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From: "Dill, Norm J" <[log in to unmask]>
To: "'IPC Technet Input'" <[log in to unmask]>
Subject: FAB: Tenting Au/Ni vias with LPI?
Date: Wed, 03 Apr 96 07:44:00 EST
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    Typically when LPI is used to tent vias, one side of the board will seal

and the other may have pin holes or openings in the mask that covers the 
via.  This creates a copper lined cup capable of trapping any subsequent 
chemistry liquids that it may be exposed to (fluxes, cleaners, etc.).
     Is this concern reduced when nickel/gold are plated over the entire 
copper board?
     Please share your via tenting experiences.  Thanks.

     Norm Dill
     RF Communications
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======== Fwd by: Tony King / N ========
I hear this concern sometimes from customers or designers,  yet as a board
manufacturer have never seen a failure in a printed circuit board attributed
to copper attack in a hole partially filled with mask. Further indication of
entrapped solutions in the hole should be detected at ionic cotamination
testing, yet again has not been seen.

There are several options to consider when designing product for plugging
holes with mask, in reference to the solder mask artwork design, a hole
should be plugged (exposed to light) from both sides of the hole or neither
side (not plugged).  The LPI process outcome is very unpredictable when
exposing a mask filled hole from one side only at image.(dependent on the
amount of mask in the holes, the degree of developing, light intensity etc.)
If the design requires the holes to be plugged completely, this can be done
prior to (or after) primary lpi coat using a screened dot pattern. The dot
pattern plugging operation can also be performed after HAL to allow solder
coating of the copper prior to plugging if corrosion is a concern.  

Some customers absolutely require holes to be plugged from one side while
leaving the other side open for testing reasons. This dot pattern screening
operation must be done after HAL to insure the partially plugged hole is
adequately cleaned after soler coat.

Though I have not seen copper fail, I would expect nickel/gold to present
better protection against possible corrosion, though the gold would be too
porous if immersion coated, electroplated gold would be better.


Tony King
Elexsys International Inc
603-886-0066
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