Once the schematic is complete and the components have been chosen, it is time to layout and route the PCB. The first step in PCB layout it component placement. There are a few key factors that will influence the placement of components.
Mechanical Constraints / Critical Placement
Whether you are creating a PCB for a hobby project or a military project there will be some components that have placement requirements. For example, connectors (USB, DSUB, Power Jack) must be placed on the edge of the board for accessibility purposes. The placement of other components, such as buttons and other interface components, will be heavily dependent on the mechanical design of the enclosure. Components with critical placement requirements will be placed first. After critical components are placed the rest of the components can be placed using a few different key guidelines.
One component placement mantra is to group components into functional groups (Voltage Regulators, Microcontroller, ADC). This minimizes the amount of long traces required to route the board. In the case of IC’s with decoupling capacitors, it is very important to place these capacitors as close to the IC power pins as possible as they become ineffective due to the added inductance of a longer trace.
Data Sheet Guidelines
Sometimes component datasheets suggest an optimal layout for a specific part. This is common for switching regulators. If a suggested layout is presented in the datasheet this makes the component placement for that block of components relatively simple and gives the designer some peace of mind.
Always remember the physical size of the components being placed. Take great care to make sure that the components do not overlapped each other and make it impossible to be assembled. Most CAD tools have DRC (Design Rule Checks) to prevent this but it is still entirely possible to fabricate a PCB with simple mistakes like this.