We recently used the following notes to attempt to do precisely what you're
describing:
12.2 All vias 0.020" in diameter or less shall be plugged from top side
prior to primary soldermask application per IPC-4761 Type III. Plug
material shall be approved by Levant engineering before production begins.
12.3 Plugs in vias shall not protude above top of hole.
12.4 All plugged vias shall be filled at least 80% with fill.
12.5 Via plug integrity, fill percentage and protrusion shall be confirmed
with a microsection of a representative quality conformance coupon

However, after soldering about 30% of the at-risk holes had solder
flow-through. We didn't see bumps, just a thin layer of wetting, but we
only had 16 boards populated in this run so I don't think we have enough
data to show that we never would have that issue.

I am currently working with our PCB manufacturer to identify what happened,
but in my opinion this isn't a fully manufacturable solution. I'm
attempting to solve the same problem that you are, so I'm absolutely
interested in any other way to do this.

--
Tom Brendlinger, CID
Electrical Engineer
Levant Power / ClearMotion Inc
Woburn, MA 01801

On Thu, Dec 15, 2016 at 11:10 AM, Jack Olson <[log in to unmask]> wrote:

> I'm looking at the various options for tenting/plugging/capping vias in
> IPC-4761 (Via Protection Guidelines). None of the options seem to address
> the thermal vias in power pads.
> We are trying to:
> 1) have a solderable pad, apply paste on that side (of course)
> 2) prevent solder from creating bumps on the heatsink side (of course)
> 3) avoid the cost of plugging/capping
>
> IPC-4671 states that all forms of one-sided via protection are NOT
> RECOMMENDED.
>
> So here's the question - I know I have seen a picture of a thermal pad in
> an IPC document where I could see vias with green mask material in them on
> the paste side (the component side), but the green stuff was only in the
> holes, which left the thermal pad exposed for soldering.
> I don't think this would be called TENTING because there's no mask on the
> pad. but is this an acceptable practice? Can I instruct a bare board
> fabricator to put mask in holes from the top side, but keep the pad clear?
> (I can't remember where I saw the picture!)
>
> If that's NOT an accepted practice, is there a way to design the hole big
> enough to plate but small enough to prevent solder flowing through the
> other side of the board? I couldn't find a discussion of that in the
> TechNet Archives
>
> thanks,
> Jack
>
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