Effect of Lamination Process-Induced Residual stress on the CTE of Advanced Prepregs Before and After Solder Reflow Process

Advanced prepreg (PPG) contains a low CTE glass fabric that is impregnated with resin filled with fillers. A typical volume fraction of the glass fabric in advanced PPG is relatively small (approximately 20%), and the resin still plays an important role in the behavior of PPG. PPGs are laminated by a thermal compression process. The residual stresses are produced in PPG, inherently by the mechanical interactions among the glass fabric, the filler and the resin during lamination as well as cooling process. When PPG is subject to the solder reflow process, the magnitude of the residual stress changes because of the stress relaxation in the filled resin. The net result is changes in the absolute dimension of PPG, and thus, affects the CTE of PPG during the additional solder reflow processes. In this study, multi-layer PPGs are fabricated by a thermal lamination press. The moire-hole drilling method is implemented to measure the residual stress at room temperature. The PPG is then subjected to multiple solder reflow cycles, and the dimensional changes associated with reflow cycles are documented by digital image correlation (DIC). The results clearly indicate CTE reduction, which can eventually affect warpage of packages.