Thermal Compression Bonding of 30um Pitch Cu Pillar Microbump on Organic Substrate with Bare Cu Bondpad

This paper reports on the development of packaging technology for the assembly of 30µm pitch micro Cu pillar bump (15µm diameter) on organic FCCSP substrate having bare Cu bondpad without NiAu or OSP surface protection. The assembly was performed by thermal compression bonding (TCB) with non-conductive paste (NCP). Finite element modeling and simulation were carried out to understand the Cu pillar structure impact on the TC reliability. The Cu pillar microbump reliability was assessed by thermal cycling (TC) and electromigration (EM) tests. It was found that by the combination of limiting substrate's thermal exposure, shortening the TCB bonding time, and use snap cure NCP can help minimizing the offset between Cu pillar and substrate bondpad during assembly. The presence of larger solder volume reduces the filler particle entrapment severity, crack initiation and propagation thus prolonging the fatigue life of interconnect joints. Given sufficient solder cap volume (16mm height), the 15µm diameter, 30µm pitch Cu pillar interconnect has passed MSL3+3x260°C reflow, high temperature storage at 150°C for 1000hours and 1,000x thermal cycle test at -55°C/+125°C extreme temperatures.