Heat sink and thermal paste

I have a question that I need some help on. I cannot seem to find a straight answer anywhere. Hopefully someone here can help me out.

If you remove the heat sink from the cpu for any reason, do you have to reapply the thermal paste every time you reassemble the heat sink back?

Lets suppose a fresh thermal paste is applied and the heat sink is installed. After 4-5 months, if for some reason you need to remove the heat sink, do you have to clean the old thermal paste and apply a new one? Does separating the heat sink from the cpu, cause the existing paste to not perform as well as before?

 

I always do. There is nothing quite like that first bond formed when the retention clip pressure works with thermal cycling.

 

You should, but you can get away with not doing it many times. Once the heatsink is removed, the contact between the surfaces is no longer the same, dirt/particles are deposited on the paste layer at both surfaces, the surface of contact is no longer symmetric, etc.

As a result of all this, the thermal conductivity coefficient is changed and it will no longer be that effective. Thus, it is recommended to re-apply thermal paste when removing the heatsink and installing it again (or installing another heatsink). You must also take care to remove the old paste when you do this.

 

you should ALWAYS replace the thermal paste without a doubt about it, its cheap enough to get high grade stuff. you can get mx2 or even better for like 3$ a tube from many places and this one small tube will give you many uses. you only need a TINY amount to do it properly. my all time fave was ocz freeze which is not made now but mx-2 and mx-3 are awesome. my current go tube thermal paste and current fave is the Prolimatech pk-1 and they also offer pk-2 and pk-3 but the pk-1 is cheap and works awesome.

 

It is recommended to replace the thermal paste but not really like do it or blow it. Hahahaha
On my previous processor (Intel E2180) i used the stock fan with the stock thermal paste and during the 5 years i used the CPU i removed the heatsink at least 3 times with no problems at all. The CPU ran fine all the time and i never experienced a single problem. Keep in mind that most of the time i was running in at stock clock speeds but sometimes i would OC it to 3.0GHZ to play some games and it never gave me any problems. (Default is 2.0GHZ)

 

Well, as long as it has good consistency, you can just re-spread it. I've done this many many times over many years and my idle and hi-speed temps have remained exactly the same.

If it starts "drying out", then you replace it.

 

I agree with zfactor. Thermal paste is so cheap that there's no reason not to re-apply it. Better safe than sorry.

You can clean up the old gunk easily with alcohol. When you apply the new paste be conservative, you only need a very thin layer. This stuff isn't what makes the contact, it just fills in the air gaps between the two surfaces created by pitting and other small irregularities.

 

YES! ABSOLUTELY! NO EXCEPTIONS!

Well - there's one exception. If you applied the TIM (thermal interface material) and NEVER - not even once - powered the system on, then you can remove the heatsink and remount it without reapplying TIM.

This is because as soon as you apply power, the CPU will immediately begin to heat up and that heat starts the curing process of the TIM and it "sets" the bond between the mating surfaces.

Once the TIM begins to cure, it cannot be reused.

Now there are some TIMs (like pure silicone "grease") that don't really cure. But most of the modern TIMs are compounds that do cure during the first several heat/cool cycles resulting in a drop of a couple degrees. And unless you know exactly which TIM was applied, don't risk it.

Remember, the absolute best heat transfer occurs between direct metal-to-metal contact. But Man cannot (yet) create perfectly flat, imperfection-free mating surfaces so the purpose of TIM is to fill in all those microscopic pits and valleys in the mating surfaces. Any excess is in the way and counterproductive to the heat transfer process.

So if you still have a bunch left over to re-smooth, that suggests to me you had too much in the first place.

BTW - TIM does not "go bad" or periodically need to be replaced. It will easily last 10, 12 or more years AS LONG AS the cured bond is never broken. It may get a little dry and crusty around the edges where it is exposed to air, but that has no effect on the cured bonded TIM, or its effectiveness between the mating surfaces.

 

Thank you for all the responses. So, the overwhelming opinion is that I must replace the thermal paste every time I remove the heat sink from the cpu. The problem I have is that I have to clean the heat dispensing fins of my laptops heat sink every 5-6 months, as they usually get clogged with a carpet of dirt. The only way to do that is to remove the entire heat sink assembly, which uncovers both the cpu and the gpu.

You can look at this attached link, where someone has posted instructions to clean the heat sink for my laptop Dell XPS M1530, to get an idea of what I mean. Scroll down to step 6 and see the carpet of dust clogging the heat sink. Mine looks exactly like this after 5-6 months and the only way to clean it is to disassemble the whole heat dissipating assembly as shown in the instructions.

By the way, I am using Arctic silver 5 and it has a cure time of 200 hours. With my typical computer use, it probably takes about 1-2 months for it to finally start working optimally and then I have to remove it after only 5 months. It just seemed a waste of good thermal paste.

My reasoning was that once the thermal paste has filled the imperfections (hills and valleys) in the cpu and the heat sink, and so provides optimal contact between the two surfaces, it should not matter if the heat sink is then temporarily removed for a short while, as there would be no chance of air or dirt to get trapped in between the surfaces.

Furthermore, since it is a laptop, so I have to use the surface spread method, recommend by the manufacturer, where you spread the Arctic silver with a credit card over the cpu and gpu. It is a very time consuming process to clean the old paste and reapply the new one. I cannot use the dot in the middle approach, as I cannot move the heat sink clockwise or counter-clockwise over the cpu, so the paste will never spread evenly over the entire surface.

In any case, thanks for the responses. I guess, I will have to replace the thermal paste, as I do not want to cause any damage to the cpu or the gpu.

 

You don't have to remove the heatsink assembly as shown in that tutorial to get the vast majority of dust out. Once you disassemble the notebook enough to see the CPU fan, stick a wooden glue stick in the fan to keep it from spinning, then blast the dust out with a (properly filtered) air compressor. If you don't have an air compressor setup for cleaning electronics, you can use a can of compressed dusting gas. Note I did not say "canned air" - that's because it is NOT air, but a poisonous gas. So take it outside regardless the method - to keep from breathing that stuff, and to keep the dust out of the house and ready to be drawn back into the computer.

I actually would recommend NOT removing the CPU fan. For one, you have to mess with cleaning the mating surfaces and applying a new layer. That's a PITA but more importantly, you risk destroying your CPU with an ESD (electro-static discharge).

Well, two problems. (1) You break the cured (or partially cured) bond and (2) when you disassemble the HSF and expose the TIM, it is likely some of that dust will get stirred up and settle on the TIM. And that dust may as well be pebbles in the way of optimal surface-to-surface mating.

Right - so don't remove the HSF and you won't have to mess with it. Just blast the dust out.

Ummm, as seen here the cure time is 50 to 200 hours. So 200 is the extreme case and it depends greatly on the thickness when applied. Since you should be applying it as thin as possible, your numbers are likely closer to 50 than to 200. And I must point out, that is the time it is cured - not the time it starts to cure, which will begin during the first heat-up/cool-down cycle.

 

You should always clean off the old thermal paste and reapply the thermal paste. Not doing so may cause your CPU to overheat and destroy it. I have built many machines.

 

I should change my wording from may to most likely will fry your CPU. I'm on my iPhone at work so I can't tell you the procedure for doing it but you may find some good material on YouTube. It's fairly simple.

 

If you read back through the entire thread, we are way past that. Raza knows how to clean the mating surfaces and apply a fresh new layer of TIM.

 

Well, if there is a way to clean the fan and the heat sink without disassembling the entire assembly I would definitely go for it. As I mentioned before, the thermal paste is fine and is doing a good job. It has been only 6 months since I last applied it. However, the heat sink vent at the back is almost 95% blocked, as I can barely feel the air when I put my hand in front of it.

I do not have an air compressor. I do have a full size blower, but I am not sure if it will be able to dislodge the dust as it does not have a small enough nozzle to fit the small gaps for the fan. I read somewhere the someone recommenced to attach a straw in front of a vacuum cleaner and vacuum the dust out, but I could not get it to work.

If someone has an idea on how to get the dust out without disassembling the heat sink and without an air compressor, as I do not have one, please let me know.

Here is how my heat sink and fan assembly looks like. See in step 4 and 5, the heat dissipating fins are very hard to reach without opening the fan assembly.

Meanwhile I will try the vacuum and straw approach one more time to make it work.

Thanks.

 

I did. You can buy cans of compressed dusting gas at most any electronics or even home improvement store. And what I do, and would do with yours, is disassemble only to the point as shown in Step 2. The fan blades are easily accessible with a can of compressed air from there - without taking the cooler apart.

I am not a fan of using vacuum cleaners although on rainy days, that is what I use. The problem with vacuums is ESD. As the air and dust particles zip past the nozzle, many bang into the nozzle and generate many 1000s of volts of static electricity. And because you need to get in much closer with the vacuum nozzle, the risk of a discharge (especially at such high potentials) is much greater.

It is important to note ESD sensitive devices (CPUs, RAM modules and most other ICs) can easily be destroy by static discharges so small, they are below the threshold of human awareness. So you can easily destroy your CPU without even realizing a static discharge occurred.

So if you must use a vacuum, I recommend doing it this way. Wrap your hand around the end of the nozzle, extending one finger out front. As you draw the nozzle in close, plant that extended finger onto bare metal of the case interior (another challenge with notebooks! ) to discharge any static in the nozzle and your body, and to prevent any buildup of static. Then use a natural fiber (not nylon or other static generating synthetics) soft brush to gently stir up the dust towards the nozzle.

Oh, and for the record, I applaud you for being concerned about heat and taking action to control dust build up.

 

i actually have a small air compressor (5 gallon) for this very thing in our shop. i do use a water filter in the line so no moisture gets sprayed onto the board or otherwise and make sure there is no oiler in line for this type of thing. but i have it set to a lower psi around 50 on the dial and use a air sprayer nozzle on the hose. it cleans all but the absolute stuck on dust or grease etc... never damaged a thing and works fast. those cans of compressed air sometimes are no where near strong enough to get between the heatsink blades for my tastes. and to clean a heatsink with a fan i simply gently hold the fan in place while im blowing out the dust. this also works awesome for power supplies where you can blow air through them they come almost spotless when done. been doing this for YEARS never a single issue and even back when i worked in robotics this is what many guys including myself used to clean out dust when they did not want to take projects apart just to get the dust out.

 

Exactly!!! Moisture WILL condense in the tank as the heated air created during compression cools. This moisture runs down the sides of the tank interior, collecting rust and other contaminates along the way, and then pools at the bottom of the tank. From there, really nasty, contaminated water droplets can be picked up during the turbulent air flow process and spewed out the nozzle - if you don't have a decent inline moisture and particulate filter in place.

Right! But to ensure no static problems, that's where I use a wooden stick to hold fans stationary. Even with cans of compressed dusting gas, you can spin the fans faster than designed limits - great for destroying the bearings.

I have my tank set to 80PSI. I used work in air traffic control radio repair facilities and we had compressed air piped through the walls for blasting out the communications equipment and 80PSI was standard. Of course the pressure at the point of contact is very dependent on the distance between nozzle and part.

I agree the canned dusting gas does not provide much pressure, but they are better than disassembling where handling components introduces potentials for damage.

 

Here is an update on this issue. I opened the back cover of my laptop to expose the fan assembly, but did not disassemble the fan or the heat sink. Then I used a thin flexible plastic nozzle, the kind that comes attached with WD-40 cans for precision spraying. The one I used was an extra one and had never been used for oiling. It will be not a good idea to get oil on the heat sink fins.

So, I carefully inserted that flexible straw thingy, through the fan opening to reach the inside fins of the heat sink to disturb the carpet of clogged dust. It was difficult as the area is hard to reach. Once the dust was partially disturbed, I then used good old lung power to blow air from the outside of the heat sink, with dramatic effect. It blew dust derbies all over the work table and carpet. So, I recommend you do it in an area, that can be easily cleaned. Make sure you hold the fan while you are doing it as the fan will spin with air pressure and blow the dust back into your mouth. I accidentally inhaled a good bit of it the first time. I then blew air into the fins multiple times and then used a Q-Tips to thoroughly clean the fan blades and the area around the fan and the inside of the heat sink fins.

The result was excellent. I can now feel a gush of air coming out when the fan runs and both my cpu and gpu are idling 10 degrees lower. This is much better than disassembling the entire heat sink, removing and reapplying the thermal paste etc. I believe if one has an air compressor, with a small enough nozzle to fit in the area, this process will go much better.

As far as my original question is concerned, it is the overwhelming opinion here that if you remove the heat sink from the cpu or gpu for any reason, you should always remove the old thermal paste and reapply a fresh layer of thermal paste before putting the heat sink back.

Thanks to everyone who responded.

 

That's why I said earlier to take it outside, regardless the method used!

And much safer too as it minimizes the potentials of ESD damage, breaking mounting mechanisms, improper remounting, improper cleaning of old TIM and improper application of new TIM.

Right. This is because the old TIM starts to cure immediately, once exposed to air and to any heat/cool cycles. Plus, as you have seen, dust goes everywhere once disturbed and you don't need that settling on, contaminating and compromising the TIM.

As an electronics technician for 40+ years, there is no doubt that is true. But again, it is absolutely essential any compressor you use on electronics have a moisture and particulate filter as shown above.

Plus, if you have an air compressor with the necessary nozzles, the very first time you wake up to flat tire on your car, and you can fill it up with air so you can drive to tire shop instead of dragging out the dirty jack and spare, and changing the tire in the drive way, you will be very happy you made the investment.

 

@Bill_Bright,
Sorry if I am highjacking the thread but I have one question. I was under the impression that it is never a good idea to spin up the fan blades with compressed air because the current generated would damage components or am I wrong? I always block the blades from spinning when I clean off the cooling and CPU fans, and the PSU fans.

 

Unfortunately I live in Minnesota. It was -11 F outside a couple of days ago. Quite pleasant today. From pleasant I mean it is 21 F. Had I taken the work outside, a technician would have had to come to rescue me

For those who operate in Celsius
-11 F = -24 C
21 F = -6 C

 

You are not hijacking but note you really should have read through the thread first for you missed where I addressed that very point - twice (this year no less! ) - and I even included a link to the wooden sticks I use to hold the fans stationary while blasting just to prevent them from spinning beyond design limits and destroying the bearings.

Don't be a wuss! I live in eastern Nebraska and we had got that arctic blast the day before you! The cold is just all the more reason to block moisture with the appropriate filter, and have a battle plan before opening the door!

If still not persuaded, consider this! What is house dust? Much of it is dander - dead skin and billions of microscopic critters that thrive on that dead skin, and all the fecal matter they leave behind. Do you really want to toss that back into the air and suck it into your lungs?

 

God, I am getting too old. Completely missed that. Thanks for the reply and the tip on the wooden sticks. If you can believe it, I actually save Popsicle sticks.

Edit: I can assume that if the CPU heatsink uses thermal tape, it must also be completely cleaned off and new thermal paste applied, correct?

 

yes 100% the tape should also be removed and replaced with either new thermal paste or new tape which i hate and dont recc. never re-use any thermal material EXCEPT the small soft pads that almost look like small foam pieces can be re-used if you are very careful with them and are kept CLEAN. but even then i replace those but they can be re-used if needed. these are found a lot of times on things like ram chips on graphics cards. and those should never be used for cpu's.

something that works easy is a plastic credit card cut down into a small scraper type tool this will not scratch the surface of a desktop cpu and scrape off most of the tape if its stuck on. do not use this however on a cpu without a heat spreader since it can cause fine scratches. also when done for the normal user i recc a simple 90% iso alcohol and some decent paper towels do not use tissues or toilet paper etc due to the dust they put off. a decent paper towel will give off very little dust which is why a lot of car window tinters use it to clean with (i used to tint windows when i owned a car audio shop).

and yes as me and bill said always hold the blades still unless using very little pressure of air. i dont use the wood though i just hold the blades and have never once had a issue with static from it in more than 26 years of doing it that way. but if it makes you comfortable go for it. it cant really hurt either just be easy with the plastic blades.

 

No problem. Yeah, I'm over 60 now and just got my first pair of trifocals. I'm missing walls, stairs and youthful eyes. And yeah, "clean" Popsicle sticks work great and I used to save and use them all the time until I discovered Hobby Lobby and other craft stores sell bags of sticks. I use them for all sorts of things.

NO! That is incorrect. As said over and over again, once they have been used even once and gone through even just one heat/cool cycle, the curing has begun. And once ANY curing has started, you MUST replace the TIM if the HSF is removed - assuming the goal is maximum heat transfer.

The ONLY EXCEPTION, also noted above, is if you apply the TIM and NEVER, not even once, apply power. As the CPU will immediately warm up above ambient temperatures and start the curing process.

Note the pads commonly used on RAM and some GPUs, and often chipsets too, are typically "adhesive TIM" - that is, they are thermal interface material, but also are used to, in effect, glue the heatsink to the device when no mechanical clamping mechanism is used. While the adhesive qualities may still be effective, I must EMPHASIZE again - and hope this is perfectly clear to all, if the protected device has had power applied - even once - that is enough to cause the device to heat beyond ambient temps and start the curing process. And once the curing process has started, you must NOT reuse the TIM. PERIOD!!!

Can you use old TIM? Yes. You can use 100% water instead of 50/50 water/coolant in your car's radiator too! But should you? NO! If no hard cured chunks are in the way, it will likely be better than no TIM at all. But it will never be as effective as a properly applied, fresh new layer of TIM.

Yes. Note the TIM that typically comes applied to HSFs from the factory consist mostly of very refined, specially formulated paraffin - wax that melts super fast and is squished out of the way (due to the pressure of the heatsink's mounting mechanism) distributing the TIM. These pads are actually very good TIM - but not as good as some of the silver or ceramic based TIM.

Yeah, I do too. Note most isopropyl alcohol is around 73 - 76%. You can use it, but it can leave a film behind where the 91 - 93% does not. Acetone also works great - but the fumes are very hazardous and combustible.

I do not recommend the use of any paper towel. There's no such thing as dust free paper towel. Paper dust is microscopic chunks of wood! Not something to interfere with looking through glass but something that certainly can interfere with maximum surface to surface contact. So instead of paper, I recommend a microfiber cloth used to clean eyeglasses, or a clean 100% cotton handkerchief. And I also give the surfaces a quick blast of compressed air just before applying TIM and mounting too.

And yes, you can use your finger to hold the fan blades still - if careful, but in tight places, your hand may get in your way. And by using a stick, you can often free up both hands - perhaps to hold cables out of the way with one. And even my boney fingers are not skinny enough to reach in and hold PSU fan blades still.

A thumbnail works fine to scrape off the majority of old TIM without the risk of scratching. Then the isopropyl alcohol and lint free cloth to get the rest.

To apply new TIM, I cut off the end of a plastic cotton swap, then bend the stick about 1/2" from the end to make a little hockey stick/TIM spreader. Then I use that spreader to evenly apply the TIM across the die.

Well, I won't hold your youthful exuberance and lack of experience against you!