Factors in component placement

Placement decides the quality of routing, noise, heat and manufacturability, so it is done before routing. Factors:

1. Functional grouping: place related components together by circuit block (analog, digital, power, RF), in the order of signal flow, to keep traces short; keep noisy and sensitive blocks apart.
2. Connector and mechanical constraints: put connectors, switches, LEDs and mounting holes where the enclosure needs them; place I/O connectors at one edge, and leave clearance for assembly.
3. Critical components first: place the processor, clock/crystal (close to the chip), decoupling capacitors (adjacent to the IC power pins) and sensitive analog parts first, with short critical routes.
4. Thermal considerations: keep heat-generating parts (regulators, power transistors) spaced and with a heat sink/air flow, and away from temperature-sensitive parts like electrolytic capacitors; do not put them in a row blocking air flow.
5. Orientation and alignment: align similar components in the same direction for easier assembly, inspection and soldering; polarised parts in a consistent orientation.
6. Routing and layers: minimise crossing ratsnest lines; leave channels for traces, and place components on one side where possible (cheaper assembly).
7. Manufacturing rules: spacing for pick-and-place, solder (wave soldering direction), and testing/rework access, test points.
8. EMI and power: place power entry filters at the connector; separate high-current paths from signals.