A comparative study on the ageing behaviour of solder alloy-cu joint assemly using lead free solder and Sn-Pb eutectic solder alloy

Abstract

In the present investigation, Sn-Ag-In ternary alloy was used to join Cu substrate (SAI-Cu) and compared with conventional Sn-Pb eutectic solder alloy–Cu assembly (SP-Cu). The melting point of the alloys was found to be 192±1 o C and 184±2 o C respectively. The re-flowing was carried out above +30 o C of the melting point of the respective alloys for 30secs. Both the joints were aged at 100 o C for 50-200hrs at a step of 50hrs. Subsequently, structural characterization and mechanical property evaluation of the joints were performed. In re-flowed condition, the shear strength of the SAI-Cu joint (~64MPa) is higher than SP-Cu joint (~55MPa). The diffusion zone of the former is decorated with Cu 6 Sn 5 intermetallic phase, whereas the latter contains both Cu 6 Sn 5 and Cu 3 Sn. The width of the reaction zone is more in case of SP-Cu joint with respect to SAI-Cu joint. Isothermal aging of the solder assembly leads to deterioration of joint shear strength accompanied by structural change in the diffusion zone. After 200hrs of aging the shear strength of SAI-Cu joint is ~44.6MPa and the same becomes ~34.5MPa for SP-Cu joint. In both the cases, formation of Cu 6 Sn 5 and Cu 3 Sn has been observed which causes lowering of bond strength. Indium plays a vital role in controlling the chemical reaction at the interface during reflowing and ageing. Hence, it is responsible for betterment in mechanical properties.