One of the first things you learn as a student pilot is that you don’t go flying unless you have a checklist to follow. At a minimum, there’s a pre-flight checklist, a start-up checklist, a take-off checklist, a landing checklist, and a shut-down checklist. Even though it’s been many years since I’ve been at the controls of a plane, I could probably still go through many of the steps from memory after having followed them from the checklists so many times.
Following checklists for flying is critical, as not doing so can result in disaster. The same is true while designing a printed circuit board. There are so many tasks that must be done in the correct order that are too easy to miss without a checklist to follow. And, missing an important item on the list during layout can result in manufacturing delays of your project. If you don’t already have one, here are some ideas on how to create a simple but effective PCB layout checklist for yourself.
Start With the Basics in Your PCB Layout Checklist
To start your checklist, start at the foundation of your PCB design and work your way through it. Some fundamental tasks must be done first to complete a successful layout, and you want to be sure to account for them. In fact, many PCB design departments will have multiple design reviews with different checklists for each review. These could include a pre-layout review, a component placement review, and a critical routing review, for example. All of these elements are included here and you can decide how best to organize the list, or lists, for your workflow:
- Complete: Has the circuitry been completed in the schematic? It is a good idea to review the schematic prior to the layout to ensure that any missing circuitry is caught before the physical layout is started.
- Checked: Have all of the schematic design rule checks (DRCs) been run and any errors cleaned up?
- Ready for layout: Have all of the components been updated to their latest library versions? Many board designs have been ruined because the schematic still contained preliminary library parts instead of their final versions.
- Board outline
- Correct documentation: Do you have the latest board information to work with? This would include mechanical dimensions, as well as connector and other mechanical part locations.
- Available data files: If you are working with mechanical CAD data that will be used in your PCB layout tools, are those 3D files ready and up to date?
- Board layer stackup: Do you have board layer stackup finalized with the correct configuration and materials? This is important for good signal integrity and manufacturability.
- Parts built: Have all the required footprints been built from current data sheets, or their model files pulled in from outside sources?
- Parts checked: Have all of the footprint models been verified for accuracy?
- Design rules:
- Updated specifications: Do you have all of the specific design data needed for rule creation?
- Rule creation: Have the design rules been created and loaded into the PCB CAD database?
- Unique rules and constraints: Have all additional design rules and constraints been added? These would include area rules, high-speed design rules, and more.
This will lay a very good foundation for the rest of your checklist, which we will start next with component placement.
PCB Component Placement and Trace Routing Checklist
Component placement, routing, and DRCs are the heart of a PCB layout. The first thing to look at is the placement of mechanical parts, such as connectors, that have specific locations:
- Mechanical parts placement:
- Verified with mechanical design: Have all connectors and other mechanical parts been placed correctly? Having a 3D view of the board design within the CAD system will help to confirm that your connectors are located correctly for the device enclosure.
- Component placement:
- Sensitive components: Have high-speed components and other sensitive parts been placed to best accommodate their signal paths? These parts may also need to be isolated from other noisy parts of the circuitry.
- Power components: Have power components been placed correctly with regard to distributing power and shielded from other sensitive circuitry on the board?
- Discrete components: Have all the decoupling capacitors and termination resistors been placed according to the active parts they are servicing?
- Thermal and vibration: Is there adequate airflow and heatsinks for cooling, and are there any unique vibration issues to consider?
- Manufacturability: Do all components have sufficient spacing between them and are rotated correctly for the best yields in PCB manufacturing?
- All parts placed: Believe it or not, it is not unusual for a PCB to be sent to manufacturing without all the parts placed and routed. This can happen in design systems where the parts are not on the screen and the operator has not queried the system to see if all the parts have been placed.
- Routing complete: As with placement, sometimes unrouted nets can be missed and an incomplete board is sent to manufacturing.
- Good routing practices: This can include different details depending on the technology of the board being routed. Here is a sampling:
- Are signal paths routed short and direct?
- Are impedance controlled signals routed at the correct width, space, and on the correct layers?
- Are clock lines and other sensitive traces separated from other routing to avoid crosstalk?
- Have adjacent layers been routed with alternating horizontal and vertical traces to avoid broadside coupling?
- Has the differential pair routing been completed with the correct spacing?
- Are there any traces routed across split planes that should be relocated?
- Routing cleanup: Have wandering traces, loops, and antennas been cleaned up?
- Power and ground routing: Do all components have sufficient connections to power and ground without violating manufacturing requirements?
- Power and ground planes: Are the planes laid out to provide clean return paths as well as equal power distribution?
- Component clearances: Are there any violations with the component-to-component, component-to-board edge, and component height (3D) clearances?
- Routing clearances: Are there any routing clearance violations to other routing, land patterns, metal planes, or other board objects?
- High-speed design rules: Have all of your high-speed traces been routed at the correct lengths and with the proper topologies?
At this point, it may seem like the board is complete, and from a layout perspective, it is. However, there are a few more details required before sending the board out for fabrication and assembly. Here are the final points to verify in your checklist:
- Test points: Have specific probe points for technicians and test points for in-circuit test been assigned?
- Silkscreen: Have all of the reference designators, text, and graphics on the board been positioned and sized so that they are readable and manufacturable?
- Drawings and documentation: Have all manufacturing drawings, bill of materials, and other documentation been generated and checked?
- Manufacturing files: As the last step, Gerber files or manufacturing databases need to be generated and packaged together with the documentation for delivery to your contract manufacturer.
Remember, this is a very generic checklist that should be customized for your workflow and the PCB technology you’re designing. A good resource for fine-tuning your checklist would be with the PCB contract manufacturer you’re working with. This can ensure you have done everything your CM needs to optimally assemble your board.