Inputs to the Layout Process
Files
- Mechanical DXF Files are provided when available
- Netlist from the schematic capture tool
- Datasheets
Mechanical Requirements Detailing
- Board outline
- Mounting holes
- connector placement
- keep out zones
- component high restrictions
More Inputs to the Layout Process
Electrical Requirements Identify
- High current signals
- Signals sensitive to crosstalk or coupling
- clocks
- isolated signals and barriers
- Transmission line signals
- Microstrip, Strip line
- Controlled impedance signals and pairs
- High impedance signals
- Desired number of layers
- Minimum trace width
- Approximate component placement, circuit sectioning
- Grounding guidelines
Milestones
The Layout Process has 2 review milestones
- The initial review occurs after the board outline, component footprints and component placement is completed.
- The second and final review of the board occurs after routing and design or rules checking prior to fabrication of the circuit board.
- The final review includes checking of the gerbers files. Schematics and layout files will be archived and the revision numbers incremented each time a board is fabricated with changes.
PCB Layout CheckList
This checklist is designed to aid with reviews.
1. Check component placement organization, component orientation, and that components are on a grid.
2. Check trace lengths.
- a. Critical lines such as USB and PCI are required to be the same electrical length
- b. keep clock signals as short as possible
3. Check trace lengths.
- a. Ensure high current traces are wide enough
- b. Generally, analog or high speed signal traces need to be wider than digital traces
4. Check Trace Separation.
- a. Ensure that coupling from adjacent traces will not occur onto sensitive signals
- b. Ensure spacing for voltage isolation requirements is met
- c. Check that guard traces are used where needed. The guard traces can be power or ground. Ground is preferred.
- d. Verify that copper pours are around analog sections, if needed.
5. Check that Power Planes and ground planes are logically organized. Make sure that they are poured and not routed.
6. Check that traces do not cut across power or ground planes unnecessarily.
7. Check that the grounding method is consistent, either:
- a. single point star
- b. grid
- c. pour
8. Check the placement of zero ohm resistors which connect ground planes.
9. Check controlled impedance trace separation and separation from ground.
10. Check that high impedance traces do not have a power or ground pour above or below them.
11. Make sure that mounting holes, board dimensions and heat sinking are correct with respect to the mechanical design.
12. Verify that the components have the correct pin outs and footprints
- a. special care is necessary with SOT-23 packages (transistors, diodes, ect.)
- How pins are numbered can vary between manufacturers.
- Verify that components with tab connections are connected correctly.
13. Verify that clock traces are short.
14. Place series resistors close to the signal source.
15. Place shunt resistors close to the signal termination.
16. Verify that the outside of the board Keep-outs (and mounting hole Keep-outs) are maintained.
17. Verify that mounting holes are grounded to the appropriate ground type, or are left floating (if the design calls for this).
18. Verify that bypass capacitors are placed close to the device power pins.
19. Verify that the correct revision numbers are displayed on the board with the silkscreen. Verify that any other items that are silkscreened are correct.
20. Make sure that the DRC/ERC is done on the board for connections vs. the schematic netlist, clearances, ect. and remove any errors that may be present.
21. Verify the packages that were placed for the board match the footprints in the datasheet and for the P/Ns in the BOM
22. Make sure any manual PCB changes are back annotated to the schematic.
23. Check Fab notes, material call outs, board and copper thickness, and plating.