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

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.