THE ROLE OF CU-SN INTERMETALLICS IN WETTABILITY DEGRADATION

Wettability of pretinned Cu decreases after long aging times. This wor k provides insight into the role Cu-Sn intermetallics play in wettabil ity degradation. This study investigates the effects of aging in air a nd argon at 170 degrees C on Cu coupons which were pretinned with 75Sn -25Pb solder. Coating was applied using an electroplating technique. T he coating thickness was controlled between 3 to 30 mu m and the speci mens were aged for 0, 2, 24 h, and two weeks. Wetting balance tests we re used to evaluate the wettability of the test specimens. Microstruct ural development was evaluated using x-ray diffraction, energy dispers ive x-ray, and Auger spectroscopy, as well as optical and scanning ele ctron microscopy. Results indicate that Cu-Sn intermetallics protected from oxidation do not contribute to a decrease in wettability. Oxidiz ed 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 f orms on Cu6Sn5. No evidence of internal oxidation was found.