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May 2014 • SMT Magazine 37 Figure 4: Doe output for copper defined (CuD) and solder mask defined (SMD) pads. this specific DOE, but the order of magnitude of the impact of the given parameter should allow for reasonable estimates to be made. Summarized can be said that the interac- tions of solder mask and underfill, or their ad- hesion properties, have a major impact to the final drop shock reliability performance. Contact angle, Surface Energy & adhesion A common method to predict the interac- tion between two materials are contact angle measurements which enable the determination of surface energy. Knowing the surface energy of two materials allows the calculation of WoA (work of adhesion) and IFT (interfacial tension), which are indicators for adhesion quality. It must be mentioned that WoA and IFT are im- portant indicators for strong and lasting adhe- sion. In simplified terms, WoA represents the initial adhesion strength, while IFT represents the force which works against it (long term). Therefore, the higher the work of adhesion and the lower the interfacial tension, the better the adhesion. Depending on the application, it has to be decided which of both behaviors have to be rated higher. The measurements were carried out with a fully automatic Kruss DSA100 drop shape measurement analysis system following ASTM D7334 [8] and ASTM D7490 [9] (Table 5). The calculation of surface energy followed Fowkes Theory [10] (Figure 8). Several types of solder mask (A–K) and two types of underfill systems (UF A & UF B) have been compared (Figure 9). The solder mask types are commonly available and there was mECHaNICaL RELIaBILITy continues feATuRe