How to Lap a Heatsink

Like it says — Maximus Nickus

First the question of why?

Most heatsinks have a relatively rough base after their machining process (this is where thermal grease comes in). Even though it may feel smooth and flat, there are microscopic peaks and valleys which impede the transfer of heat from the CPU to the heatsink.

Now obviously with a rough base, only as much as half the heatsink may be contacting the CPU; thermal grease can fill these pits, but it’s another substance for the heat to transfer through and it’s not as efficient.

So we lap the heatsink to make it smooth so you get the best possible contact between the heatsink and CPU for maximum cooling. There will always be peaks and valleys, but lapping changes Mt. Everest to a small hill. Thermal grease fills these micro-voids, but the smoother the better.

Lapping can be performed on any type of heatsink/waterblock, it can be aluminium or copper, (or a combination) just remember copper sands much much quicker than aluminium, so be careful and patient with aluminium! (Most lapping takes about an hour for a well done job).

Now for the process:

You will need some wet/dry sandpaper; if your heatsink is really bad, you’ll need a sheet of 400 grit and one sheet of 600 grit. If it’s not too bad, just a sheet of 600 grit.

Start by placing the sandpaper on a FLAT surface (a uneven surface is just defeating the object of the task – glass is often a good surface). Pour a mixture of water and a FEW DROPS of washing up liquid (stops it sticking) onto the paper.

Now grasping the heatsink, place the bottom (obviously!!!) onto the sandpaper and make circular motions about 10-15 cm in diameter (it will be difficult and the heatsink will stick at first, but as you progress, the bottom layer of the machining rubs off and it will glide over).
Remember to press with reasonable force, every 100 or so strokes rotate the Heatsink 90 degrees to make sure of an even surface.

Also, every once in a while when the sandpaper looks very orange or silvery (the metal particles), flush the paper with more water + washing up liquid. Keep doing this until the bottom of the heatsink is smooth (test by running your thumbnail over the bottom and see if you can feel any obvious lumps). When its smooth you’re done!!

A few notes + pointers:

  1. If the heatsink is really uneven to begin with (as I said above), start with a lower grit (more coarse). After a good 15-20 minutes of lapping, check to see if all those horizontal lines on the bottom are gone – if they are, proceed to the 600 grit paper.
  2. You may if you wish use a higher grit than 600 to finish with – some people even go to a 2000 grit (so fine it would only take the shine and not the paint off a Mercedes body work). However, it isn’t necessary to have it so smooth – 600 grit will give a smooth enough surface and going to a higher grit may make it shinier, but you certainly won’t get a performance gain (this has been debated many times).
  3. After you have finished, you may wish to clean the heatsink’s surface with some Isopropyl Alcohol. I prefer to use this as it evaporates, unlike polish, because you don’t want a layer of anything depleting your heatsink’s performance.

    For copper use an anti-tarnish solution you can buy – Brass-O is a popular one. It’s very good at getting dirt out and making the surface as smooth as possible.

  4. After you’re finished, apply some Arctic Silver 3 (this is the best Thermal compound) to the CPU’s core as per instructions (HERE) and mount the heatsink. You may be wondering why you’re adding a thermal compound if it’s another layer?

    Arctic Silver 3 fills any remaining micro-voids; if there aren’t any, it moves out the way, so make sure you apply an even, thin coat on the CPU core, else it will be dripping down your core (Don’t worry – it’s not very dangerous (unlikely to get a short circuit) it’s just messy!).

You’re done!

Nick Cade-Westcombe aka Maximus Nickus
Senior Member, Forum

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