PCB Layout

For this Z80 SBC, a breadboard layout was built first to prove the hardware and software design. The PCB process carries that proven design onto a manufacturable board, with the user and agent agreeing on goals, components and constraints before generating, routing and reviewing the layout.

1. Choose the PCB implementation. Adapt the proven circuit to a board: choose packages and PCB-specific details such as resistor arrays or individual resistors, connectors, and which parts are factory assembled.
2. Agree on components and constraints. The user and agent confirm the parts, component locations, board size, electrical needs, assembly, quantity and budget. The agent explains trade-offs before the user approves the design criteria.
3. Generate the layout. The agent directs KiCad's Python tools to create the schematic, component placement, board shape and routing rules. These reflect electrical needs and manufacturer capabilities; for example, on the partial-SMT board, tracks must stay at least 0.155 mm from solder-mask openings they do not connect to, matching JLCPCB's design-for-manufacturing (DFM) feedback.
4. Route the connections. The agent gives the placed design to Freerouting, which searches for copper paths and returns them for KiCad to import. The router does not choose component locations.
5. Review against the criteria. Check that connections are complete and design rules pass, then assess requirements such as keeping a component in its agreed location or matching critical trace lengths when the design requires it.
6. Iterate and deliver. If the layout misses a criterion, adjust placement or rules and route again. Once checks pass, generate and review the manufacturing files.

flowchart TD
    A["1. Choose PCB implementation<br/>packages, resistor arrays or individuals,<br/>connectors and assembly"]
    B["2. User and agent agree components,<br/>locations and design constraints"]
    C["3. Agent directs KiCad Python tools<br/>to generate placement and routing inputs"]
    D["4. Freerouting finds copper paths;<br/>KiCad imports the returned routes"]
    E["5. Review against agreed criteria:<br/>connections, design rules,<br/>locations and critical trace lengths"]
    F{"Criteria met?"}
    G["6. Adjust placement or rules<br/>and route again"]
    H["6. Generate manufacturing files<br/>and review manufacturer feedback"]

    A --> B --> C --> D --> E --> F
    F -- "No" --> G --> C
    F -- "Yes" --> H

The routing input is a DSN (Specctra Design file); the returned SES (Specctra Session file) contains the proposed routes. Gerber and drill files describe the board layers and holes. For partial-SMT assembly, the BOM (bill of materials) lists parts, and the CPL (component placement list) gives their positions and orientations. Computer checks do not replace reviewing manufacturer feedback and the assembly placement preview.