Electronic circuit board design
Electronic circuit board design is fun but challenging. We talk about circuit design and show you every step from the component choice until the schematic. Next, we look at board design and cover everything from placement to drawing the final layout.
This post is for professionals and newbies alike. You can get an overview of common electronic circuit board design processes. Additionally, it’s always valuable to share helpful tools and tricks of the trade among professionals.
I divided the circuit design phase into three different areas.
- Design concept
- Library Component Design
Let’s have a look at each of these phases and check out what workflow I used to get the circuit design of Moment-Rec done.
At the beginning of our quest to a new circuit design we have to create a concept first. I started the concept phase with drawing a block-diagram that shows all elements of our system. Then I compared the block-diagram with the specifications for Moment-Rec to check that I didn’t forget anything. Additionally, I started to write down more detailed hardware and software requirements. I added information about what protocols I plan to use (e.g. SPI) and also found a common voltage of 3.0V to power all parts of the system. As you can see in the picture below, I abused Microsoft PowerPoint to draw the block-diagram. It actually works quite well.
Another very important aspect of the design concept phase is to think about costs. But, at this stage I didn’t know yet what components I would use during development. So, I had to do a general analysis what the price of comparable products on the market is. I found that most products that succeeded in crowdfunding are priced between 60-100$. Following the advice from EEVblog (how to price hardware products), I estimated a price for Cost Of Goods Sold (COGS) between 12-33$.
Next, I needed to find out what the ball park price is for any of the individual system components. I searched for Bluetooth Low Energy Modules and microprocessors that would work and checked their price. I concluded that most BLE modules are between 4-10$ for bulk orders. As I outlined in my post about the component choice for Moment-Rec, the BLE module is a big contributor to the total bill of materials (BOM) cost. Next, I did the same research for other system components such as battery, processor, memory and peripherals. In the end, I had to change the preliminary BOM multiple times because the costs were too high.
Library component design
The next step in circuit design is the library component design. We have to create a symbol and footprint for each component we plan to use in our design. I am a big fan of taking really much care when creating the library and include as much detail as possible. The more detailed a component’s parameters are, the easier it will be to export a bill of materials or create a 3D view of the finished PCB. In the figure below, you can see all parameters that I add to every library component.
Next, I also believe that it is very important to get the footprint design right. IPC is a standardization body that regulates everything about PCB design and production. IPC-7351 in particular regulates SMD footprint design. IPC’s mission is to ensure efficient and effective communication between PCB manufacturers and designers. Therefore, it makes sense to comply to IPC rules when designing a footprint. I use LibraryExpert, a free tool provided by IPC, to generate most SMD parts and ensure all are IPC compliant. LibraryExpert also generates ready to use 3D step files for each SMD part.
Finally, we are ready to start with the most popular part of circuit design – the schematic. Today’s complex system design leads to ever more complex electronic circuit board designs. So, it is crucial to adopt a strict hierarchical approach when creating circuit designs.
I’m using a software called Circuit Studio to create the whole electronic circuit board design. Fortunately, Circuit Studio comes with a functionality called “device sheet symbols” to create hierarchical circuit designs.
In the figure below, you can see the top hierarchical sheet of the Moment-Rec’s circuit design. Every green block represents another schematic sheet which is one hierarchy level lower. Hierarchical designs also regulate the scope of net labels, ports and bus signals. The trend toward hierarchical designs is almost as big of an advancement as object oriented programming is for the software industry.
In the figure above, you can see that I separated the circuit design into two parts. On the left side, I located all peripherals and components exposed to the outside. These components and peripherals might potentially pick up ESD shocks or other types of EMI disturbances. Therefore, we will need special EMI mitigation circuitry if we want to pass CE and FCC tests. On the right side, I arranged all other system components such as the digital parts, audio amplifier and power supply related circuitry.
The crossed out parts are components which won’t be assembled for this variation of the circuit design. I added various extra component footprints to be able to react quickly, if we happen to fail any certification tests. Overall, I can alter the assembly and add nine extra EMI protection components without any change on the electronic circuit board design.
Producing a prototype takes time, effort and is expensive. So, I have to make sure that we can use the Moment-Rec prototype to gain as much information as possible about what needs improvement.
We can separate electronic circuit board design into two different areas: system and physical design. The schematic is an abstract representation of the system we try to build. Board design is about molding the schematic into a physical representation that we can fabricate.
Laying the groundwork
When I started with the Moment-Rec board design, the PCB shape was given by the casing we evaluated previously. I used the mechanical drawings and 3D case data to create a PCB shape that would fit the casing perfectly. Next, I added keep out zones to ensure enough spacing to the board edge. If I haven’t yet decided which manufacturer to use, then I usually start out with 2mm clearance. Later in the design, I can often decrease that spacing and still comply to the manufacturer’s requirements.
However, designing keep out zones to comply with mechanical requirements is not enough. We also have to ensure that no components can be harmed by EMI. Therefore, we have to keep a healthy distance to the casing walls, gaps and screws. I chose a “healthy distance” of 7.3mm to the casing walls and 9.5mm to screws. Although, that may look like a huge waste of board space, we need to guarantee enough clearance and creepage distance.
The last step is placing all components. I always try to create small groups of components and arrange all components within the group first. This bottom-top workflow ensures that components that are located close together in the schematic will also stay close in the layout.
Furthermore, we should also talk about the cost for assembling the board. Moment-Rec has a somewhat constrained budget. So, I made sure to only place components on the top side. I only used the bottom side for traces but no components. The price difference between a single and a double side assembly board is often close to a factor of two.
Drawing the circuit board design layout
I always joke that creating a board design is like a therapeutic zen meditation. We have to carefully move around different sized and shaped blocks, draw lines and arrange text. I find board design very fascinating because we have to arrange blocks and traces in order to make a design work. I love to turn on my internal PCB simulation engine and think about what might happen to a signal on its way across the board.
When I started with Moment-Rec, the first thing was defining the layer stack. It’s always awesome for the board designer to be able to use as many layers as possible. However, a higher layer count will make the board more expensive. Therefore, it’s always a compromise between how much time the board designer spends to squeeze the design in the space given and how many layers we are willing to pay for. I decided to go for a four layer board because of several reasons. First, I was sure that I could fit all components and traces on a four layer stack. Second, four layer boards offer a very big advantage over two layer boards in terms of EMI mitigation. Finally, four layer boards are probably the most often used layer stack nowadays.
We are finally ready to draw some traces and connect the components. I separated my layout workflow into local and global signals. First, I layouted all local signals that only have short connections. Second I focused on global signals like clock, peripherals etc. That’s also when I started to use Vias to pass under or over other traces. However, I kept the Mid-Layer 1 and Mid-Layer 2 completely free of any traces. These layers will later be flooded with GND and Vcc.
Finally, once I completed all local and global signals, I focused on the power distribution network (PDN). PDN is a fancy term for saying that I layouted all power and GND traces. The PDN should be very low impedance. Hence, I used one huge polygon plane on each of the Mid-Layers for power and GND. At this point we are done with the board design. Another very important factor in electronic circuit board design is Design For Manufacturability (DFM). But, DFM is a very big and important topic and is not the scope of this article.
A broad overview
I tired to outline the design workflow I use and use Moment-Rec as an example project. Electronic circuit board design is a very complex and overarching topic. It’s impossible to explain every detail in one blog post. I do regular development live streams where I show how I develop my hardware and software. It would be a pleasure to meet you in the live stream chat :).