Good PCB design is about having the perfect trade-off to make an electronic product that satisfies all the competing requirements.
During your design process there are key elements which need to be in balance and PCB design issues that can arise. This article explores these issues and considerations, so you start off prepared.
Written with the knowledge of Dr Nicholas Shattock, Embedded Electronics Engineer at Ignys.
PCB Design Fundamentals
First thing’s first. You need to understand what a PCB is and why you need to design it in the first place.
This helps you to identify the issues and risks involved in the PCB design process. A deep understanding of what you are trying to solve, the ultimate goal, and the requirements of your end product will form the foundations of a great PCB design.
What is a PCB?
Fundamentally a PCB (Printed Circuit Board) is a way of mechanically securing and providing the connections between the individual components. This could be connectors, passive components, or more advanced electronic components, such as microprocessors and FPGAs. PCBs are even used to connect multiple off the shelf modules, such as Raspberry Pis.
What is a printed circuit board made of?
A circuit board is comprised of a sheet of copper that’s attached to a suitable substrate (usually fiberglass). Then the copper is etched using an acid in the shape needed to make up the connections between the components. The holes in the board are then drilled and the connections between the layers made by plating the holes with copper.
The History of PCB Design
PCB design origins
In the beginning electronic products used a point-to-point approach to make all the connections between the individual components. Each connection had to be made individually using a soldered or wire wrapped connection. This required a huge amount of skilled labour to assemble and was an unreliable process, and as board complexity and number of connections increased this became an unviable solution.
In 1936, Austrian inventor Paul Eisler developed the first PCB to operate a radio system, based on a circuit design originally patented by Charles Ducas.
The PCB used a laminated board clad with a thin copper sheet and was etched to form traces, creating all of the connections at once.
The technology behind the etching of the copper on a PCB was based on a similar approach to that of the lithography used for printmaking. As this was a mature process, much of the technical hurdles had been resolved and the equipment and materials were already widely available.
Over the years we’ve discovered that circuit boards are the most effective and low-cost way of connecting components together. Doing things in this way allows us to make electronic products in large volumes reliably, and at a relatively low cost.
The evolution of circuit board design
Reliable volume production, low cost and small form factor are the three key metrics that have accelerated circuit board design.
These three metrics can only be optimised if we take the time to design and test our PCB layouts effectively. Over time the electronics industry has also invented different PCB types, using different substrates to allow for flexible boards, more efficient boards and board which have better thermal properties. This allows us to design products to fit in smaller enclosures or operate at higher speeds and greater powers.
The electronics design industry also uncovered many ways to omit unnecessary product unit costs. We can achieve this through careful PCB layout and production processes, by using value engineering techniques.
PCB layer trade offs
As the complexity of the system increased, people found that one layer wasn’t enough. So as the PCB productions improved and the costs decreased, the number of layers available to the designer have increased, for a product to hit a target cost.
What makes a good PCB design?
A good PCB design optimises the board in such a way as to mitigate any risks and to navigate through the delicate balance of requirements to come up with a PCB design that performs acceptably in every metric.
Creating the most efficient connections between the components in a PCB design is a mathematical problem. It is categorised as an NPs-hard problem (similar to the travelling salesman problem) and with a large number of components and connections to be made, the difficulty gets exponentially more difficult to solve.
However, PCB design is not all about making the connections in the most efficient manner. There are several aspects that are crucial and often get missed, particularly in the hobbyist market.
This is often why companies will turn to an electronics design consultancy for a deeper understanding of the PCB design issues that are involved in the development process.
How to optimise your PCB design
The best way to optimise your PCB design is to explore all the things that could go wrong with your board and hold them simultaneously in your mind, to then engineer the best design. It’s a delicate balancing act!
There is more to a circuit board than connecting it up, point A to point B. Here are some of the major considerations you need to account for:
Circuit board design considerations
For high volume projects start with this question:
‘Can we design the desired product to be manufactured with a low failure rate and at low cost?’
In many product areas, cost is key and needs to be at the forefront of the PCB engineer’s mind when they are designing your project.
Here are 3 good considerations for circuit board design:
- Cost of manufacture
- Reliability of the product
- Low failure rate during manufacture
The reliability of a product is determined by both the person who assembles and manufactures it and the design engineer. They need to work in collaboration. In addition, you need to consider designing for test (DfT). A board designed for test will allow the use of an automated test jig, reducing the testing costs during manufacture and helping to ensure that every product sold works as intended.
Manufacturability and collaboration
Every manufacturer will use different techniques to manufacture and assemble the PCBs, and this will result in different precision and tolerances, so it’s important to try and align with these.
For complex projects this is essential. Especially in high density design where you are trying to squeeze a lot into a small space.
This needs to work well for the assembly process. PCB manufacturers or CEMs (Contract Electronic Manufacturers) need to be able to produce these complicated PCB designs, with components squeezed into small spaces using their current production processes.
Designing to standards
To solve this problem, design and manufacturing standards have been created.
If a board is designed to a set standard, then it should be manufacturable by a CEM who adheres to those standards. The Ignys team work to IPC and internal standards to create a manufacturable and reliable design to maximise the chance of product success.
Product functionality and design for availability
In the past – even 20 years ago – how things were connected mattered a lot less.
Now things have got faster, use a higher density of components, and perform better in a smaller enclosure. This means that there is a much higher chance of failure from EMC and interference between components.
Many of these can be solved with the correct component selection, placement, and filtering. Of course, component selection is far more difficult these days due to the global semiconductor chip shortage, making design for availability far more desirable. Ensuring you have the right parts to make your design at high volume is paramount in today’s environment.
The importance of EMC for circuit design
Correct circuit board layout is also the most effective way of solving compliance issues such as EMC (electromagnetic compatibility).
It should be noted that schematic design also needs to look at things from an EMC point of view too.
The bottom line is, having a poor PCB layout combined with modern electronic components, is a recipe for high costs and a LOT of problems.
Consider that as your market expands you may wish to sell into new countries, all of which have their own EMC requirements, from UKCA in the UK, FCC requirements in the US, and CE in the EU. It will be easier to comply with these regulations if your initial design is created with EMC in mind.
You should bake in key design principles from the beginning such as ‘How do I reduce the noise that this radiates and is susceptible to?’ There are lots of great videos online on how to do this.
The key thing with EMC is this – anything conductive can become an accidental antenna that could radiate or pick up radio signals. Even a slot in a metal enclosure can become an antenna! The design needs to minimise the unintended electromagnetic signals that are generated, and then any signals that should be contained or dissipated to prevent them radiating out of the product.
High speed layout
Many products use high speed components with high-speed interfaces and connections. These include interfaces such as USB, HDMI, ethernet and PCIe.
All of these high speed interfaces have very tight requirements for the PCB trace length and impedance. With many modern high speed interfaces, the trace lengths are required to be accurate to within 0.1mm on a trace that could be as long as 100mm.
Impedance control is crucial and without it the signal may be reflected as it travels along the PCB trace. This reflected wave can cause EMI problems and can result in the data becoming corrupted at the receiving end.
An analogy showing the crucial role of correct impedance is that of a guitar. Without the guitar bridge the sound is barely audible and completely lacking any bass. Once you add the bridge you get a full and rich sound. In this case the body of the guitar reduces the impedance mismatch between the strings and the air, allowing the sound to propagate more easily.
Mechanically you need to look at the big picture. How does the whole product fit together?
Not only are we designing a circuit board to connect various components but also to make it fit in a specific enclosure.
You need to make sure parts don’t interfere with each other and there is access to connectors and screws aren’t obscured by any components. This means holes need to be in the right place and it needs to be assembled, and repaired, easily using low-cost techniques.
Safety and power consumption
Then there is safety and power to think about. How do we make sure nothing dangerous happens in event of a fault? It’s paramount that you avoid introducing the risk of electric shocks and other potential hazards.
Your design needs to account for the power handling required of it. You need protection, and you need it to be reliable. You also need to ensure there aren’t PCB design issues when it’s rated at the compliance stage, and that it’s maintaining the sort of power levels people are expecting of it when they use the product.
The concept of power consumption management is in almost every electronic product. Electronic products by nature use power and this brings us on to the thermal aspects.
Temperature testing and PCBs
As products use power they heat up. How do you know your product will be reliable at maximum temperatures? How do you keep them cool? Looking at the reliability of parts is a topic for another day.
You may need to do some numerical thermal modelling during the design process and experiment with the prototype product in an environmental test chamber.
Having awareness of how hot the different parts are going to get and implementing ways to mitigate or improve thermal performance of circuit board is key. Dissipating the heat over larger areas, if you have a part that is getting particularly hot, for example and placing sensitive components away from sources of heat. You may need to add some sort of heat sink, a metal-finned component that will attach to the hot components and cool them down.
Competing testing requirements
It’s the PCB engineer’s role to navigate through conflicting requirements to make a product which satisfies all of them.
For example, we are not trying to get zero EMC emissions – you merely want it to be good enough to pass pre-compliance.
You want to get a product which is satisfactory on all levels. If your product is perfect but costs a million pounds, or it’s cheap but explodes after 3 weeks, neither of these solutions help anyone.
Having the right knowledge (twinned with a little luck!)
People tend to make mistakes because it’s difficult to be knowledgeable about all these areas and to simultaneously hold these conflicting requirements – and then create a design that can fulfil the specification of the product.
This is especially true for designs operating at high speeds and to meet EMC radio emissions limits. These both can be incredibly complicated and there is a huge amount of knowledge and understanding needed here to create the best PCB layout.
The scary thing about electronic product development is you can never be 100% sure your design will work. Sometimes you get lucky, and a poor design works well, other times you create a great design, but it still doesn’t meet all of the product requirements.
This is where bringing a team or experienced design engineer on board who has the right experience can help you out.
Starting out the right way
Designing the right way from the beginning avoids a costly redesign, particularly with EMC, which can be very expensive to solve and can sometimes even require a complete redesign. Having the right design processes in place can reduce the amount of work required to solve these PCB Design issues.
No one-size-fits-all approach for PCB Design Optimisation
The best design depends very much on the project, the client, and the end user. PCB design reviews are a great way to explore PCB optimisations.
PCBs are used in such a wide variety of sectors and markets that it’s very hard to have a one-size-fits-all solution. Consider a £3 electronic device in a supermarket vs a £200,000 piece of measurement equipment or space tech – these are completely different worlds and there is no way to optimise for both of them.
One is very cost-constrained, the other has literally no upper budget but needs to perform perfectly with no room for error over a long lifespan.
The best way to tackle PCB Design issues
Ultimately, the best way to optimise your PCB design is to get an opinion from a third party. We work closely with a range of businesses on PCB layout, so if you’re looking for a second opinion, then do get in touch.
Dr. Nicholas Shattock holds a P.h.D. in Power Electronics and as an Ignys Embedded Electronics Engineer he holds a wealth of knowledge about balancing PCB design issues in order to optimise PCB design.