I used to work as a construction assistant where my primary responsibility was as a leveler of properties prior to the heavy concrete trucks laying down a level foundation. It was a tedious and taxing job, but something that is quite crucial for a building in order for it to withstand years of use and abuse. I would run through a spit of land with nothing more than a shovel ensuring that the ground is flat and level within an inch tolerance. One inch leaves very little room for error in my work. By the end of the day, you better believe I had that sucker down and looking like a site fit enough to build the great pyramids on.
But as perfect and flawless as I thought my work to be, there still remained the unavoidable one-inch bumps and dips here and there. Although at a micro-fraction of the size, your CPU will inevitably hold those tiny bumps here and tiny dips there, too. It’s the way of the world for a CPU to experience difficulties staying at pristine levelness. With the right understanding of mounting heat sinks, though, you can do better at keeping your CPU in the shape and structure that it needs to be.
Thermal Pad vs. Thermal Paste, Which Provides Better Coverage?
Realistically, there’s never going to be a perfectly level foundation—there are too many outside influences and factors that are entirely within our (practicable) control. But I still had to get it to a point for others to pour concrete all over and erect a fantastic foundation that will last for years to come. Now imagine that concrete (that will fill the small inconsistencies very nicely) as a thermal paste you pour over your CPU. This paste will fill all your micro-bumps exceptionally well. This is one of the advantages of using a thermal pad vs. thermal paste.
The thermal compound paste will hold for a very long time, and it will be a far superior option when it comes to transfer of heat to your heatsink. A layer of thermal paste will form a complete coating over the top of the component. Because it is a thick liquid, it will fill in all the gaps on the component and the bottom of the heatsink. This provides much better heat transfer to the heat sink.
Laird thermal paste (Image source)
Thermal pads, on the other hand, will still lay down nicely on this nearly flat surface (similar to laying down a huge piece of plywood over my close to level ground). However, there will still be very small gaps between the CPU and the thermal pad. These small air gaps will provide slightly lower heat transfer rate as air is not as conducive to transferring heat.
If you're still unsure if you should use thermal pad or thermal paste, both solutions will leave you with a very close-to-level surface on which to mount your heat sink. If temperature rise is a greater concern and you need that extra half a degree of temperature reduction, then go with the thermal paste. In components that consume much more power, such as a high-performance FPGA or CPU, using a thermal pad or thermal paste is not where the debate ends. You'll likely need to add a cooling fan to these components to ensure temperature is brought down to an appropriate level.
A heatsink thermal pad is much less messy than thermal paste.
Avoiding a Sticky Situation With Your Skillset
Continuing down our concrete analogy, our consideration goes far beyond simply ensuring we have level ground to mount our house (or heat sink) on. Laying down concrete is certainly a superior way to fill in the air gaps, however, it is just as equally a messy, sticky, and tricky operation. Similarly, when you apply thermal paste on your CPU, things may get a bit messy if you (or your assembler) aren’t well versed in the art. The composition of thermal paste is exactly what it seems: a liquid paste. Imagine trying to pour concrete with zero prior experience, what a mess!
On the other hand, laying down a thermal pad (again, acting like a large piece of plywood) is a much more straightforward operation which requires less operator skill. Thermal pads usually have a sticky film on either side and won’t require the steady hand that the paste demands for heat disipation. All it demands is for you to simply peel the backing off and place it on your CPU. While it might be good to practice using thermal paste, applying a thermal pad will be as simple as applying a sticker.
Thermal Pad vs. Paste: Durability
If you were to consider the longevity of your product post-build, your considerations would certainly include things such as if it will need to stand for 1000 years without any maintenance needed to the foundation, or does it only require a shelf life of 10 years? Thinking in terms of the life of a PCB, at times, our designs will be expected to last for decades to come under a variety of rough conditions. Applying thermal paste as a means to mount your CPU cores heat sink will absolutely outlast any pad placement and will be a much sturdier option in terms of wear and tear.
This is simply due to the material used in each product. Thermal paste, CPU thermal compound, or a thermal grease vs. thermal pad will adhere and solidify after application, whereas a thermal pad remains a spongy material. It will be a longer lasting solution and a less expensive option. This is particularly true under repeated thermal cycling.
Epoxy Thermal Compound vs. Thermal Pad
In contrast, a heatsink thermal pad will become brittle over time after repeated thermal cycling, at which point the heatsink can break off the component. One other thermal interface material that has a similarly short lifespan is a thermal epoxy, which dries to a soft plastic material. This has the same gap-filling capacity as thermal paste, but it has all the durability of a thermal pad. It will also become brittle over time under repeated thermal cycling.
Epoxy thermal compound vs. thermal pad: this IC has some leftover thermal epoxy (Image source).
If you’re looking for a less demanding heat sink mounting process on a product that will have a much shorter lifespan, then using a heatsink thermal pad may make sense for your design. The more expensive thermal pads, thermally conductive pads, or thermal interface pads are usually made up of a rubber material and less likely to go the distance in terms of the wear and tear factor compared to the liquid competitor.
Leftover Thermal Paste
Wrapping up the build of our house, and to conclude the mounting of our heatsink, we have determined a handful of useful considerations. Thermal paste is a far superior product when it comes to filling in your small air gap errors and will perform at a higher level when it comes to heat dissipation; however, it will certainly be a skill-dependent and possibly messy assembly step. The fact that thermal paste is more durable than thermal pads means that, if the heat sink ever needs to be replaced, the heatsink will be very difficult to remove. Some of the thermal paste will remain stuck to the component (see the image below) and it can be very difficult to remove.
Thermal pad vs. paste durability: you can see intact thermal paste on this component.
Thermal pads will always be less of a mess, but will also slightly underperform in the thermal conductivity heatsink ring. Thermal paste will also take the belt in the longevity of the life of the product compared to the thermal pad application simply due to the material they are comprised of.
Whether you choose to travel down the path of thermal pads vs thermal paste, you can be certain that each will provide ample support in your design. Using the proper PCB design software will enable you to take further considerations to the design and assembly phase of your product and Altium’s CircuitStudio® will be able to keep you informed enough to make a well-educated decision on your design choices.