Pb-Free Thin Small Outline Package (TSOP) Board Level Reliability Study

Abstract

Thin Small Outline Package (TSOP) are one of the most commonly used surface mount components due to its low overall cost. Traditionally leadframe packages such as TSOP or Quad Flat Package (QFP) are less of a concern (if assembled with SnPb eutectic solder paste) about their long term reliability and often exempted from board level qualification testing as the mechanical compliance of metal leads mitigate the stresses due to the Coefficient of Thermal Expansion (CTE) mismatch between the package and Print Circuit Board (PCB). Therefore more attention has been put on the solder joint reliability of Pb-free Ball Grid Array (BGA) packages over leadframe packages while the industry is moving away from SnPb eutectic solder materials to meet RoHS regulatory requirements. The authors have observed that TSOPs if assembled with Pb-free solder materials could fail at very early stages during qualification testing (in some case as early as 300 cycles under standard 0°C to 100°C thermal cycling). Since most Pb-free solder materials such as SnAgCu are mechanically more rigid than SnPb eutectic solder material, higher stresses are expected be induced in solder joints during temperature excursions. Pb-free solder materials’ wicking behavior may also contribute to the early failures. In this study, long term reliability of a flash memory TSOP has been investigated. These tested TSOPs, assembled on 93mil-thick PCBs with SAC305 paste, are of two configurations: one with single die and the other with stacked quadruple dies. Some test vehicles have been thermally aged under four different thermal aging conditions to study the aging effect on Pb-free solder joint life. Finite element analysis (FEA) modeling has also been employed to further investigate the impact of other parameters such as die size, package size, and the number of dies that being stacked inside one package.