Abstract
The mismatch of the thermal expansion coefficient (CTE) between the chip and the substrate, will lead to deformations or even cracks in the flip chip device in the thermal cycle, which will ultimately lead to reliability problems. The underfill (UF) filling in the gap between chip and substrate can effectively reduce the effect of the thermal mismatch. After the underfill cured, a protective layer is formed in the gap, which not only alleviates the thermal stress between the chip and the substrate, but also enhances the adhesion between the chip and the substrate. Therefore, the UF layer improves the stability of the flip chip packaging and the lifespan of the product. Fillet is formed at the edge of the chip after the underfill filling and curing is completed. The upper end of the fillet is attached to the side wall of the chip, and the lower end is connected to the substrate to protect the chip. The change in UF properties and filling method can lead to different fillet geometry. Many studies have shown that fillet geometry has significant influence on the adhesion properties between chip and underfill. However, there is still lack of studies to quantify the influence of solder ball geometry on the height of the fillet and how the fille height affect the reliability of flip chip packaging. In this study, a series of 2D finite element analysis (FEA) was performed on flip chip geometry to investigate the effect of fillet height on the stress distribution in the packaging structure. The FEA results show that chip edge point and edge solder are most prone to stress concentration, but underfill can reduce the stress concentration in the solder ball. The fillet height of 50%-75% of the chip thickness is recommended as the filling standard through the FE studies.
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