Solder ball robustness study on polymer core solder balls for BGA packages

Abstract

Restriction of Hazardous Substance (RoHS) Regulation came into effect in 2006 due to the hazardous effects of lead to human's health and toxicity for environment. As such, the leaded solder ball was replaced by lead-free solder ball which is now widely used in semiconductor industries. However, there was a concern on the robustness of lead-free solder ball especially on drop ball issues when compared to lead solder ball, especially when subjected to reliability stress tests. The polymer core solder ball was invented to solve the drop ball issue. Polymer core solder ball with an additional polymer core inside the solder functions as a stress buffer to dissipate stress better compared to the current conventional lead-free solder ball. However, a new problem arises which is the formation of Kirkendall voids in between the Copper (Cu) and solder interface which results in poor reliability performance. This formation of voids could be due to the faster diffusion from Cu to Tin (Sn) than Sn to Cu. Therefore, an additional Nickel (Ni) layer is coated on top of Cu to overcome this problem. The function of Ni is to limit / reduce the diffusion from Cu to Sn thus preventing the formation of Kirkendall voids. This enhances the robustness of the solder ball joint. The solder ball shear strength test and tray drop test were conducted in this research study under different reliability stress conditions such as temperature cycle (TC) and high temperature storage (HTS) stress to verify the robustness and the reliability of the polymer core solder balls. The solder ball shear strength experiment was conducted via Dage 4000 series bond tester and drop reliability test was carried out via the tray drop test. It is observed that the shear strength for polymer core solder ball without Ni coating layer decreased in TC and HTS stress condition and the tray drop test reliability is the worst in HTS 1008 hours. This is due to the excessive formation of Kirkendall voids resulting from the faster diffusion rate from Cu to Sn than Sn to Cu. From this research study, it can be concluded that the polymer core solder ball with an additional of Ni coating layer gives higher joint strength and better drop reliability performance compared to the polymer core solder ball without additional Ni coating layer.