An investigation of the recommended immersion tin thickness for lead‐free soldering

Purpose This paper describes the different thickness measurement techniques that enable reliable thickness assessments, and the determination of the recommended immersion tin thickness for lead‐free soldering. Design/methodology/approach Immersion tin layers were prepared with systematically varying layer thicknesses. The samples were annealed at different reflow profiles, used in assembly for tin/silver/copper (SAC‐alloy) soldering. The layers were characterized with X‐ray fluorescence, electrochemical stripping coulometry, and by examining the cross sections using a scanning electron microscope. The solderability of the samples was determined with a solder balance (Solderability Tester Menisco ST60) using a SAC‐alloy (melting point 217°C) with T (max) at Δ T =28°C and Δ T =43°C above melting. Findings If all pure tin is converted into the Sn/Cu IMC, so that no pure tin is left as solderable layer, the wetting behaviour will decrease dramatically. Especially for multiple soldering processes, two times reflow followed by wave soldering, it is essential to have a pure tin layer covering the Sn/Cu IMC before going to the final soldering process. The required amount of residual pure tin over the Sn/Cu IMC is detailed in several papers. It is stated that a minimum of 0.2 μ m of pure tin over the Sn/Cu IMC is absolutely necessary to ensure reliable wetting and solder joint formation. With the current immersion tin thickness recommendation of 1 μ m, based on the needs of lead containing solder pastes, a residual pure tin layer will not be evident or thick enough to ensure reliable assembly for multiple soldering with lead‐free temperature profiles. Originality/value Helps to enable reliable thickness assessments, and the determination of the recommended immersion tin thickness for lead‐free soldering.