Quoted from Zitt:1) Put vent holes in the PCB so those ?2W? led resistors can breath / cool better. Right under the resistors so convection heating of the resistors draws cool air up from behind the board. Looking at that layout; it should be very easy to put large holes under those resistors. The side effect is those resistors should stay cool; and reduce solder joint cracking.
Firstly ... thanks for the feedback. Any and all feedback is welcome as I'm pretty much doing it all myself at the moment.
Passive convective cooling is what's happening here. There are two drawbacks (that I can immediately think of) to putting (big) holes like you've suggested.
1) You can no longer use the board space for traces.
2) The more holes (and bigger ones) that are put in the board the weaker it becomes.
When working on boards everything is a trade-off. There are two small holes for a 0.5W resistor that overlays with the 2W resistor.
Quoted from Zitt:2) If you're laying those power resistors directly on the PCB; don't. Put at least a 10th of an inch between the PCB and the resistor's "bottom". Similar effect to 1); but both would be ideal.
I built the board with about a 1/8" gap between the body of the 2W resistor and the board for convection. I do this with all power (heat dissipating) resistors on new boards and when I re-work an existing board.
West Chester, PA
