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Reliability and Microstructure of SnAgCu Solder Joints in High Power RF Packaging
Keywords: Pb-free Soldering, Au Embrittlement, High Power RF Packaging
This study compares the reliability of SnPb solder joints to that of SnAgCu solder joints in the lead attach of high power RF packages. High-power RF packages are unique with the width of alloy-42 leads often measuring 13 mm on a side and has NiAu plating on all exposed surface. The thermal mismatch between the lead and PCB will be greater in RF devices than in other types of electronic packages with Cu leads. A larger mismatch means that the solder joints in RF packages will experience more strain during thermal cycling. The fatigue life of the solder joint is typically related to the amount of plastic deformation and creep strain in the solder joint. In RF packages the leads are plated with gold approximately between 1.0 m and 2.5 m in thickness. In the present study, the lead-to-board assembly of high power RF packages using Sn-3.9Ag-0.6Cu (SnAgCu) solder was investigated. The effect of Au concentration on the microstructure of solder joint was examined with respect to reflow profile specifications. The microstructure of solder joint was characterized in as-reflowed state and after solid-state aging for 500 hr. It was found that neither altering Au concentration in the SnAgCu solder joints nor varying reflow profiles had an effect on the resulting microstructure. Solid state aging led to a grain growth of the Sn matrix, ripening of the intermetallics (IMCs) in the solder, and a significant increase in the thickness of interfacial IMCs. In addition, the -40X/+125X C thermal cycling of SnAgCu solder joints with different gold plating thickness was investigated, compared with eutectic SnPb solder joints. It was found that SnAgCu solder joint was more fatigue-resistant than eutectic SnPb solder during thermal cycling. Also, there is some degree of Au embrittlement in SnAgCu solder joints.
Mahesh Shah,
Freescale Semiconductor
Tempe, AZ

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