Formation Mechanism of Novel Sidewall Intermetallic Compounds in Micron Level Sn/Ni/Cu Bumps

Abstract

A new kind of intermetallic compounds (IMC) were found around copper pillar in micron level bumps. To investigate the formation mechanism, three different sized Sn/Ni/Cu bumps (10 μm, 20 μm, 50 μm) were electroplated then reflowed at 230 °C for 100 s. After reflow process, a thin layer of IMC was formed around copper pillar, which is attributed to surface wetting behavior. After aging at 170 °C and 200 °C for different times, the growth mechanism of sidewall IMC was observed by scanning electron microscopy combined with electron backscatter diffraction (EBSD) technology. Surface diffusion was considered to be the main driving force for sidewall IMC growth for the activation energy of them was found to be much smaller than that in previous studies. The EBSD results showed a preferred orientation of sidewall Cu3Sn grains being perpendicular to copper periphery, which indicated direction of Cu atoms flux during Cu3Sn growth. Formation mechanism of this novel sidewall IMC was proposed based on surface wetting and surface diffusion. The findings contribute to the failure mechanism study in small size bumps and provide insights into the reliability of 3D electronic packaging.