Effect of Surface Finish and High Bi Solder Alloy on Component Reliability in Thermal Cycling

Reliability of an electronic component is typically limited by fatigue failure of one of the interconnected solder joints. For harsh applications, where high reliability is required, solder alloys with high Bismuth content, such as Innolot, are commonly used. Adding Bismuth (Bi) to SnAgCu solder joint improves the fatigue resistance and slows down the adverse effect of aging and thermal cycling on the reliability. This paper investigates the reliability of different types of electronic components in thermal cycling considering the effect of surface finish, high Bi solder material, and aging. The test board incorporates three large perimeter CABGA208 components, three small full array CABGA36 components, three small QFNs packages, and six 2512 SM resistors. Three different surface finishes were selected including Organic Solderability Preservative (OSP), Immersion Silver (ImAg), and Electroless Nickel Immersion Gold (ENIG). Three different high Bi solder alloys were used for the assembly including Innolot (3.8Ag, 0.7Cu, 0.15Ni, 1.4Sb, 3.0Bi), Cyclomax (3.4Ag, 0.5Cu, 3.3Bi), and MaxRel (3.8Ag, 0.8Cu, 3.0Bi). After the assembly, all test boards were aged for 12 months at 125oC. The boards then were cycled between -40oC and +125oC with a ramp up and down rate of 3.3 oC per minutes and dwell times of 15 minutes at +125oC and 10 minutes at -40oC. Total cycle time equals to 125 minutes. The test was completed at 5800 cycles, then statistical data analysis and failure analysis were performed. It was observed that Innolot performed the best among the solder pastes, closely followed by MaxRel. Regarding solder sphere alloys, matching solder sphere with solder paste alloy showed the best reliability followed by SAC305. The reliability of matched paste-sphere could be attributed to the comparatively higher Bi content from other sphere alloys. ENIG outperformed other surface finishes in the case of BGAs, but with SMRs, OSP and ImAg was slightly better.