During the manufacturing process of the system-in-package, it has become susceptible to defects and internal residual stresses when dies, components, electric functionality and geometric complexity have increased. The mismatch of thermal expansion coefficient (CTE) among packaging materials and devices may lead to various failure modes during manufacturing processes, such as die broken, solder crack, substrate interface delamination. In this paper, the effect of underfill on the reliability of a system-in-package has been studied. A Bluetooth package is considered. The dimension of the package is 4.84 (L) × 6.15 (W) × 1.32 (H) mm. It contains an IC chip and several passive components such as crystals and filters. The substrate is NSMD type. Three underfill materials are considered. Materials were selected based on the numerical simulation. The causes of void formation during the underfill process have been investigated. In addition, the adhesion test of die passive material and PCB solder register was performed.

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