Hi Bob - I have also tried the liquid solder mask option and had the same
"less than successful" results. I have some partial success with putting a
plastic "pin" in the PTH to the specific % level I was trying to achieve.
The "pin" needs to be snug enough to block the solder but not so tight as
to prevent out gassing of the flux materials.  You can machine a shoulder
on the pins for getting accurate placement in the PTH.

If you are going to do PTH % fill reliability testing, please consider
publishing the results as there is very very little of that type of data
available in the public forum. Good luck.


Dave Hillman
Rockwell Collins
[log in to unmask]

On Mon, Jul 9, 2018 at 9:24 AM, Bob Wettermann <[log in to unmask]> wrote:

> All:
>
> We have an assignment where we need to, amongst a variety of induced  board
> and assembly defects, come up with partially filled PTHs.
>
> The objective is to partially fill a PTH on a multilayered, fairly
> thermally massive board and make sure it is filled to approximately 25%,
> 50% and 75% of the height of the barrel. The PTHs have a 2mm hole diameter
> and are 1.57mm in thickness.
>
> We have tried to get this done by blocking a certain portion/depth of the
> holes with liquid solder mask and then getting the solder to flow down into
> the PTH. Subsequent to this we remove the mask. This has not yielded the
> results as the mask does not really get to the right level in the hole. We
> tried selectively adding solder via flux-cored wire and by the time there
> is wetting all around the component lead,  the hole fills up to too high a
> level. Using braid and excavating equipment to remove a small amount from
> the completely-filled hole has also proven to be a challenge as it is
> difficult to uniformly pull solder from the barrel all across its diameter.
>
> One of our thoughts is to have specialty made solder preform "donuts" and
> slide them into the hole.....any other thoughts from the crowd?
>
> Thanks!
> --
> Bob Wettermann
> BEST Inc
>