Bridging causes, part 3: identify and solve placement and reflow-related solder defects.

Author:Lotosky, Paul
Position:SMT TROUBLESHOOTING
 
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WE CONTINUE OUR series on solder bridging. This month, we look at potential problems caused by the placement machine and reflow profile.

If the pick-and-place machine is suspected:

Problem: Placement inaccuracy narrows gap between pads, increasing the chance of bridging.

Recommendations:

* Verify component placement pressure.

* Use x-ray to verify BGA placement.

* Use microscope for QFPs.

Problem: Excessive component placement pressure will squeeze paste out of pads. Recommendations:

* Verify actual component height against data entered in the machine.

* Component placement height should be [+ or -]1/3 of paste height.

If the reflow profile is suspected:

Problem: Extended soak will input more heat to the paste and result in paste hot slump phenomenon.

Recommendation...

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