Soft solder die attach in small outline discrete packages for both reliability and cost breakthrough

Abstract

The automotive requirements have quite many challenges regarding semiconductors reliability performance, and there was no escape for small outline discrete packages. Some epoxy die attached discrete devices could not pass MSL requirements due to die attach separation, even with leadframe surface treatment and all other solutions. The brainstorming results showed soft solder die attach would be the most probable solution. However, soft solder process has its own preferable working conditions, which were not compatible in small outline discrete packages. Some of the major package constraints were thin leadframe and small die attach flag size. These constraints could not be changed in the considerations of package outline commitment and cost factor. In early phase of soft solder development, die attach machine was prepared by performing conversion on epoxy die attach machine without incurring any capital cost. On the machine, new hardware features were introduced to stabilize soft solder performance, including motorized spanker and multi zones heater stages. Furthermore, comprehensive study was performed with details temperature profiling in order to achieve good wetting and same time controlling post heating for solder robustness. In process optimization, several evaluations were performed on most of the important factors, with major responses such as wetting, die tilt, electrical performance and even reliability performance. Some initial evaluations showed massive premature temperature cycle failures due to solder horizontal crack, with crack gap could be quite large in dimension. By doing further investigations and simulation of Finite Element Analysis (FEA), it was found that there were some interactions effect between process and material. The material selection was evaluated again, together with process optimization. There were many evaluations executed on different possible root causes including leadframe surfaces, mold compound, plating scheme, solder material, bond line thickness,…etc. Upon completion of various evaluations, some of the factors showed significant effect, including bond line thickness, mold compound, and solder selection. By performing more thorough verifications, it was revealed existence of few possible optimum interactions of significant factors. With consideration of risk management together with cost analysis, proper selection of solution could be done effectively. Confirmation runs with large sample size and reliability tests exhibited excellent performance. Even extended reliability tests up to 2.5 times of standard requirements still showing positive results. The soft solder die attached products showed high reliability performance, excellent electrical performance, and same time much lower cost in comparing to epoxy applications. More importantly, there was no change in upstream and downstream processes, mainly because leadframe thickness remained unchanged. Overall project achievement was successful development of soft solder in small outline discrete packages which was cost effective and passing all relevant reliability tests with flying colors.