Those numbers look like what I've seen calculated as well.
The solder doesn't add much to the conductivity through the board, but where the solder is needed, sometimes, is to fill the air gap/make the connection from the heat slug in the bottom of the IC to the copper via pattern on the board.
So what we have found is not to worry about via fill, but to try to avoid allowing all the solder to drain away from that interface between the IC and the board, down the holes.


Genny 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brooks, Bill
Sent: January 4, 2010 6:39 PM
To: [log in to unmask]
Subject: Re: [TN] Thermally Conductive & Plated Via Fill

Thanks for that... I called my M.E. who does the thermal calcs on the boards and he told me the same... :P (My mistake... that's what I get for opening my big mouth without having reference info available to speak to.  LOL) Ah the difficulty with Assuming... it's okay... I have messed up before and I quite probably will again. [HUMAN... doh...] But I'll keep trying to get it right. :)


At any rate... here's something to consider... 

He put a spreadsheet together to model the vias we were using with and w/out solder fill and the difference is almost negligible... 

A standard 14 mil via in an .062 thick board with 1oz copper in the barrel and air in the middle has a calculated thermal resistance of 26.92 °C/W The same via filled with solder has a thermal resistance of 25.57 °C/W

If you increase the thickness of the copper in the barrel to 2oz only... everything else being the same... the thermal resistance with air in the hole goes to 15.3 °C/W And filled with solder the thermal resistance goes to 15.06 °C/W

That's why I say the solder doesn't make much difference... If anyone needs to look at the spreadsheet I can supply it...
 
Who knows maybe he made a mistake but it's not likely... he did the calcs on a complete complex system and it predicted the results accurately within a few degrees of the proto we built. So his numbers are usually pretty good. 

Anyway... interesting discussion. 


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: Robert Kondner [mailto:[log in to unmask]]
Sent: Monday, January 04, 2010 3:25 PM
To: Brooks, Bill
Subject: RE: [TN] Thermally Conductive & Plated Via Fill

Bill,

 Air is not 1. Air is about .027.

 See: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

 The CB-100 is specified at 5.2.

 Note that any air gap is bad news.

Bob Kondner

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