Interfacial reaction of ENIG/Sn-Ag-Cu/ENIG sandwich solder joint during isothermal aging

The interfacial reaction of the ENIG/Sn–3.5Ag–0.7Cu/ENIG sandwich solder joint was studied during aging at 150 °C for up to 1000 h. (Ni,Cu)3Sn4 intermetallic compounds (IMCs) formed at both ENIG/Sn–Ag–Cu/ENIG interfaces after reflowing. After aging for 24 h, IMCs changed from (Ni,Cu)3Sn4 to (Cu,Ni)6Sn5. Until the aging for 250 h, the (Cu,Ni)6Sn5 IMC layers grew thicker with increasing aging time. After aging for 500 h, the IMCs changed from (Cu,Ni)6Sn5 to (Ni,Cu)3Sn4 once again. The phase transition could be significantly related to the diffusion and content of available Cu atoms from the solder to interface during aging. On the other hand, the results of the analyses of the interfacial reactions of the ENIG/Sn–Ag–Cu/ENIG and Cu/Sn–Ag–Cu/ENIG joints showed that the interfacial reaction on the lower Ni–P side was strongly affected by the interfacial reaction on the opposite pad. In the case of the Cu/Sn–Ag–Cu/ENIG joint, only a (Cu,Ni)6Sn5 IMC layer was observed at the lower Sn–Ag–Cu/ENIG interface during aging. A coupling effect occurred between Cu/Sn–Ag–Cu and Sn–Ag–Cu/ENIG interfaces of the Cu/Sn–Ag–Cu/ENIG joint.