Studies on Wetting Behaviour, Microstructure Evolution and Solder Joint Reliability of Sn-Cu and Sn-Ag-Cu Lead-free Solders

Abstract

The effect of substrate material, roughness and surface finish on wettability, evolution of intermetallic compounds (IMCs) and solder joint reliability of Sn-0.7Cu, Sn-0.3Ag- 0.7Cu, Sn-2.5Ag-0.5Cu and Sn-3Ag-0.5Cu lead free solders was investigated. Copper (Cu), Fe-42Ni, Cu with silver (Ag) finish and aluminium (Al) with nickel (Ni) finish were used as substrate materials. The relaxation behaviors of all solder alloys showed high spreading rates at the beginning and slower rates in the final stages. All solder alloys showed satisfactory spreading behavior with an area coefficient of spreading (Ac) ≥ 2 and the height coefficient of spreading (Hc) ≤ 0.5. Exponential power law (EPL) φ = exp(−Kτ n ) was used to model the relaxation behaviour of solder alloys, where ‘φ’ is the dimensionless contact angle and ‘τ’ is the dimensionless time. EPL parameters (K and n) decreased with increase in surface roughness. Spreading of solders exhibited capillary, gravity and viscous regimes. The increase in the surface roughness of Cu substrates improved the wettability of solders. The wettability was not affected by the Ag content of solders. The morphology of Cu6Sn5 IMCs transformed from long to short and thick needles for Sn-0.7Cu, Sn- 0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solders solidified on rough Cu surfaces. However, with Sn-2.5Ag-0.5Cu solder alloy, needle shaped IMCs completely transformed to scallop morphology. The presence of thick Cu3Sn IMC at the interface of SAC solders indicated good wetting of the Cu substrate. Wettability of all solders on Fe-Ni surfaces was found to be better than that on Cu substrates. At the solder/Fe-Ni interface, Sn-0.7Cu and Sn- 0.3Ag-0.7Cu solders exhibited (Cu,Ni)6Sn5 IMCs. Higher Ag solders exhibited mainly (Cu,Ni)3Sn4 along with (Cu,Ni)6Sn5 IMCs. Solder bonds on smooth surfaces yielded higher shear strength compared to rough surfaces both for Cu and Fe-Ni surfaces. Fractured surfaces revealed the occurance of ductile mode of failure on smooth Cu surfaces and a transition ridge on rough Cu surfaces. Solder bonds of both smooth andrough Fe-Ni surfaces showed a transition ridge characterized by sheared IMCs. The increase in the integrity of solder joint on Fe-Ni substrates was due to the presence of (Cu,Ni)3Sn4 IMC at the interface. Wettability of all the solders on Cu with Ag finish was found to be similar. At the interface, all the solders exhibited predominantly Cu6Sn5 IMCs. With higher Ag solders, large amount of Ag3Sn precipitates were found. Wettability of Sn-0.7Cu and Sn-0.3Ag- 0.7Cu solders were found to be slightly better than higher Ag solders solidified on Al substrates with Ni finish. At the interface, Sn-0.7Cu solder exhibited faceted (Cu,Ni)6Sn5 IMCs whereas, Sn-Ag-Cu solders showed the presence of both (Cu, Ni)3Sn4 and (Cu,Ni)6Sn5 precipitates. Sn-3Ag-0.5Cu solder bonds yielded higher shear strength. Fractured surfaces of all solders revealed a transition ridge on Ag finished Cu substrates. With Ni finished Al substrates, the fracture was observed in the solder matrix. (Cu,Ni)3Sn4 IMCs at the interface increased the shear strength of Sn-3Ag-0.5Cu solder/ Ni finished Al substrate system. Although, rough surfaces exhibited better wettability, bond strength of solder/rough surface was lower than that of solder/smooth surface. Hence, smoother surface is preferable as it favors failure in the solder matrix.