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Oct 13 2020

PCB Design: How to Draw Circuit Diagram of PCB?

Introduction

PCB exists in every electronic device. A fully functional PCB is mainly used to create connections between components, such as resistors, capacitors, inductors, diodes, transistors, integrated chips, etc. It is the carrier of the entire logic circuit. Sound PCB design can save production costs, and achieve good circuit performance and heat dissipation effect. PCB designs vary in complexity according to product needs. This article mainly talks about wiring, one of the basics of PCB design.

PCB Design: From Idea to Schematic to PCB

Catalog

Introduction

Ⅰ PCB Basics: Wiring Rules

Ⅱ Three PCB Wiring Methods

Ⅲ PCB Design: Wire Inspection

Ⅳ Complete PCB Design Projects Inspection

4.1 General PCB Design Inspection Projects

4.2 PCB Electrical Characteristics Checking Projects

4.3 PCB Physical Characteristics Checking Projects

4.4 PCB Mechanical Design Factors

4.5 PCB Installation Requirements

4.6 PCB Pull-out Requirements

4.7 PCB Mechanical Considerations

4.8 PCB Electrical Considerations

4.9 Electronics Inspection Before Into A PCB

Ⅴ Conclusion


Ⅰ PCB Basics: Wiring Rules

1. The area within 1mm from the edge of the PCB board and within 1mm around the mounting hole will not take wiring.
2. The power line width should not be less than 18mil; the signal line width should not be less than 12mil; the cpu input and output lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil.
3. It is necessary noted that the power line and the ground line should be as radial as possible, and the signal line must not be looped.
4. Ground circuit rules
The loop area formed by the signal line should be as small as possible. The smaller the loop area, the less external radiation and the less interference from the outside. An example is shown in the figure below:

pcb circuit loop

5. Crosstalk control
Here crosstalk refers to the mutual interference caused by long parallel wiring between different networks on the PCB, which caused by the distributed capacitance and inductance between the parallel lines. The main measures to overcome it are:
a. Increase the spacing of parallel wiring and follow the 3W rule. To ensure that the distance between the lines is large enough, when the distance between the line and the center of the line is not less than 3 times the line width (as shown in the figure below). If the line center distance is not less than 3 times the line width, 70% of the line electric fields will not interfere with each other, which is called 3W rule.

3w rule

b. Insert a grounded isolation wire between the parallel wires. Reduce the distance between the wiring layer and the ground plane.
6. The direction control rules of routing:
The routing directions of adjacent layers are orthogonal. Different signal lines in the same direction on adjacent layers should be avoided to reduce unnecessary interlayer crosstalk. When the signal rate is high, use a ground plane to isolate each wiring layer, in other words, isolate each signal line with ground line. The neighbouring wires used in the input and output end of the circuit shouldn’t be parallel to prevent the feedback, and it is best to add a ground wire between these wires.
7. Open loop inspection rules for wiring:
Generally, it is not allowed to have a floating wiring at one end, because of the "antenna effect" and unnecessary interference radiation and reception, which may bring unpredictable results.

floating wiring

8. Impedance matching inspection rules
The wiring width of the same network should be kept the same. Line width variations will bring uneven line characteristic impedance, and reflection will occur when the transmission speed is high. This situation should be avoided in the design. Under certain conditions, such as the lead wires of the connector and the similar structure of the lead wires of the BGA package, the change of the line width may not be avoided, so that the length of the middle inconsistent part should be minimized.
9. Wiring closed loop inspection rules:
Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multilayer board design, and it will cause radiation interference. As shown below:

self-loops

10. The branch length control rule of wiring:
Try to control the length of branches, and the general requirement is Tdelay≤Trise/20.

control the length of pcb branches

11. Resonance rules of wiring:
For high-frequency signal design, the wiring length must not be an integer multiple of its wavelength to avoid resonance.
12. Line length control rules:
In fact, it refers to the short-circuit rule. When designing, you should keep the wiring length as short as possible to reduce interference problems caused by unnecessary lines. Especially for some important signal lines, such as clock lines, be sure to place oscillators close to the device. In the case of driving multiple devices, the network topology should be decided according to the specific situation.
13. Parallel input and output wires on the PCB board should be avoided as far as possible to avoid parallel. It is best to place a ground wire between the two wires to avoid circuit feedback coupling.
14. Digital ground and analog ground should be separated. For low-frequency circuits, single-point parallel grounding should be used. High-frequency circuits should be grounded in series with multiple points. For digital circuits, the ground wire should be closed into a loop to improve anti-noise capability.
15. The wiring and via distribution of the whole circuit board should be uniformity. When the outer signal of the circuit board has a large blank area, auxiliary lines should be added to make the lines distribution on the board basically balanced.
16. The low-frequency circuit can be grounded at a single point in parallel, and the actual wiring can be connected in series and then grounded in parallel. The high-frequency circuit can be grounded in series with multiple points. The ground wire should be short and thick. For high-frequency components, a large area ground foil can be used. The ground wire should be as thick as possible. If the ground wire is a very thin, the ground potential will change with the current, which reduces the noise resistance.
17. Multilayer boards should be as symmetrical as possible when designing the laminated structure, as well as the wiring density and copper layout of each layer to reduce warpage and reduce EMI during soldering.
18. The signal line should not cross the power supply and ground. The signal reference plane should be as complete as possible.
19. Impedance control
The signal lines that need impedance control must be wired in strict accordance with the calculated data, in addition, it is necessary to tell manufacturers it. For signal lines that do not require it, the impedance should be calculated to prevent unnecessary interference.
20. Grid copper should be used less in low frequency circuits. Although it can effectively reduce the problem of large area copper skin blistering. When using grid copper, you need to consider the electrical length of the grid line and the working frequency of the circuit board. If using grid copper, the power supply should also be coated with solid copper as much as possible.
21. A group of buses with the same attribute should be wired side by side as much as possible, and the length should be as equal as possible.

 

Ⅱ Three PCB Wiring Methods

The wires should take the shortest route between components according to the specified wiring rules. Limit the coupling between parallel wires as much as possible. Good PCB design requires the minimum number of wiring layers, and also requires fair use of the widest wire and the largest pad size corresponding to packaging density. For example, rounded corners and smooth inner corners design may avoid some electrical and mechanical problems, therefore, sharp corners and sharp corners in the wire should be avoided. 
Here introduces three main PCB routing methods; right-angle wiring, differential wiring, and serpentine wiring to illustrate PCB layout:

pcb board


A. The influence of right-angle wiring on the signal is mainly reflected in three aspects:
1. The corner can be equivalent to the capacitive load on the transmission line to slow down the rise time.
2. Discontinuous impedance will cause signal reflection.
3. The EMI generated by the right-angle tip reaches the RF field above 10GHz. Such a right-angle is likely to develop into the source of high-speed problems.

 

B. To figure out what is differential wiring, you must first understand what is differential signal. In a word, the driving end sends two equal and inverted signals, and the receiving end judges the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces. Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:
1. Have Strong anti-interference ability. Because the coupling between the two differential traces occurs, when there is noise interference from the outside, they are almost coupled to the two lines at the same time. However, the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.
2. It can effectively suppress EMI. Due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can cancel each other out. What’s more, the tighter the coupling, the less the electromagnetic energy leaked to the outside world.
3. The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals. Unlike ordinary single-ended signals, which rely on the high and low threshold voltages to judge. Timing positioning is less affected by the process and temperature, and also more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology.

 

C. Serpentine line is a type of wiring method often used in PCB layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when a signal is transmitted on a serpentine trace, the parallel line segments will be coupled in a differential mode. The smaller the S, the greater the Lp, the greater the coupling. It may cause the transmission delay to be reduced, also the signal quality is greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following are some suggestions when dealing with serpentine wring:
1. Try to increase the distance (S) of parallel lines, at least more than 3H(H refers to the distance from the signal trace to the reference plane). As long as S is large enough, the mutual coupling effect can be almost completely avoided.
2. Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.
3. The signal transmission delay caused by the strip-line or embedded micro-strip line is less than that of the micro-strip. Theoretically, the strip-line will not affect the transmission rate due to differential mode crosstalk.
4. For signal lines with high-speed and strict timing requirements, try not to take serpentine lines, especially in a small area.
5. You can often use s-shaped routing at any angle, which can effectively reduce the mutual coupling.
6. In high speed, the serpentine line has no ability so-called filtering or anti-interference, and can only reduce the signal quality, so it is better to use for timing matching.
7. Sometimes you can consider the spiral routing method for winding. Simulation shows that its effect is better than normal serpentine routing.

pcb board

Ⅲ PCB Design: Wire Inspection

1.Wire Spacing
The minimum spacing of wires must be determined to eliminate voltage breakdown or arcing between adjacent wires. The spacing is variable, it mainly depends on the following factors:
1) Peak voltage between adjacent wires
2) Atmospheric pressure (maximum working altitude)
3) Coating layer
4) Capacitive coupling parameters
Components with critical impedance or high-frequency components should be placed very close to reduce the critical stage delay. There is something need to pay attention to. Transformers and inductive components should be isolated to prevent coupling. Inductive signal wires should be laid orthogonally at right angles. Components that generate any electrical noise due to magnetic field movement should be isolated or rigidly installed to prevent excessive vibration.
2. Whether the wire is short and straight without sacrificing function.
3. Whether the restrictions on the wire width are complied with.
4. There must be a minimum distance between wires, wires and mounting holes, wires and pads.
5. Whether to avoid all the wires (including component leads) closer to parallel wiring.
6. Whether sharp corners (≤90℃) are avoided in the wire pattern.

 

Ⅳ Complete PCB Design Projects Inspection

4.1 General PCB Design Inspection Projects

1) Has the circuit been analyzed? Is the circuit divided into basic units to smooth the signal?
2) Does the circuit allow short or isolated key leads?
3) Where must be shielded, are they effectively shielded?
4) Have you made full use of the basic grid graphics?
5) Is the best size of the printed circuit board?
6) Do you use the available wire width and spacing as much as possible?
7) Has the preferred pad size and hole size been used?
8) Are the base plate and the sketch consistent?
9) Is less cross-wiring used? Do cross wires pass through components and accessories?
10) Are the letters visible after assembly? Are their size and model correct?
11) In order to prevent blistering, is there any window on the large area of copper foil?
12) Are there tool positioning holes?

4.2 PCB Electrical Characteristics Checking Projects

1) Have you analyzed the influence of wire resistance, inductance, and capacitance, as well as the critical voltage drop on the ground?
2) Does the wire spacing and shape meet the insulation requirements?
3) Has the insulation resistance value been controlled and specified in key areas?
4) Is the polarity fully recognized?
5) According to geometric view, has the effect of wire spacing on leakage resistance and voltage been measured?
6) Has the medium for changing the surface coating been identified?

4.3 PCB Physical Characteristics Checking Projects

1) Are all pads and their positions suitable for final assembly?
2) Can the assembled PCB meet the shock and vibration conditions?
3) What is the required spacing of standard components?
4) Are the components that are not firmly installed or the heavier parts fixed?
5) Is the heating element heat dissipation and cooling normally? Or is it isolated from the printed circuit board and other heat-sensitive elements?
6) Are the voltage divider and other multi-lead components placed correctly?
7) Is the arrangement and orientation of components easy to check?
8) Has it eliminated all possible interference on the printed circuit board?
9) Is the size of the positioning hole correct?
10) Are the tolerances complete and reasonable?
11) Have you controlled and signed the physical properties of all coatings?
12) Is the ratio of via hole and lead diameter within an acceptable range?

4.4 PCB Mechanical Design Factors

The printed circuit board adopts mechanical methods to support the components, however, it cannot be used as an unique structural part of the entire device. On the edge of the printing plate, at least every 5 inches for a certain support. The factors that must be considered when selecting and designing printed circuit boards are as follows:
1) The size and shape of the printed circuit board.
2) The type of mechanical accessories and plug (seat) required.
3) The environmental adaptability of circuits.
4) According to some factors, such as heat and dust, install the printed circuit board vertically or horizontally.
5) Some environmental factors that require special attention, such as heat dissipation, ventilation, shock, vibration, and humidity, dust, and radiation, etc.
6) Physical support
7) Install and fix.
8) Disassemble

4.5 PCB Installation Requirements

According to practical experience, the distance between the supporting points of a printed circuit board with a thickness of 0.031-0.062 inches should be at least 4 inches. For a printed circuit board with a thickness greater than 0.093 inches, the distance between the supporting points should be at least 5 inches. Taking this measure can improve the rigidity of the printed circuit board and avoid possible resonance. The following factors should be considered before deciding which mounting technology they use.
1) PCB structure.
2) Input and output terminals.
3) Available equipment space.
4) Convenience of loading and unloading.
5) Type of attachments.
6) Required heat dissipation.
7) Required shieldability.
8) The type of circuit and its relationship with other circuits.

4.6 PCB Pull-out Requirements

1) The influence of plugging tools on the installation distance between two printed circuit boards.
2) When the plug-in tool used in the equipment, its size should be considered.
3) A plug-in device is required, which is usually fixed to the printed circuit board assembly with rivets.
4) As for the mounting frame of the printed circuit board, special design such as load bearing flange is required.
5) The adaptability of the plug-in tool used and the size, shape and thickness of the printed circuit board.

4.7 PCB Mechanical Considerations

The characteristics of the board substrate that have an important influence on the printed circuit assembly are: water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness. All these characteristics affect the function and the production efficiency of the printed circuit board structure. For most applications, the dielectric substrate materials of the printed circuit board are as following:
1) Phenolic impregnated paper
2) Acrylic-polyester impregnated randomly arranged glass mat
3) Epoxy impregnated paper
4) Epoxy impregnated glass cloth

Each substrate can be flame retardant or combustible. The first 3 types mentioned above can be processed. The most common used material for printed circuit boards with metalized holes is epoxy-glass cloth. Its dimensional stability is suitable for high-density circuits and can minimize the occurrence of cracks in the metalized holes. One disadvantage of epoxy-glass cloth laminate is that it is difficult to punch in the usual thickness range of printed circuit boards. For this, all holes are usually drilled and copied and milled to form a print shape of the circuit board.

4.8 PCB Electrical Considerations

In DC or low-frequency AC applications, the most important electrical characteristics of insulating substrates are: insulation resistance, anti-isolation, printed wire resistance, and breakdown strength. In high frequency and microwave applications, include: dielectric constant, capacitance, and dissipation factors. In all applications, the current carrying capacity of printed wires is important.

4.9 Electronics Inspection Before Into A PCB

1) Check the rationality and correctness of the schematic diagram.
2) Check the correctness of the component packaging of the schematic.
3) The distance between strong and weak current lines, and the distance between isolation areas.
4) Check the schematic diagram and PCB diagram to prevent the loss of the network table.
5) Whether the package of the component matches the physical object.
6) Whether the placement of the components is appropriate.
7) Whether the components are easy to install and disassemble.
8) Whether the temperature sensitive element is too close to the heating element.
9) Whether the distance and direction of the mutual inductance components are appropriate.
10) Whether the placement between the connectors is smooth.
11) Easy to plug in and plug out
12) Input and output
13) Strong current and weak current
14) digital and analog should be interlaced.
15) Arrangement of elements on the upside and downside
16) Check whether the directional component has been wrong flipped instead of rotated.
17) Check whether the mounting holes of the component pins are suitable and whether it is easy to insert.
18) Check whether the empty pin of each component is normal and whether it is a missing line.
19) Check whether there are vias between the upper and lower wiring of the same net table. And the pads are connected through the holes, to prevent disconnection and ensure the integrity of the circuit.
20) Silk screen printing should be clear, so that the operation of welding or maintenance can be easy.
21) The arrangement of power and signal lines in the socket should ensure signal integrity and anti-interference.
22) Pay attention to the proper ratio of pads and solder holes.
23) Each plug should be placed on the edge of the PCB board as much as possible and easy to operate.
24) Whether the size and distribution of the mounting holes on the PCB are appropriate to reduce the PCB bending stress.
25) Pay attention to the height distribution of the components on the PCB to ensure easy assembly.

pcb design software

Ⅴ Conclusion

Based on the above mentioned rules, drawing the PCB schematics you need becomes easier. Decide what PCB you want to and install a PCB design software. PCB software is really helpful and powerful. Also a software can check your design to make sure the design does not contain errors such as traces that incorrectly touch, traces too skinny, or drill holes that are too small. For example, run the Electrical Rules Checker (ERC) to see if you’ve made any typical errors. There is less thing stopping you from making your first PCB, right?

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