Identify and solve printer-related solder defects.

As noted last month, bridging is solder connecting or, in most cases, misconnecting two or more adjacent pads that come into contact to form a conductive path. This month, we look at instances caused by the screen printer, and how to mitigate them.

Problem: Poor gasketing (paste oozes beneath stencil during printing, increasing chance of wet solder paste bridges).
Recommendations:

• Zero print gap between stencil and PCB.
• Check paste smear underneath stencil.
• Check sufficient stencil tension.

Problem: Misaligned print will challenge the paste to pull back to pads during molten stage, increasing the potential for bridging.
Recommendation:

• Ensure print accuracy and consistency for both print strokes.

Problem: Smearing and bridging phenomenon on the next
printed board after stencil cleaning
operation.
Recommendations:

• Verify stencil is dry after cleaning and before next print.
• Standard cleaning mode is wet/vacuum/dry.

Problem: Poor print definition with dog ears, especially on fine-pitch components.   
Recommendations:

• Check board support.
• Adjust separation speed to achieve minimum dog ears. (Different paste chemistry requires different separation speed to minimize dog ears.)

Problem: Dented squeegee blades could result in uneven print pressure.
Recommendation:

• Check squeegee blade condition.

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.