Howdy, Bill:
No, I am saying that because of the thermal conductivity of a copper plane compared to the thermal conductivity of a number of unconnected traces on a substrate, as well as the very short length of the thermal spokes, the charts are probably an _additional_ 50-60% off over the 50% derating built into the 50's vintage GE nomographs the IPC "borrowed" for their standard long after it was in common use. When you also count the fact that you are only calculating based on a 10-20° rise over ambient, you can pretty safely violate the "standard" with little or no effect for the typical application. I wouldn't want to assume it with down-hole or automotive engine compartment applications, but for more normal temp ranges I am confident in saying that uprating by 50% is safe when it comes to _thermals_ calculated using the empirical formulas used by the various calculators.
This is not talking about long traces, just the short 10 mil by 15 mil or so spokes. Continue to use the values calculated for traces.
<disclaimer>As always, it is JUST MY OPINION, I don't speak for Tektronix, keep your lawyers to yourself, do your own due dilligence before using it on an airplane or other mission critical application, etc., etc., etc. </disclaimer>
--
George Patrick
Tektronix, Inc.
Central Engineering, EDS Application Support
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
Å 503-627-5272 (voice) Æ 503-627-5587 (fax)
http://www.tektronix.com http://www.pcb-designer.com
"Off grid and Proud of it!"
-----Original Message-----
From: DesignerCouncil on behalf of Brooks,Bill
Sent: Tue 5/8/2007 5:27 PM
To: [log in to unmask]
Subject: Re: [DC] Current Capacity of Thermal Reliefs
Hi George,
Actually there are papers written that describe the thermal graphs we use
from the IPC as being very conservative and 50% de-rated... with numbers to
back them up... But... If you are hoping to build a product that will have
good reliability... that means the design performance 'insurance' is already
built into the charts... That's a good thing. (No wonder boards have been
surviving so well when designed to the IPC specs)
I think I would stick to the values you have in those charts unless there is
a really compelling reason to deviate from them...
And know also that you are loosing the extra margin that is built into them
if you are design to less than what they recommend.
Bill Brooks
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California
-----Original Message-----
From: George Patrick [mailto:[log in to unmask]]
Sent: Tuesday, May 08, 2007 4:18 PM
To: [log in to unmask]
Subject: Re: [DC] Current Capacity of Thermal Reliefs
Yep, there is no thermal in the calculation.
Make sure you are using the "Internal" setting when calculating the
current.
The value provided by a calculator is probably _quite_ conservative,
considering you have a plane "heatsinking" the spokes of the thermal. I
don't have any evidence, but I would bet that since the calculator
formulas are based on a single trace, you would find their value derated
by as much as 50-60% when applied to a thermal relief.
Anybody have a better "feel" for this?
--
George Patrick
Tektronix, Inc.
Central Engineering, EDS Applications Support
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
* 503-627-5272 (voice) * 503-627-5587 (fax)
http://www.tektronix.com http://www.pcb-designer.com
"Off-Grid and Proud of it!"
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