Designers face complex issues IN meeting power requirements, which include effective thermal management, starting with the PCB design.
The entire power-electronic sector, including RF applications and systems involving high-speed signals, is evolving toward solutions that offer increasingly complex functionalities in ever-smaller spaces. Designers face increasingly demanding challenges to meet system size, weight, and power requirements, which include effective thermal management, starting with the design of the printed-circuit board.
High-integration–density active power devices, such as MOSFET transistors, can dissipate a significant amount of heat and therefore require PCBs that can transfer heat from the hottest components to ground planes or heat-dissipating surfaces, operating as efficiently and effectively as possible. Thermal stress is one of the main causes of malfunctioning of power devices, as it leads to a degradation of performance or even a possible malfunction or failure of the system. The rapid growth of the power density of devices and the constant increase in frequencies are the main reasons that cause excessive heating of electronic components. The increasingly widespread use of semiconductors with reduced power losses and better thermal conductivity, such as wide-bandgap materials, is not in itself sufficient to eliminate the need for effective thermal management.
Current silicon-based power devices achieve a junction temperature between about 125˚C and 200˚C. However, it is always preferable to make the device operate below this limit, as this would lead to a rapid degradation of the same and a reduction of its residual life. In fact, it has been estimated that an increase of 20˚C in the operating temperature, caused by improper thermal management, can reduce the residual life of the components by up to 50%.
Layout approach
An approach to thermal management commonly followed in many projects is to use substrates with standard Flame Retardant Level 4 (FR-4), an inexpensive and easily workable material, focusing on thermal optimization of the circuit layout.
The main adopted measures concern the provision of additional copper surfaces, the use of traces with a greater thickness, and the insertion of thermal via beneath the components that generate the greatest amount of heat. A more aggressive technique, capable of dissipating a greater amount of heat, involves inserting into the PCB or applying on the outermost layers real copper blocks, typically in the shape of a coin (hence the name “copper coins”). The copper coins are processed separately and then soldered or attached directly to the PCB, or they can be inserted into the inner layers and connected to the outer layers through thermal vias. Figure 1 shows a PCB in which a special cavity has been made to house a copper coin.
Copper has a thermal conductivity coefficient of 380 W/mK, compared with 225 W/mK for aluminum and to 0.3 W/mK for FR-4. Copper is a relatively cheap metal and already widely used in PCB manufacturing; therefore, it is the ideal choice for making copper coins, thermal vias, and ground planes, all solutions capable of improving heat dissipation.
Proper positioning of the active components on the board is a crucial factor in preventing the formation of hot spots, thus ensuring that heat is distributed as evenly as possible along the entire board. In this regard, the active components should be distributed in no particular order around the PCB to avoid the formation of hot spots in a specific area. However, it’s better to avoid placing active components that generate a significant amount of heat near the edges of the board. Conversely, they should be positioned as close as possible to the center of the board, favoring an even heat distribution. If a high-power device is mounted near the edge of the board, it will build up heat on the edge, increasing the local temperature. If, on the other hand, it is placed near the center of the board, the heat will dissipate on the surface in all directions, reducing the temperature and dissipating the heat more easily. Power devices should not be placed close to sensitive components and should be properly spaced from each other.
The actions taken at the layout level can be further improved through the adoption of active or passive cooling systems, such as heatsinks or fans, whose function is to remove heat from active components rather than dissipate it directly in the board. In general, designers must find the right compromise between different thermal-management strategies based on the requirements of the specific application and the available budget.
PCB substrate selection
Due to its low thermal conductivity — between 0.2 and 0.5 W/mK — FR-4 is generally not suitable for applications in which a large amount of heat needs to be dissipated. The heat that can build up in high-power circuits is considerable, compounded by the fact that these systems often operate in harsh environments and extreme temperatures. Using an alternative substrate material with higher thermal conductivity may be a better choice than using the traditional FR-4.
Ceramic materials, for example, offer significant advantages for thermal management of high-power PCBs. In addition to improved thermal conductivity, these materials offer excellent mechanical properties that help compensate for the stress accumulated during repeated thermal cycling. Additionally, ceramic materials have lower dielectric losses operating at frequencies up to 10 GHz. For higher frequencies, it is always possible to opt for hybrid materials (such as PTFE), which offer equally low losses with a modest reduction in thermal conductivity.
The higher the thermal conductivity of a material, the faster the heat transfer. It follows that metals such as aluminum, in addition to being lighter than ceramic, offer an excellent solution for transferring heat away from components. Aluminum particularly is an excellent conductor, has excellent durability, is recyclable, and is non-toxic. Thanks to their high thermal conductivity, the metal layers help to quickly transfer heat throughout the board. Some manufacturers also offer metal-clad PCBs, wherein both outer layers are metal-clad, typically aluminum or galvanized copper. From a cost-per-unit-weight point of view, aluminum is the best choice, while copper offers higher thermal conductivity. Aluminum is widely used for the construction of PCBs that support high-power LEDs (an example is shown in Figure 2), in which it is also particularly useful for its ability to reflect the light away from the substrate.
Even silver, thanks to its thermal conductivity approximately 5% higher than copper, can be used to make tracks, via holes, pads, and metal layers. Also, if the board is used in a humid environment where noxious gases are present, using a silver finish on exposed copper traces and pads will help prevent corrosion, a typical threat found in these environments.
Metal PCBs, also known as insulating metal substrates (IMS), can be laminated directly into the PCB, resulting in a board with FR-4 substrates and metal core with single-layer and double-layer technology with depth control routing, which serves to transfer heat away from on-board components and to less critical areas. In IMS PCBs, a thin layer of thermally conductive but electrically insulating dielectric is laminated between a metal base and a copper foil. The copper foil is etched into the desired circuit pattern and the metal base absorbs heat from this circuit through the thin dielectric.
The main advantages offered by IMS PCBs are the following:
Heat dissipation is significantly higher than standard FR-4 constructions.
The dielectrics are typically 5× to 10× more thermally conductive than normal epoxy glass.
Thermal transfer is exponentially more efficient than in a conventional PCB.
Besides LED technology (illuminated signs, displays, and lighting), IMS circuit boards are widely used in the automotive industry (headlights, engine control, and power steering), in power electronics (DC power supply, inverters, and engine control), in switches, and in semiconductor relays.
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