Printed Circuit Board design is a complete vertical in the electronics industry. Whether a fresh graduate or a mid-level experienced designer, we all get better in PCB design by following rules and guidelines. Every layout is unique. Not one PCB layout design will match the routing style of other designers.
With technological advancement, we have multiple categories of components that need to be fitted into compact and small board shapes. To remove the risk of board failure during the early testing phase, we can rule out the basic mistakes. PCB design and layout represent the schematic draft, where copper traces replace the wires. The existence of a type of signal on the circuit makes the layout PCB design process complex.
The following guidelines offer basic rules and steps that engineers can follow during the PCB design and layout stage. They focus on circuit design, component placement, routing, fabrication, assembly and manufacturing principles of the PCB. Hopefully, it will be useful for designing any kind of board--simple, high speed, high power and high frequency.
What is PCB design?
PCB design and layout is a step in creating a physical board. This PCB will be the platform to operate the functionality of electrical and electronic devices—everything from smartphones to high performance computers. Power supply, logic controller, memory storage, communication protocols, charging, monitoring and status indicator are all features incorporated into the board. Electronic components active and passive are used in the pre-design step to create a circuit. This circuit is then transferred to a blank canvas called a PCB board. In this board, designers place component symbols and actual footprints in the best possible locations.
Further, copper traces connect all the board features together. Designers perform PCB layout design rule checks to avoid any missing connections or short circuits. Once the board is ready, the fabrication files are exported to a folder. The PCB fabricator/manufacturer then will print the circuit on the board. Board assembly with the Bill of Material list is performed. The layout PCB design process is completed and ready for testing.
Prerequisites to PCB layout
Although many PCB fabrication suppliers provide printed circuit board layout services, students of design engineering need to learn basic PCB layout rules. The PCB design and layout step is dependent on previous stages. Clarity is crucial in product specifications, block diagram, feature list, power budgeting and circuit design. Understanding the dependent stages will help achieve high electrical performance of the layout.
Clear product requirements
Product requirements from a customer are not always a structural document. It is important in the project kickoff stage to define the product's electrical, and mechanical features. This will lead to the creation of a block diagram breakout.
Project budget
The project management team is responsible for sharing the budget for the electronics board prototype. This is crucial to clarify before the printed circuit board layout stage. Then engineers decide the bill of material, PCB substrate material, types of finish, manufacturing process, testing requirements and any other add-ons that are possible in cost.
Component selection
With the help of a block diagram, identify the major components that will participate in board functionality such as power supply IC, microcontroller, memory storage, LEDs, buttons, and switches. These parts should be selected based on the component grade like AEC and quality class. An alternate BOM list should be ready with the primary BOM to remove the risk of parts not being available in the market. Otherwise, engineers have to redesign the printed circuit board layout for replacement parts.
Circuit design
Hardware design engineers are experts in building circuits. PCB layout designers should build the circuit from their support teams. Redundancy of components should be avoided. Sections of the circuit should be simulated for the final output. The circuit should be finalized, clear and logical before the layout stage.
eCAD tools
There are multiple tool options available to design the PCB layout. It is the most essential prerequisite to begin the PCB design and layout. Many software tools are available open source, and others need professional licenses to help build the board. Examples include Altium Designer, Orcad, Eagle, Kicad, Allegro, Proteus, and Mentor Graphics. PCB layout designers can choose any tool depending on the ease of the user interface and support.
PCB design & layout guidelines
Layer stack-up
PCBs can be designed in single, double or multi-layer(4,6,8…). Layer stackup can be defined in multiple ways. Signal layers and power layers can be adjacent to each other. The placement of inner layers should reduce the crosstalk from the return path. Keeping the power layer next to the ground layer reduces EMI radiation. It is a good rule to place the ground in the center symmetry. This will help in signal shielding and impedance control.
Defined design rules
Every PCB layout design software has a section for setting the electrical and mechanical design rules. It is done before beginning the layout routing. Some of the important rules are trace width, clearance, creepage, hole size, component-specific clearance class, silkscreen rules, routing topology, power plane connecting styles, test points and more.During the printed circuit board layout stage, the design rules will keep the DFM (Design for Manufacturing) and DFA (Design for Assembly) in line.
Component placement
Mechanical drawings define the mandatory locations and other restricted areas for electronics component placement. But the rule of thumb calls for keeping the connectors close to the edges for ease of handling wire harnesses. Place bigger parts like MCU, and processors first, then related circuitry last. A good placement will ease the routing of the printed circuit board layout. Try to reduce the number of layer switches for one signal.
Following the signal path
A circuit is built with different types of sections like low voltage analog, power supply rails, digital sections, filter circuits, communication sections and more. On starting the routing, it is necessary to follow the circuit signal flow. Cross-referencing the component location in the circuit is a useful step. This method will help reduce the length of the traces from one net to another. Noise, crosstalk, and emissions from the power supply section can be avoided.
Trace width
Every circuit has multiple power rails(48V,24V,5V,3,3V etc). It is important to calculate the trace width as per the required current flow and voltage rating in the PCB layout design. Traces can be as thin as 0.3mm to 3mm thick. A 0.3mm trace will burn out if routed in the power supply section of 1A. Power planes should be dedicated to voltage regulators like step up, and step-down. Ground routing should be placed in proximity to power traces during the layout PCB design.
Component orientation
The rule of component placement suggests keeping similar parts in one direction when handling the PCB layout design. It is useful during the reflow process. Surface mount components should be dedicated on one side of the board. It will help in reducing manufacturing costs. Through-hole parts should be on the other side of the board.
Drilled holes and pad
Plating and hole size calculations should be performed earlier in the PCB layout design stage. The current flow through the vias is based on the drilled size. With heat generation in the board, the via could get damaged, causing board failure. In HDI boards, microvias, and blind vias have an aspect ratio. It is a ratio of PCB thickness to hole size. Suppliers should clarify which aspect ratio drills are available for the board fabrication process. Footprints of the surface mount parts should be standard. It will help remove the time to manage the library and costs in manufacturing.
Thermal management
In high-voltage signal flow, heat dissipation occurs as large current flows. Copper pours exposed on the board, thermal vias and heat sinks should be used in the sections during the PCB layout design where power dissipation is a few milliwatts to watts.
Filtering circuits
In switching circuits, there are transients that stay charged after the power is off (back-emf). For PCB layout design, using a flywheel diode and snubber circuits, engineers can achieve a smooth digital signal. The noise coupled with the microcontroller signal will distort the functionality of the board. Decoupling capacitors in multiple values should be placed near the Vdd, Vcc, and Vref pins of the digital IC (microprocessor or microcontroller) during PCB design and layout. This will reduce the inductive reactance value in high-frequency operations.
Performing DRC
Design rule check (DRC) is the most important rule to follow once routing is completed. It will identify the errors and warnings that need correction. All the PCB layout design rules set in the constraints toolbar need to be evaluated. Any deviation from the set values will not allow a PCB to pass for production. Clearance, missing nets, overlapping of text, the height of the component, and hole size, all will be reviewed
Summary
Suppliers for PCB manufacturing should describe their capabilities in a customer quotation to achieve high-quality board printing. Some qualified suppliers even suggest layout changes for better electrical performance and they can provide satisfactory printed circuit board layout services. Following rules and guidelines will make PCB design and layout easier. DFM and DFA rules should be practiced to verify a board's capability in the manufacturing process.
Ross Feng is founder and CEO of Viasion Technology https:// www.viasion.com with more than 18 years in the PCB industry. The company crafts Printed Circuit Boards with expertise in manufacturing, layout, assembly, and fabrication. It is based in Hong Kong.