Hello Jim.

You bring up a good point regarding the clearance of solder mask to the
plated through hole.  

I would have to state that it depends upon what the application is all about.
For example, if you are using dry film solder mask, no lead in the hole,
then perhaps one could tent the pth on both sides.  Many will argue that
solvents and other chemistries could be entrapped in the hole, etc. or that
the hole pad can no longer be used for a test point, but it still can be done.

If your using dry film solder mask and leaving the pth pad uncovered, and
there is no lead in the hole and the board is being wave soldered, then the
chances of the hole filling with solder is minimal.  We've conducted
experiments where this condition existed and what was happening was the
solder from the wave was never touching the pad so it would not solder the
pth.  If a lead was in the hole, then the lead broke the surface tension of
the solder and it would wick up into the pth.

If the solder mask is LPI, depending upon registration you could get some up
on the pad.

The main criteria in my opinion is the solder joint.  In a plated through
hole the solder joint is made in the hole not on the pad.  If you have good
transition from the lead to the barrel in the soldering operation, then you
have a good solder joint, so the solder mask could in fact go up to the edge
of the barrel.

Not much but hope it helps

Leo Lambert
EPTAC Corp.

At 01:05 PM 2/12/97 -0500, [log in to unmask] wrote:
>  
>  Speaking of solder mask encroachment, what is the minimum clearance that 
>  solder mask can be from the edge of  plated through holes? Also, what are
the 
>  preferred clearances?
>  
>  Jim Goss
>  [log in to unmask]
>
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