Effect of Combining Solder Pastes with SAC305 Spheres on Component Reliability in Harsh Thermal Cycling

There has been a strong drive to remove Lead from electronics since the early 2000s. Solder alloys based on tin (Sn), copper (Cu) and silver (Ag) were regarded as promising candidates to replace the traditional SnPb solder. Later, it was found that one year of aging reduced the life of the components by more than 50% in the case of SAC based alloys. Once the detrimental effects of aging were discovered, the industry began search for improved solder materials. Several elements such as Bismuth (Bi), Indium (In), Antimony (Sb) and Nickel (Ni) are added to SAC alloys to make them more reliable. Extensive tests are required to analyze these new alloys and thereby to filter the best solder pastes. In the study, three high reliability solder materials along with SAC305 alloy from different manufacturers have been selected and used for the test vehicle assembly. The test vehicle is a printed circuit board (PCB) of FR-4 laminate material with three CABGA208s (15×15mm) and three CABGA36s with SAC305 spheres and six surface mount resistors. In order to reduce variability, single surface finish, namely Organic Solderability Preserve (OSP) has been used for all test vehicles. Immediately after assembly, all boards are aged for a period of three months at 125°C. The boards were then thermally cycled for 4700 cycles from -40°C to +125°C with a ramp rate of 10°C per minute and dwell times of 15 minutes at high and low temperatures. Two parameter Weibull analysis is used to quantify the performance of the different alloy materials. B10 life is also used to analyze the early failures in each case. Generally, all the new alloys with new elements were found to mitigate the aging effects. It was also found that there was not much improvement in reliability when the solder alloys are used alone instead of combining them with SAC305 to form solder joints.