Steps to review a PCB Layout - Part 1

Review is one of the most important phase in the product development.Especially, in the hardware development, Schematic and PCB Layout need to be looked at very carefully. Any reworks to be done on the PCB during the testing can be catastrophic for the project and can lead to project cost over shoot. Let us in this article look at the steps to perform a PCB Layout review.

What are the pre-requisites?

1. PCB Layout file

2. Schematics

3. Component Datasheets

4. Gerber Viewer

5. PCB Stack Up

6. Project Requirement Documents (to know about the project)

7. IPC standards

8. Bill of Material

Before going further on where to start the PCB Layout, one has to remember that PCB Layout review is to be done from outside the project team. This helps to find any missing points from the project team and give more confidence to the team. Most of the PCB Layout review happens with EDA tool. so, the reviewer should have knowledge of the tool.

Follow the below check points are performed in a PCB Layout review:

1. Verify that the correct part numbers are included in the schematics and the same reflects in the Bill of Material. Even though this done in the schematic review, it is always good to start with this.

2. Next verify that the footprints are created as per the required part numbers. Ensure that the IPC standards are followed.

3. Verify that the board outline is created as per the required dimensions.

4. Before starting the PCB Layout, floor planning would have been done. So, ensure that the components are placed as per the intial plan.

5. The first step for starting the PCB Layout is to ensure that all the constraints are entered into the tool. For a beginner, to give a clarity on what constraints are, below are the contraints set in the tool:

Trace-to-Trace Spacing

Trace-to-Via Spacing

Via-to-Via Spacing

Pad-to-Trace Spacing

Pad-to-Pad Spacing (Sometimes cannot be met due to component footprint)

Minimum Track width

Minimum Via size

The above constraints are set in the tool based on the design requirements and PCB manufacturing capabilities of your FAB house. Creepage and Clerance is the terminology used for the spacing requriements. These constraints are to be manually entered in the tool and once the PCB Layout is completed, designer has to run the DRC (Design Rule Check) to esnure that Design is as per the cosntraints entered. PCB Layout should be released ideally with zero DRC errors.

6. Verify the stack up is properly entered in the tool. Designer receives the PCB stack up from the FAB house based on the impedance requirements and layer counts provided.

7. Next step is to start verifying the routing on the board. If you have any Pre-Layout Simulation done on the board, ensure that the proper recommendations are followed as per the simulation results. It is always a good practise to document the simulation results and take a sign off on the same after implementing the findings in the PCB Layout.

8. Ensure that clock signals are routed in inner layers and minimum vias present on them.

9. Ensure that the crystal/oscillator is present close to the microcontroller/processor and no vias on those nets.

10. Ensure that there is proper reference plane for all the signals. This is improtant to ensure that there is proper return path for the signals. The reference plane is very critical to the high speed signals.

11. Most of the datasheets of the chips provide routing guidelines or they shall have a evaluation board Gerber. Take notes from them and try to implement as similar as possible. On your PCB it is always difficult to replicate what is done on Evaluation board. Try to replicate as much as possible and analyze technically before deviating any routing guidelines.

12. Ensure that there are no right angle traces on the board and acute angle traces at the break out near the pads. This causes reflection and effects signal integrity.

13. Ensure that the test points on the boards are accessible and are not in the shadow of height components.

14. Ensure that SMD components around the Through hole component are placed well apart for ease of placement.

15. Check if fudicials are included on the board. Re-check the requirement whether you need global or local fudicials.

16. Ensure that silk screen is present for all the components and is visible.

17. Esnure that approrpiate board part number has been included on the PCB or not. This helps to identify the board. Board revision also is must and must be included.

18. Most of the times, the BJT foot print goes wrong, esnure that pin numbering is correct in the schematic and PCB Layout.

19. Ensure proper polarity is provided for the polarized components like the Tantalums. LED cathode indication is must.

20. Ensure that pin-1 indication is provided near the ICs.

21. Ensure that the trace widths and vias are enough to carry suficient current required for the board.

22. If there are Analog and Digital grounds on the board, esnure that grounds are clearly isolated and not routing crosses the ground voids.

23. Ensure that the FAB notes are included in the PCB Layout notes or not. These are the instructions for the fabricator on the surface finish type, PCB material to be used, board thickness, copper thcikness, constraints to be followed.

24. Even though you have done a net list verification on the Schematic, it is always a best practise to do a verification of the net list generated from PCB Layout.

25. As a last check open the Gerber and have a quick check of the Layout. Ensure that all the review comments are implemented.

26. Generate step file from the PCB Layout and import the same to the mechanical softwares to check that the hardware is designed with the mechanical constraints in mind. All the height constraints, connector placements, mounting hole locations are to be checked here.

The best practise is to have check lists at each phase of the design and have a sign off on checklist before removing to the next phase.

This post was written based on the request from Shashank Vaidya on our facebook page (embeddeddesignblog.blogspot.com). Please, write to us on embeddeddesignbloggroup@gmail.com if you have any requests or comments.