Wettability and bond shear strength of Sn-9Zn lead-free solder alloy reflowed on copper substrate

Abstract

Lead-free solders are environment friendly and are in great demand for microelectronic applications. In the present study, Sn-9Zn lead free solder alloy was solidified on Cu substrate for different reflow times varying from 10 to 1000s. The influence of reflow time on wetting, bond shear strength and formation of intermetallic compounds (IMCs) was studied using dynamic contact angle analyzer, bond tester and scanning electron microscopy (SEM), respectively. The results indicate that, the wettability of the solder alloy increased with increase in reflow time. Microstructure study revealed the presence of Cu<inf>5</inf>Zn<inf>8</inf> and CuZn<inf>5</inf> IMCs at the interface. The thickness of an IMC increased with increase in the reflow time. A mean thickness of Cu<inf>5</inf>Zn<inf>8</inf> IMC layer of about 11μm was obtained for a reflow time of 1000s. The thickness of CuZn<inf>5</inf> layer increased up to a reflow time of 100s and decreased thereafter. The bond shear strength increased up to 100s and decreased with increase in reflow time. The decrease in shear strength at higher reflow time is mainly due to the formation of thick Cu<inf>5</inf>Zn<inf>8</inf> IMC layer and diffusion of Sn from bulk solder towards the substrate. The thick IMC layer exhibited micro-cracks leading to the brittle failure of bond under the influence of shear stress. © (2015) Trans Tech Publications, Switzerland.