Ditto that.

Soldermask cover the top side (under the BGA component), and relieve the
backside soldermask at the vias, so the vias won't blow open during
assembly. I'm not a proponent for 'plugging' vias, since it adds
additional processes to the task, and I haven't personally done plugging
(always just the simple soldermask overlay). But often times people think
it's necessary for reliability (?). One thing you can do when relieving
the backside vias is to only partially open them. You can adjust the
opening so it still partially covers the vias but leave the finished hole
exposed. This gives you added coverage at the vias for protection, but
still allows the vias to 'breathe'.

Good day.

>
> I was told by one of our populators that the vias should be open on the
> bottom side and closed on the top. If both sides are sealed then one
> will blow out during heating of the PCB in reflow. This could be the
> top side and throws debris under the BGA. I seal the top and leave the
> bottom open to 'breathe'
>
> Jim Thornhill



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