From a technical novice to a qualified and excellent PCB design engineer, this is definitely a process of riding the wind and waves and overcoming obstacles. In addition to continuous learning and mastering of various skills, you have to bear psychological, economic and other pressures. Of course, on the road to becoming a PCB design engineer, learning various skills is the most important!
Five skills that PCB design engineers must master
1. Skilled use of PCB design software
There are many PCB design software, the main ones currently used in the market include the following: Cadence Allegro, Mentor EE, Mentor Pads, Altium Designer, Protel, etc. Among them, Cadence Allegro has the highest market share.
Allegro has many advantages, such as friendly software interface, fast response speed, high operation efficiency, rich secondary development functions, perfect rule manager functions, powerful high-speed design exclusive functions and so on. It supports large-scale projects well, and the response speed will not be greatly reduced due to the increase of the design scale. There is basically no pressure to use Allegro for design projects with tens of thousands of Pins. Therefore, for the communication industry, commercial servers, as well as industrial control, It is very suitable for the military field.
2. Basic English ability
English is a necessary basic ability for PCB design engineers. Why do you say that? Work required!
At present, almost all development programming tools, integrated environment software and EDA software are basically in English, and some even Chinese catalogues are not allowed. Although some software supports Chinese, there are also Chinese versions, but do you know that the results of checking the data do not correspond to the terrible English descriptions in the data~~
Cadence OrCAD/Allegro and Pads, which do PCB design, do not have Chinese versions. AlTIum Designer has localization but the translation quality is not good. If you do circuit debugging, most of the oscilloscopes, spectrum analyzers, and logic analyzers like Agilent and Tektronix have only English interfaces.
In addition, you need to be able to read the hardware manufacturer's data sheets, application notes, etc. There is no doubt that the electronics industry today is still at the forefront of the world. From hardware to software, from manuals to data manuals, most of them are in English.
3. Familiarize yourself with devices, read circuit schematics, and understand key signals
Familiar with various common electronic components, including: resistors, capacitors, inductors, transformers, diodes, triodes, field effect transistors, optocouplers (OC), sensors, crystal oscillators, relays, buzzers, rectifier bridge stacks, filters , switches, fuses, etc. Key signals include: power supply, analog signal, high-speed signal, clock signal, differential signal, synchronization signal, etc.
In the PCB routing rules, there is a principle of "priority of key signal lines", that is, key signals such as power supply, analog signal, high-speed signal, clock signal, differential signal and synchronization signal are routed first. Next, let's take a closer look at the wiring requirements for these key signals.
The main feature of analog signals is poor anti-interference, and the protection of analog signals is mainly considered when wiring.
The processing of analog signals is mainly reflected in the following points:
a. In order to increase its anti-interference ability, the wiring should be as short as possible.
b. Some analog signals can give up impedance control requirements, and the traces can be appropriately thickened.
c. Limit the wiring area, and try to complete the wiring in the analog area, away from the digital signal.
High-speed signal wiring requirements
1. Multilayer wiring
High-speed signal wiring circuits often have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. Reasonable selection of the number of layers can greatly reduce the size of the printed board, can make full use of the middle layer to set up shielding, can better achieve the nearest grounding, can effectively reduce the parasitic inductance, can effectively shorten the transmission length of the signal, and can greatly reduce the signal cross-interference, etc.
2. The less the lead is bent, the better
Lead bends between the pins of high-speed circuit devices should be as little as possible. The leads of the high-speed signal wiring circuit wiring are preferably all straight lines, which need to be turned, and can be turned by 45° broken lines or arcs. This requirement is only used to improve the fixing strength of the steel foil in low-frequency circuits, but in high-speed circuits, this requirement is met. It can reduce the external emission and mutual coupling of high-speed signals, and reduce the radiation and reflection of signals.
3. The shorter the lead, the better
The wiring between the pins of the high-speed signal wiring circuit device should be as short as possible. The longer the lead, the greater the distributed inductance and distributed capacitance, which will have a lot of influence on the passage of high-frequency signals in the system, and will also change the characteristic impedance of the circuit, causing the system to reflect and oscillate.
4. The less alternating between lead layers, the better
The less alternating between the lead layers between the pins of the high-speed circuit device, the better. The so-called "the less the interlayer alternation of the leads, the better" means that the fewer vias used in the component connection process, the better. According to the measurement, one via can bring about 0.5pf of distributed capacitance, which leads to a significant increase in the delay of the circuit. Reducing the number of vias can significantly improve the speed.
5. Pay attention to parallel cross-interference
In high-speed signal wiring, attention should be paid to the "cross-interference" introduced by the parallel wiring of the signal lines in close proximity. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal lines to greatly reduce the interference.
6. Avoid branches and stumps
High-speed signal routing should try to avoid branches or stubs. Stumps have a large impact on impedance and can cause reflections and overshoots of the signal, so we should generally avoid stumps and branches in our design. Use daisy-chain wiring to reduce the impact on the signal.
7. The signal line should go as far as possible on the inner layer
High-frequency signal lines are prone to generate large electromagnetic radiation on the surface layer, and are also susceptible to interference from external electromagnetic radiation or factors. The high-frequency signal line is routed between the power supply and the ground line, and the radiation generated will be greatly reduced by the absorption of electromagnetic waves by the power supply and the bottom layer.
Clock Signal Routing Requirements
In digital circuit design, a clock signal is a signal that oscillates between a high state and a low state, which determines the performance of the circuit. The clock circuit plays an important role in the midpoint of the digital circuit, and is also the main source of electromagnetic radiation. The processing method of the clock also needs special attention when PCB routing. By clarifying the clock tree at the beginning and clarifying the relationship between various clocks, it can be handled better when routing. In addition, the clock signal is often a difficult point in EMC design, and special attention should be paid to projects that require EMC test indicators.
In addition to the conventional impedance control and equal-length requirements, the clock lines also need to pay attention to the following issues:
a. Select the preferred wiring layer as far as possible for the clock signal.
b. The clock signal should not cross the division as much as possible, and should not be routed along the division area.
c. Pay attention to the distance between the clock signal and other signals, at least 3W.
d. For the design with EMC requirements, when the line is longer, try to choose the inner layer wiring.
e. Pay attention to the termination matching of the clock signal.
f. Do not use a daisy-chain structure to transmit clock signals, but use a star structure, that is, all clock loads are directly connected to the clock power driver.
g. All wires connecting the input/output terminals of the crystal oscillator are as short as possible to reduce the influence of noise interference and distributed capacitance on the crystal oscillator.
h. The ground wire of the crystal oscillator capacitor should be connected to the device with as wide and short as possible; the digital ground pin closest to the crystal oscillator should minimize the number of vias.
i. In digital circuits, the usual clock signals are signals with fast edge changes, and the external crosstalk is large. Therefore, in the design, the clock line should be surrounded by ground lines and more ground lines should be used to reduce distributed capacitance and thus reduce crosstalk; for high-frequency signal clocks, try to use low-voltage dependent clock signals and wrap the ground. Pay attention to wrapping the ground. Hole integrity.
Differential Signal Routing Requirements
Differential signals, some are also called differential signals, use two identical signals with opposite polarities to transmit one channel of data, and rely on the level difference between the two signals to make a decision. In order to ensure that the two signals are completely consistent, the wiring should be kept in parallel, and the line width and line spacing should remain unchanged.
On the board, the differential traces must be two traces of equal length, equal width, close together, and on the same level.
Equal length means that the length of the two lines should be as long as possible, in order to ensure that the two differential signals maintain opposite polarities at all times. Reduce common mode components.
Equal width means that the trace widths of the two signals need to be kept the same, and equal distance means that the distance between the two lines should remain unchanged and parallel.
Try to provide a dedicated wiring layer for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure the smallest loop area. Use methods such as shielding and increasing the safety distance to ensure signal quality.
4. Have a certain understanding of SI/PI knowledge
With the increase in the output switching speed of integrated circuits and the increase in PCB board density, signal integrity (SI) has become one of the must-care issues for high-speed digital PCB design. Factors such as the parameters of components and PCB boards, the layout of components on the PCB board, and the wiring of high-speed signals can cause signal integrity problems, resulting in unstable system operation, or even no work at all. How to fully consider the signal integrity factor in the design process of the PCB board and take effective control measures has become a hot topic in the PCB design industry today.
With the development trend of high power consumption, high density, high speed, high current and low voltage of electronic systems, the power integrity (PI) problem in the field of high-speed PCB design is also becoming more and more serious.
As an excellent PCB design engineer, of course, you need to have a certain understanding of SI/PI knowledge, which can be used to guide and optimize PCB design, improve power channel design, and optimize decoupling capacitor design.
5. Have a deep understanding of EMC/EMI knowledge
As we all know, the design of PCB should comprehensively consider various factors such as function realization, cost, production process, EMC, and aesthetics.
With the increasing frequency of electronic signals and processors of electronic equipment, the electronic system has become a complex equipment containing many components and many subsystems. High density and high speed will increase the radiation of the system, while low voltage and high sensitivity will reduce the noise immunity of the system. Therefore, electromagnetic interference (EMI) is a threat to the safety, reliability and stability of electronic equipment. When we design electronic products, the design of the PCB board is very important to solve the EMI problem.
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