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Thermal Design

The Most Comprehensive Principles of Thermal Design for PCBs

Serious thermal environment stress has a bad influence on the normal operation of most electronic components, leading to the quick failure of electronic components that will push the failure of the whole product. In recent years, with the application of large-scale and hyper-scale integrated circuits (ICs) andsurface mount technology (SMT), electronic products embrace the features of miniature, high density and high reliability. Electronic products in aerospace field are the most susceptible to the features of high integrity, high accuracy, high complexity and miniature, which results in the increasing requirement of thermal design of electronic system. Heat problem has been an important element in influencing the performance and reliability of electronic system.


As a significant part of electronic devices, whether PCBs have a reasonable design is directly related to the performance of the devices. If PCB design fails to meet the thermal design requirement, electronic devices will possibly be damaged. The growing circuit module integrity and massive application of ICs and multi-chip module (MCM) contribute to the improvement of assembly density, leading to the big density of heat flow on PCBs. A PCB with excellent performance not only has correct connection in terms of layout and routing, but also can ensure its thermal reliability for safe operation. Therefore, it's of great significance to implement thermal design and analysis for PCBs.

The Basic Principles of Thermal Design

Thermal design is based on the basic theory of heat transfer theory and fluid mechanics. Where there's a temperature difference, there's heat transfer from high temperature zone to low temperature zone. There are three methods of heat transfer: heat conduction, convection and radiation.


The formula of heat transfer is displayed as: φ=KAΔt, in which φ stands for the amount of heat transmission whose unit is WK stands for the coefficient of heat transmission whose unit is W/(m2 x K)A stands for the area of heat transmission whose unit is m2 and Δt stands for the temperature difference between thermal fluid and cold fluid whose unit is K.


Thermal design of PCBs refers to the process in which the thermal resistance from heat source to heat consumption space is decreased to the minimum through cooling measures by thermal transmission attributes or the density of heat fluid is controlled within the reliable range. In order to ensure the reliability, valid thermal design measures must be taken, including:


a. Natural cooling


A type of conducting heat without using external power, including heat conduction, radiation heat transfer and natural convection transfer.


b. Forced air cooling


This makes cooling air flow through electronic devices or components, transferring heat from heat source to heat sink through ventilator or ram air.


c. Fluid cooling


There are two methods of fluid cooling:


1). Direct fluid cooling refers to the process during which components are directly soaked into the fluid coolant.

2). Indirect fluid cooling refers to the process during which components are not directly in touch with fluid coolant. Cooling is carried out through heat exchanger or cold plate.


d. Evaporation cooling


This is the most effective heat conduction method up to now. Thermal transmission is obtained through the ebullition of cooling medium.


e. Other types of cooling: thermotube, cold plate, thermoelectric refrigeration.


In the process of thermal design, suitable thermal design measures can be made according to the practical conditions such as the practical operation environment (temperature, humidity, atmospheric pressure, dust etc.), thermal fluid density on board, power volume density and total power consumption, surface area, volume, heat sink and other special conditions, in order to ensure the even distribution of temperature and reasonable temperature increase within regulated limited value.

Thermal Design

The purpose of thermal design is to control the temperature of all the electronic components inside electronic products, to ensure the stability of electrical performance, to avoid or reduce temperature drift of electrical parameters, to decrease the basic failure rate of electronic components, and to make the temperature in the operation environment not exceed the highest allowable temperature. This article will discuss thermal design of PCBs in 3 perspectives: the utilization of components on PCBs, thermal design of PCBs, components assembly and PCB layout.


There are two methods of fluid cooling: direct fluid cooling and indirect fluid cooling.


a. Components Temperature Control


Temperature is the first element influencing component performance and failure rate. The highest allowable operation temperature and power consumption should be determined according to the required reliability of products and the distributed failure rate of each component. Table 1 shows the values of allowable surface temperature of components from the perspective of reliability in thermal design.


b. Components Junction Temperature Control


Component junction temperature depends on its own power consumption, thermal resistance and environment temperature. Therefore, measures of controlling junction temperature within an allowable range include:

• Components with small internal thermal resistance are picked up.

• Derating is used to decrease the temperature rising.

• Elaborate thermal design for reliability is carried out based on concerning standard manual for circuits and the components contained.


c. The Derating Design in the Utilization of Components


Based on needs, derating design can be implemented in practical usage in order to make components perform in the condition of below rated parameter (power, voltage, current) so that temperature rising and failure rate can be effectively decreased.


d. Thermal Design of PCBs


The vertical installation of PCBs is good for heat dissipation and the distance between boards should be at least 2cm. The rules of thermal design of PCBs include:


1). Material with ability of anti high temperature and high conduction parameter is picked up as the substrate material of PCBs. For the circuit with high power and density, aluminum base and ceramic can be used as substrate material for their small thermal resistance.


2). Multi-layer structure is the best choice for PCB design.


3). In order to enhance the heat conduction ability of PCBs, it's best to use heat-dissipation PCBs. Metal sandwich board is used in multi-layer PCBs to ensure excellent heat dissipation from board to strut member and sandwich heat sink. Protective coating and encapsulating material can be used when necessary to increase the heat transmission to strut member or heat sinks.



4). In order to enhance the heat dissipation ability of PCBs, a busbar can be used that can be regarded as an excellent radiator and is capable ofincreasing the anti-interference performance of PCBs.


5). In order to increase the heat dissipation ability of PCBs, the thickness of metal foil should be increased, and especially the inner conductor should use metal foil with large area. What's more, ground wire of PCBs should be properly widened because ground wires with big area are both capable of increasing the anti-interference ability and dissipating heat.


d. Components Assembly and Layout on PCBs


Components layout is quite essential to the heat dissipation of PCBs, especially those PCBs vertically placed. Placement of components should conform to the flow characteristics of coolant to provide coolant with the least resistance. The rules applying to components in terms of PCB Assembly and layout include:


1). For products with free convection air cooling method, it's best to arrange ICs or other components in lengthwise arrangement as example in Figure 2.


4). In order to enhance the heat dissipation ability of PCBs, a busbar can be used that can be regarded as an excellent radiator and is capable ofincreasing the anti-interference performance of PCBs.


5). In order to increase the heat dissipation ability of PCBs, the thickness of metal foil should be increased, and especially the inner conductor should use metal foil with large area. What's more, ground wire of PCBs should be properly widened because ground wires with big area are both capable of increasing the anti-interference ability and dissipating heat.


d. Components Assembly and Layout on PCBs


Components layout is quite essential to the heat dissipation of PCBs, especially those PCBs vertically placed. Placement of components should conform to the flow characteristics of coolant to provide coolant with the least resistance. The rules applying to components in terms of PCB Assembly and layout include:


1). For products with free convection air cooling method, it's best to arrange ICs or other components in lengthwise arrangement as example in Figure 2.



2). The components on the same PCB should be classified and arranged according to their heat productivity and heat dissipation level. Components with small heat productivity or bad heat resistance (such as small signal transistor, small-scale IC, electrolytic capacitor) should be placed at upstream (entrance) while components with big heat productivity or good heat resistance (such as frequency transistor, hyper-scale IC) should be placed at downstream. At the periphery of small-signal amplifiers should be placed components with small temperature drift and liquid medium capacitors should be far away from heat source.


3). On the horizontal direction, components with high frequency should be arranged adjacent to the edge of PCBs in order to shorten heat transmission path. On the vertical direction, components with high frequency should be arranged close to the upper part of PCBs in order to decrease their influence to the temperature of other components.


4). Components that are sensitive to the temperature should be arranged at the area with the lowest temperature such as the bottom of a product. They mustn't be placed right above components generating heat and they should be placed far from the components generating heat or be isolated from them.


5). Components with the most power consumption and heat generation should be arranged adjacent to the best place for heat dissipation. Never arrange the components with high temperature at the corner or edge unless radiators are around them. When arranging power resistors, relatively large components should be picked up and enough heat dissipation space should be left for them in the process of PCB layout.


6). Power should be distributed evenly on PCBs in order to maintain the balance and conformity and avoid the concentration of heat points. It's generally difficult to reach strict uniformity but areas with extremely high power must be avoided in case over-heated points will influence the normal operation of the whole circuit.


7). In the process of PCB design, air flow path must be taken into full consideration and components must be reasonably arranged. Air tends to flow towards the place with little resistance so relatively big airspace should be avoided when arranging components on PCBs.


8). Heat installation technology should be used on PCBs in order to obtain relatively good heat transmission effect. Over half of the heat of components such as DIL components, ICs and microprocessors is transmitted to PCBs through their own leads whose assembly holes should use metalized plating holes. These components can also be directly mounted on heat conduction stick or board to decrease the thermal resistance of PCBs caused by components.


9). Thermal resistance should be decreased as much as possible in the connections between components with high heat dissipation and PCBs. In order to meet the requirement of heat attributes, some heat conduction materials can be used under the chip and the heat dissipation of components in the contact area should be maintained.


10). Pins of components should be shortened in the connection of components and PCBs. When picking up components with high power consumption, conductivity of lead material should be considered. If possible, components should be chosen whose leads have the largest cross section and that have the most pins.


e. Other Requirement (Other aspects concerning PCB thermal design should be considered.)


1).Component Package


Component package type and heat conduction rate should be considered in the PCB thermal design. Heat conduction path can be provided between substrate and component package and air break should be avoided on heat conduction path.


2). Technique Method


Local high temperature can be caused in the areas with components on both sides of the board. In order to change the heat dissipation condition, some fine copper can be added in the soldering paste so that the solder points will rise to a certain height under components. Airspace is increased between components and PCB so that thermal convection heat dissipation can be increased.


3). Heat Dissipation Holes


Some heat dissipation holes and blind holes can be arranged on PCBs so that heat dissipation area can be effectively increased, thermal resistance can be decreased and power density of PCBs can be increased.

Thermal Analysis

Based on computational heat transfer, thermal analysis whose numerical computation methods mainly include finite difference method, finite element method and boundary element method, referring to the process of simplifying modules, establishing math modules, solving non-linear equation, making and adjusting analytical procedure and computation, measurement and test of thermal parameters.


As a basic of thermal design, thermal analysis is an important method of valuing the importance of thermal design. PCB thermal analysis refers to the process of establishing the thermal module of components and set simulation control parameters according to the structure and raw material of PCBs, package type of components and PCB operating environment to estimate values of thermal behaviors of PCBs. Thermal analysis must be carried out in the concept phase before layout and throughout the whole process of PCB design.


Values of component temperature, board temperature and airflow temperature can be obtained from thermal analysis, displaying the thermal attributes of PCBs in the form of colored pictures, temperature isotherm visual graphics or specific data.


Based on the result of thermal analysis, thermal problems of PCB can be found out quickly so as to take timely measures and eliminate high temperature dense areas, which will determine the heat conduction path, optimize the positions of key components, the shape of radiator and size to fully take advantage of heat dissipation rate, increase heat transmission efficiency of heat dissipation holes and radiators and determine the space between boards and components on boards.