Electronics > Manufacturing & Assembly

Via - how close to pad?

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shadewind:
How close is too close when talking via-to-pad clearance? With ground and power to decoupling caps, you want the via as close as possible to the pads but is it okay for the via (only the annular ring, not hole) to touch the pad? It seems like this would be the best electrically but what about manufacturing problems? I suppose I won't have any such problems since I'm hand assembling by myself.

mkissin:
You'll get away with murder if you're only going to hand assemble.

The challenge with vias near pads is that they wick away the solder in a reflow process and so the part might get very little or even zero solder adhering to it.

shadewind:
I should probably add that I tent all vias not used as test points or otherwise.

Alex:
Via-in-pad could be an option and since you are hand assembling there should be no issues.

About trace length for decaps, another option is to position the via between the pads of the decap thus reducing the enclosed loop area. You could also use multiple vias for reduced inductance, or better still use a reverse aspect ratio decap with multiple vias. Seriously though, other factors will come in play first, so it is not critical to this extreme detail.

mkissin:
Again, it's probably not a big deal in the slightest if you're going to hand assemble.

If you can, try to ensure two things: first, that there is at least some solder mask between the via hole and the pad. This will stop any solder from wicking into the via. Second, on smaller components (generally below 0603), you should try to ensure that the thermal mass on each side of the component is equal. If they're not, then in the reflow process the paste on one pad may melt before the other and the component can "tombstone".

Finally, unless you're dealing with some really fast edges, a millimeter or two of extra trace length isn't going to matter at all. You've gained more in the move from through-hole to surface-mount than you're going to give away unless your layout is really terrible.

Be careful about via-in-pad. Your board manufacturing costs would be higher as the via needs to be filled and plated -> http://i.screamingcircuits.com/docs/Via_In_Pad_Guidelines.pdf

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