It is possible to change you resin desmear process (e.g. permanganate) to 
     "roughen" the filled resin surface before the electroless plating.  Several 
     photoimageable dielectric processes (e.g. IBM SLC) uses this technique to 
     achieve improved copper peel strength.
     
     Mason Hu
     Zycon Corporation


______________________________ Reply Separator _________________________________
Subject: GEN: Blind via Fill
Author:  [log in to unmask] at corp
Date:    12/2/96 9:38 AM


Hello all,
     
I have a question on blind via which fill / partially fill with prepreg 
resin during lamination.  Assume an 8 layer board, blind via 1-6, SMOBC, 
 .062",  standard in other ways....
     
ASCII Graphic
 _______       _______  Copper Layer 1
 xxxxxx|___A___|xxxxxx  A = Copper Layer 1 in partially filled via hole 
 xxxxxx|xxxxxxx|xxxxxx
 FR-4  | resin |FR-4     FR-4 "x" and plated via hole wall "|" 
 xxxxxx| blind |xxxxxx
 -------x via x-------  Copper layer 6 
 xxxxxxxxxxxxxxxxxxxxx
----------------------  Copper layer 8
     
In area "A", above, the resin fill does not go the the surface of the 
board.  This is a partially filled blind via, and is an acceptable 
condition.  However.... The surface of "A" is plated with electroless 
and pattern plated.  The electroless/plated copper has very poor 
adhesion on the resin in the hole, and peels off in various subsuquent 
processes, especially solder coat.  I understand that the "fill" resin 
surface is very smooth which causes the adhesion failure.
     
It is my assumption that this is a typical condition for "unfilled" 
vias.  Is this a vaild assumption?
     
Are there any situations when this condition poses is a problem?
     
Do others allow partially filled vias?  IPC-RB-276 accepts a 60% fill 
for buried vias, but silent on blind vias.
     
I know that others are trying to use this filled via construction as a 
surface mount pad (God help you), what experiences do you have with 
partially filled vias?  
     
     
     
Thanks for the responces.
     
     
     
-- 
     
     
George Franck Jr
Raytheon E-Systems
Product Assurance Engineer
Falls Church Va
     
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