PCB EMC design
1）Analysis of signal spectrum
A1 = 2τ / T
A2 = 0.64 / T f
A3 = 0.2 / T tr f2
The shorter the rise (or fall) time, the richer the high frequency components of the signal.
（2）Analysis of interference sources
ΔI Noise interference
Common mode interference and differential mode interference
(a)Differential mode radiation
（b）common mode radiation
When a high frequency current flows through a signal line, the interference voltage will be induced on the signal line adjacent to the PCB board.
（3）Wiring design technology
◊ When wiring, make all signal loop areas (especially high-frequency signals and sensitive signal loop areas) as small as possible.
◊ High-speed signal lines should not be crossed over the partition area or over the unrelated reference plane.
◊ In analog circuit and RF circuit designs, as well as in dual panels without a power ground plane, the guard wires are often used to protect critical signals from crosstalk from other signals. Generally, protection line is connected to the ground network, and the two ends of the line are connected to the ground. When the frequency is high, the protection line is grounded with multiple vias. For digital circuits with complete ground planes, protection lines are generally not used.
◊ The difference corresponds to parallel equidistant equi-corresponding traces,maintain symmetry, so that the circuit has a good suppression of common mode interference.
◊ Sharp lines and right angles are not allowed for high-speed signal wiring. Signals above 1 GHz should use circular arcs wiring as much as possible.
◊ In order to reduce the radiation and interference of high frequency signals, the high frequency signals are arranged in the inner layer as much as possible.
◊ Any adjacent signal layer should adopt vertical and orthogonal direction as far as possible.
◊ Don’t lay a meaningless line on a single board; the test leads should be as short as possible.
（4）Ground wire design technology
◊ 20H principle
◊ For multi-layer boards, the integrity of the ground plane should be guaranteed and there should be no large openings in the ground plane.
① Provide a more stable electrical Level
② Provide a small signal loop area
③ Make the signal line have a certain and more uniform characteristic impedance
④ Can control crosstalk between signals
◊ Ensure that there are no isolated copper skin on the PCB; the copper areas, the metal inside the equipment (such as radiators, reinforcement strips, metal casings, etc.) must be well grounded.
◊ When there are multiple ground layers in PCB, the ground surface should be connected together with more dispersed through vias on the board, especially in the place where the signal concentrates to change the layer, so as to provide a shorter loop circuit for the signal of the change layer and reduce radiation.If the ground planes are connected together by using a via hole around the plane, the external radiation of the PCB can be effectively reduced.
Design of grounding
① Comb power supply and ground structure (double board). It is suitable for low-speed circuit and single signal wiring and low line density. To protect more important signals, after wiring is completed, the empty places are covered with copper sheet, and the two layers are connected together by multiple holes, which can improve the problem of large loop circuit area.
② The raster ground structure (double board) is suitable for low-speed CMOS and ordinary TTL circuits, but attention should be paid to adequate protection of higher-speed signals.
（5）Power integrity (PI) analysis
Problems arising from switching current
① It produces strong radiation disturbance.
② Reducing VCC,affect the normal work of the chip.
◊ Setting up decoupling capacitor
◊ Selection of Decoupling Capacitor
So：C = 2nF
Tips for designing high frequency board layout
(1) High-frequency circuits tend to have high integration and high wiring density. The use of multi-layer boards is both necessary for wiring and an effective means to reduce interference.
(2) The less the lead bend between the pins of the high-speed circuit device, the better. It is better to use full straight line for high frequency circuit wiring. If turning is needed, 45 degree break line or arc turning can be used. Once meeting this requirement, the external transmission of high frequency signals and the coupling between them can be reduced.
(3) The shorter the lead between the pins of high frequency circuit devices, the better.
(4) The fewer lead layers alternating between pins of high frequency circuit devices, the better. It means that the fewer Via holes used in the process of component connection, the better. Reducing the number of Via holes can significantly improve the speed.
(5) High-frequency circuit wiring should pay attention to the “cross interference” introduced by the parallel lines of the signal lines.Parallel lines in the same layer are almost unavoidable, but in the two adjacent layers, the directions of the lines must be perpendicular to each other.
(6) As to signal lines or partial units of particular importance,implementing the measures of ground line encirclement, i. e. drawing the outline of the selected object.
(7) All kinds of signal routes can not form a loop, and ground wires can not form a current loop.
(8) A high frequency decoupling capacitor should be installed near each IC block.
(9) High-frequency chock be used when connecting analog ground lines and digital ground lines to public ground lines.Most of the high frequency ferrite beads with central holes through conducting wires are used in the actual assembly of high frequency choke links,and it is generally not expressed on the circuit schematic, and the resulting netlist is not Including such components, the wiring will ignore its existence. In view of this reality, it can be regarded as an inductance in the schematic diagram, and a component package can be defined separately in the PCB component library. Before wiring, it can be manually moved to a suitable location near the common ground wire confluent.
(10) The analog circuit and the digital circuit should be arranged separately. After independent wiring, power supply and ground wire should be connected at a single point to avoid mutual interference.
(11) Before the DSP, off-chip program memory and data memory are connected to the power supply, the filter capacitor should be added and placed as close as possible to the chip power supply pin to filter out the power supply noise. In addition, shielding is recommended around the DSP and off-chip program memory and data memory to reduce external interference.
(12) When making the printed circuit board (PCB) of the DSP hardware system, special attention should be paid to the wiring of important signal lines such as address lines and data lines to be correct and reasonable. When wiring, try to use short and thick high-frequency line,and keep away from the signal lines that are susceptible to interference, such as analog signal lines. When the circuit around the DSP is more complicated, it is recommended to make the DSP and its clock circuit, reset circuit, off-chip program memory, and data memory into a minimum system to reduce interference.
Golden rules of PCB design
Despite the increasing integration of semiconductors, many applications have ready-to-use system-on-a-chip, and many powerful and out-of-the-box development boards are becoming more readily available, but the use of electronics in many use cases Still need to use a custom PCB. In a one-time development, even a common PCB can play a very important role. PCB is the physical platform for design and the most flexible component for the original components for electronic system design.
The following sections describe the most effective design rules that electronic design engineers should keep in mind when implementing design layout and commercial manufacturing using design software.
Rule 1: Select the correct grid, set and always use the grid spacing that matches the most components. Although multi-grid seems to have significant utility, if engineers can avoid problems when setting up the interval and maximize the application of the board if they can think more at the beginning of the PCB layout design. Since many devices are available in a variety of package sizes, engineers should use products that are best suited for their design. In addition, polygons are critical for board copper deposition. Multi-grid boards typically produce polygon fill deviations when performing copper-clad copper. Although not as standard as a single grid, they provide the required board life.
Rule 2: Keep the path the shortest and most direct. This sounds simple, but at every stage, even if you want to change the board layout to optimize the wiring length, you should always keep in mind. This is especially true for analog and high-speed digital circuits where system performance is always partially limited by impedance and parasitic effects.
Rule 3: Manage the distribution of power and ground lines as much as possible using the power plane.
Rule 4: Group the relevant components together with the required test points. For example, placing the discrete components required for an Opamp op amp close to the device so that the bypass resistance and capacitance can cooperate with it, helping to optimize the wiring lengths mentioned in Rule 2, while also making test and fault detection It’s easier.
Rule 5: Repeat the copying of the required board on another larger board multiple times for PCB splicing. Choosing the size that best suits the equipment used by the manufacturer helps to reduce prototyping and manufacturing costs. First make the board layout on the panel, contact the board manufacturer to get the preferred size specifications for each of their panels, then modify your design specifications and try to repeat your design multiple times within these panel sizes.
Rule 6: Integrate component values. As a designer, you will choose some discrete components with high or low component values but the same performance. By integrating within a small range of standard values, bills of materials can be simplified and costs can be reduced. If you have a range of PCB products based on your preferred device values, from a long-term perspective,it will be better for you to make the right inventory management decisions.
Rule 7: Perform as many design rule checks as possible (DRC). Although it takes only a short time to run the DRC function on the PCB software, in a more complex design environment, as long as you perform the check during the design process, you can save a lot of time. This is a good habit to keep. Every cabling decision is critical, and you can always remind you of the most important cabling by performing a DRC.
Rule 8: Flexible use of screen printing. Screen printing can be used to label a variety of useful information for future use by board manufacturers, service or test engineers, installers, or equipment commissioners. Not only should the clear function and test point label be marked, but the direction of the components and connectors should be indicated as much as possible, even if these annotations are printed on the lower surface of the components used in the circuit board (after assembly of the circuit board). Full application of screen printing technology on the upper and lower surfaces of the PCB reduces duplication of effort and streamlines the production process.
Rule 9: Decoupling capacitors must be selected. Do not attempt to optimize your design by avoiding decoupled power wire and depending on the limits in the component data sheet. Capacitors are inexpensive and rugged, so you can spend as much time as possible assembling the capacitors, while following Rule 6, using standard values to keep stocks tidy.
Rule 10: Generate PCB manufacturing parameters and verify them before submitting for production.
As circuit design is becoming more widely shared and internal teams are increasingly relying on reference designs, basic rules like the above will remain a feature of printed circuit board design, which we believe is important for PCB design. By clarifying these basic rules, developers have the flexibility to increase the value of their products and get the most out of the boards they manufacture.
Through hole design skills
Via hole is one of the important components of multi-layer PCB.The cost of drilling is usually 30%-40% of the cost of PCB board.To put it simply，each hole in the PCB can be called a via hole .In terms of function,the vias can be divided into two types:one is used as an electrical connection between the layers;the other is used for fixing or positioning the components.In terms of process,the vias generally divided into three types:blind via、buried via and through via.
The blind via are located on the top and bottom surface of the printed circuits board,and have a depth for the connection of the surface circuit to the underlying inner circuit.The depth of the holes usually does not exceed a certain ratio(aperture) .The buried hole is a connection hole located in the inner layer of PCB,which does not extend to the surface of circuit board.The above two types of holes are located in the inner layer of circuit board,and are made by a through-hole forming process before lamination.The third type is called a through hole,and the hole goes through the entire circuit board and can be used to implement internal interconnection or as a mounting hole for the component.Since the through via is easier to implement in the process and lower in cost,it is used in most printed circuit boards.
First, from a design point of view, a via is mainly composed of two parts, one is a drill hole in the middle, and the other is a pad area around the drilling hole. The size of these two parts determines the size of the vias. Obviously, in high-speed, high-density PCB design, the designer always hopes that the the via are small , so that more wiring space can be left on the board.In addition, the smaller the via is, the smaller its own parasitic capacitance would be, and making it more suitable for high speed circuits.However, the reduction in the size of the hole also brings about an increase in cost, and the size of the via hole can not be reduced indefinitely. It is limited by the process techniques such as drilling and plating: The smaller the hole, the longer it takes to drill holes,the easier it is to deviate from the center position; and when the depth of the hole exceeds 12 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly plated with copper.
Second, While parasitic capacitance exists in the through hole,parasitic inductance also exists. In the design of the high-speed digital circuit, the parasitic inductance of the via hole is often more harmful than the parasitic capacitance. Its parasitic series inductance weakens the contribution of the bypass capacitor and reduces the filtering effectiveness of the entire power system.
Third, in high-speed PCB design, seemingly simple vias often have a large negative effect on the design of the circuit. In order to reduce the adverse effects of the parasitic effects of vias, it is possible to do as much as possible in the design:
1. Consider the cost and signal quality, and choose a reasonable size of the via size. For vias for power supply or ground wire, consider using larger sizes to reduce impedance.
2. The thin PCB board is beneficial to reduce the two parasitic parameters of the via.
3. The signal traces on the PCB should not be changed as much as possible, that is, try not to use unnecessary vias.
4. Place some grounded vias near the vias of the signal-changing layer to provide the most recent return circuit for the signal. It is even possible to place a large number of surplus grounding holes on the PCB. Of course, you need to be flexible in design. Sometimes we can reduce or even remove pads from certain layers. Especially in the case of very large via density, it may lead to a break in the copper layer to form a partition circuit. To solve such a problem, we can also consider reducing the size of the pad with through holes in the copper layer.
The Role of Laying Copper
There are several reasons for laying copper in general.
1.EMC.For large areas of power supply laying copper , will play a shielding role, such as PGND play a protective role.
2.PCB process requirements. Generally, in order to ensure the plating effect, or the lamination is not deformed, the PCB board with less wiring is coated with copper.
3.Signal integrity requirements.Give high frequency digital signal a complete backflow path, and reduce DC network wiring. Of course, there are also heat dissipation, special device installation require to lay copper and other reasons.
First, a great advantage of laying copper is to reduce the ground wire impedance (a large part of the anti-interference is caused by the reduction of the ground wire impedance). There are a lot of peak current in the digital circuit, so it is more necessary to reduce the ground wire impedance.
Second, the significance of laying copper ：
1. Laying copper and ground wire are connected, which can reduce the return circuit area.
2, a large area of laying copper is equivalent to reducing the resistance of the ground wire, reducing the pressure drop. From these two points, both digital and analog circuits should be copper-plated to increase the ability to resist interference.