THE ROLE OF CU-SN INTERMETALLICS IN WETTABILITY DEGRADATION

Wettability of pretinned Cu decreases after long aging times. This work provides insight into the role Cu-Sn intermetallics play in wettability degradation. This study investigates the effects of aging in air and argon at 170 degrees C on Cu coupons which were pretinned with 75Sn-25Pb solder. Coating was applied using an electroplating technique. The coating thickness was controlled between 3 to 30 mu m and the specimens were aged for 0, 2, 24 h, and two weeks. Wetting balance tests were used to evaluate the wettability of the test specimens. Microstructural development was evaluated using x-ray diffraction, energy dispersive x-ray, and Auger spectroscopy, as well as optical and scanning electron microscopy. Results indicate that Cu-Sn intermetallics protected from oxidation do not contribute to a decrease in wettability. Oxidized intermetallics, however, significantly decrease the wettability of aged pretinned samples. The extent of degradation is determined by the type of oxide formed on the surface of the intermetallic. This study shows that a predominantly Cu oxide forms on Cu3Sn, while a Sn oxide forms on Cu6Sn5. No evidence of internal oxidation was found.