|Cree XP-G SMT pad with via through thermal pad and connection to component side thermal plane|
I figured that was a lot of copper to dissipate the heat. After some more reading, however, I realized that while my design was probably adequate for my current requirements, there was no way I was going to get away with running the LEDs at their maximum current. At about 300-400 mA the thermal planes are quite warm, but not alarming. At 500mA the LEDs on this design do get pretty darn hot to the touch and that isn't good as it reduces their efficiency which ultimately results in more heat, less light and reliability issues. I do have a planned application where I want to run these guys up to 1A or more, so my current PCB design needs revision.
Many power LEDs come mounted on the familiar metal-core PCB star:
|Cree LED on MCPCB star|
First, I had to rejig the XP-G solder pad, as the orignal layout created a real bottleneck for heat transfer:
The big 'I' can now be filled with vias to conduct heat away from the thermal pad. Riddling the board with thermal vias isn't necessary; just getting the heat way from the immediate vicinity of the thermal pad has the greatest effect. Here's what the PCB design looks like:
Lots of thermal vias. On the back there is a 14x14mm square solder pad to attach a copper heat sink:
These are little RAM sinks. You can get 8 for about $15. They come with their own thermal adhesive already attached but the general consensus among user reviews to to scrape that off and use something like Arctic Silver thermal adhesive instead.
Hopefully the vias will provide sufficient transfer to keep the LED cool enough to remain bright and not overheat at full current (1A+). They are the bottleneck between the LED's thermal pad and the heat sink. The application note recommends more vias in the actual thermal pad itself but my cheap PCB fabricator has a minimum drill size that doesn't allow this. Following their recommended design they claim an FR-4 PCB can have a thermal resistance close to that of a metal core PCB. In my case, I doubt I can achieve that with only 3 vias in the pad proper (they recommend something like 14!). If these aren't sufficient I might have to find another fabricator that can do smaller drill hits, although this could ultimately defeat the purpose of doing this on the cheap!
Another potential pitfall here is that BatchPCB, who I'm using for fabrication, only fabricate 1 ounce copper PCBs. Thicker copper would provide a larger conduit to take the heat away, but for the sake of not spending a fortune on prototyping, I'm kind of stuck with their 1 oz limit. I placed the order last night, so I'll know if a few weeks if this design will work.