Thermal fatigue resistance of the Sn-9Zn-xAg lead-free solders/Cu interface

Thermal fatigue resistance of the Sn-9Zn-xAg lead-free solders/Cu interface has been investigated by pull-off test, X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Sn-9Zn-xAg lead-free solders offer better thermal fatigue resistances than 63Sn-37Pb solder alloy. The adhesion strength of the 63Sn-37Pb/Cu interface decreases from 5.53±1.0 MPa to 5.33±0.9 MPa with increasing thermal cycles from 1 to 5 times. The adhesion strength of the Sn-9Zn/Cu interface increases from 9.54±1.5 MPa to 13.79±0.9 MPa with increasing thermal cycles from 1 to 3 cycles and decreasing from 13.79±0.9 MPa to 8.5±0.8 MPa when tested for 5 cycles. The other Sn-9Zn-xAg solder alloys have the similar tendency and the maximum adhesion strength of solder/Cu interface is obtained as 21.41±1.5 MPa for Ag addition is 2.5 wt% and tested for 3 cycles. The intermetallic compound forms at the 63Sn-37Pb/Cu interface is η-Cu₆Sn₅. But η-Cu₆Sn₅ and Cu₅Zn₈ formed at the Sn-9Zn-0Ag/Cu interface and are η´-Cu₆Sn₅, η-Cu₆Sn₅, Cu₅Zn₈ for the Sn-9Zn-xAg/Cu interface. The fracture path for the Sn-9Zn-0Ag system transforms from the solder bulk to the Cu₅Zn₈/solder interface when tested for 3 cycles, the fracture paths of the other solders is within solder bulk even tested for 5 cycles.