I need to make a revision to my original method of lapping.
I've since changed my method to one which I personally have found to be more efficient and leads to less errors and scratches which will cause you to go back to a lower grit to remove.
I work up through grits 200, 400, 600, 800, 1000, 1200, 1500, 2000, 2500 and then 10 microns.
The majority of the sanding will be done at 200 grit as this will get the surface flat as soon as possible (this is a good thread which illustrates the difference between a shiny surface and a flat one
), if you can't get it flat at 200 grit, you aren't going to get it flat at 400 and so on. You'll need to use something to check that you are getting a flat surface (note: it won't be anywhere near a mirror finish at this grit so using graph paper is out of the question) I personally use a machinists ruler (i.e. those metal ones for tech drawing) which usually have a perfectly straight edge. Line it up perpindicular to your lapped base with a light behind it. If you can see light through between the 2 surfaces then the base isn't flat and needs to be sanded more
Now to begin, I sanded at 200 grit backwards and fowards on the surface for a few minutes, checked if it was flat for the direction I was sanding then rotated the HS by 180" and continued. Once it was flat in that direction I rotated the HS by 90" and repeated. Until both directions of the HS where flat (i.e. N to S and E to W).
Now the hard part is over, you have a flat surface. If you are so inclined you could leave it there and use some TIM to fill in the scratchs. If you want to go that extra mile and get it shiny AND flat then read on.
Moving up through the grits, I found it took as long to get up through the grits as it did to get it flat, so getting it flat should be half of the job. The benefit of sanding in one direction per grit level then rotating 90" for the next is that you can be sure that the surface is evenly lapped because the scatches from the previous level should be completely removed, it's because of this that I recommend NOT to sand in figure 8's or circles, as you can't be sure that you are evenly sanding.
At each grit level clean the flat surface you are working on (i.e. flecks of metal on the glass under the sandpaper could cause deep scratches in the surface) also everytime you lift up the HS to check if it is sanding evenly and if it is still flat you should clean off the metal dust that is on the sandpaper you can do this by blowing on it, using a can-o-air or wiping it with a cloth.
Also once you get to a point where it is reflective start using graph paper as well as the machinist ruler to check if it is flat. Also don't worry if there is still small scratchs in the surface when you are finished. It doesn't have to be perfect, and these scratchs will be filled by the TIM anyway, the main thing is to have it flat and have the majority of the surface smooth for better contact. Some slight scratches are not going to affect your temps that detrementally. Also I found that using brasso left the surface speckled with tiny pock marks (if you look close after using it you'll see this) which is why i'd recommend NOT using brasso if possible. IMO I feel it is better to have a few tiny scratchs on an otherwise flat surface then to have a surface covered in tiny pock marks. Anyway, the choice is up to you. After this give the surface a clean with some alcohol or some Articlean and your done. Be sure to install teh HS asap after lapping as the bare copper might oxidise if left out to the air over time.
Any comments or ideas are welcome.