Engineering is a unique field as it seamlessly integrates the reliability of science with the uniqueness of self-expression. Its practitioners, particularly those of us who design electronic circuit boards, are both skilled artisans and artists. PCB designers rely on proven principles and technological devices to give ideas a canvas that is transformed into a physical embodiment, bringing those ideas to life.
However, the path from a schematic to a usable board is bounded. Specifically, your material choices, board dimensions, and other design decisions must fall within the range of capabilities of your CM’s equipment. This means acquiring and following their DFM and DFA rules and guidelines. Doing so and collaborating with your CM will enable your design intent to be incorporated into the board build process, achieving performance objectives.
For the best results, the PCB design process needs to be optimized, which requires a good understanding of the steps involved and the use of highly functional and capable tools.
PCB Designing Process Overview
For all but the simplest of designs, producing electronic circuit boards is a complex endeavor that involves many professionals and multiple companies. Typically, there is a primary board designer, a reviewer (usually a more experienced engineer that specializes in an area; such as RF analysis), an MCAD engineer to design an enclosure or integrate mechanical functionality, a testing engineer to verify performance and operation, and the fabrication and assembly house teams, often including engineers and technicians. Everyone involved relies upon the quality of the PCB design process, which includes the following stages:
PCB Design Process Stages
Each of these stages is important and builds upon the results of the previous stage, as is explained below.
1. Selecting the Components
The first stage of the PCB design process is arguably also the most important. Your choice of components directly affects the board layout, manufacturing, circuit performance, and reliability. With the many choices of parts available, and the supply chain challenges such as delays, obsolescence, and counterfeits, it is imperative that you follow best practices for sourcing and proofing the component models used for your design.
Sourcing Component Models
Component libraries should include symbols that are easily accessible or uploadable to your design software. Unless necessary — which may be the case for parts that are newly introduced or are exotic–components designed with specific functionality that is typically only made by a single manufacturer — it is not advisable to create symbols from scratch.
Today, the most used resources for sourcing components are online libraries. These free tools are great for part research and comparison; however, the amount and timeliness of data and information can vary. Therefore, it is best to follow these tips:
Proofing is also an important aspect of component selection that should not be neglected.
Proofing Component Information
Component information needs to be available and accurate. The best option is to have access to real-time parts information, especially on availability. Some of the better PCB design software packages include real-time libraries; however, the range of components may be limited. A better solution is probably an in-house CAD librarian or a software tool that integrates with your board design tool, includes real-time verified component data, and has tools for effective management.
2. Drawing the Schematic
The complexity of the schematic is directly proportional to that of your design. Often, a good schematic can consist of a single well-organized page that includes the circuit drawing, references, and pertinent information, like the title block that identifies when the schematic was created and reviewed and by whom. More common; however, your schematic will consist of multiple drawings and pages. In either case, it is important that standardized symbols — for example, ANSI Y32.2 Graphic Symbols for Electrical and Electronics Diagrams — are used to maximize readability as you step through the major stages of schematic generation. Those stages include component placement, connecting the nets, and proofing the design.
Placing the Components
Although there are no standardized rules for placing components on the schematic — as there are for the PCB layout — there are some best practices, including the list below. If followed, these best practices will aid you in creating a schematic that is easily understandable and most effective for the PCB design process.
Component Placement Guidelines for Schematics:
- ➜ Group components by signal type
- ➜ Separate subcircuits
- ➜ Use multiple pages
- ➜ Use consistent naming conventions
- ➜ Annotate well
There is no fast rule about component spacing on schematics; however, liberal spacing is helpful when connecting the nets. Another helpful hint that can help when proofing the schematic is to label your component pins in addition to numbering them.
Connecting the Nets
Once your components are placed, the next task is to connect all pins. These connections are referred to as nets, and they serve as the data for the net listing or netlist. A netlist is a text file with reference indicators identifying board elements such as components, test points, and interconnections used by CMs during PCB manufacturing. These files are typically formatted according to specifications given in IPC-D-356 Bare Substrate Electrical Test Data Format.
Connecting the nets on your schematic is a manual procedure, although there are some assisting functions like snap-to-pin that help with efficiency. As this step can be tedious, care should be taken when working with fine pin pitch parts as it is not difficult to connect to an adjacent pin by mistake. If not discovered during the design process, identifying this as the source of erratic circuit behavior can be very difficult.
Proofing the Circuit Design
Depending on your design and application, it may be necessary to run simulations to nail down parameters. For example, resistor, capacitor, and/or coil values for a filter design. You will also need to verify signal integrity. Performing these simulations can greatly reduce the time and costs associated with redesigns and board respins during development. Before moving to the PCB layout, it is necessary to proof the schematic. Fortunately, most PCB design software includes automated netlist checking, identifying common errors, such as floating pins and unconnected nets. Successfully passing this test is necessary before proceeding to the board layout.
3. Laying Out the Board
A verified schematic is a prerequisite for a good board design. The next most important consideration is to set up the constraints for your PCB layout. Virtually all design packages include defaults based on industry regulations, such as those specified for performance classes 1, 2, and 3 in IPC-6011 General Performance Specification for Printed Boards. Acquire and utilize your CM’s DFM and DFA rules and guidelines.
In some cases, you should be able to upload these specifications, which can be extensive. If not, the more advanced PCB design packages typically include a constraint manager that allows you to create and edit constraints. It is essential to follow these rules, which in many cases determine whether or not your board can be built, at least not without redesign.
Laying out your board can be divided into four stages:
These are described below.
Specifying Your PCB
This stage of laying out your board is where you specify physical attributes and dimensions. These include choosing board material, board and panel size, and the number of layers in the stackup. FR-4 is by far the most common board material, although for certain applications, thermal or other requirements necessitate a different material be used. Drilling specs for plated through holes (PTHs) like vias and non-plated through holes (NPTHs) for mounting screws, as well as aspect ratios, as shown below, are also defined during this stage.
Just as for schematic generation or capture (the commonly learned term for generating the schematic), placing components is an important stage. However, for board layout, there are important regulations to follow, such as component clearance, spacing, and board edge clearance. Other considerations that necessitate you to follow some best practices are listed below.
Following these and other good practices for component placement will help avoid manufacturing problems.
Although traces connect your board’s components, the process is quite different from creating the nets. In fact, the nets are typically used to create ratlines (guidelines that show what the interconnections should be) which are helpful for manually routing your board. Fortunately, most PCB design tools include some type of auto-routing. It does not eliminate all manual routing but can save significant time at this stage.
Traces also must adhere to certain guidelines, including limits on size and copper thickness given in your CM’s DFM specifications. Adequate spacing is also required, as shown below.
There are other important constraints and considerations that should be included during this stage. Some of these are based on board application or type. For example, when differential signals are used the traces must be identical in length, impedance, and copper weight, and be equidistant from each other.
Proofing the PCB Layout
Once the board is laid out, it must be proofed as well. The most significant testing is for DFM, DFA, and other constraint compliance. This is known as design rule checking and virtually all design tools include a checker that performs this vital evaluation. Another useful verification is layout versus schematic (LVS) testing, which compares a netlist generated from the board layout with the schematic netlist.
Other tests and simulations that may be performed are signal analysis, power delivery network; also known as power distribution network (PDN) testing, and thermal distribution evaluations. The latter two are powerful analyses that help you choose heat dissipation devices and techniques.
4. Preparing for Manufacturing
The last stage of PCB design processing is to compile all data, information, and imagery that is needed by your CM to build your board. It must reflect your design intent, and meet your performance objectives. These are encapsulated in the bill of materials (BOM), which provides data and information about the components selected, and the design file(s) that include the schematic drawing, netlist, drilling requirements, board layout, and all other important text and imagery to be used for the board build.
How to Optimize Your PCB Design Process
The PCB design process includes many well-defined steps and processes. However, the process also leaves room for your footprint. This differential is what makes every custom design unique. But it also provides an opportunity for variation in the quality of the process and the final product. Therefore, it is important to follow guidelines, such as those listed below, to ensure your design meets the highest standards.
Guidelines for PCB Design Process Optimization
Only use components from a trusted, reliable source that provides comprehensive design data, imagery, and information verified by the manufacturer
Make sure that your schematic capture is error-free and passes all proofing, such as netlist checking and simulations.
Perform DRCs often and make sure they include DFM and DFA rules obtained from your CM. For final verification, clear all flags and perform necessary simulations and assessments.
Ensure that your BOM component information is consistent with your board layout. For example, MPNs match the component package footprint on the board.
Use accurate and verified CAD data and images in the design file sourced from a reliable component library source.
As this list indicates, achieving an optimal board design requires both a sound process and the tools to implement it. Only the industry-leading PCB design package OrCAD PCB Designer and Analysis Suites has all the functionality needed to meet this requirement. However, it can be challenging to fully take advantage of these capabilities. A good option is to partner with an industry expert that can assist you in maximizing these tools quickly.
EMA Design Automation is a leading provider of the resources that engineers rely on to accelerate innovation. We provide solutions that include PCB design and analysis packages, custom integration software, and engineering expertise, which enable you to create more efficiently. For more information on the PCB design process and how we can help you or your team innovate faster, contact us.