A Study on Double Layer Non-Conductive Films (D-NCFs) for 3D-Through Silicon Via (TSV) Interconnection using Cu-pillar/SnAg Micro-Bump

Abstract

3D chip stacking packages and 3D-Through Silicon Via (3D-TSV) vertical interconnection have been popular. Usually, Cu-pillar/SnAg micro-bumps have been used for vertical interconnections of 3D-TSV chip stacking. A Thermo-Compression (TC) bonding method using Non-Conductive Films (NCFs) has performed these vertical interconnections. Heat and pressure induced molten solder wetting on the pad, and deforming too during TC bonding process. The deformed molten solder on the sidewall of the Cu-pillar results in the increase of solder and Cu pillar contact interfaces. As a result, faster Sn consumption made the Kirkendall voids at the solder joint. The novel Double layer NCFs (D-NCFs) can solve the solder sidewall wetting problem on the Cu-pillar. D-NCFs have the double layer NCFs structure, which consist of the fast curing speed top NCFs layer and the slower curing speed bottom NCFs layer. The top NCFs layer has the fast cure temperature below the melting temperature of the solder in order to prevent the molten solder movements on the Cu-pillar sidewall. On the other hand, the bottom NCFs layer helped the molten solder wet on the pads, which has the flux ability and slower curing property. In this study, D-NCFs have been investigated for wafer level (WL) processes in 40µm fine-pitch Cu-pillar/SnAg micro bump chip assembly. D-NCFs properties were adjusted for WL capability and then bonding conditions were optimized in terms of solder wetting on Cu-pillar and electrical interconnection. As a result, D-NCFs can significantly increase the remaining Sn solder between the Cu-pillar/SnAg/Cu interconnection and decrease the Sn consumption.In chapter 2, effects of curing agent in the conventional single layer NCFs on Cu-pillar/SnAg/Cu interconnection Chip-on-Chip (COC) assembly were investigated. As a result, the NCFs containing anhydride curing agent can remove the native oxide of SnAg solder well generating carboxyl acid. Finally, the content of the anhydride curing agent were optimized in terms of the NCFs formability and degree of curing after bonding process.In chapter 3, 40 µm pitch Cu pillar/SnAg solder micro-bump assembly using new D-NCFs have been investigated. At first, of the curing speed and the viscosity of the top NCF layer, the curing speed was defined as the effective factor for preventing solder wetting on the Cu-pillar in D-NCFs. Then, the thickness conditions of D-NCFs and the bonding pressure were optimized by evaluating the solder joint morphologies. The optimized thickness of the top NCF layer is 10 µm, which is the same as the thickness of Cu pillar. Finally, after thermal aging, solder joint using D-NCFs show less Sn consumption than the solder joint using conventional single NCF because of no solder sidewall wetting of Cu pillar in D-NCF packages.In chapter 4, 40 µm pitch Cu pillar/SnAg solder micro-bump assembly using wafer level processible (WL processible) D-NCFs have been investigated. WL processability of D-NCFs was optimized in terms of the adhesion and elongation properties of D-NCFs by changing epoxy formulation. Finally no delamination of the D-NCFs at the edge and corner of diced chips was obtained by using the optimized D-NCFs after the dicing process. And the thermo-compression bonding conditions of the optimized D-NCFs were evaluated in terms of, solder sidewall wetting, solder height, and electrical bump contact resistance. As a summary D-NCFs solved the solder wetting on the sidewall of Cu pillars problem, resulting in significant amount of remaining Sn contents at the solder bump joint. After reliability tests, D-NCFs show excellent reliability performance in thermal cycle test and pressure cooker test compared with the conventional single layer NCFs because of no solder sidewall wetting of Cu pillar and resulting plenty of remaining Sn at the solder bump joint.