Low Temperature Soldering : Thermal Cycling Reliability Performance

The technical and economic benefits derived from lowering the reflow temperatures have motivated the evaluation of new SnBi low temperature alloys for soldering. Eutectic Sn-Bi alloy is usually described as having a brittle nature, not being able to sustain mechanical shock and thermal cycling stresses as well as Sn3Ag30.5Cu (SAC305) solder. A new non-eutectic Sn-Bi solder with 2 wt.% additives (generally called here as alloy A) is evaluated here and compared with other eutectic Sn-Bi alloys and SAC305.Tensile tests at -55 oC, -25 oC, +25 oC, +75 oC and +125oC were performed to provide insights on their relative mechanical properties. Mechanical drop shock tests of BGA84 and LGA84 were performed as per the JESD22-B111 standard, while thermal cycling tests were performed from -40oC (10 min) to +125oC (10 min) as per the IPC 9701 standard. The BGA84 was used for thermal cycling in situ monitoring, while BGA169, SOT223, QFP44 and 1206 chip resistors were also used for evaluating the effect of thermal cycling on IMC thickness, shear strength and tin whiskers. The results show that, for the aforementioned packages, assembly and testing conditions, alloy A is a viable replacement for SAC305. The joints formed with alloy A have the mechanical (drop) shock and thermal cycling performance as good as, or, in some cases, better than the same joints formed with SAC305.