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Clearence of solder resist from QFP Leads

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#57740

Clearence of solder resist from QFP Leads | 9 January, 2009

Does anyone have design knowledge of why some manufacturers remove solder resist from the area around pads on fine pitch devices? What purpose does it serve?

I only know that it would be better for AOI as you would not have light being reflected from the solder resist coating between soldered leads as it would be a matt finish if it was removed.

Cheers, Simon UK

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#57741

Clearence of solder resist from QFP Leads | 9 January, 2009

Hi Simon,

The main reason that it would probably be removed would be do to lack of clearance for a soldermask dam from pad to pad.

typically if the clearance from pad to pad is under .008 the solder mask dam would be removed. Min recomended dam size is .003 anything smaller has a issue of flaking off.

from pad edge to soldermask min spacing is .0025, mask dam .003 to pad edge .0025 = .008 total space

The reasons for the min. .0025 spacing is done to control soldermask encroachment onto the pads.

Hope this help,

Regards, Boardhouse

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