Penny Heatsink Tutorial

April 15, 2010

 

 

I was given an old IBM T20 laptop which I was more than willing to take despite its mediocre specs compared to today's standards. Its 700 MHz Pentium III processor was amazingly slow and gets quite hot. Running benchmark softwares can easily accelerate the CPU temperature over 95° C which I read is normal. Since this was an old laptop I took the liberty to inspect the insides and clean it up a little. I removed the heatsink from the processor and found it did not use thermal paste but a thermal pad. On second thought I'm not sure it was even thermal pad. It looked like clay putty or kneaded eraser. What concerned me was how thick the layer was. There was an one millimeter gap between the processor die and the heatsink, quite a large gap I thought. The thermal pad filled in that gap. I wanted to do away with the thermal pad since it was dried up and crumbled to pieces when handled. At the same time there was no way I can use thermal grease with a gap that wide. Hmmmmm. What to do!?

 

dried thermal pad

 

I used a blade to cut the thermal pad into sections to illustrate the gap it had to fill in between the heatsink and processor.

 

wide gap

 

A little research revealed that the heatsink from an IBM T22 laptop had improved heatsink design and used thermal grease for thermal conduction. It can easily retrofit the T20 as well. That was the simple solution but I really did not want to spend any money on an old laptop. This was when the idea of using a penny heatsink came to mind. It was perfect for my purpose. The copper penny will act as a heat spreader and should perform no less than the thermalpad it was replacing. Others have used pennies for heatsinks on other projects successfully and here was another good application for it. The best part was that it costs only a penny or two to do the project, minus labor.

 

penny heatsink

 

Halfway through the project I found out that post 1982 pennies were made of 97% zinc alloy with copper plating. The price of copper had gone up and it cost more to make a penny than what the currency is worth so the Mint went to copper plating instead. This is not the penny we want to use. We need a penny consisting mostly of copper which are the 1982 pennies and earlier. As you may or may not know copper has better thermal conductivity than zinc. Just my luck I had a 1975 penny waiting to be used. Saving for a rainy day had come to fruition.

The penny needed to be flattened to provide a good mating surface so sanding it down was necessary. The image below shows a 1975 penny and to the right is a zinc penny: not a good heatsinker. I had been sanding it for half an hour before realizing there was no copper inside. I thought if I kept at it the pretty pink color would come back. It never did.

 

pennies

 

The penny is thicker than a millimeter. It's wider than the gap I needed to fill in. I figured by the time I was done I would have cut down its thickness by half.

 

penny heatsink

 

I've had experience lapping heatsinks before and it's a time consuming process. Basically, you use wet sandpaper of varying grits starting with the lowest and working your way up. You initially remove as much of the irregularities on the metal surface as possible with lower grit sandpapers and advance to finer sandpapers to buff out the fine scratches until you're left with a mirror-like finish. I used what I readily had available and used a sandstone to quickly file down the penny to a flat surface until all impressions have disappeared on both sides of the coin. I then polished with a 400 grit sandpaper and finished off with 600 grit for a mirror finish. I certainly could have gone higher but 600 grit was good enough for me. Use plenty of water during sanding to clear out metal debris from sticking to the sandpaper.

 

lapping

 

After polishing the penny was a little over 1mm thick. It was just about the right thickness I needed. I didn't want it to be the same thickness as the thermal pad since I wanted it to fit really tight between the processor and heatsink. Check out the good work I did. The 600 grit sandpaper was fine enough to produce that luster on the penny. It's got a nice reflective surface that will ensure good thermal conductivity.

 

penny heatsink

 

 

 

 

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