Thermal cycling, shear and insulating characteristics of epoxy embedded Sn-3.0Ag-0.5Cu (SAC305) solder paste for automotive applications

In this study, lead free solder alloy paste is fabricated using Sn-3.0Ag-0.5Cu (SAC305) powder mixed with epoxy based polymer and flux. The solder paste is reflowed onto Cu pad of 1608 chip, and the surface insulation resistance (SIR) of the joint is measured. The thermal shock reliability of the solder/Cu joint from −40 °C to 125 °C for 1000 cycles of aging is investigated. Shear test is also performed on the joint after thermal cycling. The results show that the SIR of the reflowed solder bump is around 2.58 × 10¹³ Ω compared to 3.21 × 10¹³ Ω of the flux. An increased growth of Cu₆Sn₅intermetallic compounds (IMCs) occurs at the solder/Cu bonding interface with increasing number of thermal cycles. There is no appreciable damage in epoxy containing solder paste up to 1000 cycles except the formation of voids and cracks. The shear force of epoxy containing solder joint after thermal cycling for 1000 cycles is ≈34 N over 29 N for SAC305 without epoxy. This shows around 14% increment in shear strength in the presence of epoxy due to the slower growth rate of Cu-Sn IMCs.