Introduction
PCB is in English (printed circuit board) PCB for short. Usually on the insulating material , according to predetermined design , made of printed circuits, printing member or a combination of both conductive patterns called printed circuits. Providing electrical connection between the conductive pattern on the insulating base material components , referred to as printed wiring . This put printed circuit or printed circuit board called the finished printed circuit boards , also known as PCB or printed circuit board. PCB we can see almost all electronic equipment without it , small electronic watches, calculators , general computer , large computers, communications electronics, aviation, aerospace , military weapons systems, as long as there are integrated circuits and other electronic yuan devices to be used in the electrical interconnection therebetween PCB, its performance is directly related to the quality of the electronic device is good or bad . With the rapid development of electronic technology , electronic products tend increasingly high-speed, high sensitivity, high density , this trend led to a PCB board design electromagnetic compatibility (EMC) and electromagnetic interference a serious problem , electromagnetic compatibility design has become PCB design urgent technical problems to solve.
An electromagnetic compatibility
EMC (Electro-Magnetic Compatibility, referred to as EMC) is a new comprehensive discipline , it is mainly studied electromagnetic interference and interference problems. Electromagnetic compatibility is an electronic device or system in the electromagnetic environment specified level , the decrease is not due to electromagnetic interference performance , while they themselves produce electromagnetic radiation is not greater than the limit defined level , does not affect the normal operation of other systems , and reach between the equipment and devices, systems and systems without disturbing each other , the common purpose of reliable operation . Electromagnetic interference (EMI) due to electromagnetic interference is generated by a source coupled to the energy transfer path caused by sensitive systems , which comprises a conductive wire and a common ground , or near field radiated through space coupling three kinds of basic forms . Practice has proved that , even if the circuit schematic design properly , improperly printed circuit board design , will also have an adverse impact on the reliability of electronic devices , so the printed circuit board to ensure electromagnetic compatibility is the key to the whole system design.
1.1 Electromagnetic interference (EMI)
When an EMI problems , need to use three elements to describe : the interference source , propagation path and the recipient . Figure 1 :
So we have to reduce electromagnetic interference , it is necessary to find ways to go these three elements . Here we focus on the printed circuit board layout techniques .
2 printed circuit board layout techniques
Good printed circuit board (PCB) layout is a very important factor in the electromagnetic compatibility .
2.1 PCB basic characteristics
Constitutes a PCB is laminated using a series of vertical stack traces and prepreg . In the multi-layer PCB , the designer in order to facilitate debugging, will signal the outermost line of cloth .
Wiring on the PCB there is resistance, capacitance and inductance characteristics.
Impedance: wiring impedance is determined by weight of copper and cross-sectional area of the decision. For example , l -ounce copper there O. 49 mΩ / impedance per unit area . Capacitance: wiring capacitance is determined by an insulator (EoEr), the current reach of the (A) and trace spacing (h) decision. Expressed by the equation C = EoErA / h, Eo is the permittivity of free space (8.854 pF / m), Er is the relative permittivity of the PCB substrate ( FR4 laminate as in 4.7 ) .
Inductance: The inductance of the wiring in the wiring distribution of the average , about 1 nH / m.
For 1 ounce copper , in O. 25 mm (10mil) thick FR4 laminate under the circumstances, is located above the ground floor of 0.5 mm (20 mil) wide , 20 mm (800 mil) long line can produce 9.8 m ∧ impedance , 20 nH between the inductance and capacitance of 1.66 pF with a couple of . The above values are compared with the parasitic components , these are negligible , but the sum of all the wiring may exceed parasitics . Therefore , the designer must take this into account . PCB layout universal principles:
( 1 ) increasing the distance between the traces to minimize crosstalk by capacitive coupling ;
( 2 ) parallel to the power line and ground cloth to make the PCB capacitors to achieve the best ;
( 3 ) the sensitivity of the high-frequency noise wire cloth in the power cord away from high places ;
( 4 ) widening power and ground to reduce the power and ground impedance.
2.2 Segmentation
Segmentation refers to the physical separation of different types to reduce the coupling between the lines , especially through the power and ground .
Figure 2 shows the segmentation technique using four different types of circuits divided examples. In the ground surface , the trench is used to isolate four non-metallic ground plane . L and C as part of the board of each filter . Reduce the coupling between different circuit power side . High-speed digital circuits due to their higher instantaneous power demand and the requirements on the power entrance. The interface circuit may require electrostatic discharge (ESD) and transient suppression device or circuit . For the L and C , it is preferable to use different values of L and C, but not with a large L and C, because it can be thought that the type of circuit to provide different filtering characteristics.
2.3 partial decoupling between the power supply and the IC
Partial decoupling can reduce the spread of noise power along the trunk . A large bypass capacitor connected to the power input port and PCB plays between a low frequency pulsating filter effects , both as a potential reservoir of power to meet the sudden demand . In addition, between each IC power and ground should have decoupling capacitors . These decoupling capacitors should be as close to the pin. This will help filter out the switching noise of the IC.
2.4 Grounding Technology
Grounding techniques used in both multilayer PCB, is also used in a single layer PCB. The goal is to minimize grounding techniques grounding impedance , thereby reducing the potential returns from the circuit between the power supply to ground loops .
( 1 ) single-layer PCB ground wire
In the single ( single ) PCB , the width of the grounding line should be as wide , and should be at least 1.5 mm (60 mil). Since the star can not be achieved on a single-layer wiring PCB, thus changing the jumper and ground should be kept to a minimum width , otherwise it will cause the line impedance and inductance changes.
( 2 ) The ground layer PCB
In the double ( double-sided ) PCB , for digital circuits priority to grill / dot wiring , this wiring can reduce ground impedance , ground loops and signal loops. As in the single-layer PCB , the ground and power lines should be at least the width of 1.5 mm. Another one is the ground floor layout aside, signal and power lines put on the other side. This arrangement will further reduce ground loops and impedance , decoupling capacitors can be placed at a distance between the IC power supply line and the ground plane as close as possible .
( 3 ) protection ring
A guard ring can be noisy environment ( e.g., RF current ) isolated exocyclic grounding techniques , because no current flows through the protective ring in a normal operation.
(4) PCB capacitors
On the plywood , separated by a thin insulating power plane and ground to produce a PCB capacitors. In the single- board power and ground laying parallel capacitance will cause this effect . One advantage is that it capacitor PCB having a very high frequency response and evenly distributed over the entire surface or the entire low series inductance. It is equivalent to a uniformly distributed decoupling capacitors across the board. No single discrete element has this feature.
( 5 ) high-speed circuits with low circuit
Laying high-speed circuit should bring it closer to the ground plane , and low power circuits should be so close to the surface .
(6) copper filling
In some analog circuit board is not used by a large area is to cover the ground , thereby providing increased shielding and decoupling capability. But if this piece of copper area is vacant ( ie not connected ) , then it may appear as an antenna and cause electromagnetic compatibility problems.
( 7 ) The multilayer PCB ground and power planes
In a multilayer PCB , it is recommended to power and ground planes placed as close as possible in adjacent layers so that the entire board to produce a large PCB capacitors. The key signals should be the fastest ground near side , the signal is non-critical deployment as close to the supply side .
( 8 ) Power Requirements
When the circuit requires more than one power supply , the use of the ground will be separated from each power supply . However, in a single layer PCB multi- point grounding is impossible. One solution is to elicit from a power line and ground power supply with other power line and ground separated . This also helps to avoid noise coupling between the power supply .
3 Conclusion
Various methods and techniques described in this article will help improve the EMC characteristics of the PCB, of course, these are just a part of EMC design , usually also considered reflection noise and radiated emission noise, and other interference problems caused by technology . In the actual design, the design should be based on objective requirements and design conditions, the use of anti-electromagnetic interference reasonable measures , designed with good EMC performance of PCB board .
