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Effects of Pre-Pack Thermal Aging on the Formation and Evolution of IMC for Sn-Pb36-Ag2 and Sn-Ag3.8-Cu0.7 and it’s Correlation to Solder Joint Strength
Keywords: Pre-pack Thermal Aging, Inter-metallic compounds, Solder joint strength
A qualitative analysis on the reliability and failure modes of the lead free solder alloy Sn-Ag3.8-Cu0.7 and leaded solder alloy Sn-Pb36-Ag2 on a particular test vehicle of BGA 388 (Ball Grid Array) using Electrolytic Ni-Au pads is presented regarding the formation of Inter-metallic compounds (IMC). The analysis is performed at outgoing prepack operations, which require thermal aging on units to remove any possible moisture trapped in the units’ mold compound so as to avoid the “pop-corn” effect during reflow operations at the SMT customer’s site. Critical reliability data along with numerous test results are presented with regard to the effects of this thermal aging on the solder joint’s reliability for both solder alloys. Fractographs depicting the types of failures from CBP and shear tests are also compared. These fractographs were able to prove the inherently better wetting characteristic of Sn-Pb36-Ag2 as compared to that of Sn-Ag3.8-Cu0.7. The relationship of IMC layer thickness as a function of pre-pack baking hours and temperature is also studied. The IMC thickness data for both solder types showed that IMC thickness cannot be a definite factor to deem if a joint should fail or not, though it is recognized that a thicker IMC layer will cause brittle failures. There seems to be a minimum level of IMC thickness which is needed for a joint to sustain impact and avoid brittle failures. The data shows that ball shear and ball pull strengths are still good at some level of IMC thickness, and drops at different values. Further investigation on the IMC structure with SEM (Scanning Electron Microscope) was able to showcase a flaky and layered structure for weaker joints as compared to solid microstructures on stronger joints.
Vemal Raja Manikam, Senior Engineer (Research & Development Engineer)
Freescale Semiconductors
Shah Alam, Selangor 40000,
Malaysia


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