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What will be the effect of additional solder paste?

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#75565

What will be the effect of additional solder paste? | 19 April, 2016

We are using Vicor SMT J-Lead VI Chip which is big component on the bottom side. The manufacturer recommends to have 6 mil stencil but the customer required 10 mil stencil. The PWB has gold contacts on both sides. We noticed some mini solder splashes or solder spot on the gold contact area on both sides of PWB randomly.

Do you think it is due to the thick stencil that we using?

Thank you in advance for your input.

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#75566

What will be the effect of additional solder paste? | 19 April, 2016

Don't blame it on the poor paste thickness requirement [although I might go on to argue that it's could be a contributor to the issue].

Most likely you're the problem. About 70% of the time solder on gold fingers is caused by poor cleaning of screen printer, staging area (table), board support tooling, conveyor, and reflow oven chain or belt and keeping boards separate from the printer cleaning process.

Other things to consider are: * Check the boards before starting the printing process. * Paste process. Are you pasting anywhere near the gold fingers? Are you leaching under the screen and contaminating the bottom side due to poor printer set up? Is your stencil under-wipe frequency proper? Is paste dropping from the squeegee blades onto the stencil? Could the paste near the edge be getting onto the conveyor? * Reflow process. Paste can splatter if your paste requirements don't match your profile, your board is full of moisture, or you're not controlling your environment. Use a slow ramp to a decent dwell then jump up to reflow and back down within a 25 second window. Keep the ramp under 2*C/sec. * Misprint cleaning. Not washing misprinted boards properly, allowing accumulated solder paste in the stencil wash re-depositing on boards washed there.

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#75567

What will be the effect of additional solder paste? | 19 April, 2016

Thank you Dave for your valuable time.

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