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Fanout usually refers to the number of devices that an output is connected to - so if the output of an inverter is connected to 10 other inverters, the fanout is 10. High fanout is usually a bad thing since it adds capacitive loading.
hi,
U can use fanouts to make routing easier and to ensure that connections are made. Fanouts are most useful for some complex SMDs, like BGAs. Layout tools places fanout vias and routes from vias to corresponding pins for ordered components or separate pins.
Fanout, as used in DXP, places short track segments and vias to make connections to inner rows of pins on high pin count devices, or to pins on a device with very closely spaced pins. It is an attempt to make it easier to connect traces to devices with close spaced, or multiple rows of, pins.
An example would be a BGA package. When routing, you need to connect pads that are arranged in concentric rings. To get traces to the inner rings of pads, you sometimes have to run traces from the second ring pads between the pads for the outside ring of pads - that is called fanning out the second ring connections.
Likewise, if the pins of a QFP device are very close together, there isn't enough room to place vias with the same spacing as the pins in order to make connections. In this case the vias would be placed away from the device pins at a distance that allowed proper spacing clearance. Traces from the actual device pins would then be "fanned out" from the device pins to the vias.
The best analogy I can think of is for you to hold out your hand and spread your fingers apart. You fingers in that position are "fanned out" from your palm. The fanout function in DXP does the same sort of thing with traces and vias to make connections easier to route.
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