Process Developments in Transient Liquid Phase Bonding of Bi-Ni for High-Temperature Pb-Free Solder Alternatives

Transient liquid phase (TLP) bonding emerges as a promising joining technique that connects dissimilar materials by employing an interlayer that provides a lower melting process. This research investigates such a TLP bonding system made of a nickel (Ni) bonding surface and a bismuth (Bi) interlayer, which can serve as a Pb-free solder alternative for high-temperature electronics. Upon melting of Bi on Ni, intermetallic compounds nucleate and grow by consuming a Ni layer on the substrate and the die side. For this bonding, the key challenge has been concerned with the brittleness of the joint and the development of porosities that consequently join together to make a continuous crack at the center line of the bond. In an effort to tailor more reliable microstructures in the joint regions, intermetallic reactions and microstructure developments were studied under various reflow conditions including reflow temperature and time, with varying thickness of Bi interlayer from
$8\ \mu\mathrm{m}$
to
$250\ \mu\mathrm{m}$
. The highest shear strength was achieved with the sandwiched samples reflowed at 330°C for 10 min under vacuum condition. Mechanical properties of the intermetallic bonds were investigated by shear testing and fracture surface analysis and the corresponding microstructural evolution of the reflowed sandwiched samples.