PCB is like the skeleton and nerve network of electronic circuits, playing a pivotal role in electronic engineering projects. However, many people have little or insufficient understanding of PCB design.
PCB is like the skeleton and nerve network of electronic circuits, playing a pivotal role in electronic engineering projects. However, many people have little or insufficient understanding of PCB design.
When I was in college, I was particularly interested in circuit board design and hardware circuit design, and I studied them quite seriously. At that time, I thought how wonderful it would be to find a job sitting in an office doing computer graphics after graduation. After graduation, I got a job at an automotive electronics company in Dongguan as expected, engaged in electronic R&D work, including making BOM tables, prototype debugging, and PCB drawing. When I graduated in 2002, there was no such job title as PCB engineer. Basically, we had to know a little bit of everything, including schematic drawing, component selection, PCB drawing, prototype soldering, debugging, BOM making, work instructions, etc. And that's how I embarked on the road of electronic R&D.
01 Schematic Diagram
Although a good schematic does not guarantee good routing, good routing starts with a good schematic. When drawing a schematic, think carefully and must consider the signal flow of the entire circuit. If there is a normal and stable signal flow from left to right in the schematic, there should also be the same good signal flow on the PCB. Provide as much useful information as possible in the schematic.
Because sometimes the circuit design engineer is not available, and the customer will ask us to help solve circuit problems. Designers, technicians, and engineers engaged in this work will be very grateful, including us. In addition to ordinary reference designators, power consumption, and error tolerances, what other information should be included in the schematic? Here are some suggestions to turn an ordinary schematic into a first-class one: add waveforms, mechanical information about the enclosure, printed trace lengths, blank areas; indicate which components need to be placed on the top of the PCB; provide adjustment information, component value ranges, heat dissipation information, controlled impedance printed traces, notes, concise circuit operation descriptions... and others.
02 Trust No One
If you are not designing the routing yourself, be sure to leave plenty of time to carefully check the designer's work. A little prevention at this point is worth a hundred times the remedy later. Don't expect the routing person to understand your ideas. Your opinions and guidance are most important at the beginning of the routing design process. The more information you can provide and the more involved you are throughout the routing process, the better the resulting PCB will be. Set a tentative completion point for the routing design engineer and conduct quick checks according to the routing progress reports you want. This "closed-loop" approach prevents routing from going astray, thus minimizing the possibility of rework.
Instructions to be given to the routing engineer include: a brief description of the circuit function, a PCB sketch indicating input and output positions, PCB stack-up information (e.g., how thick the board is, how many layers it has, detailed information about each signal layer and ground plane - power, ground, analog signals, digital signals, and RF signals); which signals are required on each layer; required placement positions of important components; exact positions of bypass components; which printed traces are important; which lines require controlled impedance printed traces; which lines require length matching; component sizes; which printed traces need to be far away from (or close to) each other; which lines need to be far away from (or close to) each other; which components need to be far away from (or close to) each other; which components should be placed on the top of the PCB and which on the bottom. Never complain about giving too much information to others - too little? Yes; too much? No.
Share a learning experience: About 10 years ago, I designed a multi-layer surface-mount circuit board with components on both sides. The board was fixed to a gold-plated aluminum enclosure with many screws due to strict shock resistance requirements. Pins providing bias feedthrough passed through the board and were connected to the PCB by soldering wires.
This was a very complex device. Some components on the board were used for test setup (SAT). But I had clearly specified the positions of these components. Can you guess where they were installed? That's right, on the bottom of the board. When the product engineers and technicians had to disassemble the entire device to complete the setup and then reassemble it, they were very unhappy. I have never made this mistake again since then.
03 Location
Just as in PCB, location is everything. Where to place a circuit on the PCB, where to install its specific circuit components, and what other circuits are adjacent to it are all extremely important.
Usually, the positions of input, output, and power supply are predetermined, but the circuits between them require "creative thinking". That's why paying attention to routing details pays off handsomely. Start with the positions of key components and consider them according to the specific circuit and the entire PCB. Specifying the positions of key components and signal paths from the beginning helps ensure that the design achieves the expected working goals. Getting the design right the first time reduces costs and pressure - and thus shortens the development cycle.
In addition, let's talk about some interesting "special pads" on PCB boards.
First, Teardrop Pads
A teardrop is a teardrop-shaped transition connection between a pad and a conductor or between a conductor and a via. The purpose of setting teardrops is to avoid disconnection at the contact points between conductors and pads or between conductors and vias when the circuit board is subjected to huge external impact. In addition, setting teardrops also makes PCB circuit boards more aesthetically pleasing.
The function of teardrops is to avoid reflection caused by sudden narrowing of signal line width, enable smooth transition between traces and component pads, and solve the problem that the connection between pads and traces is easy to break.
- In soldering, it can protect pads and prevent pad peeling during multiple soldering.
- Enhance connection reliability (can avoid cracks caused by uneven etching and via misalignment during production).
- Smooth impedance and reduce abrupt impedance jumps.
In circuit board design, to make pads stronger and prevent disconnection between pads and conductors during mechanical board manufacturing, a transition area made of copper film is often arranged between pads and conductors, shaped like a teardrop, hence commonly called teardrop addition.
Second, Discharge Teeth
This object is called a discharge tooth, discharge gap, or spark gap. A spark gap is a pair of triangular shapes with acute angles pointing towards each other, with a distance of 6mil minimum and 10mil maximum between the fingertips. One triangle is grounded, and the other is connected to the signal line. This triangle is not a component but is made using the copper foil layer during PCB routing. These triangles must be placed on the top (component side) of the PCB board and cannot be covered by solder mask.
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During switching power supply surge tests or ESD tests, high voltage will be generated at both ends of the common mode inductor, resulting in arcing. If the distance to surrounding devices is close, it may damage surrounding devices. Therefore, a discharge tube or varistor can be connected in parallel to limit its voltage, thus playing an arc extinguishing role. Placing lightning protection devices works very well but is relatively expensive.
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Another method is to add discharge teeth at both ends of the common mode inductor during PCB design, allowing the inductor to discharge through the two discharge tips, avoiding discharge through other paths, thus minimizing the impact on surrounding and subsequent devices.
Discharge gaps do not require additional cost and can be drawn when drawing the PCB board. However, special attention should be paid that this form of discharge gap is an air-type discharge gap and can only be used in environments where ESD occurs occasionally. If used in environments where ESD occurs frequently, carbon deposits will form on the two triangular points due to frequent discharges, eventually causing a short circuit across the discharge gap and a permanent ground short circuit of the signal line, leading to system failure.
In fact, there are many joys in PCB design, and only through practice can you have a profound understanding. If friends have any feelings about PCB design after reading this article, you can leave a message in the comment section to communicate together.
About Maxipcb
Maxipcb enables advanced electronic innovation. We deliver one-stop solutions including circuit design, simulation, testing, PCB fabrication, component sourcing and SMT&PCBA assembly, to boost R&D efficiency, speed up mass production and control full-cycle risks. We serve global sectors like communication, industrial automation, aerospace, automotive and semiconductor, jointly forging a safer, connected intelligent future.
