As discoveries and advances meet the demands for faster, smaller, and less expensive electronics products and gadgets, digital PCB technology has made significant advancements in the field of electronics. Multiple microprocessors and other electronic circuitry make up digital PCBs, which can do thousands of operations per second. Digital PCBs have a lot of advantages over analog circuit boards, the two main ones being better impedance matching and enhanced transmission line insertion loss management. In order to avoid issues like transmission line impedance discontinuities, inappropriate plating of the through-hole connectors, and other signal-integrity-related concerns, it is crucial that digital PCBs be carefully planned and built. A variety of characteristics of digital PCBs provide them an advantage while an electrical circuit is operating. These characteristics include: High layer count, Tight dimensional tolerances, High packaging density, complex stack-up structures, stub reduction, and other factors.
Difference between Analog and Digital PCBs
There are various similarities and differences when it comes to analog and digital routing in PCBs in terms of bypass capacitors, power supply, ground design, voltage errors, electromagnetic interference (EMI), etc caused by PCB board routing.
Fig 1:-Analog and Digital signals – Source: Author
Op-Amps, resistors, capacitors, and other electronic components utilized on a PCB make up analog circuits. An analog circuit is made up of many component combinations that can change in different ways. The following are the top two uses for analog circuits:
- Filtering signals: To eliminate all unnecessary frequencies from the circuit in the event of a continuous signal, a continuous analog filter is required. Applications for analog filtering are significantly simpler and more affordable than those for a digital filter.
- Sensors: Sensors are used to transform changeable real-world data into information that an embedded system or computer can understand. Sensors create an analog signal and transform it into a digital signal in the absence of data. Contrary to high voltage systems, these systems have low amplitude and require signal conditioning to improve the signal quality and better utilize the entire range of an ADC.
On the other hand, digital circuits use logic gates that operate on digital signals to combine logical and sequential components. Digital signals operate differently from analog circuits because they use the logic of 0s and 1s to represent data in a digital form on a single IC.
Criteria for selection of PCB materials for Digital circuits
There are certain factors that should be taken into consideration while selecting materials for digital PCBs. Some of them include:
- Dimensional tolerance stability: The essential components for digital PCBs that will ensure perfect mechanical stability during extreme temperature fluctuations, vibrations, shocks, and electrical surges.
- Outstanding thermal management: The materials must be able to transport and dissipate heat well, and they must prevent layer breakdown, delamination, and peeling at higher frequencies.
- Improved signal performance: Throughout the circuit’s operation, there should be little to no signal loss across the PCB, even when the circuit’s frequency varies. To prevent losses, designers must make sure that the chosen materials have a low dielectric loss factor (Df).
- Careful impedance control: Digital PCBs must maintain a consistent dielectric constant (Dk) throughout high-speed operations, which calls for careful control of impedance routing.
- Resistance to moisture and chemicals: To ensure that the PCB maintains the desired electrical performance with the fewest possible fluctuations, materials with low moisture and chemical absorption rates must be chosen.
Digital PCB materials
A substrate and a laminate make up the foundation or base of a PCB antenna, which also affects the PCB’s functionality. A PCB should be designed with function, durability, and cost effectiveness as top priorities; therefore, choosing the right kind of PCB material is crucial. The material that is chosen while developing a PCB can affect performance in the short- or long-term. The cost of a PCB material directly relates to how well the PCB performs. When the high performance of a PCB is not necessary, lightweight polyester might be taken into consideration because it is inexpensive and versatile. The temperature is a factor that needs to be taken into account while choosing the materials. Because an overheated circuit can malfunction, the heat resistance should be greater than the heat produced. When PCB reaches Tg (glass transition temperature), it loses its performance and stiffness. Tg should be appropriate for the assembly method being utilized; for example, lead-free assembly calls for a Tg of at least 170° C. Tg should be greater than 170°C for a high-performance PCB, whereas it is 130°C for a normal PCB. High Tg materials are better able to resist chemicals and moisture, which is a benefit. Materials like FR-1, G-10, and PTFE are frequently utilized in PCB substrates. CTE, PTFE, CEM, and a number of other substances are used to create laminates.
- FR-4: The term “flame retardant” or “FR” is frequently used in standard boards. It has two Tg points, the first of which is 135°C and the second of which is 150°C-210°C and is suitable for high-density applications.
- G-10: This fiberglass laminate is made under high pressure. For use as insulators in electrical and electronic applications, G-10 and FR-4 are available.
- PFTE (Polytetrafluoroethylene): With a Tg of 160°C and 280°C, PFTE can be an excellent option for high-frequency, microwave, and high power boards.
- CEM-1, CEM-2, and CEM-3: They are effective in applications requiring high densities. The Tg provided by CEM-1, CEM-2, and CEM-3 are, respectively, 122°C, 125°C, and 125°C.
- Polymide: This substrate has a Tg of 250°C or higher, making it suitable for high-power circuits. Since FR-4 substrates are hard, polyimide substrates should be chosen over them for flexible circuits. Although slightly more expensive than FR-4 substrates, polyamide substrates provide excellent temperature resistance.
Rules for designing digital PCBs
The majority of PCB design systems can transmit rules from the schematic to the layout. It allows the schematic to drive design rules rather than delaying the input of all restrictions on the layout side. For designers, this is a big benefit. The capacity to establish rules for precise net and component placement, which are crucial for circuit design, is provided by this level of organization. A rule set can be assigned to similar nets by using a net or net class to organize them together. The designer does not need to rely on written instructions because the net rules are already included in the design database. Here are some guidelines that must be followed in order to guarantee that the circuit board is created precisely:
- Default values. The software’s design tool will initially use default values, which are frequently left over from earlier designs or are set by the operating system. Before beginning, the designer should double-check these values to make sure they aren’t routing with the wrong trace widths or arranging the components too closely. These default values may be cleared using the settings to prevent similar issues. In order to avoid problems brought on by previous settings, it should also be guaranteed that the spacing for default values is set in accordance with the necessary circuit diagram.
- Classes – Even if the majority of the rules can be set for individual nets or components, this procedure might take a while if there are a large number of objects to deal with that each have their own set of rules and constraints. To make configuring rules and restrictions simpler, several design tools offer a method for adding classes of nets and components.
Fig 2:- Installing classes
For example, the unique trace widths and the spacing requirements can be configured for a particular value of nets, a designer can create a set of rules for one power class and add those to nets.
- High-speed design guidelines: To improve the stability of data lines, particular trace lengths can be set up and matched to neighboring connecting traces when constructing high-speed digital circuits. Differential pairs can be established at a distance by designing special trace topologies for particular net properties in order to route the traces together at set differences. Trace widths can be automatically configured for impedance-controlled routing, and net classes can be given via sizes.
Fig 3:- Trace designing
Source – https://pcbdesignworld.com/sites/default/files/main-image/PCB-Trace-Width-and-Spacing.jpg
- Selecting the appropriate PCB board spacing The required electric output is improved and costs and rework are reduced by choosing the proper distance between PCB lines and the components utilized in the circuit. A 6:1 via aspect ratio is suitable because it guarantees board manufacture everywhere it is needed, which facilitates trouble-free drilling. Also, during the design process, components and circuit schematics must be properly represented in any CAD software which allows designers to view accurate simulations before mass production.
Fig 4:- PCB spacing
- Protection from electrostatic discharge: Digital printed circuit boards (PCBs) are very susceptible to electrostatic discharge, which can happen when solid-state components like integrated circuits (ICs) and batteries malfunction. Circuits have the potential to malfunction or even blow up if not taken seriously. To prevent problems brought on by electrostatic discharge, PCB designers must incorporate ESD protectors such metal-oxide varistors, transient voltage suppression diodes, polymer-based suppressors, etc.
The circuit and the PCB must be designed with the utmost care in order for the system to operate as effectively as possible when developing digital PCBs. The proper material for the digital PCB needs to be chosen by taking into account a number of different things. For the signal to pass across the PCB smoothly and without any hiccups, the materials used must have properties such as dimensional stability, excellent thermal management control, moisture and chemical resistance, etc. Due to its outstanding signal integrity and strong dielectric strength, FR4 is one of the best materials for creating digital PCBs. To obtain the desired outputs, designers must simulate the electronic components and their operation on the PCB before mass production. Designing trace lengths is crucial for high-speed digital circuits because it enhances signal stability for better signal transmission. To prevent signal interference, the spacing of the components must be done correctly while maintaining the proper proximity. For effective board fabrication and drilling that aids in flexibility throughout the wiring and soldering process, a via aspect ratio of 6:1 can be used. The best course of action is to utilize different protectors such metal-oxide varistors, transient voltage suppression diodes, polymer-based suppressors, etc. to prevent electrostatic discharge and its impact on PCBs.