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Sensible Design: Pushing the Boundaries of Thermal Management

Thermal management plays a central role in circuit and electronic assembly design, ensuring improved reliability and increased performance of devices. But what if you could push the boundaries even further and extend the long-term stability? To explore how this can be achieved, I’m going to touch on some of the latest advances in thermal management technology.

Also, in keeping with our five-tip format, I will take a closer look at options for the automated application of thermal interface materials (TIMs) and discuss the feasibility of using a high thermal conductivity encapsulation resin instead of a TIM. There are a number of environmentally friendly thermal management materials available these days, so I will also examine ways to improve your green credentials, as well as demystify the importance of bulk thermal conductivity.

Without further ado, let’s get started with the new advances in thermal management.

1. What are the latest advances in thermal management technology, and how do these products differ from some of the more traditional pastes and greases?
Thermal pastes and greases have been leading the way in thermal management for many years and are expected to do so for many more years to come. Pastes are easy to apply and rework whilst providing a cost-effective alternative to thermally conductive encapsulants. However, make way for the new kids on the block: phase change materials (PCMs).

Once heated above their phase change temperature, PCMs become highly thixotropic liquids that perform as well as—and sometimes even better than—a traditional thermal grease. Moreover, their low phase change temperature ensures low thermal resistance over a wide temperature range and safeguards minimal bond line thickness with improved stability. With phase change technology, a key benefit is the greatly reduced effects of pump out, making PCMs an excellent choice for applications that undergo widely varying temperatures.

The application methods of PCMs for high volume production mean that most can be utilised in existing production processes with minimal—if any—changes, whilst also allowing for easy rework, offering many of the same benefits of traditional thermal pastes. PCMs offer greater long-term stability compared with thermal greases as they are better suited to thermally challenging applications where product life expectancy and reliability may be critical, such as automotive/aerospace electronics or remotely-located wind power inverters. Traditional thermal pastes/greases will continue to be a popular choice, although for some applications, especially those requiring greater long-term stability, a PCM is likely to win over the crowd.

2. What are the options for the automated application of thermal management products?
Automated applications involve the use of specialist equipment that typically consists of an applicator head where the material is fed to the applicator via dispensing equipment. Due to the high viscosities of the thermal management materials, the dispensing equipment is usually a follower-plate system that connects to the thermal paste container as supplied. In addition, automated stencil and screen applicators are widely used. For example, we work with a number of local and international equipment manufacturers.

3. When would I feasibly require an encapsulation resin with high thermal conductivity?
Electronic components and devices will produce varying levels of heat during their operation. Where significant amounts of heat are generated, intervention in the form of thermal management may be required to prolong working life and increase reliability. For certain types of applications, it may be beneficial to encapsulate the whole device in a heatsink enclosure using a thermally conductive encapsulation resin. This method offers both heat dissipation and protection from the surrounding environment, such as high humidity or corrosive conditions.

Once again, it is important to ensure that no air inclusions occur during the potting operation, as these will interfere with heat transfer to the metal case. Mineral fillers used in some resins systems have a higher thermal conductivity than the resin base, so filled resins are better than unfilled resins, as far as thermal control is concerned. The higher the filler level, the higher the thermal conductivity. However, higher filler levels will lead to higher viscosity and a greater possibility of air inclusions in the potting.

4. What options are available for more environmentally friendly thermal management products?
In recent years, we have seen demand increase for more environmentally friendly products across all our product groups. However, within the thermal management range, we have developed a high-performance thermal management paste that is entirely free from zinc oxide (ZnO). The non-silicone heat transfer compound is recommended for applications where the use of zinc oxide is restricted; for instance, in the marine industry, ZnO is a pollutant, and silicones are prohibited in places like offshore utilities. HTCX_ZF is a highly stable, non-curing paste that enables simple and efficient rework of components (if needed) and is recommended where efficient and reliable thermal coupling of electrical and electronic components is required, as well as between any surface where thermal conductivity and heat dissipation is important.

5. What is the importance of a bulk thermal conductivity value?
The initial selection of suitable TIMs for testing is often conducted based on high bulk thermal conductivity, indicating the efficiency of heat transfer through the TIM itself. However, bulk thermal conductivity alone could give a false impression of the expected performance. When tests are conducted under application conditions, low thermal resistance of the device indicates the true heat transfer efficiency of the TIM.

Conclusion
It can be a complex process deciding on the right choice of material and/or application technique regarding thermal management products. I strongly recommend getting expert advice before settling on any particular material or method. I hope this month’s column has shed some light on current thermal management issues.

Jade Bridges is global technical support manager at Electrolube. To read past columns from Electrolube, click here. Download your free copy of Electrolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series “Coatings Uncoated!”

This column originally appeared in the October 2020 issue of Design007 Magazine.