For soldering to gold, we recommend using indium-based alloys, especially those in the indium-lead (InPb) family because they are less likely to scavenge the gold surface than tin-lead (SnPb) solders. Tin-based alloys rapidly scavenge or dissolve the gold during the reflow process leading to the complete irreparable destruction of gold conduction patterns. Tin-based alloys will also result in crack-inducing platelets within the solidified solder joint.
When considering which alloy to use for your application you should take into account both the operating temperature of the device and the maximum process temperature. A good rule of thumb is to choose a solder with a solidus temperature of no less than 50°C above the maximum device operational temperature. An optimum process temperature will typically be 30-50°C above the liquidus temperature of the solder.
The InPb alloy system contains a range of useful solder alloys from pure indium to pure lead. Alloys containing an excess of 80% lead can be used, but have diminished wettability. Indium and lead form a nearly continuous solid solution.
It is possible to solder to gold without flux under certain circumstances; however, if the plated layer is thin and possibly porous, then the gold can become totally assimilated into the solder. In this situation, flux choice is then determined by the characteristics for the underlying metal.
Even though indium-containing solders solve numerous critical joining applications, certain precautions must be taken concerning metal compatibility and corrosion.
- In the operational temperature of the device exceeds 125°C, solid-state diffusion of the fold may occur, resulting in the growth of the AuIn IMC layer. In such cases, 80Au/20Sn or a high-lead alloy can be used instead, according to process or commercial constraints.
- Indium can be corroded by halides. Indium-based joints should be protected in service from halide-containing materials or if humidity will exceed 85% in the presence of halides. This can be accomplished with the use of a conformal coating.
- Fluxes based on halide activators should be avoided when using indium-based alloys. Using such fluxes can cause corrosion of the solder joint at a higher rate than for a SnPb joint.
To determine the best alloy for your application, you can order our Soldering to Gold (Au) Research Kit.
This will allow you to test a variety of alloys to ensure you achieve a strong bond without cracking.
For more information about soldering to gold, please read our application note on Soldering to Gold.