Beyond Design: High-speed Rules of Thumb
Global PCB Connections: A Technical Overview of Long-flex Printed Circuit Boards
The Shaughnessy Report: A Handy Look at Rules of Thumb
PCB Cooling Strategies, Part 1
January 19, 2018 | Bin Zhou, EDADOCEstimated reading time: 7 minutes
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
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