PCB forward annotation is a process in electronic design automation (EDA) software that involves sending design changes from the schematic capture stage to the PCB layout stage.
Schematic to PCB Automatic Forward/Back Annotation
Here's a typical flow for it:
Design Capture
During this phase, an electrical engineer uses a software tool to draw the schematic of the circuit. This includes all the components (like resistors, capacitors, micro-controllers, etc.), their connections, and values/properties. Each component is also assigned a unique reference designator, like R1, C1, U1, etc.
Initial PCB Layout
After the schematic is completed, it is transferred to a PCB layout tool. This process is called forward annotation. The software tool transfers all components, their reference designators, and connections (net-list) to the PCB layout environment. The PCB designer uses this information to place components on the board and route traces to make the necessary electrical connections.
Design Changes
Often, changes will be made to the design after the initial PCB layout has begun. Maybe a component value needs to be changed, or a part needs to be swapped out for a different one. These changes are made in the schematic capture tool.
Forward Annotation
Once changes have been made to the schematic, they need to be updated in the PCB layout. This process is also called forward annotation. The schematic capture tool will generate a report of changes, and the PCB layout tool will apply these changes to the layout.
Back Annotation
Sometimes, changes may also be made in the PCB layout tool - for example, if a component needs to be moved for space constraints. This change then needs to be reflected back in the schematic, a process called back annotation. This ensures that the schematic and PCB layout remain consistent with each other.
Note that the terms forward annotation and back annotation come from the idea that you are "annotating" or "marking" changes to be transferred "forward" from schematic to layout, or "back" from layout to schematic. Not all EDA software uses the exact terms, but the concepts are fundamental to the process of electronic design.