Accelerating the effects of aging on the reliability of lead free solder joints in a quantitative fashion

The properties of lead free solder joints continue to change over a very long period of time in service before the microstructure becomes stable. The quantitative assessment of long term service life by accelerated testing invariably misses this significant effect, and may thus end up seriously misleading. The long term goal of the present work is to establish a protocol for preconditioning of lead free solder joints before thermal cycling or mechanical testing. For this purpose, the state of a solder joint at any given time was characterized in terms of three different room temperature properties, shear strength, shear fatigue resistance, and micro hardness. These properties were measured before and after aging for different lengths of time at different temperatures. Three common lead free alloys were selected for the present study: 98.5Sn-1.0Ag-0.5Cu (SAC105), 96.5Sn-3.0Ag-0.5Cu (SAC305), and 95.5Sn-4.0Ag-0.5Cu (SAC405). The present study did not address effects of solder volume, pad size, pad finish or reflow profile, focusing on 30 mil (760 mum) diameter solder spheres reflowed onto solder mask defined OSP coated Cu pads with a typical lead free profile. Isothermal aging was conducted for up to 3,000 hours at temperatures of 70degC, 100degC, and 125degC respectively. As expected, the resulting room temperature properties all decreased with aging time, and faster so for higher aging temperatures. Some of the acceleration factors extracted for the evolution of the individual properties did, however, differ greatly for a given alloy. The only way to establish the same microstructure, and thus the same combination of properties, faster by annealing at a higher temperature is thus to fully stabilize it. This takes thousands of hours even at 125degC, i.e. it is not practical for real assemblies.