The purpose of this study is to investigate the influence of intermetallic compound (IMC) layer thickness on the reliability of plastic ball grid array (PBGA) lead-free solder joints under thermal cycling. Additionally, this study comparatively analyses the thermal fatigue life of three solder alloys: Sn3.5Ag, Sn3.0Ag0.5Cu and Sn0.7Cu. These findings provide a critical theoretical foundation for electronic package design.
The authors measured IMC layer thickness and mechanical properties via scanning electron microscopy and nanoindentation across multiple aging durations. A two-dimensional finite element model of PBGA was constructed based on actual working conditions, and the stress–strain distribution under thermal cyclic loading was simulated with ANSYS software. This study investigates the differences in IMC thickness and the material intrinsic model (Anand model) for three solder compositions (Sn3.5Ag, Sn3.0Ag0.5Cu and Sn0.7Cu). The thermal fatigue life of solder joints is predicted by the modified Manson–Coffin equation.
The thickness of the IMC layer has been shown to have a significant impact on the longevity of the solder joint. Equivalent plastic strain increases by 8.6% (from 7.56 × 10–3 to 8.21 × 10–3) when IMC thickness grows from 2 µm to 19 µm. Fatigue life decreases by 23.84% (from 9,286 to 7,072 cycles) under the same conditions. It is evident that, under equivalent IMC thickness conditions, the thermal fatigue lifetimes of the three distinct types of solders are arranged in the following sequence: Sn3.5Ag > Sn0.7Cu > Sn3.0Ag0.5Cu. The lifetimes of Sn3.5Ag (24,161 cycles) and Sn0.7Cu (9,748 cycles) are 2.97 and 1.33 times higher, respectively, than those of Sn3.0Ag0.5Cu (7,072 cycles).
This study systematically quantified the combined effect of IMC layer thickness and solder composition on the thermal fatigue life of PBGA solder joints. The optimal fatigue resistance of Sn3.5Ag at low IMC thickness is revealed. A methodology for predicting the longevity of individuals is proposed, whereby a combination of experimental methods and finite element simulations is used to consider actual IMC parameters. A novel foundation is established for the selection of materials and the optimisation of processes in the domain of high-reliability electronic packages.
