What to do when fillets aren’t forming completely.

Insufficient solder occurs when the amount of solder paste deposited on the PWB by the printer is much less than stencil opening design, or after reflow, there is insufficient solder to form a fillet at the component leads. Potential causes include the stencil, printer, reflow profile, and paste itself.

Stencil

Description: Solder paste adheres on the stencil aperture walls.
Recommendations:

• Ensure area ratio > 0.66.
• Ensure aspect ratio > 1.5.
• Ensure no burrs on stencil aperture edge.

Screen Printer

Description: Print definitions.
Recommendations:

• Verify print setup.
• Reduce print speed to provide sufficient time for paste to roll into aperture.

Reflow Profile

Description: Mismatch in CTE between component and PCB can cause solder wicking effect that may look like insufficient solder on pads.
Recommendations:

• Attach thermocouple on component and PCB.
• Apply soak profile to minimize ∆T before reflow zone.
• Set bottom zones to be higher temperature if possible, to keep PCB hotter than component leads.

Solder Paste

Description: Solder paste viscosity.
Recommendations:

• Check paste conditions such as dry paste phenomenon by verifying if paste rolls or skids along print direction.

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.