I'm doing a lighting project which requires a load of SMD high-power LEDs (9-12x Philips Luxeon LXML-PW21). I can't use star-mounted ones, because they need to be closer together than possible with star ones. Each LED will use about 2.25W (3.2V, 0.7A). I can't seem to find any PCBs which already exist for me to attach the LEDs to. I need a PCB that's reasonably thermally conductive so I can attach a heatsink to the back. I don't really understand parts of the order forms for online PCB makers and I'd rather do it myself if it's possible - it's probably cheaper (not difficult for it to be cheaper than £50). I've seen lots of tutorials for etching PCBs using various methods - do standard copper clad boards conduct enough heat to use with LEDs? I looked at a Cree XM-L datasheet, which said that FP4 circuit boards should be enough for LEDs using under 3W, which mine are. In that case, would something like this FP4 board be any good for the LEDs, and for etching at home?
You shouldn't have any issues etching that board at home, however I'm pretty sure the datasheet would be refering to how much heat the board can handle from a safety point as they are fiberglass based, thermally speaking they won't conduct much in the way of heat though the actual board - If there is that much heat being generated I expect it wouldn't hurt to attach a heatsink to there board rear (maybe screw into the heatsink from the copper side) but I'm not convinced this will have any major cooling effect. Afraid I've not used anything so high powered before so can't really comment, only used the standard type like found in remote controls and backlights etc. I've done a bit of etching at home so could be able to advice a little if needed should you get that far down the process
The datasheet was talking about what the LEDs could be used with - it wasn't for the board. I'm sure they wouldn't have mentioned it if it wasn't good enough. I can always drill holes through the board and put very short metal cylinders in to conduct the heat away to the heatsinks. Looking at all the online prices for circuit boards, I'll definitely be etching it myself, but I still need to make sure I can use that board and get my circuit designed. Which methods of etching have you done before, and if you've done more than one which gave the best results?
So far I've done toner transfer method which is ok but I got hit n miss results. I'm now using pre-coated UV photoresists boards and transparency sheets which give great repeatable result and allows more detail in the design. UV means you need to make / buy something to act as a uv exposure box As for enchant I used ferric chloride but found the times and mess painful so I'm now using hydrochloride acid and hydrogen peroxide which is much faster to etch and is clear so you can see the etch progress. I also made a small bubble tank to speed up the etching a little while using a weaker enchant solution. Don't know if you have seen this: http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/XLamp%20Application%20Notes/XLamp_PCB_Thermal.pdf Might be useful, mentioned use of vias to aid in cooling like you mention above
On you board create a ground plane then wire then up to be common cathode and don't use any thermal relief on the SMD pads, get some boards with 2oz. copper and that's about the best way to do it unless the LED has a thermal pad have you got the datasheet?
The LEDs have dedicated (fairly large) thermal pads, which is why I'm worrying about cooling. The datasheet for the LEDs is here. Look for LXML-PW21s, but all the LEDs in the datasheet have the same physical design.
