Excessive reflow ramp is just one of several spurs for these potential short causers.

After reflow, small spherical particles with various diameters can be formed away from the main solder pool. These are known as solder balls.

Possible Cause: Stencil.

Description. Paste stuck under the stencil will be transferred onto the solder mask of the next PCB.

Recommendations.

• Verify zero print gap setup.
• Check minimum print pressure used.
• Check cleaning efficiency such as wet/dry/vacuum.
• Check wipe frequency.

Possible Cause: Reflow profile.

Description. Fast ramp-up rate or preheat rate will not permit sufficient time for solvent to vaporize gradually.
Recommendations. Slow preheat rate is recommended, typically <1.5°C/sec. from room temperature to 150°C.

Possible Cause: PCB moisture.

Description. Trapped moisture may result in explosive vaporization.
Recommendations. Lower grade PCB laminates such as FR-2 and CEM-1 tend to absorb moisture. Bake at 120°C for 4 hr. if necessary.

Paul Lotosky is global director – customer technical support at Cookson Electronics (cooksonelectronics.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears monthly.