Micro-mechanical characterizations of solder mask materials

The mechanical properties of solder masks are critical in the reliability performance of flip-chip packages. Many recent studies show that the mechanical properties of solder mask materials have great influence on moisture absorption, delamination at interconnections and solder fatigue life in flip-chip packages. There are also assembly process issues related to the properties of solder masks. This article presents an experimental investigation that mechanically characterizes two polyimide solder mask materials, type A and type B, by using a micro thermo-mechanical tester. The mask samples are prepared as thin film specimens. Mechanical properties such as Young's modulus, failure strength and creep behaviors are determined based on the recorded data from the experiments. In addition, a testing procedure for measuring the coefficient of thermal expansion (CTE) is developed, which provides an alternative to the TMA method. It is shown that the experimental investigation is suitable in conducting such a characterization of thin-film solder masks.