How about IPC-4761 sec 3.4

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dharma Kemp-Bresett
Sent: Tuesday, May 06, 2008 12:51 PM
To: [log in to unmask]
Subject: [TN] single sided via tenting and immersion finishes... need
both fab and assy input

Hello everyone,
    I am usually hanging on the outskirts soaking up as much info as
possible from all the knowledgeable individuals on TechNet. It is always
enlightening and quite often amusing so thank you all beforehand for the
wonderfully entertaining education I am receiving. :-)
 
    I am looking for info from both the fabrication and assembly side
for
this one as apparently there are varied opinions on the subject. If you
can
point me to specific documentation that I can show to my customers that
would be even better! 
 
    A couple of years ago a fabricator/assembler recommended to us that
we
should tent vias (component side only) on all dogbones for fine pitch
bgas
as there was a chance of shorting to adjacent pads. We incorporated this
into our footprints going forward as standard practice and have had no
problems that I am aware of. However we recently received a suggestion
from
one of our fabricators that we should discontinue this practice on
immersion
finish boards as over the last year or so it had been observed (
throughout
the industry) that this caused via barrel degradation (due to trapped
solution) that in most cases was not caught till the assembly had been
in
the field cycling for a period of time. The recommendation given was to
now
encroach rather than tent these fine pitch fanouts. The specific
recommendation given was 2.5-3 mil encroachment on most vias with 2mil
encroachment on 8 mil vias ( the smallest we have used with this
particular
customer) with the understanding that the problem gets worse the smaller
the
via due to capillary action. The other option was plugging but that
could be
cost prohibative so i would like to stay away from that option if at all
possible.
 
What are your opinions on this subject and is there any specific
documentation i can point my customers to that might help us determine
the
best course of action?
Thanksin advance,
Dharma
 

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