The influence of temperature and humidity on the wettability of immersion tin coated printed wiring boards

This paper investigates the merits and drawbacks of immersion tin coatings as potential printed wiring board (PWB) surface finishes. Immersion tin films applied in various thicknesses (0.2 to 2 μm) to different copper substrates were characterized relative to thermal stability and shelflife. Thermal excursions included those typical in mixed technology assembly processes. Exposure to temperature/humidity was varied from near ambient (35°C/85%RH) to harsh (steam aging). A minimum thickness of ~60 μin (1.5 μm) was determined to be critical for assembly operations involving multiple thermal excursions. Even though formation of Cu-Sn intermetallic compounds (IMC) is facile, at the copper-tin interface, these compounds do not adversely affect the soldering performance, as long as the IMC phase is protected by a tin surface layer. Immersion tin finishes are relatively stable to thermal exposure, but are readily oxidized in the presence of humidity. This oxide growth is directly responsible for solderability degradation. The underlying copper substrate was also found to have a significant impact on the thermal stability of tin films. An electroless copper substrate caused significantly more intermetallic formation, that resulted in poor solderability even under moderate temperature, humidity conditions