Indium Blog

Which Off-Eutectic AuSn Alloy Do I Use?

When working with dies and/or substrates that are thickly plated with pure gold, the best way to prevent gold-rich solder joints is to use an off-eutectic AuSn alloy in the solder joint. Lowering the gold content of the preform itself allows for the solder to absorb some of the pure gold-plating, finalizing the joint at a high-tensile strength eutectic 80Au20Sn composition. The thicker the gold plating(s), the lower the gold content the AuSn alloy must be. Sure, that makes sense, but which off-eutectic alloy composition should you use?

Let’s take a step back and clarify why gold-rich solder joints are undesirable. Plating dies and/or substrates with pure gold works well to prevent the components from oxidizing. Pure gold also does not tarnish and has a long shelf life, both highly beneficial properties. However, pure gold is porous and has the ability to dissolve into the AuSn solder during reflow. AuSn has a sensitive eutectic phase, and as the solder joints become gold-rich, there can be areas that do not wet or flow properly. The mechanical and thermal properties highly sought by the eutectic composition are now less robust. Over time, the components will lose their bond strength and the substrates can actually separate from each other. This is why it is crucial for the solder joint to become complete at the eutectic composition of 80Au/20Sn, ensuring a strong joint, and reducing wetting, voiding, and delamination issues.  

A simple gold layer ratio calculation accounts for the total gold in the metallizations, compared to the thickness of the solder:

The calculated ratio can then be compared to the following chart to determine the off-eutectic AuSn alloy composition that will finalize an 80Au/20Sn solder joint.

Accommodating for these thick gold metallizations ensures the joint characteristics are optimized for high-reliability and performance. Our
AuLTRA®75, AuLTRA®76, AuLTRA®77, AuLTRA®78, and AuLTRA®79 are available now in a wide range of X, Y, and Z dimensions, and are unique to any die size. This wide range of off-eutectic AuSn alloys allows for the exact adjustment of the final solder joint composition to guarantee strong solder joints in die-attach applications, and improve wetting and voiding.