Electromagnetic interference in flexible circuit board design proofing

2020-07-03 Jing Guan Ying FPC 0

Electromagnetic interference in flexible circuit board design proofing

FPCB (Flexiable Printed Circuit Board), that is, flexible printed circuit board, is an advanced and important electronic component product and a kind of printed circuit board. In the electronics industry, flexible circuit boards play a role in the electrical connection of electronic components and the connection between circuit components and electrical devices. FPC flexible circuit boards also support circuit components and devices. The strength of the anti-interference ability of the fpc flexible circuit board is directly affected by the excellent design of the printed circuit board. In order to ensure the excellent performance of the flexible circuit board, the design layout and anti-interference ability of the circuit must be considered simultaneously. FPCB flexible printed circuit board can be divided into single panel, double panel and multilayer board according to the number of circuit layers.

 

As of now, the country has no clear requirements and rules for electromagnetic interference in the design and proofing of flexible circuit boards. Based on this, in practice, flexible circuit board manufacturers can only fully use the design principles and related rules in the circuit design process to carry out overall planning and design, especially the anti-interference design of the circuit. Achieving the above can effectively avoid serious electromagnetic interference problems in the circuit design practice, and can also effectively reduce the frequency and save the design cost, which has a very important role in effectively reducing the design time of electrical circuits.

1 Composition of the electromagnetic environment in the design of printed flexible circuit boards

Electromagnetic interference source, coupling path and receiver make up a simple electromagnetic interference model, as shown in the figure.

Circuit board manufacturers talk about electromagnetic interference in flexible circuit board design proofing

Microprocessors, microcontrollers, electrostatic discharge, transmitters, and instantaneous power actuators are common sources of interference, and the frequency of occurrence in printed circuit boards is relatively high. The clock circuit is usually the largest broadband noise generator in a micro-control system.

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Conductive coupling and radiative coupling together constitute a coupling path and play an important role in flexible printed circuit boards. Different coupling methods will naturally cause different interference problems. such as:

1) Mutual inductance occurs frequently in the wire, and the capacitance may also increase significantly when it is in a partial state;

2) Crosstalk of printed wiring;

3) The high-frequency electromagnetic field generated when the high-frequency signal passes through the printed wire;

4) Electromagnetic radiation interference caused by clock signals;

5) Reflection interference;

6) Interference caused by a series of improper operations.

In short, many objects can become sensitive components, including electronic components and wires. If you want to grasp the overall layout of the board and the position of components, you need to work hard on the wiring. Only reasonable wiring and electromagnetic compatibility standards are the best way to achieve this goal.


2 Single trace in flexible printed circuit board design

In PCB circuit design, the differential trace coupling is small, accounting for only 10 to 20% of the coupling degree, and more is the coupling to the ground. When there is a discontinuity in the ground plane. Without a reference plane area, differential trace coupling provides a return path.

It is required to avoid the occurrence of right-angle wiring in PCB layout. Right-angled traces have a negative impact on the signal, so the traces in the PCB generally use 45-degree corners or arc corners. The main differences between right-angle and non-right-angle routing are:

1) The corner can be equivalent to the capacitive load on the transmission line, reducing the rise time;

2) Corners can also resist signal reflections caused by inconsistency;

3) Electromagnetic interference will be caused by right-angled tips.

Circuit board manufacturers talk about electromagnetic interference in flexible circuit board design proofing

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Different corner lines have obvious differences in angle. Figure 2 uses the FDTD numerical method for experimentation, and compares the reflection transmission characteristics and reflection characteristics by simulation. It is superior to the other two corner lines in the reflectivity and transmission performance of the 45-degree outer chamfered corner lines. These three types of routing are worse than the corners of a circular arc, but the cost of radian scoring is relatively high. This is because the scoring of the arc requires superb plate-making technology. Superb technology will inevitably cause an increase in cost, so when choosing a route, you will usually stay on the corner of the 45-degree beveled corner.

 

3 Crosstalk analysis of multi-conductor transmission lines in the design and application of flexible circuit boards

Transmit signals and the operating frequency of the machine should pay attention to the principle of moderation in PCB circuit design. If it reaches the megahertz level, then the interference to the line is very serious. The main cause of interference between traces is crosstalk. During the design of PCB electrical circuits, attention should be paid to crosstalk issues to minimize the occurrence of crosstalk issues. In practice, it can be seen that if crosstalk occurs, there will usually be at least three conductors and two wires carrying signals. As shown in Figure 3, the third wire is only used as a reference.

In practice, it can be seen that the role between the source and the disturbed circuit usually produces a VS. The induced zs and zL will generate induced voltages and currents. The zs and the source are related to each other, and zL is mainly connected to the load. interconnected.

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Circuit board manufacturers talk about electromagnetic interference in flexible circuit board design proofing

In order to reduce the occurrence of interference, flexible circuit board manufacturers specifically put forward the following suggestions and design plans:

1) The logic device with function as the main basis to control the bus structure;

2) Minimize the physical distance of components;

3) The length of wiring should be strictly controlled;

4) The component must be far away from the I/o interface, and data interference should be avoided as much as possible;

5) To ensure the accuracy of the impedance-controlled routing path, usually the routing with higher frequency wave energy should be considered carefully;

6) Provide some intersecting traces to ensure that there is an appropriate distance between the traces to ensure that the inductive coupling is minimized;

7) The wiring layers next to each other should be vertical, which can reduce the capacitive coupling between layers;

8) Strengthen the separation and distance control between the signal and the ground;

9) The wiring layers should be separated separately, and they must be wired with the same axis to ensure that the wiring layers are separated in the pre-solid plane structure.

4 Distribution of internal components of flexible printed circuit board

Normally, the functional unit and equipment meet the electromagnetic compatibility requirements, which is mainly determined by the degree to which the basic components of the circuit satisfy the electromagnetic characteristics.

When selecting electromagnetic components, electromagnetic characteristics and circuit assembly are two factors that must be considered, otherwise the selected electromagnetic components are inferior. This is mainly because the response characteristics of components far from the fundamental frequency determine whether electromagnetic compatibility is achieved. In most cases, the external response (such as the length of the lead) and the degree of coupling between components are determined by the circuit assembly. Need to pay attention to the following points.

 

Circuit board manufacturers talk about electromagnetic interference in flexible circuit board design proofing

Flexible circuit board size is a factor to be considered first. The size of the flexible circuit board should be moderate, and too large or too small is not satisfactory. If it is too large, many lines are needed during printing to increase the impedance and reduce the anti-noise performance, but its cost will increase accordingly; if it is too small, it will lack heat dissipation capacity, and the interfered object will expand to the phase Adjacent lines. Based on this, before determining the location of special components, the actual specifications and dimensions of the PCB should be fully measured; based on the circuit function, unified planning and adjustment of all components in the circuit. In actual operation, in order to be able to

To minimize the loss of high-frequency components, reduce the complexity of parameter distribution, and avoid electromagnetic interference, we must try to separate them so that there is a distance between the input and output components. Reduce the higher potential difference between components or wires to avoid short circuit problems caused by discharge. During the circuit debugging process, if the components carry high voltages, they should be placed in a location that is not easily accessible.

At the same time, we must pay attention to the effective fixing of the bracket, if welding more than 159 components. The relatively large and heavy heating elements cannot adapt to the printed board and should be eliminated. Such components should be configured on the chassis bottom. The heat dissipation should be taken into consideration during installation. The thermal element should not be close to the heating element.

The structural requirements of the whole machine should be considered first, especially when the layout of adjustable components, such as potentiometers, switches, etc. If it is the case of internal adjustment, it should be placed in an area convenient for adjustment, such as the top of the printed board; if it is external adjustment, you need to consider the adjustment knob.

Printing board positioning holes and fixing brackets need to be freed first. When distributing and setting up all the components of the circuit, it should be based on

its functional unit. Therefore, the following points should be done:

1) In order to make the signal flow more, the flow of the circuit must be considered, and each functional circuit unit should be placed in a reasonable area, so that the signal can be maximized in a unified direction;

2) In the layout, it is necessary to closely surround the core of the core element of each functional circuit. When arranging components, attention should be paid to the principles of symmetry, no clutter and tightness. The wires used to connect each component should be minimized;

3) When the circuit is operated under high load, the actual distribution needs to be considered. Maximize the distribution of components in the circuit in parallel. The parallel distribution can make the appearance look better, it is convenient for welding, and it is also very helpful for mass production;

4) The distance between components on the edge of the circuit board and the center of the circuit board should not exceed 2 mm; for the circuit board, it is recommended to design a rectangle. The length is 1.5 times the width, or 1.3 times.

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5 Commonly used EMC design software

The electromagnetic radiation at the interface between the design of the flexible circuit board design and the outside is the factor to be considered during the analysis. In addition, the electromagnetic radiation of the power supply layer in the flexible circuit board and the radiation problem of the high-power wiring network must also be considered. Now, when designing EMC software, a large number of board-level and system-level interconnect simulations have been applied, both of which are mainly based on Cadence's technology. At the same time, the simulation analysis of SI/PUEMI is also applied.

The German INCASES company invented EMC-WORKBENCH, this software has an important impetus for EMC simulation analysis in flexible circuit board manufacturers. Therefore, INCASES has become a leader in the industry and has made significant contributions to EMC's progress. EMC-WORKBENCH provides designers with assistance, especially in the technical difficulties of electromagnetic compatibility. At the same time, the design process is changed, the workload is reduced, and some design procedures are deleted. Due to the application of EMC simulation technology, PCB design has quickly entered a new era, especially for electronic engineers using this technology to achieve short-term high-quality, high-reliability design. In the process of implementing EMC simulation analysis, it will inevitably bring greater opportunities and wider development space to the development of circuit design and PCB manufacturing industry. In practice, it can be seen that a circuit board may come from many manufacturers, and there is a big difference in their functionality. When analyzing EMC, designers need to fully understand the characteristics of the components, so that they can Carry on concrete simulation operation to it. From the traditional perspective, this operation seems to be a very difficult project. However, the emergence of IBIS SPICE has played a very important role in the analysis of EMC problems.

 

In short, in the design process of flexible circuit board, it is necessary to design a flexible circuit board circuit that meets the anti-interference principles and requirements in accordance with the relevant design specifications. The design and proofing of flexible circuit boards by circuit board manufacturers usually need to go through a rigorous process. In this process, all possible influencing factors of the terminal products should be considered in the design and proofing of the flexible circuit board to make the electronic flexible Circuit board circuit and bending performance are in the best performance state. With the increasing number of flexible circuit board design engineers in the industry, the research and design of flexible circuit boards has become more in-depth and detailed. I believe that in the near future, 3D printing will also bring new breakthroughs to the development of our flexible circuit board design and proofing.


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