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PCB DESIGN FOR THERMAL PERFORMANCE
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Thermal landing
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FIGURE 17. 4 A typical four-layer PCB cross section showing a package with an exposed pad soldered to a thermal landing that is in turn connected to the PCB ground plane through thermal vias. The vias are isolated from the power plane for electrical reasons. In this figure, the ground plane would become the thermal spreading plane assuming it was continuous over a large area.
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Thermal Planes Cu planes in the PCB can provide very effective heat spreading for the electronic components. The intent is to conduct the heat over as large an area as possible in order to optimize convection and radiation heat loss from the PCB. Figure 17.4 shows a cross-sectional view of a PCB highlighting features that make good thermal planes. These include:
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A thermal landing (sometimes called a thermal collection plate) or solder land to gather the heat that the device is shedding Thermal vias to conduct heat from the thermal landing into a buried plane (most often either the power or ground plane) Continuous Cu to spread the heat A possible thermal landing on the bottom side of the PCB where an indirect heat sink might be attached Isolation areas to ensure that the thermal vias don t short all planes in the PCB
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Many integrated circuit (IC) packages are optimized to dump power into such thermal planes in the PCB. Figure 17.5 shows images of packages with exposed pads that are intended
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FIGURE 17.5 Two package types with exposed thermal slugs on the bottom of the package intended to be soldered to thermal spreading elements on the PCB.
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to be soldered to thermal collection plates on the PCB. The die of these packages are glued directly to these exposed pads, giving a very low thermal resistance into the PCB. This thermal resistance can be found from vendor data sheets and might be listed as Theta-jc (qjc), for junction to case, or as Theta-jp (qjp), indicating junction to pad resistance. Ball grid array (BGA) packages often include thermal balls that are designed with optimized thermal conduction paths to the die inside the package. These balls should be soldered to thermal collection lands for spreading heat throughout the PCB. Often, thermal ball arrays include both power and ground balls, allowing at least two PCB planes to be used for spreading, thereby increasing the spreading efficiency of the PCB. Large thermal spreading elements that must be soldered to the PCB can sometimes present manufacturing problems. Improper control of solder paste leading to too much paste under these large thermal pads can result in floating the components on a pool of molten solder during reflow. When this happens, continuity yield suffers as the package tends to tilt one way or the other, lifting high side leads off their solder lands. Some users faced with this problem eliminated the solder between the exposed thermal pad and PCB, which led to high operating temperatures and severely degraded reliability in the field. The solution to the floating package problem is to optimize the solder paste volume, not to eliminate the thermal conduction path to the PCB. Always connect thermal management features of electronic components to the appropriate thermal collection pattern, which should then be connected through thermal vias into a thermal plane. Failure to do so will result in devices not running at the expected thermal efficiencies. Figure 17.6 shows the impact of thermal spreading plane area on the thermal performance of a 12 12 mm chip scale package (CSP) BGA-type package with 49 thermal balls. When the thermal plane size is small, the thermal performance is poor, whereas when the thermal plane size is large, the thermal performance improves by a factor of two or more. Often, suppliers of components with exposed thermal attach pads will also provide guidance as to the recommended size and shape of the thermal planes. It is best to ensure that the maximum plane area be close to the component; there shouldn t be a large thermal resistance getting from the component into the plane. Maximize the area of the thermal spreading planes to maximize thermal dissipation. It is important to point out that the plane must be continuous, that is, with few or no isolation breaks in the Cu area. Since the thermal conductivity of Cu is about 1,000 times higher than that of the FR-4, a 1 mm F-R4 break in a plane offers about the same thermal resistance
70 60 Theta-ja ( C/ W) 25 mm traces 50 40 30 20 0 20 40 60 80 Ground plane length (mm) 100 120 15 mm traces
FIGURE 17.6 The effective qja of a 12 12 mm Chip scale package (CSP) with 49 thermal balls as a function of the (x,y) length of a continuous thermal plane under the package. Two trace lengths were assumed, showing different amounts of coupling of heat from the traces into the plane.
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