| Abstract
|
:
|
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. One of the goals of the present work is to develop a fundamental understanding of the effects of preconditioning (isothermal aging) on the reliability of lead free solder joints, and thus to the establishment of a means of assessing the reliability of old lead free solder joints. 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. Four common lead free alloys were selected for the present study: 98.5Sn-1.0Ag-0.5Cu (SAC105), 96.5Sn-3.0Ag-0.5Cu (SAC305), 95.5Sn-4.0Ag-0.5Cu (SAC405) and 96.5Sn-3.5Ag (SnAg). The present study did not address effects of solder volume, pad size, or reflow profile, focusing on 30 mil (760μm) diameter solder spheres reflowed onto solder mask defined OSP coated Cu pads and Electroless Nickel Immersion Gold (ENIG) with a typical lead free profile. Isothermal aging was conducted for up to 5,000 hours at temperatures of 70°C, 100°C, and 125°C 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 and pad finish. 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 125°C, i.e. it is not practical for real assemblies.
|