The other day, I overheard my colleague and fellow Technical Support Engineer, Meagan Sloan, on the phone with a customer who had some questions regarding step soldering. I asked her to tell me about the advice she gave the caller.
Kim: Meagan, I overheard you talking with a customer about step soldering. I was wondering if you could share some of the key points of the conversation.
Meagan: Yes! What would you like to know?
Kim: What were the customer’s main concerns with step soldering?
Meagan: His main concern had to do with the temperature gap he should be using between the two different solders. He wanted to make sure that the solder in his first step would not melt in his second step.
Kim: What did you recommend?
Meagan: The absolute minimum temperature gap should be 25°C; however, this is dependent on process control. A larger temperature gap is better if you can’t control the temperature of your second step.
Kim: Why don't you recommend a gap that is smaller than 25°C?
Meagan: Typically, a solder must be heated at least 20°C above its melting point to allow for adequate wetting. So, the gap has to be at least that big. A smaller gap has the potential to exceed the melting point of the solder from the first step of the process, causing it to reflow again while reflowing the solder for the second step. Also, if you are close to the melting temperature of the first step solder, the intermetallics can grow more in the initial solder joint, due to this second heat excursion. This is because intermetallic formation is affected by time and temperature, even if the solder is not fully melted, which can result in large, brittle intermetallics.
Kim: Thank you for your insight, Meagan.