PCB 101 questions

[Christoph]

How well does a pcb transfer heat.In the case of an LS will putting the pcb on the heat sink work or should the emitter be mounted directly on the sink? I'm planning on trying my hand at making up some circuit boards for use in motorcycle lighting applications.I bought the radio shack pcb etchant kit anyone have any experience with it?
Chris

 

[Jonathan]

The PCB fiberglass board is a fairly poor heat conductor, and you are much better off mounting the luxeons 'directly' to a heat sink. For smaller LEDs, the copper layers on the PCB are enough for heat sinking, and all you need to do is leave large areas of copper connected to the cathode of each LED.

The Luxeon star devices (star, star/C and star/O) all come _premounted_ on a small PCB. This PCB is metal core rather than fiberglass core, and is supposed to be a good heat conductor. You can bolt this metal core PCB to a larger heat sink.

-Jon

 

[dat2zip]

Copper is one of the best conductors of heat.

That said, if you leave all the copper on the PCB and mount the emitter directly to the copper foil with good thermal grease like Arctic Silver III or Arctic Alumina, you will have no problem with transfering the heat from the emitter to the copper foil of the PCB. The trick is to bond the copper foild of the PCB to some larger mass of aluminum (or some other metal) to get the heat off the PCB. The thin copper foil in itself cannot be used as the heat sink.

The Luxeon Star and the /O Luxeons I've seen all have a small fiberglass PCB laminated on top of the aluminum core. While fiberglass is not that good of a conduction, I've noticed that the back side of my converters are nearly as hot as the top and if I heat sink the bottomside it substantially transfers a good amoount of the heat to the heat sink.

It would be an easy test to mount the emitter to say 1" X 1" of unetched PCB and apply 1W of power to the Luxeon and determine how much heat sinking you will need.