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Cross-Interaction of Different Pad Finishes in SMT Joints and Its Effect on Board Level Reliability
Keywords: cross-interaction, SMT joint, reliability
To meet the size shrinkage requirement of the electronic packages in ever smaller portable products, the diameters of solder joints in board level surface mount devices keep decreasing. Thus the interfacial reactions on both sides pad finishes did not proceed independently during the reflow process. The cross-interaction of the ENIG pad/solder interface and the solder/OSP(Cu) pad interface in surface mount joint was investigated. The effect of the cross-interaction on board level reliability was also studied. In this study, three kinds of typical solders Sn3.0Ag0.5Cu, Sn1.0Ag0.5Cu and Sn3.8Ag0.7Cu were selected, and the diameter of solder joint is 300 m. Just after 35 sec of reflow above liquidus, a massive spalling of Cu6Sn5 from the solder/OSP(Cu) interface occurred, some Cu6Sn5 particles were observed dissolved into bulk solder and even quite near to the opposite solder/ENIG interface. This massive spalling became remarkable after 4 reflows, much Cu6Sn5 massive were found deposited on the original (Cu,Ni)6Sn5 layer of the solder/ENIG interface. However, very little Ni could be detected in the OSP(Cu) side. This implied that Cu diffused and dissolved in the molten SnAgCu solder much faster than Ni. The driving force of the massive spalling of Cu6Sn5 toward the opposite ENIG side attributed to the Cu concentration grads and solubility difference across the molten solder, which were affected by the primary components concentration of solders. The thermal cycling test and drop test were conducted to evaluate the thermal fatigue reliability and mechanical impact reliability, respectively. The interfacial microstructure plays a critical role in the reliability issues of a solder joint. So how the cross-interaction of the opposite interfaces affects the board level reliability deserves a detailed study.
Lei Wang, Engineer
Samsung Semiconductor R&D Co., Ltd.
Suzhou, Jiangsu 215021,

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