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January 2010

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Subject:
From:
"Brooks, Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brooks, Bill
Date:
Mon, 4 Jan 2010 14:07:36 -0800
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Thanks for the comments... As far as my personal limited experience goes
for plating vias shut... we don't reject for it... but we have not seen
any vendors actually plate them shut so maybe they don't like to do that
either... Like you say Chris... it's probably not a good practice with
conventional methods. We typically use a 14 mil plated thru hole with a
24 mil pad... but for thermal vias I have used .008 in. holes (2mm) with
a 18 mil pad. Of course larger boards would have reduced Producibility
due to the tolerances but all things are compromises... 

 I have attended a couple of lectures on plated copper filled vias and
there are a few companies that claim to be able to make fully copper
filled vias with a proprietary plating process that yields something
like a solid 'copper pillar' thru the board. The sections under
microscope looked very impressive... 

I don't have the thermal analysis numbers we did here handy but should
we really describe the solder performance as 'Not that bad'? 

Solder is only 7.5 times worse than the copper?... compared to CB1 at 77
times ... and Air which is 388 times... The solder is almost 800 Percent
worse as a conductor of heat. If the majority of the medium thru which
you transfer heat is solder... you have a big resistance between the
heat and the ambient air... which btw will make the junction temp rise
significantly over a thicker copper solution. The solder would be the
biggest resistance in the path... if the copper is only 1 mil thick at
the walls of the via... the solder is the remainder of that volume... so
which material by volume has the biggest influence on temperature rise?
Add to that thought... bubbles or voids in the solder... hmmm.

Thermal dynamics as you know is not a simple comparison problem...
 I could have the best possible conduction thru the board and put a 'not
that bad' thermal pad on the back side of it between it and the metal
plate and still have the part fail from overheating at the specified
ambient temps required. Each thermal junction from the heat source to
the ambient air is a thermal resistance... and they all add up. The
highest resistance in the path is the limiting factor...making the whole
system worse.

You must look at all the thermal resistances between the heat source and
ambient air to calculate the projected junction temps at the source
semiconductor and the temps that the board is going to be exposed to.
Not to mention the effect of surrounding heat sources that are also in
the area of the part contributing to the heat load that needs to get to
ambient to protect the board and the components from over heating... 

Each component needs to be looked at together as a system... 
The amount of copper in the hole is just one part of the heat path
equation and system for getting the heat out.  Solder... according to
your numbers, is much worse a thermal conductor and is not helping the
heat transfer load much... and would be much better replaced by copper
if possible... of course. 
 
 CB-100 sounds more like and insulator than a conductor of heat. Maybe
good as spackling compound... (Drywall putty) it gets the hole closed...
so it's 'better' than air.  But all of the 'significant' heat transfer
is being conducted thru the copper plated walls of the thermal vias...
in either case :)

Best regards,

Bill Brooks | Datron World Communications, Inc.
PCB Designer/Engineer | Office: 760-602-7004| Fax: 760-597-3777 |
[log in to unmask]
1808 Aston Avenue, Suite 230, Carlsbad, CA 92008 | www.dtwc.com

Performance You Require. Value You ExpectTM


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, January 04, 2010 12:12 PM
To: [log in to unmask]
Subject: Re: [TN] Thermally Conductive & Plated Via Fill

Bill,

 Compared to copper solder is a poor thermal conductors but it is not
all
that bad. A quick check shows: (W/m-C)

  Soft Solder = 51

  Copper = 388

  CB-100 = 5

But a simple via hole can have a pretty big area.

Solder is a lot less expensive and conducts heat 10 time better then
CB-100.

Filling vias with solder could be a big help in many applications.

Thanks,
Bob Kondner

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