fillet lifting

Fillet lifting is a phenomenon mostly observed at wave soldering. It is caused by mismatch in thermal expansion coefficient (TCE) between solder and parts, and aggravated by the pasty range of solder alloys. Upon cooling, the mismatch in TCE will cause excessive stress concentration within solder joints with partial liquid phase, and eventually result in solder tearing near interface. Solder with a wider pasty range often is more prone to have fillet lifting. NCMS results suggest that addition of Bi and Cu appear to increase tendency of fillet lifting. However, fillet lifting may also occur at eutectic solder joints, such as SnAg eutectic. An exclusive picture of the exact mechanisms are still under study.

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