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40 SMT Magazine • May 2014 terials with similar polarity will also have lower interfacial-tension. The reverse should be true for the total surface energy: The higher the total surface energy, the higher the work of adhesion. Figure 10 was created using the Kruss adhe- sion tool, which enables the calculation of WoA as well as IFT based on the knowledge of the polar and dispersive part of two materials. The interaction between underfill A and the differ- ent types of solder mask inks is shown in blue, likewise the interaction of underfill B in red. The bar reflects WoA (left axis) and the point the interfacial tension (right axis). Comparing the interaction of solder mask A & B with underfill B (red) it can be assumed that the adhesion of solder mask B will be significant better due to higher WoA and lower IFT, which coincides with drop test results. Comparing the same solder mask inks and their interaction Table 5: Parameter setting. Figure 9: Surface energy comparison of different solder mask and underfill inks. feATuRe mECHaNICaL RELIaBILITy continues