Solder Joint Reliability of Middle P% and High P% Ni/Au with Low Melting Point Solder Containing Indium

Abstract

In the early 2000s, the use of lead (Pb) was banned due to environmental and human health concerns and Pb-free solder alloys have been developed. Sn-Ag-Cu (SAC solder) is the most popular Pb-free solder alloy, but it has a high melting point above 217℃. Recently, as applications become more diverse,  low-temperature soldering (LTS) alloys such as Sn-Bi and Sn-In have been required for various advantages. Since solder joint reliability (SJR) is closely related to the type of surface finish (SF), it is necessary to select an appropriate SF and then control the composition of intermetallic compounds (IMC) for LTS with good SJR.

In this study, the SJR between electroless Ni-P/immersion Au (ENIG) and low-melting-point solder containing indium was evaluated by ball grid array (BGA) substrates and high-speed shear tests (HSS). The results showed that the shear strength of ENIG with middle phosphorus content electroless Ni-P (MP-Ni) decreased after multiple reflows. On the other hand, the ENIG with high phosphorus content electroless Ni-P (HP-Ni) had a good LTS reliability and it was maintained  even after multiple reflows. The IMC observation revealed that ENIG with HP-Ni did not have excessive IMC growth, while ENIG with MP-Ni formed thicker IMC after multiple reflows. Moreover, the composition of IMC and its existence ratio at the solder joint interface also greatly varied depending on the P% of the electroless Ni-P. In particular, the IMC which was identified as InP was the key factor for the difference of LTS reliability between ENIG with MP-Ni and ENIG with HP-Ni. The role of InP and the mechanism of IMC formation were clarified by various analyses. In conclusion, it was suggested that a higher P% of electroless Ni-P might be advantageous in LTS on the ENIG when using low-melting-point solder containing indium.