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

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************