Heatsink Lapping Guide

[Mjölnir]

Heatsink Lapping Guide

What is Lapping?
If you buy a new heatsink, take a look at the base of it – you may see lots of parallel lines in a large arc shape right across the base. If you run your fingernail across these lines, you may notice that they’re a bit rough.

Lapping is the process of removing the roughness of these lines to provide a flatter, smoother surface finish for the purpose of providing greater contact area between the heatsink and the CPU - than the manufacturer’s machines are capable of producing.

Lapping is not a solution for poor cooling. It is an option for improving already sufficient cooling.

If you are looking for good cooling, search the forums and also ask people what they think. You will need a good quality heatsink, a good quality fan and good quality thermal paste.

Lapping isn't really as beneficial as it sounds. A few years ago thermal compounds were of a lower standard. They didn't conduct heat as well as todays componds do and they were less capable of seeping into even the smallest of gaps as moder themal compounds do.

Today's highest quality thermal compounds literally make the process of lapping redundant. Bear in mind it takes many hours for most pastes to cure properly and reach it’s full potential.


Why lap a heatsink?
To understand this, you have to understand the purpose of a heatsink in relation to a processor. On it’s own, a processor generates too much heat to survive, let alone operate properly. It needs some means of releasing the heat and introducing cool air. This is where the heatsink comes in.

Heatsinks are designed to conduct heat so that it can draw heat out of the processor or introduce colder temperatures, allowing the processor to run at cooler, safer temperatures.

The most effective way to transfer heat is by providing the biggest conduit or passage that you can. This means that the more surface area of the heatsink you have touching the core of the CPU, the better the capacity to cool the processor because the heat has a bigger passage to move through.

Removing the grooves in the base of the heatsink and making a nice flat surface allows for greater surface contact, and inevitably, greater ability to disperse heat through that larger “passage”.

Always use a thermal compound between the CPU core and the heatsink

Regardless of how smooth you make your heatsink, there will always be depressions or cavities in the surface of either the CPU core or the heatsink itself. Thermal compounds are designed to seep right into these gaps and maximise the contact area (the “passage”) between the CPU and the heatsink.

Top-of-the-range thermal pastes these days are highly efficient, being able to get into ultra-tiny depressions and cavities to conduct heat with almost zero resistance. Arctic Silver 5 is a good example.

As with any computer product, be sure to follow the installation directions for any paste you choose. Usually these instructions are on the manufacturer's website.

Why is this guide different?
There are many guides about lapping and quite often their directions vastly differ from others. Some mention suitable procedures and some don’t. This guide aims to provide “best practise” for all aspects of lapping. One example would be using a buffer to clear residue off the base when you have finished. This practise will actually leave residue on the surface. A chemical cleaning solution would ensure a cleaner surface.

This guide is based on preparation for applying small strain gauges to a flat surface. The same principle applies to fixing strain gauging as it does to conducting heat for processors. Maximising contact surface area. The only difference is that strain gauges require the contact area for adhesive (glue).

Unlike many guides, this one will only suggest that you require three grades of grit or emery paper. 400, 1000 and 2000. How do you compensate? By refining your technique. Pressure is the key.

[Mjölnir]

Ingredients:

Small Steel Ruler (needs straight edges)
400 Grit Sand or Emery Paper (the Wet&Dry stuff)
1000 Grit Sand or Emery Paper (the Wet&Dry stuff)
2000 Grit Sand or Emery Paper (the Wet&Dry stuff)
Small Quantity of Phosphoric Acid
Small Quantity of Ammonia Water
Small Quantity of Distilled/De-Mineralised water
Packet of sterile cotton buds
Packet of sterile Gauze
A vice
Lots of clean cloth
A source of running water
A source of light


Method:

A good trick is to use a small sheet/block of aluminium and practise on that before you start on the heatsink.

The lines in heatsinks aren't usually enough to justify 400 grit. Both the 400 and the 1000 are used purely to wear away bumps. The 2000 is mainly used to smooth and finish.

1.0 Mounting:
If you have a vice, great. Pad the vice up with lots of cloth and mount the base in the vice. You don't want to bend the fins and you don't want too much pressure on the base of the heatsink.

If you have a clamp with a micrometer fitted to it, this would be even better, as you would be able to accurately test for depressions.

2.0 Removing the grooves
Wet your more coarse Emery paper and sand in lines over the grooves. Ensure that your paper is constantly wet and does not wear too much. Use different sections of the paper to ensure that you are getting the same level of coarseness across your HS surface.

As stated before, you will probably not need the 400 grit paper. Start with the 1000 grit and see what effect that has. If it is still not enough, go up to the 400 grit.

Placing a sheet of sandpaper facing upwards on a sheet of glass, then sliding the heatsink back and forth across that is not a bad idea. It reduces the possibility of depressions, however it does introduce other problems. You risk applying too much pressure to one side of the heatsink, resulting in one side being thinner than the other. This practise is not suitable for 2000 grit, as the weight of the heatsink will provide too much pressure to produce the best finish. In any case, you shouldn’t press on the heatsink while doing this.

Sanding in circles with coarser grade grit can help you remove the lines faster and smooth out big depressions but you need to use lines using the 2000 grit.

2.1 Dealing with depressions
Once the machine lines start to disappear and the sanding lines become more obvious, wipe the surface dry and place the measuring edge of the ruler across the base of the HS and then point the heatsink towards your light. What you want to do is try and find gaps by seeing if any light is visible between your ruler and your heatsink. If you can see light, then you have a depression in the sink. Move the ruler to different positions around the heatsink, and do the same length-wise as well as across the width of the sink. The more places you test with your ruler, the better idea you will have of where your depressions are and where you need to sand in order to compensate for the depressions.

If you did happen to have a clamp unit with a mounted micrometer, using the micrometer would be a good way to double test your depressions.

You should be able to deal with the depressions using only 1000 grit paper.

Depressions can often occur as a result of your stance in relation to the sink, which in turn affects the level of pressure you apply to various parts of the sink.

Try standing directly in front of the heatsink or directly over it.

If you have deep scratches from wearing down the depressions, wet your 1000 grit paper and very gently run it across the surface of the heatsink. The reduced pressure will dig less and helps clear away deep scratches. You need it to be as smooth and as flat as possible before you move to the next step.

2.2 Getting it ultra smooth
When using 2000 grit paper, let the paper work the surface, as opposed to you working it. The idea is to apply a fairly light amount of pressure and reduce that pressure as you progress. Eventually you should get an ultra smooth finish. Applying too much pressure will result in scratches.

2000 grit paper should always be wet

The sanding process alone is the most time consuming part.

3.0 Cleaning the heatsink
You should never polish the surface, as this will leave residue that will interfere with the heat passage, since polishing materials are nowhere near as conductive as a good thermal compound.

After you have finished the sanding, and are happy with how smooth remove the sink from the vice, dry it off with some cloth. Now you have to clean the residue off it.

It may look clean and shiny, but wait until you see this:

Pour a small amount (maybe 1 or 2 sq/cm) of the Phosphoric Acid onto the surface. Use several sterilised cotton buds (one at a time) to scrub the surface. The buds get dirty very quickly, so be very liberal with them. Once they get dirty, get another one. They get dirty quickly don’t they?

Wipe the surface dry with a sterile gauze swab and then pour the same amount of the Ammonia Water onto the surface. Scrub that around the surface, again being liberal with the cotton buds.

When rubbing with the cotton buds, slowly twist it so that you use all of it and the dirt doesn’t concentrate on one part.

When wiping with the gauze swabs, use a sliding motion to avoid the same part of gauze covering too much area.

Use a NEW sterile gauze and wipe the surface dry. Repeat the Acid cleaning procedure and then repeat the Ammonia Water procedure, remembering to use NEW and sterile cotton buds and gauze swabs as you go.

Once you have wiped the heatsink dry after the second round of Ammonia Water, use the same scrubbing procedure to rinse your copper heatsink with the distilled/de-mineralised water to remove any residual ammonia water and then dry the heatsink with gauze and maybe a hair drier... Now it is ready for application. The longer you wait, the dirtier it gets.

Phosphoric acid is a weak acid (ie does not "dissociate" like sulphuris etc). It is used to halt rust in steel as it complexes with the Fe (iron) and prevents further oxidation. The idea of using a light acid, is that it will help burn away fine burrs left from the sandpaper.

Ammonia is a strong base and is quite volatile. It is a good cleaning agent and many household cleaners contain ammonia. Try and find Ammonia Water.

The ammonia will neutralise any excess phosphoric acid and as long as you remove any excess ammonia, your copper heatsink shoud be OK.

If you can't get your hands on Ammonia Water or Phosphoric Acid , you can try cleaning it with Isopropyl Alcohol. Preferably 90% or higher. Avoid Rubbing Alcohol - this contains other agents that are considered to be impurities.

Don't drink ANY of this stuff. It will kill you.

Many thanks to mule43, Jester, Blowin' Smoke, my Uncle and anyone else who contributed to this guide.