# Heat effect on FR4?

E

#### ehsjr

Jan 1, 1970
0
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

T

#### TTman

Jan 1, 1970
0
ehsjr said:
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.

G

#### Grant

Jan 1, 1970
0
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
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.

T

#### Tim Williams

Jan 1, 1970
0
ehsjr said:
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

U

#### Uwe Hercksen

Jan 1, 1970
0
ehsjr said:
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.

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
11 W to be soldered in a FR4 board too. There are also resistors for
chasis mount available for 25 or 50 W. If you think about 10 years of 24
x7 operation, you should chose a robust solution.

Bye

P

#### Phil Allison

Jan 1, 1970
0
"ehsjr"
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.

** So that is 110 mW dissipation per 1W resistor or 11% of rated.

What will happen to to the board over time (>10 years) of 24x7 operation?
(Resistors in contact with the board.)

** Probably nothing.

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

E

#### ehsjr

Jan 1, 1970
0
Phil said:
"ehsjr"

** So that is 110 mW dissipation per 1W resistor or 11% of rated.

** Probably nothing.

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

Ed

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