The Effect of Double-Layer Nonconductive Films on the Solder Sidewall Wetting and the Reliability of 40-$\mu$ m Cu-Pillar/SnAg Microbumps for Chip-on-Chip Interconnection

Abstract

Cu-pillar/SnAg microbumps have been used for the latest 3-D through silicon via chip stacking. This microbumps flip-chip interconnection method can significantly further reduce the dimensions of the package and enhance the electrical performance by reducing interconnection path. As the pitch of the Cu-pillar/SnAg microbumps becomes extremely smaller, nonconductive films (NCFs) have recently been introduced to solve interconnection problems induced by the fine size microbumps. However, the applied pressure and heat during the thermo-compression (TC) bonding processes deform the SnAg solder and cause SnAg solder wetting on the sidewall of the Cu-pillar. The SnAg solder sidewall wetting causes rapid Sn consumption at the interface between the Cu-pillar and solder and decreases the joint gap heights of the SnAg solder joints. In this paper, the effects of the new double-layer NCFs (D-NCFs) were investigated for 40- $\mu \text{m}$ pitch chip-on-chip interconnection. The D-NCFs consisted of two NCF layers. The bottom NCF layer behaves as a conventional single NCF layer, and the top NCF layer, covering the Cu-pillar, prevents the movement of molten SnAg solder to the sidewall of Cu-pillar during the TC bonding process. As a result, the gap height of SnAg solder joints was doubled without SnAg solder sidewall wetting of Cu-pillar using the D-NCFs. SnAg solder joints using D-NCFs showed excellent reliability during thermal cycle tests up to 3000 cycles and pressure cooker test up to 72 h because of the significantly remaining Sn at the SnAg solder joint interface and increased joint gap height.