Electronic systems are rapidly evolving, where electronic circuits and components are being designed to handle greater amounts of power in smaller packages. The amount of power used or transmitted per unit mass, area or volume is often referred to as the “power density.”

Higher power density systems are becoming more commonplace in several industries, including aerospace and defense, transportation, power generation and transmission, RF systems and Data Centers. These higher power density systems require more advanced techniques and materials to dissipate potentially higher heat loads that left unchecked may compromise electronic system performance or reduce system lifetimes.

This article reviews traditional heat dissipation materials and techniques and discusses why current or legacy systems may no longer be adequate for the higher heat loads developed by modern high power density systems and components. The article will also discuss more advanced heat dissipation materials and techniques, and how the newly developed Hermetic Solutions DiaCool materials can address this emerging market need.

Trends toward higher power density electronics

Higher power density electronic devices are becoming more commonplace in advanced semiconductor devices in the aerospace, defense, automotive, transportation, telecommunications and other industries. These devices may include gallium nitride on silicon carbide (GaN-on-SiC) semiconductor materials that have three times the thermal conductivity of gallium nitride on silicon (GaN-on-Si), enabling them to run at a much higher voltages and higher power densities. A review of some of these applications is provided below.

Aerospace and defense

Wide-bandgap (WBG) semiconductor materials (including GaN-on-SiC) offer features and capabilities that are orders of magnitude greater than their silicon counterparts, including 10 times voltage blocking capability, 10 times to 100 times switching speed capability and one-tenth the energy losses, while being intrinsically radiation-hardened. This technology could provide power systems with power densities up to 10 times higher than current silicon-based devices.

Automotive and transportation

WBG power semiconductor devices provide several benefits such as size reduction, cost savings and improved reliability, and further translate into reduced fuel consumption and extended operating range for electric vehicles (EVs), hybrid electric vehicles (HEVs), energy storage applications, motor drives and inverters, power conversion and upcoming urban air mobility (UAM) systems.

Telecommunications

GaN has become the technology of choice for power-hungry applications, such as power RF, where the RF systems is required to transmit signals over long distances and/or at high-end power levels. These applications include radar, base transceiver stations (BTS), satellite communications and electronic warfare (EW). Advantages of GaN include a high breakdown field, which allows the GaN device to operate at much higher voltages than other semiconductor devices, high saturation velocity, which enables GaN devices to deliver much higher current density, and outstanding thermal properties, which enable these devices to operate at lower temperatures.

Other applications

Higher power density devices are used in several other applications in manufacturing and the computer industry. For example, lasers that are used in a variety of manufacturing operations use high power laser diode submounts that come in increasingly smaller form factors. In high-performance computing and computer data centers, large power-hungry dies may be utilized that generate hundreds of watts per cubic centimeter.

Heat sink and heat spreader devices and technologies

Heat sinks and heat spreaders are needed to reliably remove heat from these higher power density devices. A heat sink maximizes the surface area by using fins or pins to allow the heat to dissipate through free or forced convection to the surrounding air. These devices are often used with dedicated cooling fans or are exposed to the atmospheric air outside of the electronic enclosure. Heat spreaders, on the other hand, have large flat surfaces, but do not have fins or fans. These devices work by transporting the heat away from the electronic device through conduction to other areas or components within the electronic system where it can be dissipated through conduction or convection. Examples of these devices are shown in Figure 1.

Figure 1. Heat sinks and heat spreaders attached to electronic devices. Source: Hermetic Solutions Group

Because heat sinks and heat spreaders are intimately connected to the electronic devices from which they must dissipate heat, it is necessary to match the coefficient of thermal expansion (CTE) of the heat sink or heat spreader to the electronic device over the operational temperature range. This is because large temperature excursions may result in large differential strains between the electronic component and heat sink. Failure to match CTEs may cause high stresses to develop, which could lead to the eventual failure of one or both components.

As an illustration, at room temperature, the CTE of GaN and SiC is about 3 x 10-6/° K whereas the CTE for copper is almost a factor of six times greater, at about 17 x 10-6/° K. To overcome this large CTE mismatch, heat sinks and heat spreaders are engineered as metal matrix or laminate composite structures using a variety of materials to obtain high heat conduction with relatively low thermal expansion. Examples of traditional heat sink materials are provided below.

Molybdenum copper

Molybdenum copper heat spreaders and heat sinks exhibit outstanding CTE and thermal conductivity. The CTE of the molybdenum copper heat spreaders can match with other components by adjusting the Mo/Cu composited ratios. The resulting reduced difference in CTE lessens the internal thermal stresses and helps to maintain the device’s operation functionality and reliability. Molybdenum copper possesses high heat resistance, good machinability and hermeticity.

Copper molybdenum copper (CMC)

CMC (Cu/Mo/Cu) laminate is a three-layered structure that consists of two outer copper layers and one molybdenum core layer. CMC has a better CT and CTE than Mo and a higher CT than MoCu at the same CTE level. CMC is also more machinable than pure molybdenum and molybdenum copper composites.

Copper/molybdenum copper/copper (CPC)

CPC (Cu/MoCu/Cu) laminate is a copper molybdenum/copper copper structure. CPC replaces its core layer from the pure molybdenum to MoCu containing 15% to 40% weight copper. The molybdenum copper core layer of CPC achieves improved thermal conductivity over CMC. At the same time, CPC keeps the CTE advantages of CMC. Compared with CMC, CPC achieves more efficient heat dissipation and distribution. In addition, it is more reliable for heat spreaders in laterally diffused metal-oxide semiconductors (LDMOS) and GaN/GaS based devices.

Cu/Mo/Cu/Mo/Cu (SCMC)

SCMC (Cu/Mo/Cu/Mo/Cu) consists of two molybdenum core layers and three copper layers. The copper layers of the advanced SCMS 51515 have distributed tunnels across the Mo layer. All copper layers can connect with each other through the tunnels. As a result, those tunnels help to consolidate the bonding effect of all layers further. Their thermal conductivity can reach 290 W/(m° K). Plus, SCMC and SCMS possess the same excellent electrical conductivity as all other types of MoCu laminates.

Tungsten copper (WCu)

Similar to molybdenum copper, tungsten copper has an adjustable CTE range. Tungsten copper with the same copper percentage as molybdenum copper has exhibited higher thermal conductivity than molybdenum copper. However, because of the high density of tungsten, tungsten copper heat spreaders are more suitable for less weight-sensitive high-power devices.

Graphene

Graphene has among the highest in-plane thermal conductivity of any known material, of about 2000 to 4000 W/m° K for freely suspended samples. However, it is a two-dimensional material, and has relatively poor thermal conductivity out-of-plane.

Hermetic Solutions DiaCool technologies and devices

Hermetic Solutions Group (HSG) has developed a new diamond-based metal matrix composite material that provides high thermal conductivity with a low CTE for use with the new generation of higher power density electronics. These materials, known as DiaCool, use diamond powders that, when combined with different base metals, such as aluminum, copper and silver, provide superior thermal management when compared with traditional heat sink and heat spreader technologies. In fact, diamond is one of the highest naturally occurring heat conductive substances in nature, having a thermal conductivity of 2200 W/m° K and a CTE of 1x10-6/° K at room temperature. When combined with high thermal conductivity metals such as copper (390 W/m° K) and silver (419 W/m° K), the DiaCool materials provide thermal conductivities that are three to four times higher than legacy metal matrix materials. Unlike graphene, DiaCool is thermally isotropic and provides identical thermal conductivity in three dimensions. The DiaCool materials have outstanding surface finish, machinability, platability and solderability, and can be fabricated into a wide variety of configurations, such as heat sinks, die tabs and heat spreaders (Figure 2).

Figure 2. DiaCool heat spreaders and heat sinks. Source: Hermetic Solutions Group

About HSG

HSG is a global developer and manufacturer of hermetic solutions for electronics packages used in the aerospace, defense, power, energy, telecom, optical and medical industries. The company is headquartered in Trevose, Pennsylvania, and has over 350,000 sq ft of manufacturing space in 11 locations across the U.S., Canada and the U.K. Their dedicated engineering staff provides value-added services, including prototype development and validation, testing, device assembly, laser hermetic sealing, final packaging and sterilization. HSG products adhere to strict quality control certifications, including NadCap, ISO9001:2008, AS9001/C and ISO 13485. More information about HSG’s products and capabilities can be found on the Hermetic Solutions website.