Hi Cor & All -

In my experience, filling vias with solder can turn into a problem.  When
the material reflows, it often forms a bead below the surface of the board
(aided by gravity).  This bead may well turn into a short or an obstacle to
placement of second side parts.  The second part of the problem occurs when
a cavity forms allowing process residues to fill the partially vacant via.

My own preference is to either plug the vias with one of the via fill
compounds or to tent them with solder mask.
Most of the circuit board shops I am accustomed to dealing with have the
ability to fill or tent.

Regards - Kelly


-----Original Message-----
From: Cor Coolen <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, April 20, 2001 1:06 AM
Subject: [DC] Paste screening of Through Hole Test Vias


Hello to  all of you,


Currently,we  are designing a double sided reflow board.
All our  assembled boards  have to pass electrical test, preferably on a bed
of nails.
Is it  good practice to close all  through hole testvias by paste screening
in order to create good
vacuum conditions when testing ?
So, I want to include these vias in the paste screen artwork.  What is a
good diameter in the paste
screen for closing vias with a finished hole diameter of 8 mil ?
Is there not a risk that the solder paste will flow through the via holes
and causing shorts at the other side
during the second step of reflowing ? The board has a thickness of 95 mil.
Anyone has some experience in this matter ?

Thanks in advance.

Cor Coolen

Board Designer


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