Solder Mask in Holes

I am researching the solder mask process in which mask is applied to the entire PCB surface, then imaged to create exposed pads and vias. In this case, is it common to have some left over mask within small diameter via holes or small nonplated holes due to not being able to completely force the mask out?

For finish hole diameter less than say 0.007" there is a possibility of the Solder mask not being completely cleared from inside the hole barrel. This will happen also when clearance is not provided in the Solder mask layers on both the sides.

If you need the holes to be clear of solder mask residues, we have process at Sierra for that.

Mask in the holes is an issue but can be controlled by the following:

  • Not over-tacking the mask prior to exposure

  • Using a Collimated light source like Laser or LED to better control exposure and registration

  • A dedicated developer that can produce higher pressure to clean out the holes. Vertical developers work well for this application.

  • A Dot stencil to help keep the mask out of the holes during screening. This should be considered a last resort as it adds time and typically requires hand screening.

Thank you both for your replies.