Lapping - October 2000

"Lapping? Isn't that what cats do?". Well yes it is, but it's also what mad overclockers do to their heatsinks and sometimes CPU's to make them flatter. The reasoning behind this lapping lies with the fact that air does not transfer heat that well, in fact, it's a great insulator. So, the flatter the mating surfaces, the less air, the less heat sink grease you need to use, the better your chip is cooled! I have lapped stuff before such as my Celeron 300a, multiple heatsinks and my GeForce (see Cooling the GeForce), but I had never envisioned lapping one of the new FCPGA chips. Until now :)

First, lets look at the heatsink I just bought to stick on my 550e. I didn't get a before shot of the Kanie Hedgehog copper heatsink, but the surface was visibly 'rippled' from the machining process. To the naked eye it seemed quite flat, however in reality the heatsink dipped quite a bit in the middle. The shot on the left was taken after lapping for while, you can see the patch in the middle that is 'untouched'. The aim of lapping is to keep going until that patch is gone, and then use finer and finer sand paper until the surface is mirror smooth.

A quick word on my lapping setup. First, you need a perfectly flat surface to lap on, otherwise you are wasting your time. I went to one of the local glass places and got an off cut, which was about 20cm x 40cm, but any decent size will do. Make sure it is thick enough that it won't break easily. I also got the guy to take the edges off with his belt sander, so I don't cut my delicate quaking fingers :)

As for the sandpaper, I just drop down to the local hardware store and get some 'wet and dry' sanding paper. I use everything from 240 through to 1200. For the heatsink I did all of the initial lapping with 240, and when nice and flat, cleaned up with 400, 600, 800 then 1200. For the CuMine, I used 400, 600, 800 and 1200. 

After doing the Hedgehog, the time of reckoning had come, I had to start on the CuMine. I knew just trying to lap that tiny CPU on its own would be very hard, and I intended to use a copper shim with it when finished anyway, so I decided to permanently affix the shim to the chip and lap the whole lot together. Time to get the trusty superglue out :) 4 dabs and a few minutes of pressure later and it was NOT coming off in a hurry.

The first shot on the right shows the state of disrepair my 550e was in before I started. Nine months of fiddling with the heatsink and not using a copper shim as a support has left it scratched and the edges and corners crushed. Still works though :)

The shot on the left shows many things. Firstly, that is what the CuMine looks like when the blue stuff is gone. You can also see the copper shim stuck in position with glue, and finally you can see how I used masking tape to cover the bits of the FCPGA package I didn't want getting dirty, such as the pins.

Lap lap lap, slow boring tedious. I didn't want to rush it after all. Anyway, I am sick of lapping now :) I made sure I applied even pressure when lapping, and rotated the chip 90^o every 20 or 30 'laps' to cancel out any bias in my hand pressure or lapping 'style'.

In the end I didn't get the entire shim level with the top of the CuMine, but a lot of it is, and certainly enough to provide good support for the heatsink. More importantly, the parts of the shim that aren't level with the CuMine are lower than it, so they won't stop the heatsink from making good contact with the chip.

The CuMine itself came up very smooth and shiny and now isn't filled with deep scratch like before! Behold the finished product:

Beast