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The Effects of Surface Finish on the Microstructure of Sintered Paste Joints
Keywords: sintering, surface finish, reliability
There are a number of different surface finishes in use on electronic substrates from lead-frames to PCBs. These surfaces include immersion tin, immersion silver, ENIG, ENEPIG, bare copper and copper with OSP. In general, all these solderable surface finishes are compatible with sintering pastes. As the breadth of applications for sintering pastes increases, the interaction between these surfaces, and others, and the sintering pastes will be need to be well understood. As with solder pastes, the thin surface metals applied as final finishes are often consumed by the sintering paste interconnect and become integrated into the microstructure of the joint. These finish metals react with both the metals in the paste, as well as with the organic vehicle and thus can manifest different behaviors with respect to behaviors like volatile generation and voiding. The differences in the joint microstructure resulting from the myriad of reaction mechanisms also manifest in how the joint microstructure changes with thermomechnical stress over time. In addition, the interface between the sintering paste and the surface underlying the finish metal should be characterized after initial processing and through thermal cycling to determine the relative reliability of these different metal interfaces. Unlike solder, sintering pastes do not contain large volumes of unreacted tin once the sintering process is complete. Therefore, it is anticipated that both the initial microstructure and interfaces as well as the changes in microstructure and interfaces with thermal cycling that are typically seen in solder will be significantly different for sintering pastes. In this study, the interaction between sintering pastes and two different surface finishes, bare copper and ENIG, will be explored in comparison to SAC 305 solder paste.
Catherine Shearer, Director of R&D
Ormet Circuits, Inc.
San Diego, California

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