Thermal conductivity: Compound, Sil-pad or nothing?

Why use anything?   

Good thermal conductivity is important to the dissipation of unwanted heat from active power components and also important to the application of thermal platforms for testing electronics.

Perfectly flat, parallel and particle-free surfaces might show better thermal conductivity directly pressed together, however, some of the real-world realities generally prevent that best-case best performance. Here are a few reasons to use thermal grease:

• Metal surfaces that appear to the eye to be perfectly flat, free of voids and parallel are usually are not.
• Electronics like cleanliness, not all products can be built in clean rooms. A small unseen particle between two surfaces can easily prevent them from mating properly
• Keeping even pressure on two surfaces is not always as easy as it might seem.  Keeping an appropriate amount of pressure on the two surfaces is important to transfer heat. Grease fills voids caused by uneven clamping pressure.

Thermal grease to the rescue?

So – given that close enough to perfect flatness, cleanliness, and parallels is hard to achieve, thermal grease has been used for decades to improve the thermal conductivity between two metal surfaces. Much has been said lately in the world of competitive computing & overclockers about heat transfer thermal grease.  Generally, the most important point is less is better. There are articles and stories reporting varying levels of improvement in heat transfer from different grades of thermal transfer grease.  In general, what I have seen says that the quality of the grease can make some difference and the high-end stuff is expensive but generally far from proportionally better.  One youtube video even suggested Nutella, the food product that worked about as well as much thermal transfer product. Well, I’d say don’t try that but is interesting that it could work at all, even as a short-term solution. I am pretty sure over time it would shrink and prove corrosive to metals to mention a few. Properly applied thermal compounds should have little risk of making a mess over the rest of the electronics or in general. This is also a good time to note that often electrical conductivity doesn’t matter that much but depending on the application, some thermal compounds are specified to be electrically conductive and some are required to be insulating. clearly, you wouldn’t want unwanted electrical conductivity on a circuit assembly.

Silpads to the rescue from messy grease.

Silpads and similar products are pretty much what the name says, silicone-based pads used in lieu of grease and designed to be either electrically conductive or not based on the application. They compress slightly to fill in voids and are often reusable providing faster, more repeatable results. Silpads are a good solution for many heat transfer requirements.  Due to their generally thicker nature, they are generally slightly less thermally conductive than a very thin layer of thermal compound.  When it is important to maintain electrical isolation, they are often better than other alternatives.

Actual “Flat” anodized aluminum surfaces at 200 X magnification

At 200x magnification, you can easily see that flat and smooth is really not flat and smooth. The one on the left might initially look and feel flat to the touch but in fact, it is not a good surface for thermal conductivity due to the voids.  This photo on the left is what the result would look like with a hard anodize treatment on cast aluminum (Cast metal generally has more voids).  Under a microscope the microscopic pitting is obvious.

The example on the right shows an acceptable surface for thermal conductivity however this is a view of a used thermal platform surface with a hard anodize that shows more true flatness as seen under the microscope but also shows minor wear scratches that can impede thermal conductivity.  A small area of missing material is not so significant but if a ridge or pockmark prevents full contact between the surfaces, it can greatly limit heat transfer.

If the surfaces can be lapped for improved flatness better thermal conductivity will clearly be achieved.

Keeping good, even force holding the surfaces together is the final note here.  It may seem obvious but in many cases, thermal conductivity is hampered by uneven pressure. Too much on one side or just not enough. More is usually better for better thermal conductivity with the limiting factors being most practical issues such as not distorting the surfaces which must have good contact.

CONCLUSION

Most surfaces are not as particle-free, flat, and parallel as they initially seem. A little thermal grease goes a long way to making for better conductivity, Silpads are less messy and also help cope with special needs such as electrical isolation and reusability but may have slightly lower heat transfer performance.  Use of thermal grease and silpads together is generally not recommended as you will end up with two layers between the surfaces. Finally don’t forget secure and even clamping.

Thermal Stress Testing

A thermal Stress Test can mean a few slightly different things.

Thermal stress we are talking about here is a form of testing applied in the design phase or production of most electronic devices when reliability is important.  It often is combined with other stresses such as power-up, operation at the margins of power, humidity, vibration, etc.

1) in the generic form, Thermal Stress Testing is any testing process that is used to verify the functionality or reliability of a device over a specified temperature range, presumably equal to or slightly more severe than what would be the range of operation expected in the field.  Also, see HALT or HAST testing which can include test-to-failure or quality verification of devices.

2) Thermal Stress Testing also can refer to an especially high rate of change temperature testing.  Used for the purpose of verifying reliability, and functionality over a range of conditions equal to or more severe than what the device would be expected to survive in use.  High rates of change are often associated with testing for the cracking of parts or material bonds. The term thermal shock is used to indicate a rapid rate of temperature change. Test requirements dictate thermal shock tests to simulate actual conditions of operation or tests that are designed to determine at what point stress causes a failure, in some cases well beyond what the product would expect to endure in normal operation.

3) Thermal Stress Testing can also apply specifically to testing where a device has two different temperatures applied.  An example of this kind of testing might be to simulate the conditions of an automobile electronic engine module with a cold-climate startup condition.

The new HBC144-N Hybrid Benchtop Chamber Combines Thermal Conduction and Convection
in one incredibly fast and efficient design.  A small benchtop chamber,  the floor of the chamber features a fully functional Hot/Cold Plate that can operate independently or simultaneously with the chamber.  Thermal platform are perfect for many types of Thermal Stress Testing.  Our standard thermal platforms can effectively perform rapid/repeated temperature cycling and verify at fast or slow rates as required.  The new Hybrid Benchtop chamber combines the gradient control of a temperature chamber with the speed and accessibility of a thermal platform.

This achieves better performance than either a chamber or platform alone. Hybrid benchtop chambers routinely cut thermal test times in half.

The Hybrid Benchtop Chamber also is especially capable of applying thermal stress of two different temperatures simultaneously applied to a device under test. For example, you can set the thermal platform to 150C and set the chamber above the platform to -40 to simulate the above-mentioned engine electronics cold-start conditions. Whatever your thermal stress test requirements are, TotalTemp can help you with the equipment to best meet your needs. Read more on the hybrid

In addition to the thermal performance of the HBC144-N, the Synergy Nano Temperature controller has advanced temperature control algorithms

that greatly enhance temperature testing speed and accuracy.  Automation features such as logging, remote control, web server, email status messaging, and network printing and can also save time and reduce errors over verification by manual methods.