| Wetting balance test is known to be the most versatile method for wettability test, because it can provide quantitative information, time dependent wetting behavior, and reproducibility. However, results for wetting balance test are only good for the specimen of which surfaces around their perimeter show uniform wettability. Therefore, this test is not applicable to wettability evaluation of UBM coated Si substrate for flipchip package. In this study, authors focused on wettability evaluation of Cu/Cr coated Si substrate with Sn Pb solder using wetting balance test. With wetting test, we found that non-wetting behavior of non-coated Si plate results in buoyancy force, which reduces overall wetting forces. In case of one-side coated Si plate, force difference between coated and non-coated surface cause tilting of axis of specimen. Therefore, it was confirmed that conventional wettability analysis using wetting forces for wetting balance could not be adaptable for wettability evaluation of coated Si plate. New equation of wetting force for coated Si plate was introduced which considers buyancy effect for non-coated surface and tilting of axis for one-side coated Si plate. With this equation, we can calculate the contact angle between Cu/Cr coated Si surface and Sn-Pb solder, which shows that wettability of Cu/Cr coated Si plate with Sn-Pb solder is superior to that of Cu plate. |
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