We are all used to policies and procedures that have to be followed both at work and outside of the office. The word guideline is often used to describe these and has become synonymous with a strict set of rules that have to be followed with negative consequences if you don’t. Interestingly enough though, guideline has another definition as well: “A cord or rope to aid a passer over a difficult point or to permit retracing a course.”
It hadn’t occurred to me that a guideline could be thought of as something positive and helpful instead of being so restrictive. Guidelines are the tools that will help us navigate our way around a difficult situation, or retrace our steps in case we get lost.
Designing a printed circuit board is an amazingly complex maze to find your way through. There are specific ways that schematics should be created, how to create library parts, how to place and route components, and finally how manufacturing output files are to be generated. This is a lot of information to digest all at once, but when laid out in a set of well-documented guidelines you can better see your way through the complex maze of requirements.
Here is a description of the different PCB design guidelines you should be aware of as a PCB designer and the kind of help that you can expect from them.
PCB Design Guidelines for CAD Libraries
Before any work can be started on the PCB design, there must first be CAD library parts to work within the design tools. For schematic capture, this will mean symbol and component information along with simulation models. For layout, there will be PCB pad and component information in footprints or land patterns. All of this data can be pulled from external sources or developed internally, and design departments usually will have guidelines for both.
For part data that is developed internally, there are industry standards to follow such as IEEE Standard 91 for the creation of schematic symbols and IPC standards for the creation of PCB footprints. Design departments usually have their own internal guidelines as well. These typically specify information such as which CAD layers are to be used for what information, and the company preferences on part outlines, text, and other details. There are usually company guidelines for part checking and verification as well.
Creating the schematic using your guidelines to help with spacing requirements
Schematic Creation Guidelines
Once the library is ready the PCB designer can begin work on the schematic. To make sure that all of the schematics have the same appearance, most companies will have internal drafting guidelines which typically will include the following:
Schematic sheet size.
Sheet border information such as company logos, name, address, date, part numbers, and revision numbers.
The size and appearance of the grid to be used. This is important so that everything is drawn to the same scale.
Symbol size and line widths.
Text heights and font size.
For the schematic development these guidelines are usually used:
Symbol spacing on the schematic sheet.
Component information such as reference designators, part numbers, values, and pin numbers.
Colors for net objects such as symbols, net lines, buses of nets, and text.
Net spacing and naming conventions.
Lastly there will also be guidelines on checking and verifying the schematic through design reviews. Once the schematic has successfully passed all of these processes, it is ready to go to PCB layout.
PCB Layout Guidelines
To start the PCB layout there needs to be coordination with the mechanical design of the circuit board. There are usually internal guidelines in place for the exchange of data between the mechanical CAD and electrical CAD systems, as well as a system of design reviews for changes. There should also be guidelines for working with the contract manufacturer (CM) that will build the printed circuit board assembly. The CM can provide a lot of information for the layout including recommendations for board layer stackup configurations, and this communication process needs to be well documented in the company guidelines.
After the board outline and layer stackup have been created in the PCB layout tools, the designer will start placing components. On top of guidelines and documentation, ensure to make use of design-rule checks (DRC) and constraint managers to eliminate design distractions. Here, again, there will be different guidelines to help the designer:
Company Placement Standards:
These guidelines will govern the specifics of parts placement. They could include rules on component rotation, clearances, and keepout zones. These may coincide with industry placement standards, but it is not unusual for a company to have refined and standardized the placement methods that work best for them, to name a few: clearance from mounting holes and slots on the circuit board, which layer is planned to place components (through hole and smd) on depending on the production line.
Power and Ground Planes:
Using a ground plane is typically the best choice when working with high-speed or high frequency signals. As interference is always a concern, placing a ground plane on one layer of a PCB will provide some level of electromagnetic interference (EMI) protection from external sources. Furthermore, a ground plane allows you to circumnavigate routing ground traces. To reduce EMI susceptibility and loop inductance, a ground return can be placed near a signal line to enable the return signal to keep on the trace.
Designing for proper signal integrity can be a much more integrative and complex process, however, here is where to start for core basics. Components must be placed to follow their signal path as depicted in the schematic, especially in high-speed designs. If this isn’t done correctly, it could have an adverse effect on the signal performance.
Trace Routing Considerations for Signal Integrity:
Signal path traces must be kept as short as possible unless specific trace lengths are required. Differential pairs must be routed together, controlled impedance lines must be routed on their specified layers, and sensitive traces must be kept cleared of other signals that might cause coupling and noise on the board.
Design for Assembly (DFA):
As part of the overall design for manufacturability (DFM) rules, the DFA guidelines are what will ensure the PCB can be efficiently assembled without error. Clearance between components and their rotations & locations are all part of DFA guidelines.
Design for Test (DFT):
Another aspect of the overall DFM rules are the DFT guidelines. These also cover component clearances as well as clearances to the board edge and ensuring that there are adequate testpoints placed for in-circuit test (ICT).
Many of the guidelines that apply here are continuations of the same principles that are practiced during placement. The goal of the PCB designer is to produce a functioning board through the design of all of its circuitry, both the component placement and their routing together.
Design for Fabrication (DFF):
The last of the guidelines that all fall under the DFM rules are those that help to produce a PCB that can be easily fabricated. These include manufacturable clearances between traces, vias, and pads as well as maintaining minimum trace widths.
As the end of PCB layout nears, there are still some more guidelines that need to be observed. The board will need markings on it in silkscreen and etch, and these are usually specified in internal company guidelines. These will include reference designators which must be readable and clear of pads, and other board markings including board name, company name, and part numbers. There will also be a host of fabrication and assembly markings that will be detailed in the guidelines as well.
Tuned trace routing in PCB layout
Guidelines for Creating Manufacturing Output Files
To get the circuit board ready to go out for manufacturing the PCB designer will run a set of design rule checks that are usually detailed in company guidelines. The board will then go through its final set of design reviews, and the company guidelines will usually contain a set of checklists that must be signed off before the board can be sent out to the contract manufacturer.
Once the board is cleared to proceed, the PCB designer will create the final manufacturing output documentation according to the requirements listed in the company guidelines. These will include bill of materials reports, manufacturing drawings, PCB image files, and netlists. The file formats may be specified in company guidelines, or they may default to the file formats that are preferred by the CM.
There is a lot that goes on in designing a printed circuit board, and fortunately we do have different guidelines that can help us through these difficult processes so that we don’t get lost. Another thing that can be very helpful for the PCB designer is to use PCB design tools that are versatile enough to handle all of the different PCB design processes. OrCAD PCB Designer is one of the best PCB design systems available, and the development team at Cadence has made sure that OrCAD as well as all of their design tools have the capabilities you need for success.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.