 55-59
The potentiodynamic polarization behavior of Pb-free XIn-9 ( SAl-Zn) -YSn solders Kwang-Lung
Lin ‘:, Fu-Chih
Chung, Tzy-Pin Liu
Received 21 April 1997: received in revised form 30 October 1997; accepted 28 November 1997
Abstract In an investigation for Jcveloping Pb-free Al-Zn-Sn solders, indium was added to lower the melting point of the solders. The melting point of a 1OIn-9( SALZn)-Sn solder was found to be 192°C when measured with differential scanning calorimetry. Meanwhile, the potentiodynamic electrochemical behaviors of thus prepared In-Al-Zn-Sn solders in a 3.5% NaCl solution were explored. The investigated solders exhibit electrochemical passivation behaviors. The corrosion products formed during the polarization study were characterized with scanning electron microscopy and X-ray diffraction. The corrosion products formed, depending on potential, during polarization were found to be ZnO, SnO, 0 1998 Elsevier Science S.A. SnO, and Sn,O,. Kqxvrds:
Potentiodynamic polarization; Solders
The eutectic 9 ISn-9Zn
1. Introduction
has a melting
point of 198°C which
The health andenvironmental concernshave called for the development of Pb-free solders [ I-31. Quite a number of Pb-free soldershave beendisclosedin the literature. Among theseare the binary systems,e.g. Ag-Sn, Au-Sn, Bi-Sn, CdSn, Sb-Sn and Zn-Sn [ 4-1 I ] and certain ternary systems such as Sn-Ag-Zn [ 121, Sn-Zn-In [ 13,14], Sn-Ag-Sb [lS],BiSb-Sn [ 16],Sn-Bi-Ag [ 171andSn-Ag-Cu [18].
is very close to the eutectic temperature, 183”C, of the PbSn system. The incorporation of Zn in the Zn-Sn solders gives rise to a poor oxidation resistance,although it resulted in a better mechanicalproperty than the conventional Pb-Sn solders.The corrosion resistanceof Zn has been improved by the addition of Al [ 23.241 in the applications for atmospheric corrosion resistance,asin the caseof the conventional galvanizing coatingsfor steel.SAI-Zn and55Al-Zn coatings
Wetting
are the most commonly
property,
melting
point,
fatigue
life and corrosion
resistanceareamongthe mostimportant considerationswhen developing solderalternatives.The melting point of the solder governs the processing conditions. The wetting property determines the bonding strength of the solder joint. The fatigue life and corrosion r&stance determine the joint life and thus the component life. The metallurgy and mechanical properties of a number of soldershave been reviewed [ 193.
commercialized
Al-Zn
coatings.
It
[ 13,14,20] will give rise to a good mechanicalproperty and fatigue life. Indium wasapplied to lower the melting temper-
was tried in a seriesof study to incorporate Al with the ZnSn soldersin order to enhancethe corrosion resistanceof the solders. Al may form solid solutions with Zn and Sn. An equilibrium phasediagram has been reported [25] for this particular ternary systemwhich hasa eutectic point of 197°C. The diffusional pathsof various Al-Zn-Sn systemsat various isothermshave also been discussed[ 251. The Al dosageis kept low in order to keepthe melting point aslow aspossible. It was attempted in this work to lower the melting point of the solderby incorporating In andto investigate the corrosion
ature of the corresponding solder, yet with a loss in mechanical properties. Indium, as much as IO%, was added to lower
resistance of thus prepared solders. The wetting and mechanical properties are also undergoing investigation and are to
the melting temperature of Sn-Zn solder [ 211. The soft indium solder also served as a die attachment medium in a multichip module design [ 221,
be discussedin other work.
Among
all of the alloying
elements of the Pb-free solders, Zn
* Corresponding author. Tel: + 886-6-2757575-6290016-238-3866:
fax:
+ SS6-6-233-6290. 0254-0584/98/$19.00 PI180254-0584(
0 1998 Elsevier Science S.A. All rights reserved 97 )02062-2
2. Experimental The Al and Zn were added with a SAI-Zn masteralloy. The In, Sn and SAl-Zn were melted in a furnace to give Xln-
56
K.-L. Lin el cd. /Mnteriais
Chemistry
9(5AI-Zn)-Sn solders, The In contents are 5%’ and 10%. The metals were kept melting for a few hours in the furnace to assure a uniform composition. The potentiodynamicpolarization experiments were conducted from - 1500 mV ( SCE) to - 100 mV (SCE). The specimens were cathodically treated at - 1500 mV ( SCE) for 10 min in the 3.5% NaCl testing solution, which was deaerated with N2 for 1 h, prior to the polarization scanning. The specimens were investigated for their surface corrosion products formed after the polarization scanning up to the potentials of interest, refer to the polarization curve. The corrosion products were characterized for their morphology, compositions and microstructure, respectively, by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction ( XRD) The melting points of the solder alloys were measured by differential scanning calorimetry (DSC) under an Ar atmosphere at a 10°C min-’ of heating and cooling rates.
3. Results and discussion Figs. 1 and 2 are the DSC results of the 5In-9(5Al-Zn)Sn and lOIn-9( SAL-Zn)-Sn solders, respectively. The DSC curves of both solders exhibit a single peak upon heating from ambient temperature to 400°C. This behavior delineates that these two solders have a eutectic property. The eutectic temperatures are 191.9”C and 196/K, respectively, for 10 In- and 5 In-containing solders. The melting point of the lOIn-9( SAl-Zn)-Sn solder is around 9°C greater than that
and Physics 53 (1998)
55-59
of the conventional eutectic 63Sn-37Pb -solder. The DSC cooling curves show distinct sharp peaks at 172.4”C and 175.2”C for 10 In- and 5 In-containing solders, respectively. The deviations of the solidifcation temperatures from the melting temperatures are believed to be caused by supercooling 1261. A tiny peak occurs for both solders at a temperature somewhat greater than the detected solidification temperature on the DSC cooling curve. This is assumed to be due to a precipitation behavior as discussed in another work
v71. The polarization curves of the In-containing solders are presented in Fig. 3 along with that of the 9(5Al-Zn)-Sn solder. All three polarization curves possessverysiitnilarprofiles. The corrosion potentials are all in the range of - 1350 N -1250 mV (SCE) wherein the 5In-9( SAI-Zn)Sn solder is the most subjective to corrosion while the 9( 5AlZn)-Sn solder is the most noble one. These solders exhibit passivation behaviors in the potential range between - 1000 and - 500 mV (SCE). The passivation current densities are around the order of lo-’ A cm-? with the descending sequence of 5In-9(5Al-Zn)-Sn> lOIn-9(5Al-Zn)-Sn > 9 (5 Al-Zn)-Sn. The variations in the polarization curve reflect the different corrosion reactions. It is of interest and of importance to characterize the corrosion products formed at every particular stage of the polarization curve so as to provide a sketch of the corrosion behavior. In a series of XRD investigation, it was found that the corrosion products of the solders investigated in this present Cwe A : Temperature CweB : Dsc (mW)
Sl~-9(5Al-Zn>S~ loin-9(5Al-Zn)-Sn
Curves4 : Tempaature CweB : Dsc (mW)
(c) VS.Tii (ti) vs.Tii (mitt)
(‘C ) vs. Time ( min ) vs.Tiw (mill)
DSC
-===P
TCXXIp
DSC
00.00
,mzi--goo.oo
00.00
00.00
00.00
a,.,+,
0.00
100.00 Time[min]
200.00
Fig. I. The DSC curve of rhe lOIn-9(5
. 0.00
,
,
,
,
.
100.00
,
,
)
)
.
*
,
.oo
200.00300.0
TimcLmin]
Al-Zn)-Sn
solder.
Fig. 2. The DSC curve of the .51n-9(5
Al-Zn)-Sn
solder.
K-L.
-a 00
-a 00
4 00
Lin et al. / Maierials
-2 00
Chernisrry
53 (1998)
55-59
57
most of the In added forms InSn,. ZnO and SnO, were detected in the as-prepared solder. However, In and Al were not seen to form a corrosion product either in the as-prepared solder or during the potentiodynamic polarization. The surface morphology of the solder polarized to - 900 and - 700 mV (SCE) are shown in Figs. 5 and 6, respectively. The metal compositions (Table 1) analyzed with EDS on the surface of these two specimens show an enrichment in In. 13.5% and 16.2% compared to 10% of the as-prepared solder. It is evident from Figs. 5 and 6 that the corrosion products became visible when polarized up to -700 mV (SCE). These products consist of ZnO and SnOz as revealed by the XRD results of Fig. 4. The primary passivation behavior may be due to the formation of thick, compact corrosion products of SnO, and ZnO which slow down the diffusion of corrosives and metal ions. The XRD results show that the corrosion products formed at the potentials from - 500 to - 150 mV (SCE) consist of SnO and Sn,04 in addition to ZnO and SnO,. The surface composition of the specimen polarized up to -500 mV (SCE) has an even lower content of Zn compared to the Zn content achieved at -900 and -700 mV (SCE). It is believed that the ZnO tends to dissolve especially when the potentials move to a further anodic direction. The dissolution of the ZnO is further confirmed through the fact that only 2.0 - 2.4% (Table I ) of Zn was detected at the surface when polarization moves to - 300 and - 150 mV (SCE). In the meantime, the surface Sn content increases gradually from 84.6% to 9 1.8%. The dissolution of the ZnO at - 300 mV (SCE) from the surface corrosion product layer results in a porous product layer as revealed in Fig. 7. The porous product
0 00
log(l/Area)(Akm”Z) Fig. 3. The potentiodynamic polarization solution of the 9(5 Al-Zni-Sn, 5In-9(5 Zn)-Sn solders.
rind Pizysics
curves, in deaerared 3S%NaCl Al-Zn)-Sn and lOIn-9(5 Al-
work are of similar types. Fig. 4 shows the XRD curves of the lOIn-9(5Al-Zn)-Sn solder as a representative. after being potentiodynamically polarized to particular potentials of interest. The XRD of the as-prepared solder shows that the In and Sn can foml intermetallic compounds InSn, and In,Sn. However, In,Sn and pure In were not detected for all the samples other than the as-prepared solder. This means that
.
0
-15OOmV--150mV
Sn
-1500mV--300mV
In sno : Sn,O, v sno, v zilo * I&n, A In,Sn 0
zn
l
Ii
-1500mV--5OOmV
-1500mV--9M)mV 4
0.W
Fig. 4. The XRD analysis
20.W
I
40.w 28
60.00
6O.W
ot’thcpotentiodynamically polarized solders atdifferentpotentials, andas-prepared lOIn-9(
5 Al-Zn)-Sn
solders.
58
K.-L. Lin et cd. /Mnrehls
Table 1 The hut-face composition
In Al Zn Sn
of the IOIn-91.5
Al-Zn)-Sn
Chnnimy
solder at different
polarization
mri Physics 53 (IY98J 55-59
potentiais
-
IOin-9(5Al-Zn)-Sn
- 900 mV
- 700 mv
- 500 mV
- 300 mv
- IS0 mv
10 0.45 8.55 81
13.5 0.75 1.5 1 81
16.2 0.083 5.88 77.8
11.3 0.12 4.04 84.6
10.0 0.20 2.0 87.8
5.09 0.73 2.35 91.8
Fig. 5. The SEM surface morphology p01; irked to - 900 mV (SCE)
of the lOIn-9(5
Fig. 6. The SEM surface morphology polarized to - 700 mV f SCEj.
of the lOIn-915
Al-Zn)-Sn
Al-Zn)-Sn
solder, Fig. 7. The SEM surface morphology polarized to - 300 mV (SCE),
of the IOIn-915
Al-Znj-Sn
solder,
Fig. 8. The SEM surface morphology polarized to - 150 mV (SCE).
of the lOIn-9(5
AI-Zn)Sn
solder.
solder,
layer will not provide any protection to the underlying metal against corrosion. Accordingly, a peak current density is achieved at the potential of around - 300 mV CSCE). and similarly for the other two solders as seen in the polarization behaviors of Fig. 3. On the basis of the above discussion a reaction sequence of the potentiodynamic polarization behavior is proposed as follows. The as-prepared solder exhibits
in the passivation between - 900 and - 500 mV. At potential From - 500 to - 300 mV, the ZnO dissolves or falls and the current density increases, ZnO-+Zn+‘+O’At potentials above -300 mV, the reaction starts forming SnO and Sn,O,
Zn+ l/2 0,+&O
Sn+ l/2 02+Sn0
Sn + O2 3 SnO,
3 Snf20,+Sn,O,
The growth of these products to a thick, compact layer results
and thus the second passivation occurs.
The further dissolution of the ZnO product and the tremendous growth of the oxides of Sn up to - 150 mV (SCE) gave rise to the flake products as seen in Fig. 8. At this stage. the surface composition analysis shows a diminishing of In down to as few as 5. I ‘%. In other words. the dissolution of In may not occur significantly until the potential approaches as anodic as - 150 mV ( SCE) This is understandable as most of the added indium form intermetallic compound. mainly In,%,, which is relatively noble in comparison with the constituents of the investigated solders, as shown by the XRD results (Fig. 4).
4. Conclusions The 5In-9( SALZn )-Sn and lOIn-9( SAl-Zn)-Sn solders exhibit eutectic behavior. The melting points of the 5In9(5Al-Zn)-Sn and lOIn-9(SALZn)-Sn solders are, respectively, 19 I .9 and 196.3”C. The potentiodynamic polarization behaviors of these two solders are very similar to that of the 9( SALZn)-Sn solder. The 5 wt.% In-containing solder is slightly more susceptible to corrosion than the 10 wt.% In-containing solder. The corrosion products formed during the potentiodynamic polarization in a 3.5% NaCl solution are SnO, and ZnO in potentials from - 1500 to - 700 mV (SCE). SnO and SnxO, were formed when polarized to - 500 mV (SCE j and above. Iridium form In,Sn and InSn, compounds in the solders. The In and Al did not form detectable corrosion products during the polarization investigation; In did not dissolve until - 150 mV (SCE).
Acknowledgements The financial support of this work from the National Science Council of the Republic of China under NSC85-22 16E-006-03 1 is gratefully acknowledged.
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