Assessing the effect of surface roughness on the wetting of Cu and Pd by Sn/Pb solder
Here we examine the effect of varying substrate surface roughness on solder wettability (area of spread) and the time required to reach terminal area of spread. Results are given for solder wetting experiments that were performed on copper (Cu) substrates having chemically etched surfaces, as well as, Alumina (Alz03) substrates electroplated with various thicknesses of palladium (Pd). The effect of etching on the A1203/Pd specimens was also examined as related to surface roughness and solder
... hness and solder spread. These surface treatments were found to significantly alter wettability. Substrate surface roughness properties were quantified using an optical profilometer. Area of spread and time to terminal area of spread were quantified using video recording and image analysis equipment. Substantial improvements were observed in both solder wettability and time to wet with the uniformly etched Cu surfaces used in this study. For the Cu substrates, the average terminal area of spread is shown to be directly related to the substrates root mean square (RMS) surface roughness. The rate of wetting of the Cu surfaces is also shown to increase when chemical surface treatment is used, Maximum wetting on the A1203/Pd specimens was found to be directly related to surface smoothness (Le., inversely related to surface roughness). The average terminal area of spread of A1203/Pd specimens is inversely related to the vertical distance from the highest surface peak to the deepest surface valley (Le., peak-to-peak variation) The rate and area of spreading of 63Sd37Pb solder on chemically treated Cu substrates show a significant increase over those substrates that have not been surface treated. The opposite effect is observed on the spreading of 63Sd37Pb solder on A1203/Pd substrates, The rate and area of spreading are greater for the smoother surfaces in that case.