Microstructure and adhesion strength of Sn–9Zn–xAg lead-free solders wetted on Cu substrate

The microstructure and adhesion strength of the Sn–9Zn–xAg lead-free solders wetted on Cu substrates have been investigated by differential scanning calorimetry, optical microscopy, scanning electron microscopy, energy dispersive spectrometry and pull-off testing. The liquidus temperatures of the Sn–9Zn–xAg solder alloys are 222.1, 226.7, 231.4 and 232.9 °C for x = 2.5, 3.5, 5.0 and 7.5 wt%, respectively. A flat interface can be obtained as wetted at 350 °C at a rate of 11.8 mm/s. The adhesion strength of the Sn–9Zn–xAg/Cu interfaces decreases from 23.09 ± 0.31 to 12.32 ± 1.40 MPa with increasing Ag content from 2.5 to 7.5 wt% at 400 °C. After heat treatment at 150 °C, the adhesion strength of the Sn–9Zn–xAg/Cu interface decreases with increasing aging time.