Friday, October 14, 2011

getting the heat out of LEDs (as cheaply as possible)

Well, it turns out designing a PCB for power LEDs isn't as simple as I'd previously thought.  Now, to be fair, with the intention I had in mind, I was only going to run my power LEDs at about half of their rated maximum current. With that in mind, I'd put a single via through the thermal pad to connect it to a large plane on the bottom side. It was also connected to a plane on the component side. Like so:

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
MCPCBs have a metal core that conducts heat much more efficiently than the standard FR-4 PCB laminates from the top layer to the bottom layer (where a heat sink can be attached).  In other words, the metal core has a much lower thermal resistance than FR-4; 5-10 times lower.  MCPCBs seem like a great solution for mounting LEDs, but they are very expensive to have custom made.  Fortunately, there are several application notes out there that describe how to design an FR-4-based PCB using thermal vias to get a top to bottom layer thermal resistance closer to metal core.  I used this one from Cree to guide my design.  The application note is summarized thusly:  instead of using a metal core to get heat away from the LED's thermal pad, use as many thermal vias as you can fit in the thermal pad to transfer the heat away from it through to the bottom layer, where a heat sink can be attached.

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.

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