I added the recommended copper flood areas and I am pleased to report 
that the temperature is significantly decreased, to the point where one 
can now put a finger on the bottom of the PCB w/o discomfort from heat.

The SMT resistors are in 2512 packages. It actually took a lot less cu 
area than I thought it would to make a significant difference. Some of 
the cu flood areas are less than the size of the resistor itself. Of 
course, I made them as large as I could, but the point is the space 
available made a big improvement.

Thanks for the input.
 
Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
www.competitionelectronics.com


Dwayne Reid wrote:
> At 08:47 AM 3/18/2010, Mark E. Skeels wrote:
>
>   
>> But my question is, will 105 deg C max be good long term for the FR4 pcb?
>>     
>
> What solder are you using?  Is the PCB subject to high vibration or 
> sudden high-G drops?
>
> My experience is that temperatures up to about 90C are fine.
>
> Question: do you have enough room on the PCB that you can do copper 
> floods on each of the resistor leads?  More copper area that each of 
> the leads is attached to equals lower resistor temperature.
>
> Holes under the resistors may actually increase the temperature - a 
> closed box has little air circulation and air is a better thermal 
> insulator than the FR4 substrate.
>
> dwayne
>
>   
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