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42 SMT Magazine • May 2014 with underfill A (blue), a clear statement of a preferable solder mask would get difficult due to reverse behavior of WoA and IFT. Furthermore, it can be assumed that solder mask C and the interaction with underfill B causes remarkable adhesion, supported by excellent drop shock re- liability. Conclusion The first part of this paper was a drop test DOE focusing on four factors and their influ- ence to the drop shock reliability of assembled PCBs. The second part has focused on the main influence factor, the interaction between sol- der mask and underfilling system, including a method to predict the efficiency of such addi- tional production step without the need of time and cost intensive drop tests. It was proven that the drop shock performance of two different sol- der mask inks without an underfilling step are quite comparable, independent of pad type or surface finishing. The use of underfill provided a reliability improvement for both solder mask types, but the efficiency strongly depended on the specific solder mask. Consequently, under- fill/solder mask interaction (adhesion) has a major impact to the final drop shock reliability. The different ink and underfill types were measured by contact angle. Based on these re- sults, surface energy, work of adhesion and the interfacial tension of each sample was calculat- ed. The calculated adhesion fits quite well with the drop test results—the better the adhesion, the better the shock resistance. Finally, it should be mentioned that contact angle measurements strongly depend on factors like contamination, pre-treatment or environ- ment, therefore, comparison tests should only figure 10: Work-of-adhesion and interfacial tension of solder mask and underfill. feATuRe mECHaNICaL RELIaBILITy continues