Solder joint strengths and interfacial reactions in various Pb-free solder joints

Two kinds of Pb-free solders, Sn8Zn3Bi and Sn9ZnAl, were used to mount passive components onto OSP and ENIG PCB using a reflow soldering process. The component mounted boards were stored at 150degC for 200, 400, 600, 800, and 1100 h. The microstructures of the interfaces between the solders and pads of the aged assemblies were observed using scanning electron microscopy (SEM) and the chemical compositions of the intermetallic compounds (IMCs) formed at the interfaces were analyzed using energy dispersive spectrometry (EDS). After shear testing, both solders showed acceptable solderability on the OSP and ENIG layers, but the shear strengths of the Sn8Zn3Bi solder joints were better than those of the Sn9ZnAl solder joints. All of the solder joint strengths deteriorated during the aging process, but the degradation of the OSP solder joints was more evident than that of the ENIG solder joints. Scallop-shaped epsiv-Cu_0.2Zn_0.8 IMC and flat gamma-Cu₅Zn₈ IMC formed between the solders and Cu pad of the OSP board after reflowing with an increase of aging time, all of theepsiv-Cu_0.2Zn_0.8 layers transformed into gamma-Cu₅Zn₈ and the IMC layer became thicker. The Zn-depletion zone and Cu₆Sn₅ IMC were observed in the solders due to the interaction between the tin and zinc compounds. Both solder joints on the ENIG boards had similar interfacial microstructures; very thin gamma₂-AuZn₃ layers were formed at the interface of the solders and Ni-P layers. The gamma₂-AuZn₃ layer transformed into gamma₂-AuZn₈ IMC with a consistent thickness during aging. The Zn phase redeposited above the IMC layer and the size of the Zn phase grew with increasing aging time.