lapping!

ok i have heard when i lapp a HS and and if i go too high grid and polish it out too much it can have an opposite effect on cooling so if i start out with 600 grid than 800 when should i stop? i just got a slk-800 u HS the bottom looks pretty nice and flat!
so should i go up to like 1000-1200 or go all out and use 1500-2000 and 10000 at the end.


thanks

Most people say that 600 to 800 grit is as high as is needed.

Lapping is meant to make the base flatter; polishing does nothing for temps, but it will make your elbows sore.

Make sure you use the method BillA recommends: place heatsink on sandpaper, move across sandpaper, lift and return to starting position, repeat. Always move the heatsink in the same direction. Rotating the HS or circular motions will give you a convex base and make HS to core contact worse. If your HS uses clips you also don't want to sand the base down too much or you will not have the appropriate amount of pressure holding the HS down.

I just read up a bit on this previously. There didn't really seem to be a consensus on how much benefit you get after using 800 grit. But to me, that was saying go *at least* to 800 grit and if you want more beyond than, then it's personal preference.

I just did my HS yesterday. Started at 350 grit (they were out of 400), moved to 600, and then moved to 1000. It was my first time lapping and I actually would have felt comfortable stopping at 600 grit. My HS base was plenty flat and smooth by then. Much smoother than it was when it was shipped to me, at least.

For most uses you really don't need to go to more than around 1000 unless you want to see you reflection in it.

Thank You,
Daniel

IMOG said:

Rotating the HS or circular motions will give you a convex base and make HS to core contact worse.

Click to expand...

Many folks will tell you that unless you are very careful to keep even pressure on the Heatsink as you sand it, you'll end up with an uneven job no matter what "pattern" you choose to use...
If you choose to use the one direction method and lift and start at each pass, you still stand a chance of having it get uneven if you aren't careful with the way you apply pressure. Going in one direction is no guarantee of keeping it flat, and can actually be detrimental to your effort if you apply uneven pressure...
Aircraft mechanics apply the tried and true "figure eight" method, and car mechanics will usually use the circular method. Instructors from both schools will tell you to avoid using the one direction method because you can easily fubar the job by applying uneven pressure repeatedly in the same direction, which results in removal of more material on the front edge than on the back edge...thus the thought that multi-directional sanding using even pressure will give you better results.

I think that where most folks foul up is when you get impatient and try to apply more pressure in an effort to speed up the process...That's when you can apply uneven pressure and get yourself into trouble...Keep it even-handed and you'll be OK.

Everyone has to come to their own conclusions tho, so whatever works best for you...I'm a multi-directional sander, either circles or figure eights...sometimes both...

ok thanks

basicly it won't mess up your HS to go high, the only reason why getting it "too flat" would be bad is if you apply alot of AS3 and such. The thermal greeses are made to fill in the little gaps, and if you have almost/no gaps then it has nowhere to go and it justs sits in the middle and gets in the way by keeping the HS and CPU from completly touching. So just remember, the higher you go the less thermal you need.

The phrase "apply pressure", in it's many forms, bugs me because from my experiences in lapping, I never apply "pressure". I let the weight of the heatsink be the "pressure". I move the heatsink back and forth and all that good stuff, but I try to apply as little force towards the sandpaper as possible. Maybe this is because I've always worked with heavy heatsinks or I'm too scared to apply pressure because I'll screw the job, but this method has worked for me.

If my thinking is wrong, I'm always open for correction.

In response to Wolf's PM: You do not want to rotate the heatsink 90 degrees. Many people will tell you to, and many guides on the internet will tell you to, but this will cause the bottom of the heatsink to not be flat. That's not good.

I would lap with only 600 grit, but if you really want to go higher then I guess 800 grit would be fine. I would not go any higher than that. Lap until you are satisfied with the smoothness basically. Sometimes when you get a heatsink there are visible dents, ridges, or markings on the base and these are what lapping is intended to remove; sometimes you can even run your fingernail across the base of the heatsink and feel ridges from when the saw blade cut the metal.

A good way to gauge how much to lap is to color the bottom of the heatsink with a permanent marker... When lapping you can look at the bottom to see if you are applying even pressure and how much of the base you have sanded off. Once all of the marker is gone your base should be flat unless there were severe dents in it to start with.

Keep in mind that the marker on the front edge of the heatsink will wear off first, but this is intended - if you see this do NOT rotate the heatsink or then you will give your heatsink a convex base. When you sand anything, no matter what method you use, the front edge of the material you are lapping will always get the most sanding. If you use a circular or figure eight motion then this means that all edges of the heatsink will be the front edge through the motion, and the middle will be the only area that benefits less from the sanding - hence the convex base. If you use the straight method, then there is only one front edge and the base of your heatsink will remain flat, though it will have a slight slope.

Here is the important part: With heatsinks, it is better for heatsink to die contact if you have a flat slope than if you have a convex surface. It only makes sense.

Here is some more information on lapping from a couple people who have the experience and resources to accurately comment on this topic:

Graystar in respect to [url]http://www.systemcooling.com/modules.php?name=News&file=article&sid=449[/url][/i] [B]I believe that a lot of what this person had written is wrong. Lapping.... First said:

follow the procedure that Greystar described above
its the same that I use

one direction, lift and return - never rotate
close to impossible to actually get it flat by hand, I never have
(and I can, and do, inspect for such)

be cool

Click to expand...

Like Dago basically said though, to each his own... You gotta do what your comfortable with.

Here's a good rule (pun intended)

Buy a precision straight edge and lay it against the base of the HS to tell if it is dipping/bulging or not during lapping.

Cypress said:

basicly it won't mess up your HS to go high, the only reason why getting it "too flat" would be bad is if you apply alot of AS3 and such. The thermal greeses are made to fill in the little gaps, and if you have almost/no gaps then it has nowhere to go and it justs sits in the middle and gets in the way by keeping the HS and CPU from completly touching. So just remember, the higher you go the less thermal you need.

Click to expand...

Close. Actually lapping comes no where close to removing the tiny gaps that thermal grease is designed to fill. There will always be bumps and ridges in the base, so flat is a very relative term... But the point is that lapping is intended to increase the percentage of direct heatsink to die contact.

You are right that you don't want to use too much thermal grease, but lapping your heatsink will not cause the thermal goop to become trapped any more than it would normally be trapped, it will just squish out to the sides.