PCB Assembly Services - Screaming Circuits: Via in pad methods


Via in pad methods

Here is another method for dealing with QFN heat slugs with thermal vias. While the best option is to plug the via holes with metal early in the board fab process and then plate over with copper, there are a number of alternatives. We've discussed several of them in previous posts.

Qfn_w_soldermasked_vias Here we have another option. This is for a 9mm X 9mm QFN on a board using an ENIG (Electroless Nickle Immersion Gold) finish. The larger size of this part gives a little more flexibility than some of the real small parts. In this case, while the vias are left open, the area around the vias is masked to keep solder away.

This method can work well on larger QFN and QFP parts, but it is absolutely critical that your solder paste stencil be segmented and not put any solder over the masked parts. (see this post). If the stencil is left fully open or deposits solder paste over the vias, that solder may go down and mess up the bottom side of the board. With a well designed stencil, this may be an easy and reliable method for dealing with your thermal vias.

Duane Benson


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The thermal transmisivity is a challenge. Most component manufacturers seem to recommend that the solder mask only be about 100um - 125um larger than the via being masked. That will help mitigate the issue you bring up.

A while back I ran across a design guideline that discusses the thermal transmisivity of various materials used to plug vias. If I can find it again, I'll post a link to it.

This is an interesting and novel approach.

Our concern is: Often board designers use the via under the heat slug to conduct the heat from the device to thermal plains located on various layers of the bare board. If you mask over via of this type, regardless of the size component, that thermal path is partially broken by the lack of good contact between the via and the heat slug and the insulating effect of the mask.

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