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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