The risk is that components can be partially re-flow, significantly weakening their attachment, when a neighbouring component is being re-worked or added after re-flow.
Hot gas pencils can be the worst offenders for partial re-flow, he said, where the heat affected zone from hot gas can cover several millimetres around the central heat source, heating adjacent
The issue is that grain boundary weakening occurs, making the solder more compliant and weaker, when heat comes within two thirds of the melt temperature of the solder alloy.
During initial assembly, difficult-to-solder components can be another cause of the problem, where the extended dwell (heating) time can impact components on the other side of the board as well.
McAlpine recommends that engineers avoids placing components on the underside or too close to difficult component in the first place, and recommends a re-design if this has already happened.
“We had a difficult situation where a battery terminal had to be hand fitted and soldered after re-flow,” he said. “There was a 0201 diode close by [top diagram], with one trace connected to the terminal and the pads and component running adjacent to the battery terminal area to be soldered. To make things a bit more complicated, the device was bottom terminated, it’s easy to see that the re-flowed solder of the diode was compromised by the heat-affected zone, which made this component easy to knock off the board.”
Potential solutions proposed Dynamic included:
Redesign to move the device out of the heat affected zones
Use a low melting indium solder
Solder the 0201 diode after hand-soldering the terminal
Resistance soldering
Pre-heat the board prior to soldering
Conductive glue
