Today’s electronic devices are smaller, more component-dense and need more power than ever, especially for handheld devices. Heat loads are increasing as a result – threatening the functionality of the devices we’ve come to rely on.
Engineers have historically used active and passive cooling techniques (ex., fans and heat sinks) with thermal interface materials to provide a reliable heat flow path between component and thermal transfer surface. These materials traditionally include thermal greases applied between power dissipating and cooling components, such as a microprocessor and its heat sink.
Today however, manufacturers can turn to thermally conductive adhesives, sealants and potting compounds to not only solve heating issues, but to also avoid losing crucial product “real estate” and extend component life.
One- and two-component epoxies, silicones, solvent-based compounds and epoxy films can be used for thermal, environmental and structural stability requirements. These materials can optimize heat transfer across interfaces for electronic devices in industries from medical to aerospace.
Thermally conductive adhesives have also been used in the past for IC die attachment, heat sink bonding for PBCs and power electronics among others. Thermal potting compounds can not only improve heat transfer of the components they encapsulate, but also protect them from shock, vibration and other environmental threats.
This all sounds promising, but determining the right adhesive, compound or chemistry for your application depends on your requirements – there is no master solution for every application.
The Chemistry Conundrum of Thermally Conductive Adhesives
Modern adhesive chemistries can combine the best properties of different materials to find ideal solutions for any application. Thermally conductive adhesives, sealants and compounds can be designed for flexibility or rigidity, as well as with customizable viscosity and cure rates.
“For example, if there are thermal shocks involved, you’d lean toward something flexible – silicones are ideal in this scenario,” said Rohit Ramnath, senior product engineer at Master Bond Inc. “On the other hand, if you need something structural to withstand vibration and mechanical loads, you’d lean toward epoxies. It varies on the application and the conditions that the product may be subjected to.”
Historically, adhesives and sealants are typically effective electrical insulators and poor heat conductors. This need no longer be the case.
“The base polymer is so thermally insulating that if you were to use just an epoxy with a typical curing agent by itself, you’d be looking at 0.1 to 0.2 W/mK for bulk thermal conductivity,” said Venkat Nandivada, manager of technical support at Master Bond. “Based on the filler, you can achieve at least 10 to 20 times that result for really good heat transfer.”
Filled epoxies can normally achieve a bulk thermal conductivity 1 W/mK or potentially even higher with some being > 6 W/mK. Formulators can achieve higher values by increasing the mass fraction of filler particles in the matrix.
In cases where both thermal and electrical conductivity is required, specially formulated adhesives can be made. Formulators select a specific resin, hardener, filler type and concentration as well as additives, and control the degree and method of curing. Thin layers of filler can be applied to achieve bond thicknesses of 10 to 20 microns for the best heat transfer, Nandivada added.
Loading adhesives with very high filler content may result in decreased bond strength, however, as they have proportionally less epoxy available for cross linking. A similar principle applies to silicone products.
“It’s always important to maintain the right balance,” said Nandivada. “You don’t want to be adding filler or loading the epoxy with a ceramic-based filler to the point where it behaves like a ceramic system. There are many cases where ceramic filler will improve the adhesion properties of the base epoxy, but the loading has to be right.”
The conductivity-bond strength trade-off is not a significant concern for most electronics assemblies, as there is enough margin to cope with the stress and strain typically seen with commercial and consumer products.
Thermally conductive epoxy systems offer grades with excellent moisture resistance. Some are cryogenically serviceable, while others can resist temperatures above 260° C (500° F). There are also formulations that can withstand thermal cycling and grades that meet NASA low outgassing standards. Epoxy films and one- and two-part compounds with different viscosities, moduli and cure rates offer additional application options.
Assembly and Curing with Thermally Conductive Adhesives
The proper application of the adhesive is just as sensitive as its creation. Adhesive compounds need to be applied carefully, yet fully, to prevent the forming of voids while also minimizing curing times.
Automation is an excellent tool for the application process, ensuring greater accuracy than with a manual application.
“You definitely need to automate,” said Nandivada. “If you go with a one-part epoxy adhesive, you can use a syringe with an automated dispenser and get a tiny dot or whatever amount that’s needed. The components can also be laid out and dispensed with a volumetric dispensing profile. Even on smaller scales, rather than using a manual gun applicator, you can substitute with a pneumatic system.”
When potting PCBs, Nandivada recommends the “glop top” technique to cover individual components or the use of a dam and fill approach.
“When you want to encapsulate a part, you can use an epoxy to dam first and then use a lower viscosity material as the filling agent,” he said. “It’s like a selective encapsulation process on the same board.”
Encapsulation has multiple advantages, but carries a price where there’s an excessive difference between the coefficient of thermal expansion (CTE) between compound and substrate. In the worst cases, temperature rise can fracture and damage components, especially in high density or sandwich type construction. By specifying a compound with the right CTE, engineers can reduce thermal expansion, reducing stress.
“If there’s hardly any temperature change, the material is operating around ambient temperatures and the board is not too fragile, you can go with something like a rigid curing epoxy,” Ramnath advised.
Post-application, curing conditions can impact the thermal conductivity of an adhesive. Too low a cure temperature will not only slow the cure rate, but also lower crosslink density. Too high a temperature can produce exotherms that could cause the adhesive to expand. Master Bond recommends special curing fixtures and controlled environments for production applications.
With an ideal cure, filler particles will meet each other and allow for more efficient heat conduction.
Air gaps in heat sinks will create a thermal barrier to heat flow. Thermally conductive epoxies can help, for example, as a thermal bonding agent between the die and the heat spreader in a BGA package.
Several dispensing methods can also accelerate curing.
“We can package two-part epoxies as a one-part in a syringe, so it’s already pre-mixed and degassed, so there’s no need for measuring, mixing and degassing by the end user,” said Nandivada.
When pot life is a concern, chemistries can allow for a working life of 12 to 24 hours or even longer, Ramnath explained.
“There are one-component systems that need heat curing, where the heat activation would take place only at temperatures of 80° C (176° F) or slightly higher, with certain specialty systems. There are some two component systems that offer at least 12 to 24 hours of pot life at room temperature, which helps in assembling many components at a time. Such one- and two-component systems which need moderate heat for curing are useful for large volume production of heat sensitive components.”
Working with Thermal Adhesive Experts
When asked what the most common mistakes manufacturers make with respect to the use of thermally conductive adhesives, Ramnath and Nandivada spoke of issues including:
- Processing
- Handling
- Incorrect measurement
- Insufficient surface preparation
- Improper heat curing
It’s easiest to prevent mistakes and ensure production processes go correctly by speaking with adhesive specialists like Master Bond early. At Master Bond, Nandivada and Ramnath work together with their customers on custom solutions to determine the right chemistries, filling and other variables for the perfect balance for the application.
“If you’re at an early stage, you can afford to customize a little more and it’s possible to tweak things depending on what kind of conditions the product might see in service,” Ramnath said. “We can even help optimize your process.”
For more information about thermally conductive adhesives and Master Bond’s services and advice, visit the Master Bond website.
Master Bond Inc. has sponsored this article. All opinions are mine. –Kagan Pittman