The IPC 4553 spec for immersion silver gives a "typical" immersion silver  
thickness for the vendors of "thick" plating as 8-12 MICROinches.  That  means 
that with both sides of the hole plated, you could get 16-24 MICROinches  of 
hole diameter reduction.  That is .016 to .025 MILLIinches of hole  reduction, 
or .016 to .024 mils of reduction from the silver - that is 1/60th to  1/40th 
of a mil.  I cannot imagine that since you are specifying holes in  mils, that 
you can even see the reduction in hole diameter due to silver.   HASL 
tolerance may cause a hole diameter to be reduced a mil or slightly more  due to the 
curvature due to solder surface tension, as HASL therefor might be  say 40 
times as thick in the center of the hole barrel as immersion silver, but  you/your 
board fabricator has problem somewhere else - drilling, copper  
electroplating, etc.  
 
Denny Fritz
MacDermid, Inc. 
 
In a message dated 5/27/2008 10:58:07 A.M. Eastern Daylight Time,  
[log in to unmask] writes:

We've  been using immersion silver on our boards for quite a while with
great  success, but in a recent run of a new product, we had the boards
come in  with the PTH's used to mount RCA and audio connectors smaller
that  usual.  The parts came in on the large end so it became very
difficult  for the operator to insert them into the board.  Both holes
and parts  were in tolerance and the holes were specified at the
recommended  diameters.  Our CM has a woman who spent 20 years working in
pcb fab  houses and she tells me that it's not at all unusual to have IS
boards with  reduced ID's on the holes as opposed to HASL.  Has anyone
else  experienced this?  If this is the case, we'll have to adjust for
this  in the future, but it would be useful to know if this was an
anomaly or a  normal condition?


 



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