Solder Beading under Cap...Need help

Dear Masdi,

Perhaps you should check again your screen print process. Small amounts of solder paste under stencil can cause your problems. This paste is from previous printed pcb.

When printing next pcb this paste attachs to places that it doesn't belong, and after that in reflow owen you get solder balls. Try to wipe stencil clean more often.

-hmal-

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Dear Masdi,

Solder beading (as opposed to solder balls) is usually caused by volume of solder paste, displacement of solder paste when component is placed and sometimes the proximity of the solder depositions for that component. Basically, a small amount of solder paste is squeezed out under the body of the component when it is placed. During reflow, the solder balls coalesce to form larger solder beads. These are usually located under or beside the component body.

Options:

Remake your stencil with "Home plate" appertures. This puts less volume of solder paste directly under the component body. See the archives for more info.

Reduce the placement force of the machine. This sometimes helps but I have not found it to be a remedy.

In some cases, increasing the squeege pressure at solder print gives a slightly better gasketing to the PCB reducing the solder paste bleed. This should be visible on the PCB prior to reflow however and is probably not the problem here as you have inspected your print quality.

I hope this helps.

My $.02

Chris

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Masdi: Are you sure you have your washer set-up correctly? We never have problems with solder balls. Ta. Dave F

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