Novel solder ally with wide service temperature capability for automotive applications

A novel lead-free solder alloy 90.6Sn3.2Ag0.7Cu5.5Sb, designated as Indalloy276, was developed targeting for high reliability with a wide service temperature capability. The alloy exhibited a melting temperature range of 223 to 232°C, reflowable at profile with peak temperature 245°C and 255C, with ambient temperature Yield stress 60MPa, UTS 77 MPa, and ductility 28%, and a higher stress than both SAC305 and Innolot, the latter two alloys were used as controls. When tested at 140°C and 165°C, the die shear stress was comparable with Innolot but higher than SAC305, and the ductility was higher than both Innolot and SAC305, with Innolot exhibited distinct brittle behavior. When aged at 125°C and 175°C, the die shear strength was comparable or higher than both controls. When pretreated with a harsh condition, TST -55°C/155°C for 3000 cycles, the die shear strength of 276 was 8 times of that of Innolot and SAC305. When pre-conditioned at TCT (-40°C/175°C) for 3000 cycles, the die shear strength of 276 was 11 to 20 times of Innolot and SAC305, depending on the flux type used. For SMT assembled BGA solder joints with both ball and paste using the same alloy, 276 showed higher Eta value than Innolot and SAC305 when tested at TCT with -55°C/125°C and - 40°C/150°C. For 1% failure, 276 was 2 to 3 times higher in cycling life than Innolot, mainly due to the Beta value of 276 being much higher than Innolot. SAC305 performed the poorest in these two tests. Both 276 and Innolot are alloys based on SnAgCu, but reinforced with precipitate hardening and solution hardening, with the use of additives including Sb, Ni, and Bi. 276 exhibited a finer microstructure with less particles dispersed, while Innolot exhibited more particles with some blocky Ag
₃
Sn plates or rods. 276 is rigid and ductile, while Innolot is rigid but brittle, Under the harsh test condition where ΔT was high, the dimension mismatch between parts and substrate became very significant due to CTE mismatch. This significant dimension mismatch would cause a brittle joint to crack quickly, as seen on Innolot. The challenge was more tolerable for a ductile joint, as shown by 276. Accordingly 276 showed a much better reliability than Innolot under harsh condition, including high testing temperature and large ΔT. Overall, to achieve high reliability under a wide service temperature environment, a balanced ductility and rigidity for solder alloy is critical for success.