Interfacial microstructures and solder joint strengths of the Sn–8Zn–3Bi and Sn-9Zn–lAl Pb–free solder pastes on OSP finished printed circuit boards

Abstract

Two kinds of lead-free solders, Sn–8Zn–3Bi and Sn–9Zn–lAl, were used to mount passive components onto printed circuit boards via a re-flow soldering process. The samples were stored at 150 °C for 200, 400, 600, 800, and 1100 h. The microstructures of the samples after aged at 150 °C for various times were characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and the analyzed of solder joint shear strengths. The joint strength between Sn–8Zn–3Bi and Cu pad was about 4.0 ± 0.3 kg, while the strength between Sn–9Zn–lAl and Cu pad had values of 2.6 ± 0.1 kg. Both kinds of solder joints exhibited reduced strengths with increasing aging times. After aging at 150 °C for 1100 h, the joints strengths of Sn-8Zn-3Bi and Sn–9Zn–lAl were 1.8 ± 0.3 and 1.7 ± 0.3 kg, respectively. Both the Sn–8Zn–3Bi and Sn–9Zn–lAl joints showed brittle fracture behaviors. A flat layer of Cu 5Zn 8 intermetallic compound (IMC) was formed between Sn–8Zn–3Bi solder and Cu pad after reflow. When the aging time was increased to 400 h, Zn-depletion and formation of Cu 6Sn 5 IMC were observed in the solders due to the interaction between the tin and zinc compounds. The interaction between Sn–9Zn–lAl solder and Cu pad had similar behavior, however, Cu 6Sn 5 IMC formed in Sn–9Zn–lAl solder when after aging at 150 °C for 600 h. As the aging time increased, both types of solders generated clear IMC spalling layers with large and continuous voids. Those voids substantially decreased the joint strength.