You may not know that you are step soldering already, but you probably are! Step soldering is a process in which different melting point solders are applied from highest melting point to lowest melting point in a system of soldering steps. This is very common in PCB assembly. The reason solder is used in this way is to prevent the re-melting of the solder joints effected in the previous processes.
When I first learned about step soldering, I didn’t understand how widely it was used in the PCB assembly industry. But then I started thinking: there are solder joints in components! It makes perfect sense that step soldering is part of our industry. The inside of components are soldered with a high-temperature solder and then subsequently soldered to a PCB with a “lower” temperature solder like SAC305. Sometimes, after that, during rework, or in an additional step, solder is added with an even lower melting temperature. The lower temperature solder is typically an indium- and/or bismuth-based alloy.
One of the main questions concerning step soldering is what should the temperature gap be between the different steps. I discussed this with my colleague Kim Flanagan in this blog post (Real-Life Scenario of a Step-Soldering Application). She also discusses the challenges of Pb-free step soldering in this post (Pb-free Step soldering).
Our engineering team can help with alloy selection for step soldering. Email us at AskUs@Indium.com.