Characterization study for polymer core solder balls under AC and TC reliability test

Abstract

Since Restriction of Hazardous Substance (RoHS) Regulation came into effect in year 2006 due to the hazardous effects of lead to human's health and toxicity for environment, Ball Grid Array (BGA) semiconductor chip are widely used for many electronic applications including portable, automotive and telecommunication products that require stringent thermal and mechanical requirements. However, dropped balls in lead-free BGA products due to poor solder joint strength caused by reliability stress are a major concern in the semiconductor industries. A new technology with polymer core inside the solder ball (polymer core/Cu/Sn) is integrated to improve the solder joint strength. The polymer core inside the solder ball is function to dissipate the stress better as compared to the lead-free solder ball. The diffusion rate of Cu is faster than the diffusion rate of Sn, thus Kirkendall voids are tends to form in between the interface at the Cu and Sn layer, especially after subjected to the high temperature reliability stress. This would affect the solder joint strength and causing drop ball issue. To overcome this, an additional of 1 μm Ni layer is coated on the Cu (polymer core/Cu/Sn/Ni) to reduce the diffusion from Cu to Sn, to avoid Kirkendall voids formation. This research work studies the performance of the solder ball shear strength and IMC thickness of two types of polymer core solder balls applied to BGA device. In this research, polymer core solder balls were went through under AC (Autoclave) and TC (Temperature Cycle) reliability test up to 144 hours and 1000 cycles, respectively. Solder ball shear strength test was conducted via Dage 4000 series bond tester and IMC thickness measurement via cold mount cross-section. From the results of the two types of polymer core solder ball, observed that, the ball shear strength were decreased with increased of aging time, while IMC thicknesses were increased with increase of aging time. This is probably due to the rapid Cu diffusion into the Cu core interface resulting in lower shear strength and thicker IMC. From this research work, it can be concluded that the polymer core solder ball with an additional Ni layer showed better performance than the polymer core solder ball without Ni layer, after subjected to the AC and TC reliability test. This is due to the Ni layer could limit the Cu diffusion into the solder thus resulting in good solder joint strength and drop reliability performance as well as reduced crack issues caused from Kirkendall voids. © 2014 IEEE.