Advances in the development of high power and high performance electronic devices demand innovative approaches for heat transfer materials. While it is given that thermal performance must be maximized, matching the other physical properties of the thermal management materials to the die and its packaging must be given serious consideration. Performance, cost and ease of integration of thermal management solutions are key driving factors for widespread acceptance in consumer electronics.
Considering most consumer electronic circuits are built on FR4 epoxy circuit board materials, metal core boards compare favorably. They offer an attractive price point and utilize standard assembly processing making possible a near-zero NRE cost of implementations. These metal-core circuit boards, however, have poor thermal dissipation capabilities compared to substrates such as direct-bond copper (DBC). DBC, a stack of copper/alumina/copper, has excellent thermal properties and therefore can be implemented in circuit designs requiring higher thermal load dissipation. However, DBC is expensive when compared with standard circuit boards and imposes some limitations on circuit design. Advances in thermal management materials can deliver high thermal management performance, excellent physical properties, and be competitively priced while still being compatible with existing circuit manufacturing techniques.
Applied Nanotech (ANI) has developed a group of high-performance graphite-based thermal management materials, known collectively as CarbAl™, and the corresponding surface functionalization techniques for the purpose of achieving the tough thermal management requirements of power electronic devices. When properly engineered and functionalized, these new concepts in thermal management can maximize performance of heat conduction by:
- Reducing the number of thermally-resistant layers between the device and the environment.
- Reducing the thermal resistance of the layers.
- Using high thermal conductivity materials between each layer of the heat dissipation substrates.
This process is able to provide new methods of thermal management for substrate and package designs with improved overall power electronic device performance. Furthermore, utilizing Xenon Corporation’s photosintering equipment and technology to create printed circuit traces, we describe the proof of concept process for fabrication of novel graphitic substrate based PCBs with integrated thermal management that optimize thermal conductivity and promises thermal coefficient of expansion (CTE) matched close to most power electronic semiconductors.
Integrated Thermal Management PCBs
Integrated thermal management substrates such as metal core boards and DBC are currently used for power electronics applications. Each of these boards share a basic architecture with a base layer material, middle layer of dielectric and final conductor layer on the top. However, despite their ease of use, compromises need to be made related to their thermal management performance. For example, the alumina dielectric layer in a DBC substrate is quite thick, based on the limitations of the thermal press method used to bond the outer layer of copper. Metal core boards that are clad with epoxy-based circuit board materials can have high thermal resistance. Advances in dielectric materials and processing of these materials allow for reduction in the thickness of this cladding, the most thermally-resistive component of any dielectric stack.
Most high thermal conductivity materials are also good electrical conductors. Phonon propagation, the key to transferring heat, is similar to the mechanism of electron movement in a conductive material. Therefore, special methods and materials are required to passivate the base thermal conductor to provide electrical isolation and prevent short circuits on the top conductor layer. There are many examples of dielectric layers on top of thermal conductors; however, few are well matched with respect to thermal conductivity and CTE with the substrate and its packaging.
ANI’s integrated thermal management solution provides a lightweight base material with thermal conductivity similar to copper, a high-thermal conductivity dielectric layer with minimized thickness and a final copper circuit conductor trace. Additionally, for DBC there are often interface layers between different materials that increase the overall thermal resistance of the completed circuit or stack. The ANI approach provides an integrated material set that minimizes the total number of interfaces and maximizes the thermal performance of each layer.
Fig. 1 compares standard laminated PCBs attached to a heat sink (left) to the ANI integrated direct-print circuit boards on graphitic carbon-substrate solution (right). The ANI concept illustrated in Fig. 1(right) is a promising candidate for power electronics applications because it reduces the number of interfacial adhesive layers and leverages the matched CTE properties of the CarbAl™ material. This technique minimizes the total number of layers in a passive thermal management solution. Eliminating any interface between passive components always helps in obtaining the lowest resistance to thermal transfer. Optimizing the integration between the active component and thermal properties of each individual passive component is the key factor for the thermal management of power electronics packaging, and as a result, fully-integrated power electronics PCBs as well as DBC-like substrate materials are realizable.
The base material of our integrated thermal management is CarbAl™, an optimized graphitic carbon matrix that combines high thermal conductivity, low density and low CTE. Table 1 compares the properties of two types of CarbAl™ materials to other commonly-used thermal management materials. CarbAl™ materials have a thermal conductivity of 300 - 350 W/m-K vs. 200 W/m-K for Al and 390 W/m-K for Cu. When compared with commercial thermal management graphite foams, CarbAlTM provide much higher thermal conductivity and mechanical strength at a significantly lower cost. In terms of thermal diffusivity, CarbAl™ materials have a thermal diffusivity of 2.5 – 2.9 cm2/sec, compared with 0.84 cm2/sec for aluminium and 1.1 cm2/sec for copper.
Fig. 2 compares CTE and thermal conductivity of various thermal management materials. CTE and thermal conductivity values for each material have been normalized to Si, which was assigned a value of 1. The ideal material would have a high thermal conductivity and low CTE. Fig. 2 shows that commonly-used semiconductor materials such as Si, GaN and GaAs have a thermal mismatch when interfaced with materials currently used for heat sinks and heat spreaders (i.e. Cu, Al). These metals have high CTE, and as a result, induce stress due to differences in expansion rates as function of temperature. This has a detrimental effect on the lifetime and reliability of electronic devices.
Fig. 2 shows that CarbAl™ materials have a much closer match in CTE to commonly-used semiconductor materials when compared to Cu and Al. Besides eliminating thermally-induced delamination and cracks, a lower CTE allows scope for direct bonding of bare chips on the composite materal and further minimizes the number of material interfaces and maximizes heat dissipation.
Enhanced Dielectric Materials
As mentioned previously, reducing the number of thermally-resistant layers between the power devices and the ambient environment is very advantageous. To fulfill this purpose and address the possibility of thermally-integrated PCBs, specially-designed ceramic dielectric inks have been developed that can enable direct printing of a dielectric layer on CarbAl™ substrate to build fewer layered circuit boards by eliminating the highly thermally-resistant organic adhesive layer.
DTC-C is a line of thermally-conductive, electrically-insulating inks that are based on dielectric ceramics (aluminum nitride in nature) modified to maximize thermal conductivity with excellent adhesion and wettability to low CTE substrates (e.g. CarbAl™). DTC-C has a thermal conductivity of 4 W/m-K and a CTE smaller than 7 ppm/K. This thermal conductivity far exceeds the performance of regular epoxy-based dielectric films used in metal core circuit boards.
Conductive Photosinterable Inks
The copper inks developed by ANI are homogeneously-dispersed copper particles that are compatible with photosintering processes at regular atmosphere at room temperature. The inks can be patterned by screen or inkjet printing and, after photosintering, form conductive traces which adhere to various dielectric layers and substrates including THERCOBOND™-type materials. Fig. 3 shows photosintered Cu patterns on a PET substrate. The photosintered Cu inks have a typical resistivity value range of 10-6 - 10-5 oh m-cm and are suitable for direct soldering using common solders. The copper patterns can also act as a seed layer for further metal deposition using electroless plating techniques.
In a typical circuit board, traces are made of conductive metals that are laminated onto a substrate and then etched away using a chemical photolithographic process. The problem with trying to print these conductive layers using inks is that the resistivity is unacceptably high due to the inks’ porous nature. Sintering melts the ink particles to form a homogenous metal surface with good conductivity. As the melting point of metals is typically significantly higher than that of the substrate, normal particulate metals are not suitable. When these particles are on the nanometer scale, however, a phenomenon known as melting-point depression occurs. This is where the surface-to-volume ratio is such that the atoms on the surface are able to become more fluidic and hence, have a lower melting point. Because nanoscale particles are smaller than the wavelength of light, their interaction with light also differs from that of bulk. Nanoparticles of copper may look dull or black in contrast to the shiny metallic color of bulk. Both melting point depression and spectral absorption changes allow the use of high energy light to sinter these inks on relatively low temperature substrates to form conductive traces. Once sintered, these links lose their nanoparticle properties and behave identical to that of bulk. Compared with standard PCB manufacturing, the additive process requires only the regions that needing a trace to be printed.
The photosintering of ANI copper inks was performed using a Sinteron 2000 tool manufactured by Xenon Corp. Photosintering involves a short, high-intensity light pulse using a xenon flash lamp. The Sinteron system can adjust both the pulse duration as well as the pulse amplitude of the light flash. This allows controlling the amount of optical energy incident onto the surface. Peak power on the Megawatt range can be realized. Because the pulse durations range between a few hundred microseconds and a few milliseconds, substrate temperature rise associated with this process is low. This allows photosintering on low temperature substrates such as PET. When compared with oven-based thermal sintering, photosintering is orders of magnitude faster.
Fig. 4 shows a prototype circuit on CarbAl™ as an example of the concept of an integrated thermal management PCB. These substrates provide improved thermal performance and therefore do not require the large heat sink pads normally associated with power electronic components. The construction provide superior thickness control, thus reducing thermal resistance compared to alternative substrates.
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