PCB News - There are several common types of electromagnetic interference

(1) RF Electromagnetic interference. Due to the proliferation of existing radio transmitters, rf interference poses a great threat to electronic systems. Cellular phones, handheld radios, radio-controlled units, pagers and other similar devices are now very common. It does not take a lot of generating power to cause harmful interference. Typical faults occur in the range of 1 to 10V/m rf field intensity. In Europe, North America and many Asian countries, avoiding rf interference from damaging other equipment has become legally mandatory for all products.

(2) Electrostatic discharge (ESD). Modern chip technology has advanced greatly and components have become very dense at very small geometric sizes (0.18um). These high-speed, millions of transistor microprocessors are highly sensitive and easily damaged by external electrostatic discharge. Discharge can be caused directly or radiatively. Direct contact discharges usually cause permanent damage to equipment. Electrostatic discharge caused by radiation may cause device disorder and abnormal operation.

(3) Power interference. As more and more electronic devices are connected to the power backbone, there are potential disturbances to the system. These disturbances include power line disturbances, electrical rapid transients, surges, voltage changes, lightning transients, and power line harmonics. For high frequency switching power supplies, these disturbances become significant.

(4) Self-compatibility. A digital part or circuit of a system may interfere with analog equipment, causing Crosstalk between wires, or a motor may cause disorder in the digital circuit.

In addition, an electronic product that works well at low frequencies will encounter some problems at higher frequencies that low frequencies do not. Such as reflection, string winding, ground projectile, high frequency noise and so on.

An electronic product that does not conform to EMC specifications is not a qualified electronic design. In addition to meeting the functional requirements of the market, appropriate design techniques must be adopted to prevent or remove the influence of EMI.

EMC considerations for PCB design

There are two ways to solve EMI problems in Printed Circuit Board(PCB) design: one is to suppress EMI influence, the other is to shield EMI influence. There are many different manifestations of these two methods. In particular, the shielding system minimizes the possibility of EMI affecting electronic products.

Radio frequency (RF) energy is generated by switching currents within a printed circuit board (PCB), which are by-products of digital components. Each logical state change in a power distribution system produces a transient surge. In most cases, these logical state changes do not generate sufficient ground noise voltage to cause any functional effect. But when the edge rate of one component (rise time and fall time) becomes very fast enough rf energy is generated to affect the normal operation of other electronic components.

1. Causes of electromagnetic interference on PCB

Improper practices often cause out-of-specification EMI on the PCB. Combined with the characteristics of high-frequency signals, THE EMI related to PCB level mainly includes the following aspects:

(1) Improper use of packaging measures. For example, devices that should be wrapped in metal are wrapped in plastic.

(2)PCB design is not good, the finished quality is not high, cable and joint grounding is not good.

(3) Inappropriate or even wrong PCB layout.

Including improper setting of clock and cycle signal wiring; PCB layering and improper setting of signal wiring layer; Improper selection of components with high frequency RF energy distribution; Common mode and differential mode filtering are not considered enough. Ground loop causes RF and ground bomb; Bypass and decoupling deficiencies and so on.

To achieve system-level EMI suppression, some appropriate methods are usually needed: shielding, padding, grounding, filtering, decoupling, proper wiring, circuit impedance control, etc.

2. Electromagnetic compatibility shielding design

Nowadays, the electronics industry is increasingly concerned with the need for SE/EMC(Shielding Effectiveness), and electromagnetic compatibility is becoming more and more important as more electronic components are used. Electromagnetic shielding is a method to control electromagnetic interference from one area to another by induction and radiation. It usually includes two kinds: one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic field and constant magnetic field; The other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric field, alternating magnetic field and alternating electromagnetic field.

EMI shielding can make the product simple and effective in compliance with EMC specifications. When the frequency is below 10MHz, electromagnetic waves are mostly in the form of conduction, while electromagnetic waves with higher frequencies are mostly in the form of radiation. New materials such as single layer solid shielding material, multi-layer solid shielding material, double shielding or more than double shielding can be used for EMI shielding during design. For low frequency electromagnetic interference needs to use thick shielding layer, the most appropriate is to use high permeability materials or magnetic materials, such as nickel copper alloy, to obtain the maximum electromagnetic absorption loss, and for high frequency electromagnetic waves can use metal shielding material.

In actual EMI shielding, electromagnetic shielding effectiveness largely depends on the physical structure of the chassis, that is, the continuity of electrical conductivity. The joints and openings on the chassis are leakage sources of electromagnetic waves. Also, the cable that runs through the box is the main reason for the shielding effectiveness to decline. The electromagnetic leakage of the opening on the chassis is related to the shape of the opening, the characteristics of the radiation source, and the distance from the radiation source to the opening. The shielding efficiency can be improved by properly designing the opening size and the distance from the radiation source to the opening. Electromagnetic sealing gaskets are usually used to solve the electromagnetic leakage in the cracks of the chassis. Electromagnetic sealing gasket is a kind of conductive elastic material, which can keep the conductive continuity in the gap. Common electromagnetic sealing gaskets are: conductive rubber (in the rubber mixed with conductive particles, so that the composite material has both the elasticity of rubber, and the electrical conductivity of metal)., dual conductive rubber (it is not in all parts of rubber mixed with conductive particles, which maximum benefits is to maintain the flexibility of rubber, and guarantee the electrical conductivity), metal mesh grid set (with rubber core metal mesh grid set), spiral pipe liner (stainless steel, beryllium copper or tin plated beryllium copper rolled into a spiral tube), etc. In addition, when higher requirements for ventilation rate, ventilation must be used by the waveguide plate, the plate is equivalent to a high-pass filter, the above a certain frequency electromagnetic wave attenuation through, but for less than the frequency of electromagnetic wave attenuation, had a lot of reasonable application by this feature of the waveguide can be very good shielding EMI interference.

3. Reasonable PCB design with electromagnetic compatibility

With the large-scale improvement of system design complexity and integration, electronic system designers are engaged in circuit design of more than 100MHZ, and the working frequency of bus has reached or exceeded 50MHZ, and some even exceeded 100MHZ. When the system works at 50MHz, transmission line effect and signal integrity problems will occur. When the system clock reaches 120MHz, PCB designs based on traditional methods will not work unless high-speed circuit design knowledge is used. Therefore, high speed circuit design technology has become the electronic system designers must take the design means. Only by using the design techniques of high-speed circuit designers can the design process be controlled.

It is generally considered that if the digital logic circuit frequency reaches or exceeds 45MHZ~50MHZ, and the circuit operating above this frequency has accounted for a certain amount of the entire electronic system (say, 1/3), it is called high-speed circuit. In fact, the harmonic frequency of the edge of the signal is higher than that of the signal itself. It is the rising edge and falling edge (or jump of the signal) that causes the unexpected result of signal transmission. To achieve EMC compliant high-frequency PCB design, the following technologies are usually used: bypass and decoupling, grounding control, transmission line control, and wiring terminal matching.