Bill,

Do you know if that calculator can adjust the thermo relief used to attach the via to a plane? I would be interested to see if changing the number and or size of the spokes would have a greater impact then having it filled in with solder? I would also be interested to see how much more thermally conductive a direct connect via vs. a thermo relief via can be. 

I have often wondered if the formula for thermo reliefs could be tweaked to provide the best of both worlds.

Ted T.  


-----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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