Roughness evolution of Cu6Sn5 intermetallic during soldering

The roughness evolution of the Cu6Sn5 intermetallic compound during the soldering reaction of 40/60% (by wt %) PbSn solder on Cu and its effects on wettability were studied due to their importance in the reworking of microelectronic components. The roughness parameters investigated were Rrms and the ratio, Rrms/λa, where Rrms is the root mean square roughness and λa is the average distance between asperities. It was found that Rrms increased with soldering time for the range of soldering times studied. The evolution of Rrms was found to follow a parabolic relation with soldering time—which is similar to that observed for the intermetallic thickness. However, the ratio Rrms/λa was found to increase very gradually for the range of soldering times investigated. From the ratio Rrms/λa obtained, the average inclination of the intermetallic morphology was calculated. The average inclination of the Cu6Sn5 intermetallic morphology was found to be greater than the apparent contact angle observed during the wetting of the 40/60% PbSn solder on Cu substrates. This inconsistency was attributed to the difference in the roughness of the Cu6Sn5 intermetallic in the inner regions and at the reaction band at the edge of the solder cap.