Understanding the PCB Design Process

Your PCB Assembly at VSE Starts with a Good PCB Design

In the world of printed circuit board manufacturing, it is fairly straightforward to explain how a PCB is made. Using a bare board as an example, we can explain the different etching and lamination processes that the board went through in order to be fabricated. Then with our own assembly processes here at VSE, we can describe the component procurement, soldering, test, and QA processes that are required to build the board. But how the electronic circuitry in the board is designed in the first place takes a little more explaining.

If you are new to the world of PCB design, then phrases such as “schematic capture,” “trace routing,” and “copper pour” may be new as well. They are however all very real and important parts of the PCB design process. Since our goal here at VSE is to build the best products possible for our customers, we want to answer any questions that you might have about PCB design. Here we will break down the PCB design process into its basic steps from start to finish to give a clear picture of how this is done.

Schematic Capture: the First Step in the PCB Design Process

Everything that gets built needs a blueprint, and for a circuit board that blueprint is the schematic. The schematic is the logical representation of the electronic circuitry of the circuit board to be built and uses industry-standard symbols and notations to represent different components and their values. Each physical component that gets used on a circuit board, such as a resistor, will have an identifying symbol representing that component on the schematic.

The schematic is created in an electronic CAD system specifically made for designing printed circuit boards. Each logical symbol will have one or more pins on it to represent the actual pins of the real component which will eventually be reported in the bill of materials. The PCB designer will place these symbols onto a schematic sheet within the CAD system, and then draw lines between the pins to connect them together. This connection is referred to as a net, and a net will have two or more pins all connected together.

From the Schematic to the PCB Layout

Once all of the symbols are placed and the nets are connected in the schematic, the circuit board is ready for physical design in a process called “PCB layout.” The component information from the schematic symbols as well as the net connectivity will all be converted into the data needed for PCB layout. There are a number of other steps that also have to be done however before layout can begin:

Library parts:

Models of the physical components, such as a resistor, need to be created within the layout tools. These models are referred to as footprints or land patterns, and will contain a representation of the metal pad that the component pins will eventually solder too. They will also contain the shape of the component as well as specific electrical, pin, and 3D data.

Design rules:

The PCB layout must be completed without any of the nets coming in contact with each other, or else those nets would be shorted together when the board is built. To prevent this, PCB design CAD tools have extensive rules and constraint systems built into them to govern the size and spacing of metal objects. These rules must be completely set up or copied from a previous design before the layout begins.

Board outline:

The physical shape and structure of the circuit board also need to be set up in the layout database. This requires creating the outline of the board in the CAD system and setting up how many layers the board will have in it and in what order they are to be stacked up.

Once these steps are complete, the design is ready for layout.

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PCB Component and Parts Placement

The symbols that were placed in the schematic are now associated with the PCB footprints in the layout database to become PCB components. The designer now places these component footprints on the board in the CAD system, usually in the following order:

1. Fixed parts:

Components such as connectors, switches, or other mechanical parts are usually required to be in a specific location and should be placed first. This ensures that they will mate correctly with plugs or openings in the system enclosure. Once these fixed parts are placed in the layout database, the remaining parts can then be placed in order from them.

2. Critical parts:

Microprocessors, memory chips, power supplies, or other main components of the board are usually the next parts to be placed. They need to be placed close to the fixed parts that they are associated with yet with enough space to place other parts around them. It is also important to balance the needs of circuit performance with thermal management when placing these components.

3. Supporting parts:

Power supply components will have additional parts associated with them, as will microprocessors and memory devices. These supporting parts are often discrete components such as capacitors, resistors, and inductors, and they must be placed close enough to work directly with the critical parts.

4. Remaining parts:

The final parts in the layout may not necessarily support a critical part directly, but their placement may still be important for the correct functioning of the overall circuit. These could include terminating resistors or bulk decoupling capacitors.

Another critical aspect of component placement is making sure that the board can be manufactured without any errors. Design for manufacturing, or DFM, is essential to lowering the production costs of building the board. Parts that fail the DFM requirements are prone to automated assembly errors forcing the board to be built by hand, which is more expensive and time-consuming.

Connecting Signal and Power Nets Between the Components

With the components in place, the next step is to connect the nets between the pins in a process known as trace routing. These traces will ultimately become the metal connections in and on the circuit board when it is fabricated. The PCB Design CAD system will display the unconnected nets as straight lines, and the designer will use one of many different routing features in the CAD system to create the trace routing:

Manual routing:

All design tools give the user the ability to pick one of the nets, and draw its trace in manually. This is done using straight or curved lines, right angles, or placing a hole in the board called a via to transition to another layer.

Semi-automated routing:

Many CAD systems give their users various versions of automated routing. These different features may route a signal net, a portion of a net, or groups of nets at the same time.

Auto-interactive routing:

These specialized features combine manual routing with automated functionality. This gives the user the ability to direct where they want the routing to go, but rely on the system to do the actual routing and adhering to signal integrity rules. These features are useful for pushing other traces out of the way, and winding through dense areas of routing that would take a lot of manual effort to complete.

Batch auto-routing:

These tools will automatically route the entire board for the user. Care has to be exercised though because the CAD system may not always yield the desired results. Here is where the experienced designer will pre-route areas by hand first, and set up a complete list of routing rules before using the auto-router.

While some nets can be simply connected together, the majority of nets may have specific rules that have to be followed. These can include the trace width, what layer they are routed on, areas of the board to avoid, and their length. In some cases, trace lengths have to match other trace lengths, while other traces may have to be routed tightly together in pairs. All of these requirements can be set up in the initial design rules.

Another important part of connecting nets together is the use of large areas of metal called fills, planes, or poured copper. Large areas of metal for the power and ground nets provide a simple way to connect various components to those nets. In addition, most designs will use the ground planes for a return path for the signals that are conducted through the trace routing. Here again, is where the skill of the designer becomes very important. These planes must be designed to provide adequate power and ground coverage, a clean signal return path, EMI shielding for the traces, and thermal dissipation for hot components.

At this point, the circuit board is fully placed and routed, but there is still more work that has to be done.

The Final Steps to Complete the PCB Design

Although the circuit board is functionally complete at this point, there are still some additional tasks that need to be done before it can be manufactured:

Test:

To verify the assembly process and the functionality of the board, the completed board will be run through a variety of testing. Test points must be assigned during layout to create the test documentation needed to create the test fixtures.

Silkscreen:

In order to identify the completed board and its components, ink markings and reference designators are silk-screened onto the board. The silkscreen layer is prepared in the CAD system as one of the last steps of the PCB layout.

Drawings:

Fabrication and assembly drawings are also created by the designer using the PCB design tools. These drawings give detailed instructions on how to manufacture the circuit board.

Final manufacturing files:

All of the circuit board image files, drawings, test files, and other documentation will be gathered into one set of manufacturing files for the PCB contract manufacturer.

At this point, the circuit board is ready to be manufactured, and that is where we at VSE will help.

Working Together with VSE on Your PCB Design

At VSE our commitment to our customers is to make sure that we can build your circuit boards at the highest levels of quality. This means that you will have full access to all of the technical resources that we have to offer:

Design for manufacturability review and corrections
Bill of materials (BOM) review with recommended alternatives
Component procurement expertise
Precision inspection and QA processes
State of the art PCB manufacturing
Trained and experienced rework technicians
PCB testing processes and engineers
Box build, wire harness, and cable assembly capabilities

In addition, we realize that not everyone has a full PCB design group at their disposal, and therefore we invite you to leverage the services of our engineering team. Not only will they conduct BOM and DFM reviews of your design, but they are also available for the following:

Schematic capture
PCB layout
Mechanical engineering
Wire harness and cable design

Give us a call, we have the design and engineering resources that can help you on your next PCB project.