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PCB DESIGN FOR THERMAL PERFORMANCE
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17.1 INTRODUCTION
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The reliability and electrical functionality of electronic components is partially determined by the temperature at which they operate.As such, control of the component temperatures in the system is an important design consideration. Factors that impact device temperatures include the power at which they operate, the air flow surrounding them, heat generation upstream to them, the environment in which the system operates (either indoors or outdoors), the system orientation (either vertical or horizontal), and a variety of printed circuit board (PCB) layout and design properties. These PCB design factors include the design of the copper (Cu) traces contacting the components, the number and area of Cu planes that are connected to them, any thermal vias that might be designed between them and the spreading planes, the proximity of other devices that dissipate power, and any cuts in the thermally conducting layers.Additional PCB features that impact the thermal performance of components include chassis screws, connectors, edge guides, and shields. To control component temperatures, PCB factors that impact thermal energy flow must be considered in the design phase of the PCB layout. These factors include many complex interactions that make application of simple equations to calculate system temperatures impossible. For example, the historical equation used to calculate component temperatures from a thermal resistance parameter called Theta-ja (qja) as shown in Eq. 17.1 is not applicable in modern systems. This is stated clearly in the Joint Electron Device Engineering Council (JEDEC) standard for qja.1 qja is not a constant. It is a function of the PCB onto which the component is placed and can vary by a factor of two or more as a function of the PCB design layout. Therefore, if component temperatures are calculated using Eq. 17.1, wildly erroneous estimates may be accepted, which might lead to a system design that fails thermally. Tjunction = Tambient + (qja * Power) where Tjunction = the temperature of the active portion of the component Tambient = the temperature of the air at a specified location qja = the thermal resistance of the component as defined by JEDEC Power = the power of the component (17.1)
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Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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This chapter describes best-design practices that enable the PCB designer to achieve the best possible thermal performance for a given design. As there is no method to calculate analytically the combined impact of the methods described, the designer is encouraged to use sophisticated tools to model the final component temperatures.
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THE PCB AS A HEAT SINK SOLDERED TO THE COMPONENT
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The PCB can be considered to be a heat sink that has been soldered to the leads or solder joints of the electronic component. The physical design of the PCB dramatically impacts its efficiency as a heat sink and the temperatures at which the components operate. Figure 17.1 helps to illustrate this point. Here, a packaged component (shown in cross section) is attached to the PCB. Heat is generated by current flowing through electrical resistances on the active surface of the die. This raises the temperature of the surface, resulting in a thermal gradient. Thermal energy (heat) flows from regions of high temperatures to regions of lower temperatures. For the component illustrated in Fig. 17.1, heat flows from the die through the die attach, then through any Cu metallization in the package substrate, then through the solder joints into the PCB. If there are good thermal conduction paths in the PCB, the heat spreads out over a large area of the PCB, allowing the potential for efficient convection and radiation into the environment. If there are few heat transfer paths in the PCB, the component is insulated and the temperature increases.
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FIGURE 17.1 Cross section of an electronic component on a PCB. The arrows indicate heat conduction paths.
How important is the PCB to the thermal performance of components Depending on the PCB design, up to 60 to 95 percent of the thermal energy can be dissipated by the PCB. This type of performance can be achieved by PCBs that meet the following criteria:
Large spreading planes to conduct heat away from the components Sparsely populated PCBs with large areas for convection and radiation Long traces interconnecting the components, again to conduct heat away from the components Sufficient spacing of PCBs in a system rack to enable adequate convection
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