Size effects of lead-free solder joint thickness under shear creep based on micro-electrical-resistance strain

Abstract

Single shear lap creep specimens with a 1 mm2 cross sectional area between thin copper strips were developed and fabricated using a lead-free solder (Sn–3.5Ag) to quantify their electrical resistance. The electrical-resistance strain of solder joints with different thicknesses (0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, 0.50 mm) were measured by a tailor-made in situ micro-electrical-resistance measurement. The results showed that the solder joints with 0.25 mm thickness yielded a minimum electrical-resistance strain and the longest creep life. Thicker solder joints produced a larger electrical-resistance strain and a shorter lifetime if the thickness >0.25 mm. In contrast, thinner solder joints produced a larger electrical-resistance strain and a shorter lifetime if the thickness <0.25 mm. A quantificational relationship between the electrical-resistance strain and the solder joint thickness was obtained according to the experimental data. On the other hand, solder joints with different thicknesses (corresponding to those used in the experiment) were studied by the finite element method (FEM). The results showed that the creep strain of solder joints was the lowest with 0.25 mm thickness, and the creep strain increased with the increment in the thickness if the thickness >0.25 mm but decreased with the increment in the thickness if the thickness <0.25 mm. The quantificational relationship between the creep strain and solder joints thickness was obtained according to the data of FEM. Finally, the quantificational relationship between the creep strain and the electrical-resistance strain was obtained by combining the experimental data and the FEM data.