The impact of Sn-Oxide on the solder wetting of immersion tin and how to overcome possible solderability defects to ensure constant solder wetting performance

Immersion tin is a final finish which is widely used in the printed circuit board (PCB) industry. It provides a cost competitive surface protection with excellent corrosion resistance and has the capability for multiple reflow soldering. The tin is deposited on copper by immersion reaction with a typical layer thickness of 0.8 – 1.2 μm. On top of the tin layer an oxide layer is formed, which can influence the properties of the final finish. During the assembly process, an intermetallic compound (IMC) grows connecting the copper substrate and the tin of the solder alloy. The growth of the IMC is a complex function of temp and time. The IMC formation and well distributed pure tin remnants are key factors to obtain a reliable solder joint. With increased aging or reflow of the immersion tin deposit, the role of the tin oxide layer becomes of particular interest influencing properties of solder and immersion tin in liquidus process at reflow step. Due to the IMC formation, the tin layer is facing increasing internal stress which is potentially released via the oxide covered surface. The quality and thickness of the tin oxide layer impacts the solder wetting performance of the final finish, the risk of whisker formation and in particular the stability and appearance of the layer after reflow aging. In this paper, an introduction on typical failure mechanisms and root causes for solder wetting defects of immersion tin will be given. Such defects can become e.g. visible by solder dewetting in certain areas of the soldered pads or by partially shiny appearance of the tin surface after reflow cycles. The mechanisms introduced in this paper are supported by correlating tests to identify possible root causes for the solder wetting defects. Various methods are presented which allow the determination of the tin oxide layer thickness. Different factors are investigated for their impact on the tin oxide layer formation, and various approaches are studied to modify the thickness of the oxide layer. Based on the test results, the properties of the tin oxide layer could be identified as critical parameters for the immersion tin layer performance. The application of a dedicated post-treatment solution can modify the tin oxide layer and improve the performance of the immersion tin deposit regarding appearance and solderability. This is confirmed by optical inspection, different types of solderability tests and whisker evaluation.