INFLUENCE OF TEMPERATURE AND HUMIDITY ON THE WETTABILITY OF IMMERSIONTIN-COATED PRINTED WIRING BOARDS

Immersion tin films applied in various thicknesses (0.2-2 mu m) to dif ferent copper substrates were characterized relative to thermal stabil ity and shelf life, Thermal excursions included those typical in mixed technology assembly processes, Exposure to temperature/humidity was v aried from near ambient (35 degrees C/85% RH) to harsh (steam aging), A minimum thickness of similar to 60 mu in (1.5 mu m) was determined t o be critical for assembly operations involving multiple thermal excur sions, 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 protect ed by a tin surface layer, immersion tin finishes are relatively stabl e to thermal exposure, but are readily oxidized in the presence of hum idity, This oxide growth is directly responsible for solderability deg radation. The underlying copper substrate was also found to have a sig nificant impact on the thermal stability of tin films. An electroless copper substrate caused significantly more intermetallic formation, wh ich resulted in poor solderability even under moderate temperature/hum idity conditions.