Nearly all printed circuit boards are rated by Underwriters Laboratories.? Dan - you are touching on the 
maximum operating temperature for the lowest rated material in the build.? Epoxy and the coverlay are 
UL rated at only 105C, so that is the maximum you can safely operate the board, while getting a UL 
certification.?? Your board shop will have a UL maximum temperature for its product boards, through its
initial certification, and will have had its boards sampled annually to remain certified.? Here is a pretty
good presentation from UL to get you started.? Maximum operating temperature is in the middle. 

http://www.ul.com/global/eng/documents/offerings/industries/chemicals/plastics/ThermalManagement_April2008.pdf


-----Original Message-----
From: Dan Skweres <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, Jun 29, 2009 3:59 pm
Subject: [TN] Operating Temperatures



I have a question regarding operating temperatures. Specifically regarding bare 
board level. It's actually opened up dialogue here as well as with my material 
suppliers. What is taken into consideration when determining the operating 
temperature of the board? I understand most base materials have a maximum 
operating temp. but what characteristics (electrical, mechanical etc.) are taken 
into consideration when determining the operating temp of a bare circuit board 
with mask, legend etc.? Is it just the base material that is a concern or a 
combination of all co-mingled elements on the board? I have spoken to a 
representative at DuPont and they, for example, stated that AP material alone 
has a maximum operating temperature of 180C but when FR or LF coverlayer 
material is applied then the operating temp goes down to 105C due to the acrylic 
adhesive present. If this is true then wouldn't a mask applied to either a 
flexible or rigid board influence the operating temp? If so, is testing on each 
and every option(s) of materials then necessary? Or... is there a simpler method 
of determining the operating temp. of the bare board?  Just curious.
Thanks in advance!
Dan S.

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