Dust Bunnies are Choking Heat Sinks to Death

My current notebook is about 18 months old and had been running slower and slower for a few months. It was also noticeably hotter causing the fan to run almost continuously. I knew it was that time. About every year and a half I have to take apart my PC to clean out dust from the fin stack and to replace the thermal grease if I want it to run at peak performance. When I mention this to friends they always look at me strangely. Here’s what I found, and expected to find, when I opened it up.

Fouled Notebook Heatsink

Notebook Heat Sink After 18 Months

 

Yes, that’s the heat sink. And, for the most part this notebook is used on my desk and most people think I am anal about keeping things neat and tidy. So, this is not from a harsh environment!

And since the processor has been running hot for a few months the grease has started to harden so that needed replacing as well. Whoever put the grease on in the first place used enough for three processors!

Thermal Grease

Thermal Grease

 

This reminds me of several papers I saw presented some years ago. David Moore of HP showed results in both desktop and laptop systems with failures showing up in less than 1 year1&2. The process begins when hair, and/or thin fibers of fabric or paper begin to layer on top of one another forming an intertwined network. Smaller flakes, usually skin, are then trapped by the matted fibers. Here’s what the process looks like2.

Magnified Dust Bunny

Long Fibers Intertwine then Small Particles Become Trapped

 

Wanting to understand more about the process he and an HP team created a “Dusty Environment Simulator”, complete with manmade dust – a combination of finely shredded and ground recycled newspaper and attic insulation, with a fire retardant added. The material would be introduced into an enclosed test chamber using a standard flour sifter.

The next step was to test small form-factor desktop systems, each with a different CPU heat sink design. The two I’ll touch on here are a folded fin radial heat sink with a top mount fan and an extruded straight fin heatsink, also with a top mount fan. I pulled these pics off the web for illustrative purposes so show roughly what these sinks look like.

Folded Radial Fin and Extruded Straight Fin Heat Sinks

Folded Radial Fin and Extruded Straight Fin Heat Sinks

 

His results bowled me over. The radial heat sink had a delta-T which was 3 degrees Celsius better than the extruded fin sink when the systems were tested without dust. However, the radial sink became more quickly clogged with dust causing its delta-T to rise above that of the extruded sink1.

Heat Sink Fouling Test Results

Heat Sink Fouling Test Results

 

David concluded that while environment is a significant risk factor in heat sink fouling, heat sink design also plays a key role. Here are some of the non-environmental factors that increase vulnerability.

  • Finer pitch heat sinks – provide a shorter distance for fibers to bridge
  • Sheared fin construction – sharp surfaces retain fibers
  • High impingement velocity
  • Close proximity of fan

Hopefully, every one of you is not only reaching for a screwdriver to take apart your own system, but also thinking about these killer dust bunnies when designing your next thermal solution.

Sources

  1. Moore, David A. “The Dust Threat” Presentation to the IMAPS Thermal ATW, October 23rd, 2003
  2. Moore, David A. “Characterization of Fiber Accumulation Fouling in Fine Pitched Heat Sinks” Paper for 25th IEEE Semi-Therm Symposium.