Reliability Study on the Polymer Core Solder Balls under Multiple Reflow and HTS Stress Tests
Authors: Dr. Yap Boon Kar, Tan Cai Hui, Dr. Agileswari, and Calvin Lo Company: College of Engineering, University Tenaga Nasional (UNITEN) and Freescale Semiconductor, (M) Sdn. Bhd. Date Published: 4/18/2012
Abstract: Drop ball reliability for BGA package on lead-free product is a major reliability concern due to the different environment stress. Integrating a polymer core in the solder ball may be a good strategy to dissipate stress better compared to the purely metallic solder ball. However, the diffusion rate of the copper (Cu) is faster than the diffusion rate of the Tin (Sn). Hence, Kirkendall voids starts forming and causing crack between the interface at copper and solder. This could affect the solder joint as well as the solder ball drop reliability especially when subjected to high temperature stress. The new solder ball with 1 µm thickness of Nickel (Ni) coated on the copper (polymer core/copper/nickel/solder) could offer better solder ball joint and drop reliability performance. This work studies the solder ball shear strength and drop test reliability between these two types of polymer core solder balls. Solder ball shear test was carried out to measure the solder joint performance after multiple reflow and HTS stress tests via the Dage 4000 series bond tester. Drop reliability test was carried out via the tray drop test. From this study, we could conclude that the polymer core solder ball with an additional Ni layer coating demonstrates better performance than the polymer core solder ball without Ni layer in terms of the solder ball shear strength and drop reliability performance in multiple reflow and HTS stress tests. This is most probably because of the Ni could limit the Cu diffusion into the solder, thus resulting in better reliability performance.
Polymer core solder balls, solder ball shear, tray drop test