Good thermal via placement puts copper close to the source of the heat to be removed. An array of thermal vias is commonly placed under a surface mount component.
Extend copper areas out from the part where practical, and place additional thermal vias in the immediate vicinity. AVOID THERMAL RELIEF ON VIAS.
Vias in the pad should be small enough to inhibit solder wicking during reflow. Vias not in pads should be large enough to plate up well.
Higher layer counts, thicker copper foil, and larger pour/flood areas will all improve thermal conductivity.
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