PCB Cooling Strategies, Part 1
With the development of communication and IT industries and the ever-increasing demand for information analysis, many chip makers have racked their brains trying to provide customers with better technology, such as increased computing power and storage capacity of chips as well as diversifying their product offerings.
For example, Huawei unveiled the Kirin 970 processor and Apple introduced their A11 processor to entice potential customers. However, the heat generated by the chips during operation, especially during high-speed operation, causes the internal temperature of the mobile phone to rise rapidly. If the heat is not effectively dissipated, the internal parts of the mobile phone will fail due to overheating and the reliability will decline. This issue must be addressed properly, or you will see disastrous results similar to Samsung’s Galaxy S7 phones. The circuit board heat management is very important, not just for cell phones, but for other electronics as well.
Traditionally, electronic equipment was cooled through technical means, structural modes and design techniques to meet the requirements of reliability and service life. With the improvement of communication and IT products and the ever-increasing demand for portability, the power consumption of information equipment is on the rise while the volume tends to decrease. Device density is high, so high heat flux density cooling needs are becoming more urgent, and thermal design will face enormous challenges.
Each electronic product requires a specific thermal design methodology, from the early architecture design and device selection, through PCB design, final assembly and packaging. Each section has a corresponding heat management plan. This requires thermal design engineers to use their theoretical knowledge combined with practical experience to develop a reasonable thermal design.
PCB design has a direct impact on product performance and time to market. Devices on the PCB have their own temperature range, and any temperature outside of this range will greatly reduce the efficiency or failure of the device, resulting in damage. Therefore, heat dissipation is a key issue to consider in PCB design.
The PCB substrate is in direct contact with the components, so its cooling capacity directly affects the cooling of the entire system. We know that the heat of components is not conducted by the PCB itself, but by the surface of the components from the ambient air. So, the heat dissipation characteristics are dependent on the size of electronic products. Today, the miniaturization of product components and high-heat-generating assembly mean that we will need to alter our cooling methods. Heat dissipation on the surface will not be enough for conventional PCB designs.
At the same time, due to the use of QFP, FPGA, BGA and other highly integrated surface mount components, the heat generated by the components will be transmitted to the PCB. Therefore, the best way to increase the PCB’s cooling capacity is by conduction through the PCB itself. This means the choice of the PCB laminate is particularly important.
When selecting the PCB laminate, we must analyze the work environment. Plate heat dissipation capacity and thermal conductivity & heat resistance are closely linked. Thermal conductivity (K, °C) is the quantity of heat transmitted under stable heat transfer conditions.
Currently, there are five types of substrates used in the PCB market. They are: paper substrates, composite substrates, epoxy glass fiber cloth substrates, adhesive-coated copper foil (RCC), and special substrates in HDI.
- Paper substrates (FR-1, FR-2, FR-3) are the cheapest, but the soldering temperature is more stringent and is easily dampened and blistered. Any temperature of more than 260°C will cause it to turn yellow and it has poor heat characteristics. The thermal conductivity is much lower than 1.0W/mK.
- Composite substrates (CEM-1, CEM-3) are made of two kinds of materials: fiberglass cloth base and wood pulp paper base. It is an upgraded version of the paper substrate, with improved performance & mechanical machinability. However, it can only maintain 50 seconds at 260°C (Figure 4). Compared to the paper substrate alone, the thermal shock resistance does not greatly improve, and the thermal conductivity is less than 1.0W/mKK
- 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.
Aluminum 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.
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.