Bob Willis – Defect of the Month video guide

Welcome to our latest Defect of the Month – Solved, this month we illustrate secondary reflow on Low Temperature Soldering. Secondary reflow is when part of a solder joint gets close to reflow temperature or is in a semi liquid state. One of the reasons I have seen happen with traditional lead-free SAC technology and I’ve also seen it with low temperature in lead contamination. In both situations it is very uncommon for it to happen today, however it’s better to know about the problem then not actually be aware and experienced something similar. If part of the joint at one interface starts to go into a semi liquid state but at a lower temperature its lead that lowers the temperature at which the material goes from a liquid to a solid stage and vice-a-versa. 

Images show joint separation and surface of a joint that has separated with non-brittle tear this is secondary reflow or liquid tearing

If you introduce lead contamination into part of the joint from the termination or pad surface coating the interface will actually reflow or become semi liquid at a much lower temperature than the reflow temperature of the main alloy. If we consider low temperature solder SnBiAg around 138oC introduced into the equation, then the actual potential reflow temperature with the liquidus temperature is around 100oC with lead. If you then subject a board assembly to a temperature anywhere close to that lower end and it has lead contamination as part of the interface, then the joints can actually separate and that’s what we call secondary reflow. I said this is very uncommon, it shouldn’t happen today, but I illustrate it and show you examples. Our video simulation shows what is going on when this does occur in manufacture

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This is just one of the Defect of the Month videos previously featured by IPC, SMTA, SMART Group and Circuits Assembly magazine helping to provide better understanding of process issues and help support training of younger engineers. We have recently celebrated being a 100, that is the number of videos produced not Bob’s age Bob!

A company can sponsor our videos and photographic defect guides

Here is your opportunity to sponsor our regular Defect of the Month videos on materials, soldering, assembly, cleaning and PCB design. Produced by Bob Willis over the years there are many topics which have been covered and are unique to the industry for further information on sponsoring Defect of the Month videos for either 6 or 12 months, contact

Here is a link to an example video

In addition, we also have our popular Photographic Process Defect Guides for engineers to download free after registering. Our first guide featured Solder Paste Print Inspection & Defect Guide 

Our next two guides will feature: 

Low Temperature Solder Joint Inspection & Defect Guide

Robotic Soldering Guide, Inspection & Defect Guide

0201 & 01005 Chip Component Reflow & Defect Guide