Roughness evolution of Cu6Sn5 intermetallic during soldering

The roughness evolution of the ${\mathrm{Cu}}_{\mathrm{6}}{\mathrm{Sn}}_{\mathrm{5}}$ 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 $R_{\mathrm{rms}}$ and the ratio, $R_{\mathrm{rms}}{\mathrm{/\lambda }}_a,$ where $R_{\mathrm{rms}}$ is the root mean square roughness and ${\lambda }_a$ is the average distance between asperities. It was found that $R_{\mathrm{rms}}$ increased with soldering time for the range of soldering times studied. The evolution of $R_{\mathrm{rms}}$ was found to follow a parabolic relation with soldering time—which is similar to that observed for the intermetallic thickness. However, the ratio $R_{\mathrm{rms}}{\mathrm{/\lambda }}_a$ was found to increase very gradually for the range of soldering times investigated. From the ratio $R_{\mathrm{rms}}{\mathrm{/\lambda }}_a$ obtained, the average inclination of the intermetallic morphology was calculated. The average inclination of the ${\mathrm{Cu}}_{\mathrm{6}}{\mathrm{Sn}}_{\mathrm{5}}$ 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 ${\mathrm{Cu}}_{\mathrm{6}}{\mathrm{Sn}}_{\mathrm{5}}$ intermetallic in the inner regions and at the reaction band at the edge of the solder cap.