PCB Assembly Services - Screaming Circuits: QFN solder paste stencil guidelines


QFN solder paste stencil guidelines

To better illustrate the proper way to make your solder paste stencil for QFN parts, as I discussed in the post just before this one, I went to the back room and took a couple of photos of good and bad.

Qfn_float This is essentially what happens with too wide an opening for the center pad on the QFN. The part high-centers and never gets the opportunity to contact the signal pads. In some case, the part will cock a little sideways and contact some of the signal pads but not all.

Qfn_stencil_full_open_aperture_moddedThis is what a worst-case stencil would look like. Note how much surface area that the center pad has compared to the row of side cut-outs. Actual size for this part is 5 x 5 mm. By default, we typically reduce the size of such cut-outs already, but in a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area.

Qfn_stencil_four_cut_outsThis is one example of a recommended practice. We would actually suggest even a little less coverage. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of high-centering and other problems associated with large paste areas, such as outgassing and spattering.

Just make sure that the openings match your copper layer underneath the stencil openings. Some parts require that the copper pad be segmented also. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.

Qfn_stencil_lots_of_small_cut_outsThis is a pretty good example of how to do it properly. This will give good solder distribution with little chance of high-centering or outgassing problems.

Dfn_stencil_cut_outs_1 It works for DFN packages also.




Duane Benson


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Ha, just wished to reply for prev. comment, but noticed it was mine :D

Please, post recommendation for size of "holes" in stencil according to pads on board.

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