Methods for high-power PCB heat dissipation

The entire power electronics industry, including RF applications and systems involving high-speed signals, is moving towards solutions that provide increasingly complex functions in increasingly smaller spaces. Designers face increasingly demanding challenges in meeting system size, weight and power requirements, including effective thermal management, which starts with the design of the PCB.

Highly integrated active power devices, such as MOSFET transistors, dissipate a lot of heat, so the PCB is required to transfer heat from the hottest components to the ground or heat dissipation surface to operate as efficiently as possible. Thermal stress is one of the main causes of power device failure because it can cause performance degradation and may even cause system malfunction or failure. The rapid growth of device power density and the continuous increase in frequency are the main causes of overheating of electronic components. Although semiconductors with lower power losses and better thermal conductivity, such as wide bandgap materials, are becoming more and more widely used, they are not sufficient in themselves to eliminate the need for effective thermal management.

The junction temperature that can be achieved with current silicon-based power devices is between about 125℃ and 200℃. However, it is best to always operate the device within this extreme condition to avoid rapid aging of the device and shorten its remaining life. In fact, it is estimated that if the operating temperature increases by 20°C due to improper thermal management, the remaining life of the components will be reduced by up to 50%.

Layout Methods

The common thermal management method adopted in many projects is to use a substrate with a standard flame retardant grade 4 (FR-4), which is an inexpensive and easy to process material, and focus on the thermal optimization of the circuit layout.

The main measures adopted involve providing additional copper surface, using thicker traces, and inserting thermal vias under the components that generate the most heat. A more radical technique that can dissipate more heat involves inserting an actual copper block into the PCB or applying it to the outermost layer. This copper block is usually in the shape of a coin, hence the name “copper coin”. After the copper coin is processed separately, it can be soldered or directly attached to the PCB, or it can be inserted into the inner layer and connected to the outer layer through thermal vias. A special cavity is made in the PCB shown in Figure 1 to accommodate a copper coin.

Copper has a thermal conductivity of 380W/mK, compared to 225W/mK for aluminum and 0.3W/mK for FR-4. Copper is a relatively cheap metal that is widely used in PCB manufacturing; therefore, it is an ideal choice for copper pendants, thermal vias, and ground planes—all solutions that improve heat dissipation.

The correct placement of active components on the board is a key factor in preventing the formation of hot spots, thereby ensuring that the heat is distributed as evenly as possible across the board. In this regard, active components should be placed around the PCB in no particular order to avoid the formation of hot spots in specific areas. However, it is best to avoid placing active components that generate a lot of heat near the edge of the board. Instead, they should be placed as close to the center of the board as possible to facilitate even heat distribution. If a high-power device is mounted near the edge of the board, heat will accumulate at the edge, increasing the local temperature. On the other hand, if it is placed near the center of the board, the heat will be dissipated in all directions along the surface, making it easier to cool and dissipate the heat. Power devices should not be placed close to sensitive components and should be properly spaced from each other.

Measures taken at the layout level can be further improved by using active cooling and passive cooling systems such as heat sinks or fans, which remove heat from active devices rather than dissipating it directly into the board. In general, designers must find the right compromise between different thermal management strategies based on the requirements of a specific application and the available budget.

PCB Substrate Selection

FR-4 is generally not suitable for applications that need to dissipate a lot of heat due to its low thermal conductivity (between 0.2 and 0.5W/mK). 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 than using traditional FR-4.

For example, ceramic materials offer significant advantages for thermal management of high-power PCBs. In addition to improved thermal conductivity, these materials also have excellent mechanical properties, which help compensate for the stress accumulated during repeated thermal cycling. In addition, ceramic materials have low dielectric losses at frequencies up to 10GHz. For higher frequencies, there is always the option of hybrid materials such as PTFE, which offer equally low losses but with a modestly lower thermal conductivity.

The higher the thermal conductivity of a material, the faster it transfers heat. Therefore, metals such as aluminum, in addition to being lighter than ceramics, offer an excellent solution for transferring heat away from components. Aluminum in particular is also an excellent conductor, has excellent durability, is recyclable, and is non-toxic. Due to its high thermal conductivity, metal layers help to quickly transfer heat throughout the board. Some manufacturers also offer metal-clad PCBs, where both outer layers are metal-clad, usually aluminum or galvanized copper. Aluminum is the best choice from a cost-per-weight perspective, while copper has a higher thermal conductivity. Aluminum is also widely used to make PCBs that support high-power LEDs (as shown in the example in Figure 2), where its ability to reflect light away from the substrate is particularly useful.

Even silver, with its approximately 5% higher thermal conductivity than copper, can be used to make traces, vias, pads, and metal layers. Additionally, if the board is used in a humid environment with toxic gases present, using a silver finish on exposed copper traces and copper pads will help prevent corrosion – a known threat in such environments.

Metal PCBs, also known as Insulated Metal Substrates (IMS), can be laminated directly into PCBs to form a board with an FR-4 substrate and a metal core. Single and double layer technologies are used with deep controlled routing to transfer heat from on-board components to less critical areas. In IMS PCBs, a thin layer of thermally conductive but electrically insulating dielectric is laminated between the metal substrate and the copper foil. The copper foil is etched into the desired circuit pattern, and the metal substrate absorbs heat from the circuit through the thin dielectric.

The main advantages provided by IMS PCBs are as follows:

· Significantly higher heat dissipation than standard FR-4 construction.

· The thermal conductivity of the dielectric is typically 5 to 10 times higher than that of ordinary epoxy glass.

· Heat transfer is much more efficient than with traditional PCBs.

In addition to LED technology (illuminated signs, displays and lighting), IMS circuit boards are also widely used in the automotive industry (headlights, engine control and power steering), power electronics (DC power supplies, inverters and engine control), switches and semiconductor relays.