How heat sink fin spacing plays a key role in natural convection cooling and how you can optimize it

We wrote a three part series, “How to Apply Natural Convection Cooling for the Thermal Management of Electronics” during the week of August 22nd. Many of you liked our technical post so we thought we’d follow that up with a little focus on fin spacing.

Fin spacing is also known as channel spacing, the channel being the width from one fin to the next in a heat sink as illustrated in this drawing:

Here we see that the “B” dimension is the channel width or the fin spacing of this heat sink.

So what’s the big deal on fin spacing? Well, as usual in thermal engineering quite a bit more than meets the eye. The fin spacing of a heat sink, if not optimized for your application, can actually  reduce the thermal efficiency of your heat sink. If the fins are too narrowly place then the channel is too narrow and air has a harder time getting through!  Think of air as it really is: a fluid. Don’t forget that in physics, a fluid can be either liquid or a gas. That should help you to visualize what’s going on.  One case in particular was very interesting

The project we were working on was  for a rather large heat sink, (about the size of a hardcover dictionary if anyone remembers those!) for a wireless network remote radio head (RRH) application. The customer was another Tier 1 Telecommunications Equipment OEM. The customer actually had developed their own heat sink for natural convection cooling. It’s specifications were:

35 Fins

5mm base

Thermal Resistance of 1.39 C/W at 10LFM 55C ambient

 

While the design was basically on target, the thermal performance just wasn’t there to cool their RRH. ATS performed a series of experiments and, based on our data, developed a new heat sink that had these improved specifications:

17 fins

5mm base

Thermal Resistance of 1.05°C/W thermal resistance at 10LFM 55C ambient

17% fewer fins than a standard RRH HS

60% Lighter than a standard RRH HS

25% less expensive than similar solutions due to reduced material cost.

 

Here’s a comparison photo showing the two side by side; the fin spacing is clearly seen between the two:

The results are dramatic! And that’s what can happen when solid thermal analysis is applied to a natural convection problem.

How about some tools to help you in this?

There was a great article in “Power Electronics Technology” magazine by Chris Soule of Thermshield that covered this issue very nicely. You can get a copy of Chris’s article by clicking to Power Electronics at this link: “Optimize Fin Spacing, how close is too close?”

Intel has a nice article providing an example of how to do an optimized plate fin extruded thermal solution spacing calculation. Chun Howe Sim and Loh Jit Seng of Intel wrote the article and it’s a quick read. You can get your copy by clicking to EE Times here: Fanless Design for Embedded Applications

You should also review the ATS series, “How to Apply Natural Convection Cooling for the Thermal Management of Electronics” You can read our three part series here:  part 1, part 2, and part 3.

 

Finally, give us a call at 781-769-2800 or drop us an e-mail at ats-hq@qats.com or submit your questions and project for natural convection cooling via our online project form on our main web site here.