When solder mask isn’t a viable via capping solution, there’s still hope.

Pad parts change and so do vias. Our standard policy is that open vias in pads are bad. We from time to time recommend ways to plug them. Generally, you have several options, such as capping the top or bottom of the via with solder mask. However, with vias in the pads of really small parts, those solder mask options will probably not work. Solder mask generally isn’t put down with enough precision to cover holes on tiny pads, and further, the solder mask would probably mess with the clearance, especially with very small QFNs. Immersion silver finishes may develop corrosion in sealed vias.

Figure 1 is an example of a small QFP with open vias in the pads. Those are some small vias.

If solder mask isn’t going to work, what will? Filling and plating over them, that’s what. You really have only two choices: fill and plate, or live with a bunch of voids under the part and solder slopped on the bottom side of the printed circuit board.

Figure 2 shows two illustrations representing the issue. In the top half of the image, the vias represented have copper plugs and are plated over by the board fabricator. As with all parts of this sort, there may still be tiny voids. IPC and the manufacturer have guidelines on the maximum allowable voiding. On the bottom, see what happens with the vias left open. Two problems: big voids and solder on the underside of the PCB.

Certainly there are some applications where this doesn’t matter. That’s why there is a second choice: “Live with a bunch of voids and slopped solder.” If you can’t live with voids and solder slop, you have to bite the bullet and pay extra for a PCB with filled vias. Board fabricators that do this have a variety of materials to use, including copper, electrically conductive epoxy and thermal conductive epoxy. Let your board fabricator know what your thermal requirements are, and they can help you choose the right fill material.

Ed.: Read Duane’s blog each week at circuitsassembly.com/blog/.

Duane Benson is marketing manager at Screaming Circuits (screamingcircuits.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column appears bimonthly.

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