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January 2010

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From:
Robert Kondner <[log in to unmask]>
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Date:
Mon, 4 Jan 2010 17:39:54 -0500
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Bill,

 

 You wrote:

 

 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.

 

Air thermal conductivity depends on temperature but is about .027 W/m k or
about 14,000 times worse than copper. Sounds like a lot but those are the
numbers I found with a quick look on the web.

 

 Yes copper is about 8 times more conductive of heat than solder. A lot
depends on the hole size.

A .014 hole plated at 1/2 oz exposes a copper cross section about 1/5 the
hole surface area. As the hole gets larger they rations grow by a square. At
about 40 mil hole the rotten solder conducts almost twice the heat as the
copper wall.

 

 If anyone can correct these numbers please let me know.

 

 Filling a few vias with selective solder or wave is not that difficult.
Protos are easy build by hand. 

 

 Filling the vias with air is easy and cheap. Solder filling is better, it
almost buys you twice the thermal conductivity for a small hole (.014) and a
lot more conductivity at larger holes.

 

 Has anyone seen any small copper studs that can be placed and soldered
using pin and pasts techniques? That would really conduct some heat!

 

Bob Kondner

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks, Bill
Sent: Monday, January 04, 2010 5:08 PM
To: [log in to unmask]
Subject: Re: [TN] Thermally Conductive & Plated Via Fill

 

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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