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 heatsinks, though, you can do better at keeping your CPU in the shape and structure that it needs to be.
Will Thermal Pads or Thermal Paste Mind the (Air) Gaps Better?
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 in exceptionally well. This thermal paste will not only hold for a very long time but will be a far superior option when it comes to the actual transfer of heat to dissipate through your heatsink.
Thermal pads vs Thermal paste, on the other hand, will still lay down nicely on this close-to-flat surface (similar to laying down a huge piece of plywood over my close to level ground); however, very small gaps will remain between your CPU and your pad. These small air gaps will slightly slow the transfer of heat (as air is not as conducive to transferring heat) making the pads a slightly lesser-performing product. Whichever solution you choose will ultimately leave you with a very close-to-level surface on which to mount your heatsink.
There is no such thing as ‘flat’ when dealing with CPU’s and there will be (air) gaps to mind.
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 heatsink) 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.
Consider What Your Design Needs to Last Through
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 heatsink 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 of each product. Thermal paste, CPU grease, thermal compound, or any of the other names it tends to go by, is a liquid format that of course adheres and solidifies after application and will be a longer lasting solution as well as a less expensive option.
If you’re looking for a less demanding heatsink mounting process on a product that will have a much shorter lifespan, then using a 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.
The direction you choose is up to you, but consider the options before you do so.
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. Thermal pads will always be less of a mess, but will also slightly underperform in the 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.