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Overprint, solder flow back

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#45976

Overprint, solder flow back | 5 December, 2006

Good afternoon,

Due to problems with power connector soldering I have to overprint paste on pads for this connector. Stencil thickness is 125um and I cannot make it thicker. Area ratio of chip resistor assemblies limits stencil thickness to 125um. Reflow profile is ok as well. So it seems to me that overprint is only one possible solution. What is the maximal distance from the edge of the pad to the edge of the overprinted paste that surely guarantees that solder will flow back to pad leaving no residues? Thanks in advance.

BR, Pavel

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#45981

Overprint, solder flow back | 5 December, 2006

How about a stepped stencil? Not ideal but ...

As far as overprinting goes, try: http://www4.uic.com/wcms/WCMS2.nsf/index/Resources_44.html

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Loco

#45998

Overprint, solder flow back | 6 December, 2006

Would say about the same as above, 8mm according to a cookson presentation we had, of course it would depend a bit on the paste you are using.

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#45999

Overprint, solder flow back | 6 December, 2006

Successful solder paste overprint is a function of the solder paste rheology, solder mask and through hole component stand off. * Solder mask with a high surface energy will permit a larger overprint than a solder mask with a lower surface energy. In large overprint a solder bead can be formed with a low surface energy solder mask. * In addition, more solderable land finish (e.g. HASL versus OSP), lead finish and a solder paste with higher wetting strength, all assist in making the overprint reflow process more robust. * Solder paste print should cover the entire land, in order to maximize the wetting force exerted by the land on the overprinted areas. [Stencil Design for Mixed Technology Through-hole / SMT Placement and Reflow; Coleman, WE; Photo Stencil, Colorado Springs, CO]

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David

#46001

Overprint, solder flow back | 6 December, 2006

Hi Pavel, We regularly use over printing on a multitude of problem parts including fine pitch QFP's. In an Automotive Environment the only thing you can change without a mountain of paperwork is the stencil.

best way to work out how much you can get away with or need is expermentation. Design a couple of different stencils and see which works best for you.

you will get a witness mark left in flux residue where the paste was though

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#46026

Overprint, solder flow back | 6 December, 2006

Normally, we try to limit the over print of the paste to the width of the pad on both sides. If you are using 500um pad, then I wouldn't try over a 1500um aperture.

Mario Scalzo, CSMTPE, Dartmouth 6 Sigma Green Belt Technical Support Engineer - Indium Corporation

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