Cu Powder/Sn Powder Mixed Paste for High Heat-proof Lead-free Joint

摘要

A mixed paste of copper powder and tin powder was developed as an alternative material to lead-based solder. This paste attains goodsolderability through the molten tin, and gives good joint stability at high temperature due to a network formed from copper particles and copper-tin intermetallic compounds with high melting point. The wettability of this paste is at the same level as conventional pastes, with the wetted area corresponding to about 90% of the paste printing area. The bonding mechanism of the copper/tin powder mixed paste was investigated by optical and scanning-electron microscopes. Voids in bonding interface were decreased by adjusting boiling temperature of flux. Joint strengths at 260°C were measured by tension-gauge shear tests. The test results show that high joint strength at 260°C is due to connection of copper particles and copper-tin intermetallic compounds. Moreover, the joint strength increased with increasing weight ratio of copper powder, rising bonding temperature, increasing bonding time, and decreasing particle diameter of copper powder. The joint strength of this paste after 1000 temperature cycles at -55/125°C is about 90% of the initial value, and therefore the joint reliability of this paste is also good.