I don't think anyone has provided the information that tenting vias is
required if you want to draw a vacuum for the tester.  If there are a lot
of holes open and you do not use a rubber "blanket" to close the holes,
then you get "whisteling" and not a tight draw for pin contact.
The true way to tent is to use dry film.  If this is not acceptable then
a dot pattern of UV cured ink on the bottom side is ok if there are no
test points that need to be open.  Fab vendors have a lot of methods
with LPI.

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