A Study on the Double Layer Non Conductive Films (NCFs) for Fine-pitch Cu-pillar/Sn-Ag Micro-Bump Interconnection

Abstract

3D-Through Silicon Via (TSV) with Cu-pillar/SnAg micro-bump vertical interconnection becomes popular for 3D chip stacking packages. Among various Cu-pillar/SnAg micro-bump bonding methods, NCFs combined with a thermo-compression bonding method have been introduced for fine pitch bump TSV assembly. However, NCFs thermo-compression bonding has some problems of molten solder wetting not only on the Cu pad but also the sidewall of the Cupillar. Increasing the contact areas between molten SnAg solder and Cu pillar results in faster Sn consumption and the kirkendall void formation at the solder and Cu interface. In order to solve the solder wetting on the Cu bump sidewall problem, the novel Double layer NCFs(D-NCFs) has been introduced. D-NCFs are consisted of a fast curing speed top NCF layer and a slower curing speed bottom NCF layer with lower viscosity. The top NCF is cured below the melting temperature of solder to prevent the molten solder flowing and the Cu bump sidewall wetting. In this study, the wafer level packages(WLPs) using D-NCFs have been investigated for 40 mu m pitch micro-bump. NCFs properties for WLPs were optimized by adjusting the ratio of liquid type resin. In addition, bonding conditions were also optimized in terms of Cu bump sidewall wetting, remaining solder height at the joint, IMC growth rate, and electrical resistance. As a result, D-NCFs process significantly increased the remaining Sn solder content remaining between the Cu-pillar/SnAg/Cu-pillar interconnection and results in slower Sn consumption.