Microsructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying

摘要

Slow cooling (1-3degC/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X(X=Fe,Co) solder-joint specimens,made by hand soldering,simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microsructures for realistic shear strength testing,using Sn-3.5 Ag(wt.%) as a base line.Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study,either Sn dendrites,Ag_3Sn primary phase,or Cu_6Sn_5 primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys.Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu_6Sn_5 phase,whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase.Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure,expecially coarse dendrites in solute poor Sn-Ag-Cu,e.g.,Sn-3.0Ag-0.5Cu,while Sn-3.7Ag-0.9Cu with Co and the Fe additions have increased shear strength.At elvated (150degC) temperature,no significant difference exists between the maximum shear-strength values of all of the alloys studied.