Heat effect on FR4?

Mormal practise is to space the resistors off the board with ceramic beads...... then you don't have the problem. If you can put a copper pour all under the resistors that will help dissipate the heat evenly as well, as opposed to leaving localised 'hot spot' sources.

I'm inclined to put the resistors in parallel and with lots of copper under them. If they're those small body type 1W, the heat goes out through the leads, need copper there to spread the heat.

Sorry, my crystal ball is bust.

Grant.

C'mon, you must have some Consumer Electronics on hand! I see it all the time, discolored boards under diodes/resistors. Phenolic and epoxy both turn brown over time. I don't know how much this affects mechanical stability; it's not like it's crumbling under the hotspot.

(Although I did once overload an ATX power supply, which had the result of turning the output filter choke into a heating coil, merrily charring the PCB to a combination of carbon and glass fibers.)

Tim

ehsjr schrieb:

Hello,

why do you want to use 11 one watt resistors to spread about 1.2 watts? If it is only 1.2 W, two or three resistors will do, if it is 12 W, 11 of those resistors will not be enough. There are resistors for 4, 5 and

Bye

Is that out on the bench outside fo the intended enclosure? If you it will be much hotter when you enclose it.

"ehsjr"

I see old equipment all the time with resistors laying flat on the PCB that dissipate more than 25% of rated and there is not even a mark left on the PCB.

You could space the body up a few mm from the surface for even greater immunity.

.... Phil

Thanks, Phil. I was hoping someone would answer based on experience with the long term effects, so your answer is particularily appreciated.

Ed