Techies,
Dupont sells a product called CD-100. It is a silver-filled conductive
epoxy. The via holes are filled via a flood coat or silkscreen, but has to
be pushed into the hole. Any epoxy on the surface must be removed to allow
subsequent etching. The epoxy goes thru a cure, then has a copper flash
plate to cover the silver. (migration issues ?) The final cure of the epoxy
occurs during subsequent lamination cycles. Final copper thickness is
increased during final pattern plate.
The smallest hole size you want to design for this application is one that
is drilled with .0135 drill. There are aspect ratio issues which also
apply. The process can also be applied to Thru holes, with modification.
We design such boards when blind vias are present IN surface mount pads, AND
J-Std-001 solder standards are required on the assembly. It is the only way
we have found which can produce a reasonably flat, void free surface mount
land with a Via under it.
Hole fill is a very manual operation. It can be spotty depending on
operator ability.
You are near the leading edge with this technology. Expect variations from
lot to lot with the same supplier. Expect surprises from supplier to
supplier.
The other option is to allow the holes to fill with pre-preg material. In
fact, it is hard to keep them from filling. The holes fill fine, but do not
metalize well, due to the VERY slick surface profile of the pure resin which
fills the hole. This is prefered when vias are not in pads, or if you do
not care if the tops of the filled holes are plated or not.
These comments are from a design and procurement view point. The
manufacturing info is rumor, hear-say, and in no way is to be taken as a
formula as how to use, manufacure, or market PWB's with CB-100. Dispose of
all materials properly and consult your doctor before changing any of your
medications.
George Franck
Raytheon Systems
Falls Church Virginia
"My Opinions are just that."
----------
From: [log in to unmask]
To: [log in to unmask]
Subject: [TN] Overplated blind via's
Date: Tue, 16 Jun 1998 19:22:30 GMT
Hello Technet,
Our company is busy designing some printed circuit boards with
"overplated blind via's" (I mean blind via's, made in a sequential
plating process, where the via is filled with epoxy, after which it is
covered with copper and a solderable plating).
All our vendors claim they can build such boards, but as far as I
known they do not use this technology very often, and I don't want to
be their "guinea pig".
The question :
What kind of questions should I ask our bare board vendors in order to
check wether they are capable of building these boards with good
relaibility in a reproducible way ? What are the potential problems
that the board vendors should be aware of, and should have solved ?
Any ideas ?
Mark Roach
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