Evaluation of wettability for microelectronic materials by reflow-mode wetting balance test

The reflow-mode wetting balance test is a promising method for evaluating the wettability of various kinds of chip devices and newly developed solders for Pb-Sn solder substitution. In this method, solder metal is applied as paste on to a micro pod, and the geometric boundary constraint limits the meniscus shape and varies with shape and size of specimen. It is crucial to understand the meaning of force curve and wettability parameters extracted from curves acquired by this method from a physical viewpoint. In this report, we introduce our test instrumentation and some experimental results. Moreover, extensive numerical analysis is conducted to realize the meniscus geometry and wetting force in the equilibrium wetting state of solder liquid on copper lead models. Both calculated wetting height and force acting on the specimen are increased with decreasing contact angle as for boundary conditions. These relationships are found to be insensitive to specimen dipping depth, even for constant solder volume. Although dipping depth slightly influences overall solder meniscus geometry, it drastically increases the maximum contact angle that could be formed at an early force curve stage just after solder melting; consequently, nonwetting force also increases. These results show good agreement with experimental data. It is concluded that the internal pressure force which is proportional to lead cross section and inversely proportional to global radius of curvature of the solder surface should be taken into account to evaluate wettability.