Maybe somebody at IPC HQ can corroborate this but I've long been under the 
impression that the venerable IPC-D-275 current handling charts, which came 
from the Mil standard of the same number, was generated not from a mathematical
formula, but from plotting measured temperature rise vs current data on the 
different widths and weights of traces.  Can anybody out there support or 
refute this statement?  

AMP has in isolated instances tried to duplicate these measurments and 
quickly came to the conclusion that "2 OZ copper thickness" statements 
don't guarantee a well controlled conductor thickness.  To do justice to 
this type of measurement, one would have to verify every test panel as to 
copper thickness and trace width, such that an accurate cross-section of each
tested conductor could be documented.  Definitely tedious. If you didn't,
you'd have to design a test which included a relativley large sample population
to ensure that a "nominal" copper weight and trace width were being used for 
each measured situation of each variable. VERY tedious.  Hence people like 
myself fall back on the "official" table in IPC-D-275.  And that works for 
most situations.  

If that chart WAS generated by a formula, I'd think some of us would like to 
know that some empirical data was measured and analyzed to verify said
formula.  I'd like to know how or if that was ever done.  


The above statements are my own thoughts and opinions and do not necessarily
reflect those of my employer.

	Leonard F. Bendiksen
	AMP Incorporated 
	Test Engineer and 
	PCB/Fixture Designer
	Americas Regional Laboratories
	[log in to unmask]
	(717)780-6493 
	
 

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