Resistors encased in Resin

[Falcon69]

Okay, I have a project (I've posted it on this forum) and i will be encasing the circuit board in resin to protect it from debris and what not.

Now, in simulation, a single resistor for example, is dissipating 43mW under it's load.  Now, being that the resistor (and the entire circuit board which measures about 10mm by 16mm) is encased in resin to protect it from debris, would a 62mw resistor be okay? Would I need to step it up to 1/8 watt? Maybe even 1/4watt? I need to stay with an 0805 or even preferred 0603 case for the resistor.  Smaller the better.  Now I know they make 1/4watt 0603 resistors, but they are about twice the cost of the 62mw 0603 and 1/8watt 0805.

Thanks in advance,
Jason

[valentinc]

     If I were you I would try to put a voltage o that resistor that dissipates ~43mW and measure the temperature... 62 mW should be okay in theory, but in practice I have seen that resistors tend to become hot even at half the rated wattage, plus that you are ecasing the circuit in resin, that will make the resistor even hotter... I think that an 1/8W resistor will be ok (it's less than half the rated wattage, and if it get a little warm is not a problem) to keep the cost down...

     Or you can do another thing, if the space requirements permits: you use 62 mW resistors, with double the value, and put 2 in parallel (soldered one over another)... Or use even 4 (and 4 times the value)... It doesn't look fancy, but it works...

[pickle9000]

Solder a short length (10-15mm) copper wire to one side of the resistor and encapsulate it. The thicker the wire the better but not so thick as to cause any grief. Leave the insulation on the wire if needed and run close to the exterior of the encapsulation.

[Andreas]

Hello,

a SMD-resistor usually distributes its heat to the PCB board.
So I would not worry about. Except when you use elevated temperatures.
In this case you would probably have to do a derating above 70 deg C.

With best regards

Andreas

[peter.mitchell]

a SMD-resistor usually distributes its heat to the PCB board.
So I would not worry about. Except when you use elevated temperatures.
In this case you would probably have to do a derating above 70 deg C.

Exactly, the heat is transferred to the PCB, so the resistor doesn't have to dissipate much heat at all. An 0603 will dissipate it's 1/8w when soldered to a board, safely, over quite a wide operating temperature. That said, by virtue of thermodynamics, an 0805 resistor rated at 1/8w will dissipate it's heat over an even larger operating temperature because it is a larger package with a larger surface area and a better thermal junction to the board.
I wouldn't worry about it, even with an elevated temperature; you'll run into issues with other components first most likely.

Additionally, to a certain extent, potting/resin can IMPROVE localized heat dissipation, by offering a lower thermal resistance path, eg a single resistor in the center of an empty 10x10mm pcb will cool quicker in epoxy than in free air, because the epoxy will pull the heat away from the resistor quicker than air. However, if the board is a densely populated SMD board with power devices, the natural convection currents of air would provide vastly better cooling.

[Falcon69]

Hmm, that is good to know.  The 100mW resistors in 0603 size is really cheap compared to the 1/4 watt.  Maybe I will stick with those. a 5,000 roll of 0603 1% 100mW are .003 each ($15). A roll of 5,000 0603 1% 250mW are .045 each ($225).  Big difference in price.  Oddly enough, the 63mW ones are more money than the 100mW ones.

[T3sl4co1l]

Use fatter traces, extra copper, etc.

Depending on resin type, there may also be more dissipation through it than through air.

Tim