Datasheet

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AFE030

SBOS588A –DECEMBER 2011– REVISED DECEMBER 2011

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Some heat is conducted from the silicon die surface through the plastic packaging material and is transferred into

the ambient environment. Because plastic is a relatively poor conductor of heat, however, this route is not the

primary thermal path for heat flow. Heat also flows across the silicon die surface to the bond pads, through the

wire bonds, into the package leads, and finally into the top layer of the PCB. While both of these paths for heat

flow are important, the majority (nearly 80%) of the heat flows downward, through the silicon die, into the

thermally-conductive die attach epoxy, and into the exposed thermal pad on the underside of the package (as

shown in Figure 53). Minimizing the thermal resistance of this downward path to the ambient environment

maximizes the life and performance of the device.

Figure 53. Heat Flow in the QFN Package

The exposed thermal pad must be soldered to the PCB thermal pad. The thermal pad on the PCB should be the

same size as the exposed thermal pad on the underside of the QFN package. Refer to Application Report,

QFN/SON PCB Attachment, literature number SLUA271A, for recommendations on attaching the thermal pad to

the PCB. Figure 54 illustrates the direction of heat spreading into the PCB from the device.

Figure 54. Heat Spreading into PCB

The heat spreading into the PCB is maximized if the thermal path is uninterrupted. Best results are achieved if

the heat-spreading surfaces are filled with copper to the greatest extent possible, maximizing the percent area

covered on each layer. As an example, a thermally robust, multilayer PCB design may consist of four layers with

copper (Cu) coverage of 60% in the top layer, 85% and 90% in the inner layers, respectively, and 95% on the

bottom layer.

Product Folder Link(s): AFE030