Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density |
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Authors: | B Y Wu H W Zhong Y C Chan M O Alam |
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Affiliation: | (1) Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, P.R. China |
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Abstract: | Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate
current density of 6.0 × 102 A/cm2 at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement,
microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this
moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating,
which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly
apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic
compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected
with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain
boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity
of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current
stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening,
volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing. |
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