Heatsink Lapping Guide

[Mjölnir]

HEATSINK LAPPING GUIDE

Introduction


1.0 What is lapping?

If you buy a new heatsink, look at the base of it – you may see lots of parallel grooves in a large arc shape right across the base. Lapping is the process of removing these grooves to provide a flatter, smoother surface for the purpose of providing greater contact area between the heatsink, thermal paste and CPU.

If you are looking for good cooling, you will need a good quality heatsink, a good quality fan and good quality thermal paste. Ask around the forums if you want ideas as to what might be best for you.

Lapping isn't always 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 almost make the process of lapping redundant. Bear in mind it takes many hours for most pastes to cure properly and reach their full potential. Many new heatsinks are released from the factory smoother than you'd be able to get them by lapping.

In recent times, some people have had impressive results lapping their CPU's. Not all CPU's are suitable for lapping, plus you void the warranty if you do lap a CPU. If you do want to lap your CPU, be absolutely sure that it's safe to do so before starting. Some of the practises described in this guide can be used for lapping a CPU, but I take absolutely no responsibility if you damage your CPU (or heatsink).

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

1.1 Why lap a heatsink?

To understand this, you have to understand the purpose of a heatsink and thermal compounf in relation to a processor. On it’s own, a processor generates too much heat to survive, let alone to operate properly. It needs some means of releasing the heat and introducing cool air. Enter the heatsink and thermal compound.

Heatsinks are designed to conduct heat so that they draw heat out of the processor and in some cases, introduce colder temperatures. Thermal compound simply bridges any gaps that exist between the CPU and the heatsink. By having a flatter and smoother surface, thermal compound contacts with more of the heatsink and CPU and less compound is required to bridge the gaps. Thermal compound is typically less capable of transferring and dissipating heat as most heatsink metals are, so the less compound heat has to travel through, the better. That said, regardless of how smooth the surface is, there will always be tiny cavities in any surface, so themal compound should always be used. Modern thermal compounds are made up of super-small molecules of highly (thermally) conductive material, allowing them to better penetrate the smallest of gaps with minimal thermal resistance. Arctic Silver 5 is an example of this type of thermal compound.

The most effective way to transfer heat is by providing the biggest and most direct conduit or passage for heat transfer possible. This means that the more surface area on the heatsink and CPU available for contact, the more heat that can be transferred.


1.2 How is this guide different?

There are many guides about lapping with different instructions to each other. Some mention suitable procedures and some don’t. This guide aims to provide “best practise” for all aspects of lapping and explain some of the principles behind certain techniques.

This guide is based on the same methods that professionals use to applying strain gauges to flat surfaces. Strain gauges don't require heat transfer, but they do require maximum contact area.

[Mjölnir]

Ingredients:

• 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)
• Isopropyl Alcohol (90% purity or higher)
  (or Phosphoric Acid and Ammonia Water)
• Distilled/De-Mineralised water
• Packet of sterile cotton buds
• Packet of sterile gauze
• A bench mounted vice
  (or a solid bench and smooth block of glass/perspex or laminated wood)
• Clean rags/cloth
• A source of light

[Mjölnir]

Method:


2.0 What's the best way to lap?

There are two common ways to lap a surface. One is to mount the object in a vice, the other is fix the grit paper to a flat, smooth surface. Ideally you should to be able to get your centre of gravity over the heatsink/CPU. If you're standing beside where you're sanding rather than standing over it, the pressure you apply will almost always become uneven as you push the sanding block or object away from your body. This is how the edges and corners of lapped objects can end up being sanded down more than the center - leaving the object convex rather than flat. At least if your head and shoulders are directly over the object, applying even pressure with your thumbs and forefingers to the sanding block or object should be much easier, reducing the chance of rounding the edges.

Whether you use a bench or a vice, a good height for the object or sanding surface is between waist and chest height. If it's too low, you'll probably feel the strain on your back and if it's too high you might not be able to get your body over the sanding area. In any case, the bench should be stable. If it moves when you're trying to sand, it could cause uneven sanding.

A good trick is to practise on a small block of aluminium before you start on the heatsink.

2.1a Using a vice
If your heatsink has a fairly chunky base, it should be OK to mount into a vice. However, if the base is pretty thin or the heatsink has heatpipes or the fins protrude a great deal, forget about the vice. If you do use a vice, pad it up with a fair bit of cloth and then mount the heatsink at the base. You don't want to bend the fins, so think carefully about which way you're going to clamp it and where the pressure from the vice is going to be applied to the heatsink. Try and get the heatsink as level as possible.

Once you have it mounted, find a smooth block of wood or meta to wrap some paper onto as a sanding block - or buy a sanding block. It will be much easier to grab and will also provide more even finish than just using the grit paper in your hands to directly apply pressure.

Don't ever mount a CPU into a vice!

2.1b Using a flat surface
Many guides suggest using a sheet of glass. The main principle behind this is that the glass is smooth. Glass can still flex, so it should still be put on a solid flat surface. A laminated sheet of plywood or a thick sheet of plastic/perspex should be just as suitable as glass. I find that laminated wood is more practical because you can't shatter it when clamping it to the bench.

When you do use a flat surface, make sure that when you tape/clamp the grit paper down, it is taut in all directions. If the paper is loose, the object is more likely to catch and jump as the paper ripples.

[Mjölnir]

Shaping the Surface


3.0 Removing the factory grooves

The grooves in heatsinks aren't usually enough to justify 400 grit, but it's worth having some just in case. Both the 400 and the 1000 are used purely to wear away bumps. The 2000 grit is mainly used to smooth and finish. Wet your Emery paper and sand in straight 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. You will probably not need the 400 grit paper, so start with the 1000 grit and see what effect that has. If it is not enough, go up to the 400 grit.

Sanding in circles with coarser grade grit can help you remove the lines faster and smooth out big depressions but for the finer grade grit, you need to use straight lines to get the best results.

The sanding process alone is the most time consuming part.
Be prepared to spend plenty of time on this to get it right.

3.1 Dealing with depressions

Once the lines from factory machining 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 heatsink and then hold the heatsink and ruler up to your light source. If there are gaps present, the light should be clearly visible between the ruler and the heatsink.

Assume length and width a represented as X and Y axis, move the ruler to various positions on X axis and do the same for Y axis. 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 there is warping, also check the diagonals. A good way to iron out warping is to sand diagonally as well. You should be able to deal with the depressions using only 1000 grit paper.

If you're not sure about how straight the metal ruler might be, use two rulers and use different sections of the edge of each ruler. If you did happen to have a micrometer mounted to a clamp, that would be a good way to double test for depressions.

Sometimes you might end up with deep scratches from sanding. Wipe the grit paper down to make sure there aren't any beads of grit rolling around between the two surfaces and then wet your 1000 grit paper and gently run it across the surface of the heatsink. The reduced pressure will dig less and helps clear up deep scratches. You need it to be as smooth and as flat as possible before you move to the next step.

There will always be some light between the ruler and heatsink.
You be the judge of how much light is acceptable.

[Mjölnir]

Smoothing the Surface


4.0 Removing the sanding lines

Once the machine lines are gone and the heatsink surface is flat, you can start smoothing out the sanding lines. When using 2000 grit paper or higher, let the paper work the surface, as opposed to you working it. The idea is to apply a light amount of pressure and reduce that pressure as you progress. The pressure should amount to no more than the weight of the heatsink or sanding block. Applying too much pressure will result in scratches. Eventually you should get smooth finish.

For best results, wet the grit paper.

Some sources suggest that you should end up with a reflective surface and to put the heatsink onto a sheet of graph paper to spot any uneven spots on the surface. A couple of things to note - A reflection with sanding like this will probably only be visible when the surface is at an angle to your line of sight. The surface needs to be flat and smooth, not polished and reflective. Secondly, if using graph paper to spot unevenness on the surface, you're wasting time and fine grit paper. If the surface is uneven, you'll have to go back to the shaping stage and then start the smoothing stage all over again. Checking for grooves should be done before you smooth the surface (when you're shaping it), not after.

[Mjölnir]

Cleaning the Surface


After you have finished the sanding, and are happy with how smooth the surface is, wipe it down with a clean rag. There are two cleaning and sterilising methods described. Using Isopropyl Alcohol is probably the easiest and safest method, since Isopropyl Alcohol is easy to obtain and it evaporates quickly.

Some guides suggest polishing the surface to make it nice and reflective. Don't do it. Polishing materials are designed for polishing and not for heat transfer. They're nowhere near as conductive as a good thermal compound and the polishing material will inhibit the flow of heat.

Cleaning the surface should be done in a fairly dust free environment.
Don't use the same room that you used to sand the surface.

5.0a Using Isopropyl Alcohol

Dampen a clean bit of gauze with a small amount of distilled water, and wipe it down. This rinsing process should pick up any excess residue from the sanding process. After "rinsing", use high concentrate (90%+) or pure Isopropyl Alcohol to scrub it by wiping it several times. You should only need to drop a tiny amount of alcohol onto the surface. Each time you wipe, use a different section of the gauze or a use new gauze so that you're not putting the same residue that you've removed back onto it.

Repeat the scrubbing process two or three times. Each time you wipe, make sure you're using a clean section of cloth. A good way to do this is to roll the cloth with your finger as you move it across the surface. It may take more than two or three attempts to get it fully clean and that should be apparent if the gauze is a bit dirty after each wipe.

Once it's clean, give it a minute or two to fully dry up before mounting the heatsink to the CPU.

Try not to touch the surface with your fingers when you're done.
Your skin will leave grease on the surface which will also inhibit heat flow.

5.0b Using Phosphoric Acid and Ammonia Water

The idea of using this combination is to combine two fairly aggressive cleaning agents that will both clean the surface well and neutralise each other. The surface may look clean and shiny now, but wait until you see this:

Pour a small amount (maybe 1 or 2 drops) of the phosphoric acid onto the surface. Use several bits of gauze (one at a time) to scrub the surface. They get dirty very quickly, so be liberal with them. As you move the gauze across the surface, roll the gauze with your finger so that you're using clean sections and not smearing dirty stuff back onto the surface. Wipe the surface dry with a dry gauze swab and then put the same amount of ammonia water onto the surface. Scrub the surface, again being liberal with the gauze and again using the same rolling motion with your finger.

Use another dry gauze swab and wipe the surface dry. Repeat the acid cleaning procedure and then repeat the ammonia procedure again. This time you want to be more careful about not wiping the surface with dirty parts of the gauze. Once you have wiped the heatsink dry after the second round of ammonia water, rinse your heatsink with the distilled/de-mineralised water to remove any residual ammonia water and then dry the heatsink with gauze. Make sure you use the same rolling motion with the gauze (you get the idea).

Let it sit for several minutes to dry or if you want overkill, get a hair dryer... Just hope no nits get jet-propelled onto the surface. Once it is dry, it is ready for mounting. The longer you wait, the dirtier it gets.

Try not to touch the surface with your fingers when you're done.
Your skin will leave grease on the surface which will also inhibit heat flow.

6.0 About the Chemicals

Alcohols are best because they evaporate quickly. Isopropyl alcohol is particularly good because it has a fairly neutral pH level. It should be available at chemists or some hardware stores. Be wary of what other agents might be mixed in with it (such as in rubbing alcohol). Some of these agents may not evaporate properly and can inhibit heat transfer. Ethyl alcohol (commonly found in methylated spirits) can be used as a substitute for ispropyl alcohol, but like ispropyl alcohol it should be as pure as possible. Ethyl alcohol may take slightly longer to evaporate than ispropyl alcohol.

Ammonia water is a strong base with the right alkalinity to neutralise phosphoric acid. It is a good cleaning agent and many household cleaners contain ammonia. Ammonia can melt copper, so I like to use it first on copper heatsinks and neutralise it with phosphoric acid. Phosphoric acid is a weak acid (ie does not "dissociate" like sulphuris etc). It is used to halt rust in steel and can be found in many rust prevention solutions. The idea of using a light acid, is that it will help melt away fine burrs left from the sandpaper. As long as phosphoric acid and ammonia water are neutralised and any excess is rinsed off, there should be no adverse effects on the metals or the thermal compound.

Please don't drink ANY of this stuff. It will probably kill you.

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