Newly developed in-situ formation of SnAg and SnAgCu solder on copper pillar bump

The trend to smaller and lighter electronics has accentuated efforts towards high density, increased performance and miniaturization in packaging technology. Fine pitch micro bump interconnection can greatly improve the interconnect density, thereby becoming a mainstream technology of high-density packaging. A new method to fabricate binary and ternary solder bump based on electroplating and micro-alloying are introduced in present paper. The formation of SnAg and SnAgCu alloys on copper pillar bump is demonstrated. The new manufacturing process is explained as follows. Firstly, alloying metals such as Ag and Cu are deposited at the surface of the electroplated Sn layer over copper pillar by physics vapor deposition (PVD). Then, the multi-component solder bump is formed through a reflow process. By scanning electron microscope (SEM) and Energy Dispersive Spectrum (EDS) analysis, it was confirmed that Ag and Cu layers over Sn have dissolved into the solder and formed intermetallic compound (IMC) in the solder after first reflow. After 1000 temperature cycles up to -50~85°C, Ag3Sn phase precipitation was observed having stick or island shapes. But we did not observe tin whisker on the surface of the solder bump.