Heat effect on FR4?
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From: ehsjr on 11 Jun 2010 01:54 I have a resistance circuit that dissipates ~1.2 watts. If I use 11 one watt resistors in series (physically parallel) I can spread the heat out across the width of the board. I measured ~66C under each resistor at worst case ambient (~32C) in the test setup. What will happen to to the board over time (>10 years) of 24x7 operation? (Resistors in contact with the board.) Does FR4 behave like wood that is exposed to temperature over time, slowly charring? TIA, Ed
From: TTman on 11 Jun 2010 02:56 "ehsjr" <ehsjr(a)nospamverizon.net> wrote in message news:husj1c$g5l$1(a)news.eternal-september.org... >I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. I measured ~66C under > each resistor at worst case ambient (~32C) in the test setup. > > What will happen to to the board over time (>10 years) of 24x7 operation? > (Resistors in contact with the board.) Does FR4 behave > like wood that is exposed to temperature over time, slowly charring? > > TIA, > Ed > Mormal practise is to space the resistors off the board with ceramic beads...... then you don't have the problem. If you can put a copper pour all under the resistors that will help dissipate the heat evenly as well, as opposed to leaving localised 'hot spot' sources.
From: Grant on 11 Jun 2010 07:25 On Fri, 11 Jun 2010 01:54:00 -0400, ehsjr <ehsjr(a)nospamverizon.net> wrote: >I have a resistance circuit that dissipates ~1.2 watts. If I use >11 one watt resistors in series (physically parallel) I can spread >the heat out across the width of the board. I measured ~66C under >each resistor at worst case ambient (~32C) in the test setup. I'm inclined to put the resistors in parallel and with lots of copper under them. If they're those small body type 1W, the heat goes out through the leads, need copper there to spread the heat. > >What will happen to to the board over time (>10 years) of 24x7 >operation? (Resistors in contact with the board.) Does FR4 behave >like wood that is exposed to temperature over time, slowly charring? Sorry, my crystal ball is bust. Grant. -- http://bugs.id.au/
From: Tim Williams on 11 Jun 2010 07:47 "ehsjr" <ehsjr(a)nospamverizon.net> wrote in message news:husj1c$g5l$1(a)news.eternal-september.org... >I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. I measured ~66C under > each resistor at worst case ambient (~32C) in the test setup. > > What will happen to to the board over time (>10 years) of 24x7 operation? > (Resistors in contact with the board.) Does FR4 behave > like wood that is exposed to temperature over time, slowly charring? 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: J.A. Legris on 11 Jun 2010 08:52
On Jun 11, 1:54 am, ehsjr <eh...(a)nospamverizon.net> wrote: > I have a resistance circuit that dissipates ~1.2 watts. If I use > 11 one watt resistors in series (physically parallel) I can spread > the heat out across the width of the board. I measured ~66C under > each resistor at worst case ambient (~32C) in the test setup. > > What will happen to to the board over time (>10 years) of 24x7 > operation? (Resistors in contact with the board.) Does FR4 behave > like wood that is exposed to temperature over time, slowly charring? > > TIA, > Ed |