The Effects of Surface Finish Roughness on Intermetallic Layer Growth, Intermetallic Interface Roughness, and Solder Joint Reliability

The intermetallic (IMC) layers which form within a solder joint are necessary for a robust, high reliability bond between the joining materials. A key contributing factor affecting the strength of the joint is the roughness of each interfacial boundary and thickness of each IMC layer. Under annealing and/or temperature cycles, the IMC layer thickness and roughness evolves with time. Since the IMC layers are the most likely places for crack propagation, it is necessary for electronics manufacturers to produce solder joints with controlled, low-growth IMC layers. In this work, SAC lead-free solders were bonded to a variety of board finishes in order to examine the effect of interfacial roughness on joint reliability during isothermal annealing at 150°C The results show that the IMC thickness as a function of time obeyed the expected power law relation y ~Bt^C, with an average growth exponent of C = 0.4. The texture of the board finish does not notably influence the growth rate of the layers. All three intermetallic interfaces coarsened with annealing time. As the IMC layers increase in thickness, the interfacial roughness increases. The data support the notion that smoother initial board finishes lead to stronger and more reliable solder joints.