Design engineers typically prescribe eutectic AuSn solder (80Au20Sn) preforms for die-attach and lid sealing on high-reliability electronics packages. Here is a reminder that designing a eutectic AuSn alloy into a soldering application can be a bit tricky.
Many variables impact a solder joint, but the one I'm focusing on in this post is the plating on the bonding surface. A common practice is to cover a low-solderability surface, such as Kovar®, stainless steel, or ceramic, with Ni and then add a gold plating over the Ni to protect it from oxidation. During the soldering process, the Sn in the solder preform scavenges (dissolves) the Au plating into solution, increasing the gold composition in the solder joint.
Gold scavenging during the reflow process results in the solder joint having an increased melting temperature, causing premature freezing, and creating small voids in, the final solder joint. These voids are poor (thermal and electrical) conductors and create hot spots in die-attach bond line.
One way to prevent this from happening is to use an off-eutectic (lower Au concentration) solder preform alloy to compensate for the plating. Indium Corporation offers several alloys for this purpose, including 79Au21Sn, 78Au22Sn, and some experimental alloys. We also use tools that calculate the composition of the surface plating and solder preforms to give you a final solder joint composition. Remember, the dissolution rate and amount are affected by many conditions, including the total amount of energy (temperature and time) above liquidus.
If you have a new project, contact me or Indium Corporation’s tech support team to discuss a solder preform composition that works best for your application. I am Bernard Leavitt (firstname.lastname@example.org), and I'm ready for your questions.