Nanomechanical Characterization of Various Materials within PBGA Packages Subjected to Thermal Cycling Loading

Abstract

Electronic packages are frequently exposed to thermal cycling environments in real life applications. Particularly, the plastic ball grid array (PBGA) is one of the most widely used electronic packages, and consists of various component materials, e.g. solder joint, silicon die, die attachment adhesive, mold compound, solder mask, etc. All of these materials play a significant role on the reliability of the overall PBGA assembly. Hence, it is important to study their mechanical property evolution under the thermal cycling environment. Our prior work has shown that cyclic temperature variations cause the solder joints to change their mechanical behavior. However, the other component materials can also change their mechanical properties during cycling. These changes must be evaluated in order to understand and predict their failure behavior in operation.In our previous study, evolution of mechanical properties of SAC305 solder joints in a PBGA package up to 250 thermal cycles was evaluated using the nanoindentation technique. In this work, the nanoindentation technique was utilized to understand the evolution of mechanical properties (modulus, hardness, and creep behavior) of the die attachment adhesive, silicon die, and solder mask material for various durations of thermal cycling. Test specimens were first prepared by cross sectioning a PBGA package to reveal the different materials, followed by surface polishing to facilitate SEM imaging and nanoindentation testing. After preparation, the package samples were thermally cycled from T = -40 to 125 °C in an environmental chamber. At various points in the cycling (e.g. after 0, 50, 100, and 250 cycles), the package was taken out from the chamber, and nanoindentation was performed on above mentioned materials to obtain modulus, hardness, and creep behavior at room temperature (T = 25 °C). From the nanoindentation test data, it was found that die attachment and solder mask materials showed increases in modulus and hardness up to 250 cycles. As expected, the silicon die material does not show any significant change in mechanical behavior during same duration of thermal cycling.