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Solder Beading under Cap...Need help

Masdi

#1964

Solder Beading under Cap...Need help | 11 February, 2000

Hi,

Need help. We are starting to see a lot of small solder under capacitors and some of them can be bigger than 10 mils. For current action, we have checked through screen print process and component placement but still don't have any clue on how to get rid of them. For temporary action,we are doing touch up and remove those balls manually...pretty slow process. Any info is greatly appreciated.

Rgds, Masdi

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HMAL

#1965

Re: Solder Beading under Cap...Need help | 11 February, 2000

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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#1966

Re: Solder Beading under Cap...Need help | 11 February, 2000

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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Joe

#1967

Re: Solder Beading under Cap...Need help | 14 February, 2000

check the solderablity of the board and the thickness of the solder before screening, if you are using HASL finished boards.

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#1968

Re: Solder Beading under Cap...Need help | 14 February, 2000

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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MARK ALDER

#1969

Re: Solder Beading under Cap...Need help | 16 February, 2000

Please check Circuit Assembly March 1998, article "Solder balls and aperture shapes" and August 1999 "Guiding stencil design".

They can be contacted at www.circuitassembly.com

The main cause of mid chip solder balls is the volume of paste that is printed onto the pcb. Try using lazer cut stencils with relevant aperture reductions. Should you require further information on reductions please specify components and stencil thickness.

Are you presently using metal or rubber squeegees.

Regards Mark Alder

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