Identify and solve placement and reflow-related solder defects.

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 ±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: Adopt a straight ramp to spike profile, without soak zone if possible.

Paul Lotosky is global director - customer technical support at Cookson Electronics
(cooksonelectronics.com); plotosky@cooksonelectronics.com. His column appears monthly.

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