When a circuit board is designed, the components soldered to it are represented by schematic symbols and footprint models in the PCB design CAD system. Each schematic symbol type has a unique part number assigned to it to differentiate it from other similar symbols, as in two resistors with different values. This part number identifies the component’s type and value and is forwarded into the bill of materials (BOM) to purchase the correct parts for PCB assembly. When a component is replaced in the design, that schematic symbol must be updated with the new part number and device attributes. Simultaneously, the corresponding PCB footprint will be altered to model the new package if the physical part has changed.

Components are often swapped around during the design process until the final circuit configuration is decided. Once the design is completed, however, components are usually not replaced except for the following reasons:

*Design problems: Errors discovered during production may require the replacement of a part. Issues like these could be related to the performance of the board or in how it is manufactured. The replacement could be something as simple as changing the value of a capacitor or as complex as physically redesigning some critical circuitry layout.

*Unavailable parts: During production, a part may become unavailable for several reasons:

*Another OEM has bought out all the stock of a specialty part.

*The lead-time is too significant for the project’s needs.

*The component manufacturer is unable to keep up with demand.

*The part was phased out in favor of a more common component with similar functionality.

*The part was discovered to have a defect or design flaw.

*The manufacturer obsoleted the part due to other business reasons.

*Expense: Components may be replaced on a production circuit board to reduce manufacturing expenses. If a part begins to escalate in price, it’s sometimes more cost-effective to redesign the board. In some circumstances, a part will be replaced with less expensive components or redesigned circuitry.