The Truth about CPU Soldering


Skylake delidding seems to be very common by now. Every day I read postings from people complaining about Intel and the thermal paste between IHS and die. Even tho Skylake is performing great, people are not satisfied with the temperatures on load. Compared to older generations there is conventional thermal paste between the IHS and the die while Sandy Bridge and older CPUs were soldered. Why did Intel change the production and is the thermal paste really that bad?

Solder what and why?

Nowadays CPUs are produced in the C4 flip chip package (Figure 1). The silicon-wafer is the base to create the CPU. Due to it’s monocrystalline structure it’s possible to create perfect layers on an atomic level. So after creating the integrated circuit on top of the substrate (Figure 1: bottom of the substrate), metalized pads will be placed on the chip as a connection. Solder balls will eventually connect the chip with the PCB.

The die itself will produce a quite large amount of heat compared to its size – that’s one reason why you need an integrated heatspreader (IHS). Another reason is the LGA socket. The Skylake PCB is about 0.78 mm thick and will bend if you only apply pressure on the die (Haswell PCB is around 1.17 mm thick). So you also need the IHS to keep the PCB straight for a perfect connection to the pins in the LGA socket.

The key is how to connect the IHS with the die for a perfect thermal connection. The semiconductor material is usually silicon and the IHS is made out of copper because of its good thermal conductivity of about 400 W/(m*K) – plus it’s reasonably affordable. There are several theoretical ways to connect both materials but in the real world you are limited by a lot of factors such as the maximum temperature the chip can sustain and the thermal conductivity of the joint-material.

Figure 1: C4 Package


Figure 1 shows how a current Intel CPU looks like (Ivy Bridge, Haswell, Skylake…). The substrate is connected via solder bumps with the PCB which eventually connects the CPU with the LGA socket. The underfill is necessary because PCB, substrate and bumps have different thermal expansion coefficients. Thus the underfill is protecting the CPU from destroying itself due to thermal expansion. The heatspreader will conduct the heat from the substrate to the heatsink which will sit on top of the heatspreader. The adhesive is flexible and will alow thermal expansion without damaging the CPU.

Depending on the type of CPU you can either use a normal thermal compound between substrate and heatspreader or connect both with a solder TIM.

How to solder silicon and copper?

You have to understand that silicon (Figure 2) and copper are completely different materials. Silicon (Si) looks like a metal but behaves and feels more like glas (e. g. SiO2). The thermal conductivity is quite good with about 150 W/(m*K) and the thermal expansion is relatively low 2.6 µm/(m*K).

Raw silicon []

Figure 2: Raw silicon []

Copper (Cu) on the other hand is a ductile metal with very good electrical and thermal conductivity. However, the thermal expansion with 16.5 µm/(m*K) is more than 6 times higher than silicon. You can solder normal copper wires with several tin (Sn) based materials such as Sn60Pb40 (Figure 3), which is a common tin alloy.


Figure 3: Sn60Pb40 solder []

 However, tin based solder won’t stick to silicon at all. In addition, solidifying tin will shrink by a large factor which results in a big thermal tension inside the material. This tension could already damage the substrate of the CPU.

Indium (Figure 4) is the only known material which can stick to both, copper and silicon. At the same time, solidifying indium doesn’t shrink much which leads to a small factor of thermal tension inside the TIM.

Indium []

Figure 4: Indium []

The thermal conductivity is not as high as copper but higher than any other thermal interface material 81.8 W/(m*K). Common thermal compounds have a conductivity of about 5-10 W/(m*K).

In addition, indium is very ductile which allows thermal expansion of substrate and heatspreader without damaging any of the components involved. Indium melts at 157 °C.

Before we start with the soldering details there are few things you need to know about indium. Similar to aluminium, indium forms a small oxide layer exposed to ambient air. Indium is a post-transition metal and very rare on earth. Compared to the worldwide gold production of about 3000 tons per year, indium is extremely rare. The world production of indium does not even reach 1000 tons per year. The average price of indium was about 800 USD per kilo in 2014. The price dropped now to about 400 USD, but the material is still pretty expensive. Depending on the size of the CPU, the raw indium will already cost about 2-5 USD.

CPU Soldering – The Real Deal

Most people have a normal soldering iron in their mind whenever they talk about soldering. However, soldering DIE and Heatspreader is a complete different story. Indium will stick to both, silicon and copper but the bond strength has a huge influence on the long duration performance and durability. Thus you need to prepare heatspreader and substrate before soldering.

The Heatspreader

Delidded CPU. Heatspreader on the left, CPU on the right []

Figure 5: Delidded CPU. Heatspreader on the left, CPU on the right []

The heatspreader (Figure 5) is covered in a nickel (Ni) layer. Nickel will act as a diffusion barrier to prevent any atoms to form an alloy with the copper. Indium also sticks to Nickel but not really well. So to improve the adhesion you have to apply another layer on top – preferably using a noble metal because they provide the best wetting conditions. Examples would be gold (Au), silver (Ag) or palladium (Pd). The melting point of silver and gold is quite similar at around 1000°C. The melting point of palladium is at 1555 °C which makes it much harder to apply on the heatspreader. So choosing between gold and silver, gold easily forms an alloy with indium and has great wetting conditions. So before you can think of soldering you have to apply a gold layer on the heatspreader. The gold layer has to be around 1-3 µm thick.

Heatspreader with selectively plated gold

Figure 6: Heatspreader with selectively plated gold

The solder

As described above, indium is the only material you can use. Depending on the shape of the indium you have to remove the oxide layer before soldering. This can be done by selective etching using hydrochlorid acid.

The indium layer has to be about 1 mm thick to provide enough material for thermal expansion without cracking after few thermal cycles.


The DIE is made out of silicon (Si) but you can’t solder directly to silicon. Otherwise indium would diffuse into the silicon which would result in a different doping characteristic or damage the chip over time. So you need another diffusion barrier on top of the CPU. The diffusion barrier is formed out of several layers made out of Titanium (Ti), Nickel (Ni) and Vanadium (V).

On top of the diffusion barrier you need another gold layer as wetting layer for the indium connection.

The Soldering Process

Let’s take a look at what we have before soldering heatspreader and DIE.


Figure 7: Stack before soldering process

The nickel plated heatspreader is sitting on top. The gold layer underneath the heatspreader will provide the best wetting conditions for the indium. The Indium solder sheet is sitting between heatspreader and DIE. The DIE is covered with 3 layers made out of titanium, nickel+vanadium and gold.

The soldering temperature has to be around 170 °C. Lower might result in a bad soldering connection and too high temperatures will damage the CPU permanently or result in a bad yield rate.

Some of the materials will form alloys during the soldering process. After soldering the result will look like this:


Figure 8: Stack after soldering process

You can see that gold, indium and nickel formed alloys with different thicknesses. DIE and heatspreader are now joined and ready to go.

So far so good. But there are a lot of negative aspects which we will discuss now.


Deformed Heatspreader


Figure 9: Solidifying indium is pulling heatspreader and DIE together

Keep in mind that you also have to glue the IHS to the PCB during the soldering process. Solidifying indium will shrink during the soldering process. As a result, DIE and heatspreader will be pulled together. The result is an uneven heatspreader which you might have read about before.


Typical failure mechanisms

Intense thermal cycling will damage the solder preform significantly. Tensile stress inside the solder preform will lead to voids (Figure 10).


Figure 10: Voids inside the solder preform, caused by thermal cycling

The voids will decrease the thermal conductivity and increase the thermal resistance (Rjc). Eventually micro cracks will occur which usually originate from the DIE corner (Figure 11).

Figure 11: Micro-cracks caused by intensive thermal cycling.

Figure 11: Micro-cracks caused by intensive thermal cycling. Total length of the shot is about 400µm.

The micro cracks will also decrease the thermal conductivity but will especially increase the thermal resistance at the corner of the DIE. Without the gold layer between diffusion barrier and solder preform, delamination of the solder preform would occure after few thermal cycles. Micro cracks occur after about 200 to 300 thermal cycles. A thermal cycle is performed by going from -55 °C to 125 °C while each temperature is hold for 15 minutes. The micro cracks will grow over time and can damage the CPU permanently if the thermal resistance increases too much or the solder preform cracks completely.

Void and micro crack occurrence is mainly affected by the solder area – thus the DIE size. Small DIE size (below 130 mm²) e. g. Skylake will facilitate the void occurence significantly. However, CPUs with a medium to large DIE size (above 270 mm²) e. g. Haswell-E show no significant increase of micro cracking during thermal cycling (Figure 12).

Figure 12: Thermal resistance (Rjc) is increasing after intensive thermal cycling. Small DIE is affected much more than medium size DIE.

Figure 12: Thermal resistance (Rjc) is increasing after intensive thermal cycling. Small DIE is affected much more than medium size DIE.

This failure mechanism is one reason why small DIE CPUs like Haswell-DT or Skylake are not soldered while the large Haswell-EP CPUs are soldered.

Skylake Soldering

I spent months and some $$$ on figuring out how to solder a recent Intel Skylake CPU. After reading all the above you can imagine that it’s by far not as simple as it sounds like. I’m not going to publish all info on Skylake soldering for now because I’m still waiting for the thermal cycling result. So far I can say that it worked great on air.

Figure 13: Soldered Skylake CPU tested with air-cooling.

Figure 13: Soldered Skylake CPU tested with air-cooling.

During Prime95 AVX Load the CPU core temperature is at about 50 °C. The CPU was cooled by a Prolimatech Megahalems with one fan at an ambient temperature of 24 °C. Compared to stock, the temperature dropped by about 18 °C. The result is pretty similar compared to liquid metal thermal compounds. However, liquid metal thermal compounds are not working subzero that’s why I’m trying to find a way to solder Skylake.

Skylake CPU with indium solder preform

Figure 14: Skylake CPU with indium solder preform

Skylake IHS with indium solder preform

Figure 15: Skylake heatspreader with indium solder preform

Thermal cycling is a significant problem for overclocking with liquid nitrogen because you go from +30 °C to -190 °C. Normal thermal compounds can’t stick properly to the DIE at e. g. -180 °C so extreme overclockers have a lot of trouble getting the TIM to work. The main reason is the different thermal expansion coefficient. Silicon, copper and compound have different coefficients and a sudden heat load can lead to separation of these components.

I sent a soldered 6700K to my friend Splave who is a famous overclocker in the US. Unfortunately the first LN2 test failed. At the moment I’m trying to figure out where exactly the solder preform failed – will keep you updated!



Whenever I read sentences like “What a ripoff – Intel doesn’t even solder a 300 USD CPU” or “Why does intel save 2 USD on soldering” I’m thinking


Stop hating on Intel. Intel has some of the best engineers in the world when it comes to metallurgy. They know exactly what they are doing and the reason for conventional thermal paste in recent desktop CPUs is not as simple as it seems.

Micro cracks in solder preforms can damage the CPU permanently after a certain amount of thermal cycles and time. Conventional thermal paste doesn’t perform as good as the solder preform but it should have a longer durability – especially for small size DIE CPUs.

Thinking about the ecology it makes sense to use conventional thermal paste. Gold and indium are rare and expensive materials. Mining of these materials is complex and in addition it’s polluting.

After soldering one of my 6700K CPUs I can tell it’s a pretty complex process. I’m still working on it and trying to make it available for extreme overclockers. However, I doubt that Intel will come back with soldered “small DIE CPUs”. Skylake works great even with normal thermal paste so I see no reason why Intel should/would change anything here.

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  • You could have a lot of use on gold printing , or gold electrodepositing , printing could be easier .

  • Jon Snow

    An excellent guide that was extremely informative but I have always had |another| arm-chair metallurgist cry about the TIM vs. Solder debate:
    “Why doesn’t Intel spring for some damned fine TIM if they are going to go that route!?”

    Can you speak to the quality of the TIM used as I have easily seen double digit temp drops when high quality TIM is used instead of whatever gunk Intel uses.

    • usercfg

      this makes me so wet ;–;

    • Ripio Suelto

      Consumer products are designed for high margins of reliability and durability, and since nothing is free, this is achieved at the expense of performance. Silicon TIMs such as Gelid Extreme, MX-4 Artic and others degrade when are directly exposed to the DIE temperature. The Intel TIM has a lower thermal performance, but a high resistance to degradation. That is all. If you want high performance, delid, liquid metal and periodic maintenance. That is for you and me, but not for the general public who thinks “I buy, I use, I forget everything else”

  • Sgt Bilko

    Great read and very informative, thank you :)

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  • Well said der8auer. This is very informative indeed and a lot of research was done in this article. A few years from now this whole TIM / solder thing would most probably be replaced by a new and superior way to cool a CPU. and Jon Snow lives to continue his journey hehe

  • Nick Dedman

    Is there any reason why they can’t make the gap between the die and the IHS smaller?

  • giggitypuff

    You also need to prove that the TIM Intel uses(both (pre-)Haswell and (post)Devil’s Canyon grade TIM) is much more reliable and long lasting than most of the aftermarket solutions(in trade of short term performance perhaps), because that is the main problem here. While you have nicely backed up with evidence, as to why TIM is preferred, it only answers half of the problem.

    • der8auer

      Intel is using Dow Corning TIM and Dow Corning already did long term tests with normal thermal paste. It basically shows that the performance might decrease within the first year but stays the same for years afterwards without any changes.

  • Luumi

    Nice work Roman asways!

    Send a soldered cpu to intel for RMA and ask their comment about it ;), maybe they get inspired by you and give you a new one.

  • TehUnit

    whoa. Nice work. I want to suggest that instead of solder why not use liquid metal and relid the cpu with its IHS? Wouldn’t that perform about the same as soldering minus the issues? just a thought

  • Vellinious

    Hmmm… they’re not cheaping out, but still use solder on the enthusiast and server CPUs. Considering that the Sandys are still some of the best overclocking Intel CPUs around, in part, due to the soldered TIM, I’d say your apologetic tone for Intel is not very well deserved. They earned the “cheap bastard” flag fully…..

  • magik

    I was interested in the problem of the diffusion of Indu, how do you make security nickel + vanadium and gold? Is it always necessary? If I do not I have the cover they will certainly damage the processor? and you may only need to use some other alloy may INDALLOY 1E 52% In, 48% Sn?
    thank you

  • Daniel

    Why plate the IHS nickel with gold when you can just lap it back to the copper?

    • der8auer

      You need nickel as a diffusion barrier. And gold for better wetting conditions to the nickel. Soldering directly to copper wouldnt be good.

  • Natulo

    Very interesting read! The passage about that Indium having direct contact with the silicon might damage it made me wonder about liquid metal paste. What is it made of and is it safe to use? (For the regular user, so no sub-zero OC involved.)

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  • Gordon Lenz

    This is awesome.

    If one was to make a custom IHS what would be the best materials to use?

    Copper as core
    silver as diffusion surface?

    I’m a machinist and was thinking of trying to make a better IHS for fun

  • FoxX

    der8auer, I swear, you are the only one I actually listen to when it comes to overclocking advice.

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  • Rcihmodo

    This essay is a lie.
    i7-E and E5/E7 still choose solder.
    So just intel wanna earn more.

    • Very good article, but I still have my doubts. Like mentioned before Intel still solders some high-end (workstation) and server CPU’s. These needs to be reliable as much as they can be also. So why no paste on these?
      Also, as far as I know, Intel didn’t mention this argument to all the questions (hate) there were when they started using paste. Why not?

      Really hope you answer this.

      Many thanks in advance.

  • Elipsus

    My point of view :
    In the past, Intel always used Indium.
    Intel still use it for Xeon processors, who are for the professional market, and thus aim for better longevity.
    I’ve never heard of processor failure due to Indium induced stress (in non LN2 applications)

    Intel just want to earn more.
    Intel is being a ripoff.

  • Ken

    I noticed that you didn’t put the silicon oxide layer in your diagrams. Shouldn’t the passivation layer on the cpu prevent the indium from migrating into the silicon?

  • Platypus maximus

    Is there some reason to not glue some water container using the CPU as one of it’s wall and let the cooling water flow through?

    Underfill will cover the pads (some epoxyde resin can be added probably), the PCB can be glued using epoxyde resins too, so there is way to change the CPU to cube with pipes.

    Would water damage the chip? If yes, would ethanol, silicone oil, pure mineral oil or pure gasoline too?

    For liquid nitrogen overclocking – is there some reason to not let the nitrogen to flow on the chip? Would it migrate into the chip’s structure?

    • HeresJohnny!

      You wouldn’t be able to dissipate that much heat through such a small amount of surface area. So basically the die would instantly shoot up over 100c and only a tiny area would be bubbling away. Even with water and liquid nitrogen you need a waterblock or a pot to spread the heat out so there’s more area for the water/ln2/air even to cool. Hope that makes sense :)

  • Bruno


    So the hypothesis mof iridium was to bound copper and silicon directly but after this is not the case as many elements were used in between. So there has to be a simpler solution.

    Also what about graphene or vaporchambers…

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