Defect of the Month – Secondary Reflow

Hello, my name’s Bob Willis and welcome to Defect of the Month. Every month I try and provide an example of a particular defect and hopefully some solutions. I’ve made over a hundred different defect videos over the years. So hopefully one will solve one of your problems. Remember that if you like this video content, please subscribe to the channel. If you like this individual video, then just like it. And of course, please share it with some of your other engineering colleagues within your company. 

Now, secondary reflow is where a solder joint reflows for a second time, and there can be a number of different variations of this. It just depends on where it’s happening or the solder alloys that are being used on the product. So let’s just take it step by step. First of all, we can have secondary reflow during double sided assembly because a lot of parts will reflow again, when you pass them through the reflow process, you know, that’s part of the process and it happens. 

If your parts have been reflow soldered and then you’re going through a wave soldering process or a selective soldering process, what you’re trying to do is make sure that the topside board temperature does not go anywhere near the reflow temperature of the solar alloy you’ve used on the top side of the board. And this sometimes can be a struggle with certain applications, but you must do temperature profiling to make sure that you’re not getting anywhere near the semi-liquid state of the alloy being used. If you don’t then open circuits or intermittent joints can occur. The solar joint is reflowing and if there is any tension in the pins or the components, or there is warpage of the printed circuit board, that then puts strain on some of the joints that are going into this semi-liquid stage, you can get joints that are separated. They may appear solid, but there is an interface there that is separated.

So again, it’s that experience you need to look at another side issue if you like, if you are using lead-free technology. And for some reason there is lead contamination, either on the components or on the PCB, which is generally unusual today, I have to say any lead in a lead-free alloy will drop the temperature at which the solid joint goes into a semi-liquid stage. So if for some reason you had contaminated parts and then you reloaded those parts because the lead brings down the liquidous temperature of the alloy of the original joint, then the component terminations can separate and ping apart. So again, that’s how you get open or intermittent joints. Of course, if you’ve got joints reflowing for a second time, it’s possible the components can move during reflow it’s kind of  less likely, but it’s always a possibility.

So those are some of the reasons. More recently with my work on low temperature alloys, I’ve been able to easily demonstrate if you have got lead contamination in the original joints, the temperature at which those joints reflows, again, certainly does drop quite significantly 

Thank you very much for listening to defect of the month. Hopefully it’s been useful and possibly you’ve found a solution to one of your process problems. So just a reminder that if you want to listen to more defect of the month, subscribe to our channel, if you just like this individual video, then please like it. And of course share it with some of your colleagues in manufacture and in other companies, it all helps to bring information to both production and engineering staff. If this short introduction to this particular problem was of interest to you, then of course we can hopefully provide you a solution to aid. You eliminate more process defects within the future. Finally, if you’ve got a defect and you’d like it covered in a future Defect of the Month, then let us know. Thank you very much