A Comparison of the Solderability of Wrought and Plated Nickel Surfaces: The Selection of a Reliable Pin for Second Level Packaging

The solderability of a wrought nickel pin directly overplated with gold and that of a wrought kovar (tradename of an Fe-Ni-Co alloy manufactured by Carpenter Technology Corporation) pin plated with thick nickel and then overplated with gold was evaluated. The kovar pin was determined to exhibit very high solderability whereas the nickel pin was found to have extremely poor solderability. The poor solderability of the nickel pin as compared with the kovar pin is attributed to the contamination of the nickel surface prior to gold plating. A primary cause of the poor wetability of the nickel pin was due to embedded alumina particles on the surface of the pin (this occurs during the tumbling operation to remove any metallic slivers) prior to gold plating. The high solderability of the kovar is related to the clean nickel plating prior to overplating with gold. The cleanliness of the nickel layer was found to strongly influence the formation of the Ni-Sn intermetallic phase layer, having a nominal thickness of 42 μin (1.1 μm) after the kovar pin was tinned with eutectic Sn-Pb solder. For the tinned wrought nickel pin, a thin but continuous Ni-Sn intermetallic layer was also present; however, the nominal thickness of this layer was only 15 μin (0.38 μm). This large variation in solderability of both types of pins is attributed to differences in the intermetallic phase layer.