High-Speed Cu Electrodeposition and Reliability of Cu Pillar Bumps in High-Temperature Storage

Abstract

This study was conducted to investigate the microstructure, impurity distribution, and mechanical reliability of copper pillar bumps (CPBs) by means of scanning electron microscopy in conjunction with an electron backscatter diffraction analysis system, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and high-speed ball shear (HSBS) testing. After soldering reaction and subsequent isothermal annealing at 180°C, a typical Cu6Sn5/Cu3Sn dual layer formed at the solder/Cu pillar interface when the pillars were electrodeposited using a low j (2 A/cm2). Interestingly, an abnormal, interwoven lamellar structure consisting of Cu6Sn5, Cu3Sn, and Cu10Sn3 phases accompanying numerous voids and impurities (including Cl and CN) could be obtained in the high j case, particularly for j = 10 A/cm2. The propagation of this abnormal microstructure and voids at the interface seriously deteriorated the shear resistance of CPBs in HSBS testing. These investigations revealed that j is an important factor of the CPB microstructure and might greatly affect its mechanical reliability after high-temperature storage. Based on the crystallographic and chemical analysis, we proposed a unified argument associated with the formation mechanism of the undesired joint microstructure to explain this rather unusual phenomenon.