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Effect of External Stress on Sn Whisker Growth
Keywords: whisker, Sn, stress
We have studied an effect of external stress on whisker growth with fabricated sample under various conditions. For experimental, we fabricated Cu alloy and FeNi42(Alloy42) substrates of strip type, and then, these substrates were electrolytic-plated with matte Sn and Sn-3wt%Bi of 15 um thickness, respectively. To make change of plated surface morphology, Sn grain size and IMC thickness, pre-treatments were conducted such as annealing, temperature humidity storage(THS) and soldering with Sn3.0Ag0.5Cu, respectively. Finally, external compressive or tensile stress was induced by mechanical bending of each fabricated strip sample. After 1500 temperature cycle(-55C to +95), whiskers of Sn plated on Alloy42 substrate were much grower than others(Sn on Alloy42 > SnBi on Alloy42 > Sn on Cu alloy > SnBi on Cu alloy). Especially, whisker of 55 um maximum length (average of 42 um) was founded at Sn plated on Alloy42 sample with pre-treatment of THS. However, whisker wasn’t found at all samples with pre-treatment of soldering(THS > as plated > aging > soldering). This might be due to the fact that internal stress related with surface morphology, Sn oxidation and IMC thickness was different with variation of pre-treatment conditions. After all, internal stress was relived by rearranged Sn grain after soldering process. Sn whisker of sample induced by external tensile stress was much grower than that of stress-free, regardless of pre-treatment conditions. However, whisker was mitigated at Sn(Bi) on Alloy42 substrate induced by external compressive stress. These were explained by difference of whisker growth mechanism between Cu alloy and Alloy42 substrate. Namely, in case of Cu alloy substrate, whiskers were formed by internal stress induced from IMC, but weren’t mitigated by external stress. However, whiskers on Alloy42 substrate were formed by internal stress induced from mismatch of CTE, and affected by external compressive and tensile stress.
Jae-Hoon Choi,
Samsung Electronics
Asan, Chungcheongnam-Do 336-851,
South Korea

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