Hi, James,
Hearing of what you're trying to do, I furrow my brow in anxiety. If you're
plating the entire hole closed, what surface copper weight are you
expecting as a result, especially with 21 mil diameter holes? My estimate
is about 8 oz. What trace widths and spacings are involved? If they're
fairly fine, the undercut from the etching process will probably sever them
from the board, or make them extremely weak. It is possible to grind excess
copper of the board surface, but this is a brutal and risky process as
well, since if the board isn't dead flat, you can take off traces and pads
adn go through to the substrate material.
If it's only the thermal holes you're trying to plate shut, how are you
going to prevent the other holes in the board from being filled as well
without a lot of selective masking? Also, I suspect that the holes will
close at the top and bottom first, leaving a void in the centre that will
contain plating solutions and cause all kinds of corrosive and expansion
problems.
This is not a good idea!!! My strong recommendation is that you use silver
loaded epoxy to fill the thermal via holes prior to final plating. It's
thermal conductivity is not as good as that of copper, but is relatively a
lot less hassle and will be a lot cheaper than all that plating. There are
a number of epoxies available, though Dupont's CB100 is one that is
commonly cited. It has a thermal conductivity value of 5.23 W/mK, so is
pretty good for this type of material. Other makes are similar.
Let us know how it goes. Good luck!!
Peter
"Marsico, James"
<James.Marsico@D To: [log in to unmask]
P.AIL.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask] Subject: Re: [TN] VIAS PLATED CLOSED
>
01/11/02 08:09
PM
Please respond
to "TechNet
E-Mail Forum."
Hi, Peter, thanks for the response.
Let me try to explain further. We're using ceramic leadless chip carriers
which give off a lot of heat. In an effort to dissipate the heat to the
ground planes, we're placing thermal vias underneath. We wanted the
thermal
vias to be plated closed, resulting in a solid plug of copper, which we
then
can deposit solder paste on top which would contact the bottom of the
ceramic package. The heat path would be from the component through the
solder bumps on the thermal vias through the solid copper via. Our board
supplier claims that he can plate the vias closed, but when I asked about
voids and entrapped plating chemicals, he said he was never asked that
before and would have to look into it. I'm getting concerned with this
design, thinking that an epoxy-filled via which is plated over on top would
be better. I now need recommendations for a THERMALLY conductive adhesive,
it doesn't have to be electrically conductive but I don't really care.
Thanks again,
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
-----Original Message-----
From: <Peter George Duncan> [SMTP:[log in to unmask]]
Sent: Thursday, January 10, 2002 9:28 PM
To: [log in to unmask]
Subject: Re: [TN] VIAS PLATED CLOSED
Hi, James,
I'm trying to picture your "thermal reasons" and how plating over
the holes
will help, reliably. I haven't encountered a process for plating
over
holes, and 21 mils seems quite a canyon to bridge. I'ld be very
interested
to know how this is done.
I'm very involved with conduction-cooled MLB's 60 mils thk, that
require a
lot of thermal management. 'Thermal' via holes are punched through
the
board and plated to internal thermal planes. To improve the
conductive
cross-sectional area of these holes, I had them filled with
silver-loaded
epoxy, which was then Cu-plated over. This process also served the
secondary purpose of filling holes that otherwise, as you suggest,
would
contain entrapped plating solutions.
Depending on your substrate material and operating environment, I
could
imagine a plated 'cap' splitting over time through temp cycling and
mechanical stresses, assuming it survives the hot assembly
processes
with
the expansion of entrapped gasses. Not serious, thermally, perhaps,
but it
would allow the outside atmosphere to circulate inside the holes as
well,
which might not be so good inteh long run.
Peter
"Marsico, James"
<James.Marsico@D To: [log in to unmask]
P.AIL.COM> cc: (bcc: DUNCAN
Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask] Subject: [TN] VIAS
PLATED CLOSED
>
01/11/02 05:55
AM
Please respond
to "TechNet
E-Mail Forum."
Good day technet:
We're considering designing a multilayer polyimide PWB with a
number
of
vias
(approximately .021" diameter holes, .062" thick PWB) that are
plated shut,
for thermal reasons. Can anyone comment on pitfalls regarding the
fab
process? How about voids in the solid copper via? Entrapped
plating
solution?
All comments are appreciated. Thanks in advance...
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
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