The demand for thermal management materials and adhesives is driven by the unwanted and potentially harmful heat generated by ever-shrinking electronic components and systems in all areas of the electronics market, including medical. In recent years, more functionality has been included in a single device and the size of each new device has gotten smaller. Miniaturized components in tight packages with high power output create the challenges that design engineers are now facing.
Thermal management needs are also expanding in hot niche markets like batteries and LEDs, where increased complexity, density, and intensity are causing manufacturers to look for new materials and designs for better thermal conductivity, dissipation, and insulation.
Factors in Thermal Management
There are three important basic factors that need to be considered in any approach to thermal management. These include:
- Good heat sink or heat pipe design and proper airflow
- A high, thermally conductive interface material that is as thin as possible
- Elimination of voids along the interface material so that no trapped air remains between the interface material, power device surface, and heat sink surface
Supporting high-power integrated circuits (ICs) on today’s printed circuit boards (PCBs) requires working within the limitations of thermal conductivity, coefficient of thermal expansion (CTE), weight, and rigidity. Balancing these different considerations is tricky. For example, where copper can be very useful in thermal management, it is not able to manage a board’s CTE, and it substantially increases board weight.
Thermal Interface Materials
As the miniaturization trend continues in medical electronics and other industries, the importance of good thermal interface design has led to development of new thermal interface materials Choosing a thermal interface material that will work in an application, an engineer might consider power density, heat dissipation, bond line thickness, processing requirements, and reworkability.
Thermal interface materials can be broadly categorized as one of the following types:
- Polymer matrix composites — different types of carbon fibers combined with a variety of thermosetting and thermoplastic resins, including epoxy, cyanate ester, liquid crystal, nylon, polycarbonate, acrylonitrile butadiene styrene, polybutylene terephthalate, and polyphenylene sulfide
- Metal matrix composites — silicon carbide particle-reinforced aluminum, beryilia particle-reinforced beryilium, carbon fiber-reinforced aluminum, copper-tungsten, copper molybdenum, aluminum silicon, and Invar silver
- Carbon/carbon composites — carbon nanofibers, vapor-grown carbon fibers, nano-graphene platelets, pyrolitic graphite, and other carbon/ carbon mixes
Advanced thermal interface materials can provide better thermal management characteristics as well as improvements in weight and strength and are being integrated into a range of thermal management solutions, including insulating, hybrid, and nonwoven papers; insulating solders; grease; phase change materials; and conductive adhesives.
Insulating hybrid and nonwoven papers. Lightweight carbon composite laminates, aramid papers, and nonwovens can all be used for heat insulation/ dissipation. Easily die-cut to complex shapes, they provide excellent high temperature, abrasion, and chemical resistance; smooth surfaces; high torsional rigidity and stability; and high or low conductivity for absorbing, reflecting, or conducting heat.
Solder. New formulations in solder are delivering added thermal control to today’s high-performance, high-energy, and high-heat applications. Advances in solder thermal management include:
- Lead-free die attach solder alloys
- Active solder — lead-free solder with titanium or rare-earth elements added
- Eutectic bonding/soldering where silicone is alloyed with metals such as gold or aluminum to offer better heat dissipation/management
Thermal grease. The traditional interface material in electronics is thermal grease. Available in silicone or non-silicone varieties, thermal grease provides thermal resistance through excellent gap filling and an extremely thin bond line. It is reworkable, and it is easy to apply, including automated dispensing. It provides good reliability at a low cost. Thermal grease also can be applied as a thermal interface pad, where the grease is impregnated in the pad. The thermal pad offers the same wetting capability as thermal grease, but can be die-cut to custom shapes for drop-in-place assembly. Issues with grease can include contamination (for silicone grease), pump-out during thermal cycles, and imprecise or inconsistent application.
Phase change materials. Phase change materials are often used to supplement some of the issues related to grease. These materials are solids at room temperature but change to liquid once the excess heat of a device pushes the material past its melting point. Typically composed of a coating of phase change compound on an aluminum or polyimide substrate, new phase change materials can be coated directly onto a release liner without using a substrate. This improves performance by creating a better flow when the phase change material is in the liquid stage, as well as better gap and void filling. The interface is thinner without the substrate, resulting in more efficient heat transfer.
The phase change material doesn’t pump out of the interface like grease and it is more manufacturing friendly. There is no messy application or cleanup necessary, and in many cases, phase change materials provide more reliable thermal management over the long run than grease.