Solderability degradation models for fusible tin alloy coatings on copper substrates

Ion milling is used to investigate the role of surface corrosion products in degrading solderability of tin and tin alloy coatings over oxygen-free high-conductivity copper substrates. The coatings were applied by electroplating from a low organic stannous sulfate bath and by dipping into molten 300 degrees C Sn and Sn-40Pb alloy. Ageing took place in an 85% relative humidity, 85 degrees C environment for 100 days, and in a 190 degrees C environment with less than 6 mm (Hg) of water vapor for 10 hours. Solderability was measured by edge dipping the samples into molten 260 degrees C Sn-40Pb alloy. The effectiveness of ion-mill cleaning in removing the surface corrosion products was determined using Auger depth profiles. A small reduction in solderability due to surface corrosion is seen regardless of the coating type or ageing condition. The majority of the solderability degradation is due to the modification of the metal system below the surface corrosion layers, and surface corrosion plays only a small role.<>