Do you know how to dissipate heat from high-power PCB (MCPCB)?
The entire power electronics industry, including RF applications and systems involving high-speed signals, is moving towards solutions that provide increasingly complex functionalities in ever-smaller spaces. Designers face increasingly stringent challenges in meeting system size, weight, and power requirements, which include effective thermal management, starting from the PCB design.
l Thermal Management Requirements
Highly integrated active power devices, such as MOSFET transistors, generate a significant amount of heat, requiring the PCB to efficiently transfer heat from the hottest components to ground planes or heat dissipation surfaces for optimal operation. Thermal stress is one of the primary causes of power device failures, leading to performance degradation and even system failure. The rapid increase in device power density and frequency are the main reasons for overheating of electronic components. While wide bandgap materials with lower power losses and better thermal conductivity are being used more widely, they alone are not sufficient to eliminate the need for effective thermal management.
Currently, the junction temperatures achievable with silicon-based power devices range between approximately 125°C and 200°C. However, it is best to always keep the devices operating below these extreme conditions to avoid rapid aging and reduce their remaining lifespan. In fact, it is estimated that if poor thermal management leads to a 20°C increase in operating temperature, the resulting reduction in component lifespan can be as much as 50%.
l Layout Methods
A commonly adopted thermal management method in many projects is the use of substrates with standard flame retardant grade 4 (FR-4), which is an inexpensive and easy-to-process material, focusing on thermal optimization of the circuit layout.
The main measures involve providing additional copper surfaces, using thicker traces, and inserting thermal vias under components that generate the most heat. A more aggressive technique for dissipating more heat includes embedding actual copper blocks into the PCB or applying them to the outer layers. These copper blocks, often coin-shaped, are known as "copper coins." After machining the copper coins separately, they can be soldered or directly attached to the PCB, or inserted into inner layers, connected to the outer layers through thermal vias. Figure 1 shows a special cavity made in the PCB to accommodate a copper coin.
Copper has a thermal conductivity of 380 W/mK, while aluminum has 225 W/mK, and FR-4 has 0.3 W/mK. Copper, being a relatively inexpensive metal and widely used in PCB manufacturing, is an ideal choice for making copper coins, thermal vias, and ground planes—all of which can improve thermal dissipation.
Proper placement of active devices on the board is a key factor in preventing hotspots, ensuring that heat is distributed as evenly as possible across the entire board. Active devices should be distributed around the PCB without a specific order to avoid forming hotspots in particular areas. However, it is best to avoid placing high-heat-generating active devices near the edges of the board. Instead, they should be placed as close to the center of the board as possible to facilitate even heat distribution. If high-power devices are mounted near the edge of the board, heat will accumulate at the edge, increasing local temperatures. On the other hand, if they are placed near the center, heat will spread out in all directions, making it easier to lower the temperature and dissipate the heat. Power devices should not be placed near sensitive components, and there should be an appropriate spacing between them.
Further improvements to the measures taken at the layout level can be achieved by incorporating active and passive cooling systems, such as heatsinks or fans, which remove heat from the active devices rather than dissipating it directly into the board. Generally, designers must find a suitable compromise between different thermal management strategies based on the specific application requirements and available budget.
PCB Substrate Selection
FR-4, with its low thermal conductivity (between 0.2 and 0.5 W/mK), is generally not suitable for applications that require substantial heat dissipation. The heat generated in high-power circuits can be considerable, and these systems often operate in harsh environments and extreme temperatures. Using alternative substrate materials with higher thermal conductivity may be a better choice compared to traditional FR-4.
For example, ceramic materials offer significant advantages for thermal management in high-power PCBs. In addition to improved thermal conductivity, these materials have excellent mechanical properties, which help to compensate for the stresses accumulated during repeated thermal cycles. Furthermore, ceramic materials have low dielectric loss up to 10 GHz. For higher frequencies, hybrid materials (such as PTFE) are always an option, offering similarly low losses but with moderately reduced thermal conductivity.
The higher the thermal conductivity of a material, the faster the heat transfer. Therefore, metals like aluminum, in addition to being lighter than ceramics, provide an excellent solution for transferring heat away from components. Aluminum, in particular, is an excellent conductor, durable, recyclable, and non-toxic. Due to its high thermal conductivity, metal layers help to quickly distribute heat across the entire board. Some manufacturers also offer metal-clad PCBs, where both outer layers are clad with metal, typically aluminum or galvanized copper. From a cost-per-unit-weight perspective, aluminum is the best choice, while copper has higher thermal conductivity. Aluminum is also widely used in the manufacture of PCBs supporting high-power LEDs (as shown in the example in Figure 2), where its ability to reflect light from the substrate is particularly useful.
Even silver, with its thermal conductivity about 5% higher than copper, can be used for making traces, vias, pads, and metal layers. Additionally, if the board is used in a humid environment with toxic gases, a silver finish on exposed copper traces and pads can help prevent corrosion—a typical threat in such environments.
Metal Core PCBs (MCPCBs), also known as Insulated Metal Substrates (IMS), can be directly laminated into the PCB, creating a board with an FR-4 substrate and a metal core. Single and double-layer technologies with deep-controlled routing are used to transfer heat from onboard components to less critical areas. In IMS PCBs, a thin, thermally conductive but electrically insulating dielectric is laminated between the metal base and the copper foil. The copper foil is etched into the desired circuit pattern, and the metal base absorbs heat from the circuit through this thin dielectric.
l Key Advantages of IMS PCBs:
ü - Significantly better heat dissipation than standard FR-4 structures.
ü - The thermal conductivity of the dielectric is typically 5 to 10 times higher than that of conventional epoxy glass.
ü - Much more efficient heat transfer compared to traditional PCBs.
In addition to LED technology (lighting signs, displays, and lighting), IMS boards are widely used in the automotive industry (headlights, engine control, and power steering), power electronics (DC power supplies, inverters, and motor control), switches, and semiconductor relays.
