The coolest compounds

Thermally conductive adhesives and potting compounds are an important weapon in the war against heat

BY ROBERT MICHAELS
Master Bond, Hackensack, NJ
www.masterbond.com

Electronics are hot right now — literally. Chip makers have significantly upped the ante on microprocessor power and density over the past decade. And these powerful microprocessors are shoehorned into tiny mobile and embedded devices that make thermal management all the more difficult. Design engineers charged with keeping these devices cool have a big challenge on their hands.

How big a challenge? Today’s microprocessors already have to dissipate up to 100 W. And that number is bound to go up as chip makers continue to increase the operating frequency and number of CMOS devices within each package. In fact, semiconductor industry projections have microprocessor power dissipation densities rising to 200 W/cm2 over the next decade or so. What’s more, modern electronics packaging techniques will exacerbate the thermal issues caused by rising power dissipation requirements. For example, some of the new space-saving system in a package (SiP) designs stack chips and other electronic components on top of one another, which can make it more difficult to remove heat.

Smart engineers have long taken a systems approach to thermal management, employing a wide variety of active and passive cooling measures to get the heat away from sensitive electronic components. Thermal interface materials, which fill in the air gaps between thermal transfer surfaces, are one of the key cooling measures. Among these materials are the thermal greases applied between power dissipating and cooling components, such as microprocessor and its heat sink. But there’s a lot more to thermal interface materials than greases.

The lineup of thermal interface materials also includes a variety of thermally conductive adhesives and related potting compounds — including one- and two-component epoxies, silicones and solvent-based compounds. Also available are thermally conductive epoxy films that address some of the most common application issues.

From a design standpoint, thermal adhesives have a lot going for them because they do double duty: Not only do they help manage heat but they also bond components to create electronics assemblies. Likewise, thermal potting compounds improve heat transfer of the components they encapsulate, and they also work to protect those components from shock, vibration and other environmental threats.

Here’s a closer look at the wide range of thermal adhesives and potting compounds and what they can do in your application.

Balance of properties

In general, it’s a good idea to think of adhesives and related compounds not in terms of a single property but in terms of how well they balance a variety of desirable properties. That’s true with thermal management applications too.

What many engineers don’t realize is modern adhesive chemistry can combine thermal conductivity with other useful or even essential properties. To take a couple of key examples from just the epoxy family, Master Bond offers one- and two-part thermally conductive products that have been certified to NASA’s low outgassing specifications. Products that meet USP Class VI biocompatibility standards are pending as are grades that meet aerospace specifications.

Fig. 1. Thermally conductive adhesives.

Thermally conductive products offer a host of desirable physical and mechanical properties too. Among them are chemical, moisture and temperature resistance—with the latter ranging from cryogenically serviceable grades to products able to withstand temperatures in excess of 500° F. Grades that do a good job in thermal cycling applications are also available as are grades optimized for shock and vibration resistance. Thermal products are finally available with many different moduli—from flexible to rigid—as well as different viscosities and cure rates. (See Fig. 2 for a list of popular grades and their properties).

Fig. 2. Thermally conductive products are available in different viscosities and levels of flexibility.

As for electrical properties, the vast majority of thermally conductive products are formulated to be electrical insulators which is a desirable property when bonding or potting most types of electronic components. In those cases where thermal and electrical conductivity is required, there are specially formulated adhesives that conduct both heat and electric currents.

From insulator to conductor

One obvious barrier to using any adhesives or potting compounds to help dissipate heat is that polymeric materials in their natural state are actually good thermal insulators. Unfilled epoxy, for example, has thermal conductivity of 0.14 W/mK. Compare that value to a world-class conductor like aluminum, which is approximately 200 W/mK.

With the addition of various metallic, ceramic or even nanotech fillers, formulators can improve on an adhesive’s baseline thermal conductivity values by a factor of 10 or more. The resulting products, while never approaching the conductivity of metals, do conduct enough heat to become a valuable part of a comprehensive thermal management system. And remember that thermal adhesives or potting compounds do eliminate another troublesome insulator—the thermally insulating air gaps that would otherwise exist between heat transfer surfaces.

Many thermal adhesives and potting compounds have dozens of individual grades and conductivity values (for example Master Bond offers products that have conductivity from 1.5 to 3 W/mK). That conductivity range covers the vast majority of commercial electronics bonding and potting applications (see sidebar 1). In special cases, however, there may be epoxies with thermal conductivity values 4 W/mK and above without significantly compromising the mechanical performance of the adhesive.

And that emphasis on mechanical performance is important with thermal adhesives because the same inorganic fillers that impart conductivity tend to reduce bond strength. Compare an unfilled and filled version of the same epoxy, for instance, and the unfilled will have higher bond strength every time. The reason why is simple: The more highly filled products have proportionally less epoxy available for bonding. The same reasoning applies to silicone products as well.

Fortunately, this trade-off between strength and conductivity is not an issue in the majority of electronics applications. Even when filled with thermally conductive additives, epoxies and silicones still offer more than enough bond strength to withstand the minimal forces seen by most power dissipating components. For example, potted components may have to withstand some level of stress and strain during the manufacturing process and in use, but they don’t often experience the high forces that characterize a true structural adhesive bond.

There are some applications in which the adhesive plays both a structural and thermal management role. In these cases, engineers need to keep the strength-conductivity trade-off in mind—and potentially take steps to design around it. ■

Side Bar 1:

Bond line considerations

While engineers should pay attention to the bond line characteristics in any structural bonding application, thermal adhesives make that attention all the more important. One oft-forgotten consideration in these applications is the particle size of the thermally conductive fillers.

In some cases, the size of the filler particles can be the limiting factor in bond line thickness (BLT). If the particles, for example, are 40 µm across, the bond line can’t be thinner than that without some reduction in the structural strength of the bond. In ordinary bonding applications, the effect of fillers on BLT would not necessarily matter because optimal structural bonds often favor thicker bond lines. Thermal applications, however, require relatively thin bond lines for the sake of heat transfer. Thus, the particle size of the conductive fillers can contribute to a trade-off between optimal bond strength and optimal heat transfer.

This problem pops up less and less as suppliers develop more advanced fillers. For instance, Master Bond has new proprietary fillers that measure just 3 µm across, rather than the more typical 40 µm. These smaller fillers enable a better balance between bond strength and the thin bond lines required for the best heat transfer. Since it’s difficult for engineers to find out how large the particles are in a given adhesive product, the best thing to do is discuss your desired BLT with a technical service engineer, who can flag any size conflicts during the adhesive selection process.

Side Bar 2:

New films

Not all thermally conductive materials come in tubes and cans. Thermally conductive epoxy films are an increasingly popular application option—and for good reason: Films improves the uniformity of heat transfer. Depending on the type of film chosen, uniform bond lines as thin as 0.003 in. are easily achievable. With traditional adhesives in electronic assemblies, the bond lines are hard to control and can vary between 0.003 to 0.006 in.

Films can also be die cut to intricate shapes that closely match and fully cover an electronic component’s thermal surfaces, which again has a beneficial effect on heat transfer. And unlike thermal greases, films don’t flow or run as temperature and assembly forces increase.

Despite these benefits, films have in the past had some trade-offs in the ease-of-use department. If the films need to be cut into intricate shapes, there’s more upfront engineering work than non-film adhesives. For instance, engineers may have to design and commission a die. What’s more, most thermally conductive films require cryogenic storage and heat cure temperature requirements that make them more complicated to use. The trade-offs, however, are diminishing as new films come on the market. For example, Master Bond offers the FL 901 AO film, which comes in many pre-cut shapes and can be stored at room temperature or in an ordinary refrigerator. ■

Bond line considerations

While engineers should pay attention to the bond line characteristics in any structural bonding application, thermal adhesives make that attention all the more important. One oft-forgotten consideration in these applications is the particle size of the thermally conductive fillers.

In some cases, the size of the filler particles can be the limiting factor in bond line thickness (BLT). If the particles, for example, are 40 µm across, the bond line can’t be thinner than that without some reduction in the structural strength of the bond. In ordinary bonding applications, the effect of fillers on BLT would not necessarily matter because optimal structural bonds often favor thicker bond lines. Thermal applications, however, require relatively thin bond lines for the sake of heat transfer. Thus, the particle size of the conductive fillers can contribute to a trade-off between optimal bond strength and optimal heat transfer.

This problem pops up less and less as suppliers develop more advanced fillers. For instance, Master Bond has new proprietary fillers that measure just 3 µm across, rather than the more typical 40 µm. These smaller fillers enable a better balance between bond strength and the thin bond lines required for the best heat transfer. Since it’s difficult for engineers to find out how large the particles are in a given adhesive product, the best thing to do is discuss your desired BLT with a technical service engineer, who can flag any size conflicts during the adhesive selection process.