Key Steps in Designing Printed Circuit Board Layouts
Recent advances introduced by Rush PCB UK Ltd have had sweeping effects across the Printed Circuit Board (PCB) design industry. PCBs form the core of any electronic product today. They serve to hold the innumerable minuscule electronic components together, while allowing them to interconnect and function as the designer intended. Design and layout of PCBs is therefore, and important stage in the development of any product—often deciding the success or failure of the product.
Rush PCB UK Ltd has been in the forefront of this innovation, constantly evolving this technology. As a result, these designs have reached new heights in complexity and expectation, with PCB design rules and production processes evolving to achieve new layouts and capabilities. For instance, mass-produced PCBs commonly have very thin tracks and multiple layers. Even a few years ago, no one would have believed the feasibility of such designs. The availability of advanced PCB design software has also helped this progression tremendously.
However, such improved capabilities alone are not sufficient to effectively design printed board layouts. Experienced electronic engineers routinely struggle with creating layouts on a PCB or when designing a PCB following the best practices in the industry. Often, it becomes really difficult creating a quality board that will fully meet the customer’s need. As each design is different, balancing the functionality of a PCB while following the best design practices is an ever-involving process. To make the process easier, Rush PCB UK Ltd has outlined the process of designing PCBs and included some essential PCB design rules.
Determining the Basic Needs
Although requirements are broadly spelled out by the customer, the design engineer must translate these requirements into mechanical, electrical, and electronic terms. For instance, the design engineer must decide if the equipment requires a single PCB or several, and the functionality of each of the PCBs. Depending on the functionality and the interconnection between the PCBs, the design engineer will apportion a certain part of the total circuitry to that PCB.
Determining the Material for the PCB
The basic needs of the project also help in determining several important aspects, one of them being the materials of which the PCB should be made. For instance, the project may require the PCB to handle high frequencies, and therefore, the designer will use materials with controlled impedance. On the other hand, applications involving medical or automotive devices often require components that will fit into tiny spaces, thereby requiring a flexible PCB. Additionally, the final size of the PCB and its functionality often determine whether it will be made of a single layer or with multiple layers.
Based on the above needs and the functionality of the PCB, the designer will decide the initial schematic of the PCB, and hence the Bill of Materials or BOM.
Determining the Schematics and BOM
While the schematic determines the functions of the PCB, the BOM lists the components that the PCB requires, their source, and their footprints. Essentially, the schematic forms the blueprint engineers and manufacturers use during the development, production, and the assembly processes. The schematic and the BOM together contain details of the hardware for the PCB, material, components, and details of other materials the manufacturer will need during the production process. The designer must ensure the schematic refers to each component with a unique identifier, and they all have the proper footprints associated with them. Additionally, the information in the schematic must also tally with the BOM. In fact, designers typically use PCB design software to generate the schematic and the BOM.
The design engineer must ensure the schematic and the BOM contain the manufacturer’s part number for the components, and where possible, alternatives for each component, as alternatives help in speeding up the procurement process.
Determining the Net List and the Layout
After finalizing the schematic and the BOM, the designer has to generate the Net List. This is a list of interconnections of the components on the PCB. Each PCB will have its own Net List.
The Net List serves two purposes. The first is to generate the layout, and second is to allow the PCB manufacturer verify the interconnections when fabricating the PCB. The designer imports the Net List into another part of the PCB design software, which recalls all the footprints from its library and connects the pins of the components according to the Net List using a series of straight lines also called the Rats Nest. In the next step, the design engineer must follow some best practices for achieving an effective layout:
- Implement the Appropriate Panel Size for the PCB
- Implement the Proper Grid
- Implement Design Rule Checks or DRC
- Implement Design for Manufacturing or DFM
Implementing the appropriate panel size of the pcb is a vitally important step for the design engineer to follow, as the PCB has to fit into the space allocated in the equipment. Moreover, it also marks the areas on the PCB allocated for mechanical attachments, and hence, the designer has to be careful of not placing any electronic component in these areas.
Implementing a proper grid is essential for maintaining proper spacing between components and tracks, and the distance components must maintain from the edge of the PCB.
DRC is another essential requirement that a design engineer should start implementing right from the start of the design rather than check at the end of the design process. This allows addressing problems identified by DRC quickly, thereby minimizing requirement of massive changes at the end of the design process and saving time.
From the beginning of the design process, the design engineer should also be concerned about the manufacturability of the PCB. A discussion with the Contract Manufacturer during the design process helps to address manufacturing and assembly constraints right at the beginning, thereby avoiding requirements of changes at the end of the design process.
Determining Placement of Components
Placing each component at its designated spot on the PCB is tricky and depends on numerous factors and considerations. These include the overall PCB functions, thermal considerations, EMI/EMC considerations, and many more. However, design engineers typically place components following best practices order such as:
- Major Connectors and Mechanical Attachments
- Power Components
- Precision and Sensitive Circuits
- Critical Components
During placement of components, the design engineer must also keep Design for Assembly or DFA requirements in view. For instance, if a connector requires manual operation, it must have space around it for finger insertion.
A requirement the design engineer must address at this stage is the position of appropriate test points to ease fault detection. Another requirement is the silkscreen should show clean and unambiguous marking for the components. This is a very important requirement as the information on the silkscreen is useful not only to the designer, but also to the manufacturer, assemblers, and testers. If possible, the design engineer should mark the functionality of the circuit, put test markers, and add component and connection placement directions. Manual assembly gains from silkscreen markings on both sides of the board, while simplifying the production process.
Once the design engineer has satisfactorily placed all components, it is a good idea to conduct a design review along with a round of DFM and DFA analysis. Although this step may look unnecessary, it helps to locate mistakes and anomalies for timely correction.
Printed Circuit Board Routing
Most PCB design software have capability to auto-route traces connecting the components based on the Net List. The program does this by using the number of layers available for the connection and taking advantage of the available space by calculating the position of vias.
It is always a good idea to cross-check the interconnections after the computer has finished auto-routing the board. Most design software can produce a second Net List after routing, and the design engineer can verify the connections by comparing with the original Net List. Some areas may be difficult for the auto-router, and the design engineer may have to alter the layout to some extent to generate more space to allow proper routing in a dense environment.
Sometimes, the computer may produce results the design engineer may not approve. This is especially true for high-speed traces, where the designer may have to control the impedance for maximizing energy transfer.
DRC is an important parameter the design engineer must use during the routing process. It should place restrictions on the minimum spacing between traces and pads not only on the outermost layers but also on the inner layers as well. Trace width determines current carrying capacity, and the design engineer must allow wider traces where higher current is flowing.
Power and Ground Planes
One of the advantages of a stackup with multiple planes is to reduce the physical size of the PCB. Another advantage is to distribute power and ground lines as planes. PCB designers preferably dedicate one or more circuit layers for use as ground plane, and others as the power plane. Designers allocate the power and ground planes in pairs.
There are several advantages of using dedicated ground and power planes in multi-layered PCB. Planes have much lower inductance as compared to thinner traces, and act as low source resistance connection. They also act as shields on either side of the circuitry sandwiched in between the two. This helps to reduce EMI/EMC largely, while improving the operational efficiency. Being large tracts of copper, ground and power planes also act as efficient heat dissipators, especially as designers assign them to the top- and bottom-most layers of a PCB.
PCB Heat Management
Electronic components carrying current will generate heat. The extent of heat generated depends on several factors, which may include variation in copper thickness, the number of layers present, and absence of thermal paths. The design engineer must arrange to distribute this heat evenly, thereby preventing heat spots.
Design engineers follow several methods to manage heat in PCBs. One of them is to use the power and ground planes to the hot component to dissipate heat. As planes contain more copper, they dissipate heat better. Another method designers use for reducing heat generation is by using effective high-current routes and optimizing heat transfer. In some cases, designers also maximize the area they use for transferring heat. However, designers must do this early on in the design phase, and it can also affect the final size of the PCB.
Final Design Review
The final design review of the printed circuit board design must contain many other checks apart from the regular DRC check. At the very least, it should include among physical verification processes a layout-versus-schematic check, an electrical rule check, an XOR check, and an antenna check. Advanced PCB manufacturers such as Rush PCB UK Ltd use additional rules and checks for improving the yield, apart from basic checks manufacturers and designers typically employ.
For the design engineer, one of the good practices to follow is verifying the manufacturing parameters before submitting the PCB documents to the contract manufacturer. By personally generating and verifying the manufacturing parameters before submitting the final design of the PCB, the designer can save the OEM countless hours of confusion, misunderstanding, and losses. This is an important verifying step to expedite the manufacturing process, as it decreases the time necessary for correcting and rectifying the design before manufacturing can start.
According to Rush PCB UK Ltd, implementing the above key techniques and best practices leads to an effective and efficient method of designing Printed Circuit Board layouts. In fact, the process is further simplified by partnering with a contract manufacturer who works with the OEM to make the best and most cost-effective PCBs.