Research Highlights
 
 
 
Bi-induced interfacial voids in solder interconnects

The interfacial voids have been widely observed in eutectic SnPb/Cu solder joints. They are often associated with the growth of Cu3Sn intermetallic compound (IMC) and are attributed to the Kirkendale effect. Another type of voids were found in the interconnect made from Pb-free eutectic SnBi alloy. This type of voids is different from the Kirkendall void in that they appeared along the Cu3Sn/Cu interface. only following the necessary segregation of Bi to that interface. Transmission electron microscopy evidence was obtained that supports Bi-induced interfacial void formation in solder joints. After thermal aging, Bi segregated to the Cu3Sn/Cu to form fine particles on the interface in SnBi/Cu solder joints. The interfacial voids were found near the interfacial Bi particles.
(a) HRTEM cross-sectional image from the eutectic SnBi/Cu sample aged for 24 hours and (b) Low-magnification HAADF cross-sectional image from the eutectic SnBi/Cu sample aged for 240 hours.

Reverse Polarity Effect During Electromigration in Eutectic Sn-Zn Solder Interconnect
Electromigration tests were performed on the eutectic SnZn interconnects at 150°C and 3.87×104A/cm2 for different times. The tests were designed to examine how the disparity in the effective charge between alloy constituents might affect the electromigration behavior of solder interconnects. Under electric stressing, Zn was driven to the cathode while Sn was forced to accumulate at the anode. The electromigration of Zn resulted in a reverse polarity effect in the IMC development. A back stress model showed that the back stress gradient, produced by Sn electromigration, was critical in driving Zn migration to the cathode.
Kinetic analysis of the model (Fig.1) yielded the following criteria for migration direction for individual species where subscript i refers to the dominant diffusing species.During electromigration, IMC growth at the electrode interfaces followed three different stages. In the first stage, the chemical potential gradient controlled the IMC growth. In the second stage, the back stress gradient plays a critical role in separating migrations of individual species with disparate effective charges. Beyond the second stage, the IMCs were converted into different types of intermetallic compounds at both electrodes.
Components of Zn flux in a sample undergoing electromigration
Solderability of electrodeposited Fe-Ni alloys

Solderabilities of electrodeposited Fe-Ni alloys with the SnAgCu solder were examined by wetting balance measurements and compared to those of pure Ni and pure Fe platings. Excellent solderability was found on the Ni-52Fe plating as both the wetting force and times approached or exceeded those on the pure Ni. However, upon further increase of Fe content to 75 pct, the solderability of the alloy was severely degraded even though it was still better than that of the pure Fe plating. X-ray photoelectron spectroscopy showed that such a strong dependence of solderability on Fe content is related to the much thinner, incomplete oxide coverage of Ni-rich plating surface. On the surfaces of Fe-rich platings, the Fe oxide layer was much thicker. The inferior solderability of the Fe-rich platings was related to slow and incomplete removal of the Fe oxide layer on the surface, the residual of which acted to pin down the wetting front.
TEM image and analysis of the intermetallic compound formed on Fe-Ni alloy substrate.