In a message dated 6/2/2008 3:58:59 P.M. Eastern Daylight Time,  
[log in to unmask] writes:

KEVIN: Specifically, is it possible, impossible, or just difficult to  apply 
finish plating to blind and/or soldermask tented vias?


BOB: Possible but difficult for blind vias, no problem for  tented. Blind 
vias can be plagued by air entrapment. An air bubble in blind  via keeps the 
finish solution out. For tented vias, simply do the surface  finish prior to 
tenting.

KEVIN: What are the negatives to tenting both sides of a via vs just  one 
side?  We have PCBs with .015" dia vias.  The vias are tented on  one side 
w/soldermask.  The other side terminates as via-in-pad.   Finish is ImAg.
 
BOB: When you tent both sides of a via you create a drum-effect. When the  
via subsequently goes through a thermal excursion (Fab House HASL or Assembly  
House Reflow) the air in the via rapidly expands, sometimes to the point of  
blowing a tent. That frees solder mask fragments onto your assembly (or into  
your assembly environment) and makes the via vulnerable to entrapment of  
contamination (we used to call them "flux buckets").
 
KEVIN: Should these vias also exhibit ImAg or be exempt from it due to  the 
tenting?
 
BOB: Good question. Exposed copper is never a good idea. But immersion  
silver is not intended to be a final finish. Ask any supplier of immersion  silver 
what the post-assembly service life of immersion silver is, and they'll  
likely reply that it's only rated as an interim solderable protectorate (prior  to 
assembly). PCB finishes that are good for before-and-after assembly  
protection include HASL (Sn/Pb and Pb-Free) and ENIG. As for anything else  being used 
as a final finish (e.g., ImSn, ImAg, OSP) ask before you  assume to avoid 
disappointment.
 
ROBERT LAZZARA
VP, Marketing & Tech Support
Atlanta  Office: GMT -5
Tel: 800.560.9457 ext. 1
Fax: 800.878.5566
Fax: +1  603.880.7975
Cell: +1  706.498.9138






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