A good PCB design is crucial for circuit performance and fabrication ease. The main areas of board design are the layout and routing. These give immense flexibility to the designer.
To be a better designer, there are some practices you should follow, that can be found here in PCB Design best practices guide.
PCB Design Best Practices To Improve Your PCB Designing Skills
Good component placement is very important, as this can affect the time taken for routing, but the routing is equally critical. Moreover, your design technique affects the performance, feasibility of manufacturing and cost to build the PCB.
Similarly, there are multiple tips that you can refer to for specific tasks, but the techniques we have offered in this article will be useful for all your projects, as these are more general and applicable for essential PCB designing also.
Let’s take a look at 13 of the best practices for PCB design.
Technique #1: Strategic Component Placement
One of the first steps on the path to a good design is, understanding the importance of component placement. This begins with an in-depth knowledge of the available board area and how to use it optimally for extracting the best performance from each part down to every single via.
There are many ways to do this. You can begin with finding ideal grid spacing. You might want to go for multiple grids, but that can potentially lead to problems later on in some cases. Plus it is often better to opt for a single grid spacing that works well for a majority of your components, and use it throughout.
While working on the layout, there are some rules you can follow for ensuring a good design process.
These focus mainly on improving the symmetry in your design, organizing components according to their functions and placement; furthermore, having a good knowledge of assembly and manufacturing procedures will help you place components in order to optimize process times and make them more cost-efficient.
Technique #2: Deciding Ideal Trace Sizes
Another important guideline to keep in mind while working with traces is that resistance exists in copper traces and has its own set of effects. Voltage drops across a trace, and attenuation occurs in the form of power dissipation. The resistance also generates heat when current flows through a trace.
Considering the factors that affect resistance, such as length, thickness and area of cross section it can be seen that resistance can be controlled by trace size.
The standard for measuring trace thickness is copper ounces. One oz. of copper is the trace thickness when 1 oz. Cu is spread uniformly over an area of 1 sq. foot. Commonly used values of thickness are 1oz. or 2oz., however, most manufacturers can provide up to 6oz.
Optionally, you can go for a trace width calculator to find an ideal thickness for your board. Heating should also be factored in, and you should try to keep the temperature rise around 5 degrees Celsius.
For designers working on multi-layer boards, another important thing to consider is that traces you make on outward layers or external layers are better vented and exposed to the surroundings for easier radiation of heat. Hence, they will be cooler than traces on the internal layers.
Technique #3: Efficient Routing
Routing is one of the most important steps in PCB design. To route a PCB basically means connecting signal traces according to the schematic. It is usually best to place traces as directly between components as possible and use short trace lengths.
An important rule to follow is that: if according to your placement, you need to trace horizontally on one face of the board, then make sure to route traces vertically on the opposite face.
With an increase in the number of layers in the stack-up, the complexity of guidelines to follow also increases. You will need to place horizontal and vertical traces alternately, or will have separate signal layers with reference planes.
For advanced boards that are even more complex, a set of particular guidelines will have to be followed, which will be specific to the application.
Technique #4: Power Plane Setting
It’s an easy option to use pours on the power plane. It is a good method to ensure even power flow and reduce impedance and voltage drop; and it also helps in checking if the ground return paths are sufficient.
Another tip to follow is, if you have multiple supply lines, try to run them nearby and in the same area. By running the ground plane over a larger section of the same layer, you can reduce cross-talk from lines on the layer above.
Your ground and power planes should be on the internal area of your PCB, and it’s better if they are placed symmetrically and centrally. This distributes the bending load better and provides better placement options.
This is however, not possible on a double layer board, as there will be less space for components. It is also recommended to have common rails for each power supply.
Technique #5: Ground Plane Setting
A commonly known and followed design practice is to have a common ground plane. This provides a uniform reference point where you can measure voltage at.
This can help beginners who are working on an analog design. Users who opt for traces to route to ground, solving and understanding the circuit can become really complicated.
Instead, it is recommended to create a single ground plane on the layout. You can do this with a copper area on a PCB with a single layer, or even as an entire layer on multi-layer boards. Once this is done, you can simply connect all the parts that need grounding through vias.
Technique #6: Avoiding 90 Degree Corners
This is a common practice even in mechanical design. Sharp edges and corners such as 90 degree corners should try to be avoided as they are points of stress concentration. Similarly in PCB design as well, you should try and avoid 90 degree corners.
Traces with sharp turns or angles equal to or greater than 90 degrees can cause problems while etching. This is because they can create acid traps, and will have a higher probability of being etched narrower as compared to the standard trace width. In a worse scenario, you could have shorts due to traces not etched properly.
It’s better to use 45 degree angles, and angles up to 80-85 degrees on the higher side. Smaller angles affect the trace width less. Angles that are extremely narrow can cause EM radiation and in some cases, even copper migration. Hence, those should also ideally be avoided.
Technique #7: Understanding Thermal Resistance
Heat distribution across the board affects performance a lot. To start with, you can identify which components are dissipating the maximum heat on the PCB. This can be done by referring to the Thermal Resistance ratings in the datasheets of your components, followed by adhering to guidelines on heat dissipation and distribution.
External devices such as heat-sinks and cooling fans can be added. Try to also ensure that you keep the components away from high heat sources.
Hot spots can form on the board if multiple heat generating components are placed adjacent to each other. Hence distributing such parts over the board is extremely important. This must be done strategically, in order to balance heat generation and having shorter trace lengths.
Technique #8: Thermal Relief
Once you have understood the heat signature of the board, the next step to ensuring optimal thermal performance is thermal relief. This is the process of connecting a trace or a fill to a component pin, and thus making it easier to solder. This connection is short to help in reducing the effect of electrical resistance.
If you do not use thermal relief on the pins of components, it might result in a slightly cooler component, as there is now a better source of heat dissipation available to the traces or fills. But this can make the process of soldering and de-soldering harder.
There are some methods you can use to disperse heat better and improve the thermal performance of the board.
Soldering is one of the main heat-intensive processes that the PCB undergoes. To make the process easier, and to protect the internal components of the PCB from heat damage, thermal reliefs can be used on through-hole components.
A guideline to keep in mind here is that excess heat generation will take place in case a power or ground via is near an IC with a fast switching speed. Hence you must try to design in such a way that heat dissipation from near the IC is easier to maintain the IC at a regular operating temperature.
Since the ground plane can act as a heat sink, you do not need to use a thermal relief pad for vias that are connected to the ground plane. These planes distribute heat evenly over the board.
Another method to reduce thermal stresses is using teardrops at the points where pads and traces come in contact. This provides better copper foil support. This technique also helps in reducing mechanical stresses in the board structure.
Technique #9: Auto-Routing and When To Use It
The auto-routing feature might appeal to beginners, and they may feel it’s going to save them the cumbersome task of routing. However, the auto-router is not a substitute for manual routing. You may use it on certain occasions and for certain reasons, as mentioned in the following points:
- Validation and Precision: The autorouter can be used once you have placed all the components to check the completion rating percentage of the software returns. An average range of 85% and less can mean that there is still some scope for improvement in your component placement.
- Bottlenecks: Users can also turn to the autorouter, as it can help them identify bottlenecks, and other such crucial points that might have been overlooked during part placement.
- Idea Generation: You can also take the help of auto-routing in case you are stuck and feel like you can’t come up with any good ideas for making paths.
Apart from the above three reasons, try to avoid using the auto-router. The reason is, it hardly ever gives symmetric outputs, and will, in general, not be able to create routes as good as you can, with a bit of effort.
Your paths will be based on the kind of performance you want, and other such factors like costing etc. The auto-router will not consider these parameters. Hence, it’s always better to go for manual routing, because that gives you a chance to come up with creative and innovative solutions.
Technique #10: Bypass Capacitor Placement
The purpose of using Bypass capacitors is to have a filtering process for AC components and DC components. Additionally, they help in reducing noise, garbage signals, ripples and other such unwanted AC signals.
This is achieved by passing the AC fluctuations to the ground, and hence they are named bypass capacitors. As a general practice, you should connect the ground and wherever you wish to filter the voltage.
As a suggestion, you can place such a capacitor at the board’s power inlet. The connecting wires in this region are long and receive numerous RF signals. Another application for these Bypass Capacitors could be near the ICs, and close to the ground and power pins, to filter out much of the noise that may be added internally on the board.
You can apply this similarly almost any pin where a stable voltage is necessarily required, or at reference pins.
Technique #11: Working With Mixed Signal Circuits
- To ensure a good PCB design for those of you working with mixed signal circuits, try to ensure that you keep digital and analog ground planes separate.
Similarly, while working with power circuits, it is better to keep the digital and analog grounds separate. The reason it is important to do so, is that voltage and current spikes from digital circuits might generate noise in analog circuits, which will have an impact on their performance.
If the situation requires you to place them together, it is recommended that you do so in the area of the supply path end, preferably in a point near the ground connection of the PCB. You might have other solutions to this; however, you can stick with this as a tried and tested method.
- While working with mixed signal circuits, another rule to remember is that you should shield analog grounds from interference and noise.
Interference affects analog grounds the same way as they affect signal lines. The principle applicable here is the same: the differential voltage between the ground and any point.
Although you can reduce the resistance by having a larger ground plane, this also increases the effects of capacitive coupling from lines that have been routed above or below the plane.
To effectively reduce capacitive coupling in mixed-signal circuits: try to have only analog lines crossing the analog plane, and similarly for digital.
Technique #12: Optimal Silkscreen Utilization
The Silkscreen is a widely used, standard layer in PCBs globally. This is extremely useful for labeling and displaying important information, such as: component labeling, application of the board, details about the author, revision number and so on.
You can use the silkscreen to convey useful details to the board fabricator, test engineer, or other people involved in dealing with the PCB.
Make sure you label clearly the test points, functions, and try to include orientation of parts and connectors wherever you can. This is a good design practice.
A judicious usage of the silkscreen on both sides of the PCB improves the workflow during fabrication and production, and can also help you reduce reworking.
Some guidelines for annotations that you can refer to are: If you have line resolutions of 0.008” and font heights of 0.040”, you will have to develop and provide exposure additionally. It is usually a good idea to have the minimum line resolution at 0.010” and font heights at 0.060”.
Technique #13: Quality Control and Design Verification
Rushing through the final steps in the design process can ruin all the work you have put in. And a little bit of extra effort at this stage can really help make your project fabrication a success.
You can initiate the quality control process by applying the DRC and ERC systems, or, the Design Rule Check and Electrical Rule Check systems. These will verify if your circuit is according to the constraints you were supposed to follow. Additionally, users can input custom constraints and define rules specific to the PCB application they are designing for.
Although you can run the rule checks after the design phase is complete, it is often considered a better practice to employ them during designing itself. This way, you can identify errors and constraint violations early-on and correct them.
Not only will this help you avoid unexpected errors before manufacturing, but also save the effort of changing blocks of design built upon some incorrect component.
Furthermore, there are some methods in addition to the rule checks that you can use to ensure design validity and reliability.
Once you have been thorough with the accuracy as per the rule checks, the next step is checking whether the routing for every signal is complete and that nothing has been missed; this can be done by checking every wire of your schematic.
You can make use of your design tool’s masking and probing features to ensure that the PCB layout matches the schematic and check if the layout has been done exactly according to the schematic.
So now we have looked at 13 of the PCB Design Best Practices, which will help you become a better designer along with improving your technique and skills.
Of course, there are more specific tips and guidelines for your particular design tasks, but the guidelines and techniques mentioned here will always be useful to you.
Having a good theoretical knowledge is also very important for a good design, as is an understanding of the components, assembly and fabrication of the PCB. This will be beneficial in creating innovative designs and solving any PCB design related issues you might face along the way.
In addition to the theoretical knowledge, hands-on experience and practical knowledge in designing circuits is just as important. So keep these best practices for PCB design in mind and keep improving.