PCB Cooling Strategies, Part 1


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  • FR-4 (epoxy fiberglass cloth substrate) has high mechanical and dielectric properties, good heat resistance and moisture resistance, and good machinability. The heat resistance is superior to other substrates, and they can resist delamination and foaming for 150 seconds at a temperature of 288°C. The peel strength of the thermal stress test is also larger, reaching 1.5 N/mm. Its thermal conductivity is about 1.0W/mK. High-Tg FR-4 is even more tolerant to high temperatures. Because of these improved specifications, FR-4 is priced higher than the previous two materials.
  • High-density-interconnect (HDI) designs usually use resin-coated copper (RCC), also known as coated copper foil. With increased durability and high anti-peel strength, it is easier to manufacture, and its smooth surface makes it suitable for smaller lines. However, due to the thin copper surface, and the media contains only resin, not glass fiber, so the hardness and heat transfer capacity is not as good as the other substrates.
  • Ceramic substrate is a ceramic medium embedded in the copper foil, forming a special CCL. This material has excellent electrical insulation properties and high thermal conductivity, excellent solderability and high adhesion strength. It is generally used in the military and aerospace industries and is more expensive than the other substrates.
  • Aluminum substrate is a type of metal CCL with good heat dissipation capabilities. It is generally designed for single-sided boards, mainly used in the design of LED light boards and low-end power boards. It is also used for high-end double-sided boards. There are very few applications for multilayer designs. Aluminum substrates minimize thermal resistance, with excellent thermal conductivity, electrical insulation properties and machining performance. Aluminum substrate voltage can take up to 4,500V and thermal conductivity levels of above 3.0W/mK.

It’s clear that paper-based and composite substrates are no longer suitable for current heat treatment applications, even when considering their lower cost. RCC is limited in HDI use, and its cooling capacity is very weak. Ceramic and aluminum substrates are the most recommended of these materials, with absolute advantages in heat dissipation and heat resistance, but these two materials are expensive and costly and need to be carefully selected based on the condition of their products. That leaves FR-4. While FR-4 does not have the same cooling capacity as ceramic and aluminum substrates, its price is much lower, and its thermal performance is also moderate enough to deal with the general circuit design. It is the most widely used PCB substrate.

After the PCB plate material is selected, the stack-up setup will begin. Each project has its own stack-up, the number of cascades required, and the cooling performance we have been discussing. Before planning the stack-up, we select the substrate with the lowest loss, highest stability, and highest thermal conductivity for good thermal management.

We must consider several things when designing the stack-up. First, we must consider the copper thickness. A thicker copper core layer can improve thermal management. Printed wire has a certain resistance and the passage of current through it will produce heat and cause a voltage drop. The higher the current through the wire, the higher the temperature. If the wire is heated for a long time, the copper foil will fall off due to the decrease of adhesive strength. Increasing the copper thickness can suppress the increase of the junction temperature of the components.

Second, in a multilayered PCB design, the number of power and GND planes will be considered. Heat can be dissipated through a large area of copper foil. Since the heat won’t be concentrated in a small area, the components will not be damaged. During the design process, the electrical interconnection between different layers is achieved by adding a via hole on the PCB board. The multilayer GND planes are connected to enlarge the heat dissipation area, thereby greatly improving the heat dissipation capability on the PCB. Figure 5 depicts a 2mm thick 8-layer laminate as an example: In addition to two outer and two inner layers, we designed four flat plane layers. The three GND layer not only ensure that the signal line aligns to the reference plane, but also maximizes the number of planes to achieve the best flat heat dissipation.

fig5-eda.jpgAluminum substrates are commonly used in single-sided boards, sometimes with double-sided boards, and rarely in multi-layer boards. To improve its cooling capacity, we must increase the thickness of the aluminum because that allows for better heat dissipation. Under normal circumstances, single-layer heat dissipation is better than the double-sided. This is because, as the single-layer aluminum is exposed to air, the heat dissipation area increases, creating a direct heat exchange with the air. On the other hand, double-sided aluminum is caught in the middle by the thermal conductivity of thermal plastic and insulation effects, so the heat cannot be directly distributed. Aluminum laminate stack-up is more fixed, as seen in Figure 6.

fig6-eda.jpg

Editor's Note: Part 2 of this article will appear in next week’s issue of the Inside Design Newsletter.

Bin Zhou is a senior PCB design engineer for EDADOC. He has 10 years of experience in high-speed design. His responsibilities include high-speed PCB design solutions, HDI, R&D and training.

 

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