PCB is just like the skeleton and nervous system of electronic circuit, which plays an important role in the electronic engineering project.
When I was in college, I was particularly interested in circuit board design and hardware circuit design, and I studied them carefully. I thought it would be great to get a job sitting in an office doing computer graphics after graduation. After graduation, I also entered an automotive electronics company in dongguan as I wished, and engaged in electronic research and development, including BOM table making, prototype debugging and PCB drawing. When I graduated in 2002, I didn’t have a job as a PCB engineer. I basically knew a little about everything, including schematic drawing, device type selection, PCB drawing, sample welding, debugging, BOM production, and work instruction, etc., and I started my career in electronic research and development
01
Schematic diagram
Although a good schematic does not guarantee good wiring, good wiring begins with a good schematic. The schematic must be carefully drawn and the signal direction of the entire circuit must be considered. If there is a normal steady signal flow from left to right in the schematic diagram, there should be an equally good signal flow on the PCB. Give as much useful information as possible on the schematic.
Because sometimes the circuit design engineer is absent, the customer will ask us to help solve the circuit problems, the designers, technicians and engineers who are engaged in this work will be very grateful, including us. In addition to common reference identifiers, power consumption and error tolerances, what information should be given in the schematic? Here are some Suggestions for turning an ordinary schematic into a first-class one. Add waveform, mechanical information about shell, printed line length, blank area; Indicate which components need to be placed on the PCB; Give adjustment information, element value range, heat dissipation information, control impedance printed lines, notes, brief circuit action description… (and others).
02
Don’t trust anyone
If you are not designing your own cabling, be sure to give yourself plenty of time to double-check the cabler’s design. A little prevention at this point is worth a hundredfold remedy. Don’t expect the wiring people to understand you. Your advice and guidance are most important in the early stages of the cabling design process. The more information you can provide, and the more involved you are throughout the wiring process, the better the PCB will be. Set a tentative completion point for the cabling engineer — check it out quickly according to the cabling progress report you want. This “closed loop” approach prevents wiring from going astray and minimizes the possibility of rework.
Instructions to the wiring engineer include a brief description of the circuit function, a schematic diagram of the PCB with input and output locations, and PCB cascade information (e.g. The layers need those signals; The position of important components is required; The exact location of the bypass element; Which printed lines are important; Which lines need to control impedance printed lines; Which lines need to match the length; Dimensions of components; Which printed lines need to be kept away from (or near) each other; Which lines need to be kept away from (or near) each other; Which components need to be kept away from (or near) each other; Which components should be placed on top of the PCB and which on the bottom. Never complain that you need to give too much information to others — too little? Is; Too much? Not at all.
Let me share a learning experience: about 10 years ago, I designed a multilayer, surface mounted circuit board with components on both sides of the board. A number of screws are used to hold the plates in a gold-plated aluminum case (due to the strict shockproof specifications). The pins providing offset feed-through are passed through the board. The pin is connected to the PCB by welding wire.
This is a very complicated device. Some of the components on the board are used for the test setup (SAT). But I have specified the location of these components. Can you guess where these components are installed? Oh, it’s under the board. Product engineers and technicians are unhappy when they have to take the whole thing apart, set it up and then reassemble it. I haven’t made such a mistake since.
03
location
As in PCB, location is everything. Where a circuit is placed on a PCB, where the specific circuit elements are located, and what other circuits are adjacent to them are all important.
Usually, the location of the input, output, and power supply is predetermined, but the circuit between them needs to be “creative”. This is why paying attention to the details of wiring can yield huge rewards. Starting from the position of key components, according to the specific circuit and the entire PCB to consider. Specifying the position of the key elements and the path of the signal from the beginning helps ensure that the design meets the desired objectives. Getting the design right once can reduce costs and stress — and shorten the development cycle.
And also, interestingly, the special pads on the PCB.
First, the teardrop pad
The teardrop is the excessive connection between the solder pad and the wire or between the wire and the guide hole. The purpose of setting the teardrop is to avoid the disconnection between the wire and the solder pad or the contact point between the wire and the guide hole when the circuit board is subjected to great external force. In addition, setting the teardrop can make the PCB circuit board more beautiful.
The purpose of teardrop is to avoid reflection caused by the sudden decrease of signal line width, so as to smooth the transition of the connection between the wire and component welding pad and solve the problem that the connection between the wire and the welding pad is prone to fracture.
1, welding, can protect the welding disk, avoid multiple welding is the shedding of the welding disk
2. Strengthen the reliability of connection (production can avoid uneven etching and cracks caused by hole deviation, etc.)
3. Smooth the impedance and reduce the sharp jump of impedance
In the design of circuit board, in order to make the bonding pad stronger and prevent the disconnection between the bonding pad and the wire during mechanical plate making, a transition area is usually arranged between the bonding pad and the wire with copper film, which is shaped like a teardrop. Therefore, it is often called Teardrops.
Second, discharge teeth
It is called discharge tooth, discharge gap or spark gap. A Sparkgaps is a pair of triangles pointing at an acute Angle to each other, with a distance of 10mil at the largest and a distance of 6mil at the smallest. One triangle is grounded and the other is connected to the signal wire. This triangle is not a component, but is made from copper foil layers used in PCB wiring. These triangles need to be set in the top layer of PCB board (componentside), and can not be covered by welding coating.
1. During the surge test or ESD test of the switching power supply, high voltage will be generated at both ends of the common mode inductor and flashover will occur. Close spacing with surrounding devices may cause damage to surrounding devices. Therefore, a discharge tube or a piezoresistor can be connected in parallel to limit its voltage, so as to play the role of arc extinguishing. Lightning protection devices work well but cost more to place.
2. Another method is to add discharge teeth at both ends of common mode inductor in PCB design, so that the inductor can discharge through two discharge tips and avoid discharging through other paths, so as to minimize the influence on peripheral and post-stage devices.
The discharge gap does not need additional cost, it is ok to draw it when drawing PCB board, but it should be noted that the discharge gap in this form is the discharge gap in the form of air, which can only be used in the environment with occasional ESD. If it is used in the occasions where ESD often occurs, carbon deposition will be generated on the two triangular points between discharge gaps due to frequent discharge, and eventually short circuit will be caused on the discharge gap and permanent short-circuit of signal lines to the ground, thus causing system failure.