Experts guide you on how to make a good PCB board
Everyone knows that making a PCB board is to turn the designed schematic diagram into a real PCB circuit board. Please don’t underestimate this process. There are many things that work in principle but are difficult to achieve in engineering, or things that others can achieve but others cannot. Therefore, it is not difficult to make a PCB board, but it is not an easy task to make a good PCB board.
The two major difficulties in the field of microelectronics are the processing of high-frequency signals and weak signals.
In this regard, the level of PCB production is particularly important. The same principle design, the same components, and different people will produce different results. So how can we make a good PCB board? Based on our previous experience
I would like to talk about my views on the following aspects:
One: To clarify the design goal
When receiving a design task, we must first clarify its design goal, whether it is an ordinary PCB board, a high-frequency PCB board, a small signal processing PCB board, or a PCB board that has both high frequency and small signal processing.
If it is an ordinary PCB board, as long as the layout and wiring are reasonable and neat, and the mechanical dimensions are accurate, it will be fine. If there are medium-load lines and long lines, certain means must be used to deal with them, reduce the load, and strengthen the drive of long lines.
The focus is to prevent long line reflections.
When there are signal lines exceeding 40MHz on the board, special considerations must be given to these signal lines, such as problems such as crosstalk between lines. If the frequency is higher, there will be stricter restrictions on the length of the wiring. According to the network theory of distributed parameters, the interaction between high-speed circuits and their connections is a decisive factor and cannot be ignored during system design.
As the gate transmission speed increases, the opposition on the signal line will increase accordingly, and the crosstalk between adjacent signal lines will increase proportionally. Usually, the power consumption and heat dissipation of high-speed circuits are also very large, so they should be given enough attention when making high-speed PCBs.
When there are weak signals at the millivolt level or even microvolt level on the board, these signal lines need special care.
Because small signals are too weak, they are very susceptible to interference from other strong signals. Shielding measures are often necessary, otherwise the signal-to-noise ratio will be greatly reduced. As a result, the useful signal is drowned by noise and cannot be effectively extracted.
The commissioning of the board should also be considered in the design stage.
The physical location of the test point and the isolation of the test point should not be ignored, because some small signals and high-frequency signals cannot be directly measured by adding probes.
In addition, other related factors should be considered, such as the number of board layers, the package shape of the components used, and the mechanical strength of the board. Before making a PCB board, you should have a clear idea of the design goals of the design.
2.Understand the requirements of layout and wiring for the functions of the components used
We know that some special components have special requirements for layout and wiring, such as the analog signal amplifiers used by LOTI and APH. The analog signal amplifiers require a stable power supply with small ripple. The analog small signal part should be kept as far away from the power device as possible. On the OTI board, the small signal amplifier part is also specially equipped with a shielding cover to shield the stray electromagnetic interference. The GLINK chip used on the NTOI board uses the ECL process, which consumes a lot of power and generates a lot of heat. The heat dissipation problem must be specially considered during the layout. If natural heat dissipation is used, the GLINK chip should be placed in a place where the air circulation is relatively smooth, and the heat dissipated should not have a big impact on other chips. If there are speakers or other high-power devices on the board, it may cause serious pollution to the power supply. This should also be given enough attention.
3.Consideration of component layout
The first factor to be considered in the layout of components is electrical performance. Components with close connection relationships should be placed together as much as possible. Especially for some high-speed lines, they should be as short as possible during layout, and power signals and small signal devices should be separated. On the premise of meeting the circuit performance, the components should be placed neatly, beautifully, and easy to test. The mechanical size of the board and the location of the socket should also be carefully considered.
The grounding in the high-speed system and the transmission delay time on the interconnection line are also the first factors to be considered in the system design. The transmission time on the signal line has a great influence on the overall system speed, especially for the high-speed ECL circuit. Although the speed of the integrated circuit itself is very high, the increase in delay time caused by the use of ordinary interconnection lines on the bottom board (about 2ns delay per 30cm line length) can greatly reduce the system speed. Synchronous working parts such as shift registers and synchronous counters are best placed on the same plug-in board, because the transmission delay time of the clock signal to different plug-in boards is not equal, which may cause the shift register to produce errors. If it cannot be placed on the same board, the length of the clock line connected from the common clock source to each plug-in board must be equal where synchronization is critical.
4.consideration of wiring
With the completion of the design of OTNI and star fiber optic network, there will be more boards with high-speed signal lines above 100MHz that need to be designed in the future. Here we will introduce some basic concepts of high-speed lines.
(1)Transmission line
Any “long” signal path on a printed circuit board can be considered a transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the reflections generated during the signal rise will be submerged. No more overshoot, kickback and ringing. For most current MOS circuits, the ratio of the rise time to the line transmission delay time is much larger, so the line can be measured in meters without signal distortion. For faster logic circuits, especially ultra-high-speed ECL
Integrated circuits, due to the increase in edge speed, if no other measures are taken, the length of the line must be greatly shortened to maintain signal integrity.
There are two ways to make high-speed circuits work on relatively long lines without serious waveform distortion. TTL uses Schottky diode clamping for fast falling edges, so that the overshoot is clamped at a level one diode drop lower than the ground potential, which reduces the subsequent kickback amplitude. Slower rising edges allow overshoot, but it is attenuated by the relatively high output impedance (50~80Ω) of the circuit in the “H” state. In addition, due to the greater immunity to interference in the “H” state, the kickback problem is not very prominent. For HCT series devices, if Schottky diode clamping and series resistor termination methods are combined, the improvement effect will be more obvious.
When there is fan-out along the signal line, at higher bit rates and faster edge rates, the TTL shaping method introduced above is somewhat insufficient. Because there are reflected waves in the line, they will tend to synthesize at high bit rates, causing serious signal distortion and reduced anti-interference ability. Therefore, in order to solve the reflection problem, another method is usually used in the ECL system: line impedance matching. This method can control reflections and ensure signal integrity.
Strictly speaking, transmission lines are not very necessary for conventional TTL and CMOS devices with slower edge speeds. For high-speed ECL devices with faster edge speeds, transmission lines are not always necessary. However, when transmission lines are used, they have the advantages of predicting connection delays and controlling reflections and oscillations through impedance matching. 1
There are five basic factors that determine whether to use transmission lines. They are: (1) the edge rate of the system signal, (2) the connection distance (3) the capacitive load (the amount of fan-out), (4) the resistive load (the termination method of the line); (5) the allowable backlash and overshoot percentage (the degree of reduction in AC immunity).
(2)Several types of transmission lines
(1) Coaxial cable and twisted pair: They are often used to connect between systems. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and the twisted pair is usually 110Ω.
(2) Microstrip line on printed circuit board
Microstrip line is a strip conductor (signal line). It is separated from the ground plane by a dielectric. If the thickness, width and distance of the line from the ground plane are controllable, its characteristic impedance can also be controlled. The characteristic impedance Z0 of a microstrip line is:
Where: [Er is the relative dielectric constant of the printed circuit board dielectric material
6 is the thickness of the dielectric layer
W is the width of the line
t is the thickness of the line
The transmission delay time per unit length of a microstrip line depends only on the dielectric constant and has nothing to do with the width or spacing of the line.
(3) Stripline in a printed circuit board
A stripline is a copper strip line placed in the middle of a dielectric between two conductive planes. If the thickness and width of the line, the dielectric constant of the medium, and the distance between the two conductive planes are controllable, then the characteristic impedance of the line is also controllable. The characteristic impedance B of the stripline is:
Where: b is the distance between the two ground planes
W is the width of the line
t is the thickness of the line
Similarly, the transmission delay time per unit length of a stripline is independent of the width or spacing of the line; it only depends on the relative dielectric constant of the medium used.
- Terminating a transmission line
When a line is terminated at the receiving end with a resistor equal to the characteristic impedance of the line, the transmission line is called a parallel terminated line. It is mainly used to obtain the best electrical performance, including driving distributed loads.
Sometimes, in order to save power consumption, a 104 capacitor is connected in series to the terminated resistor to form an AC termination circuit, which can effectively reduce DC loss.
A resistor is connected in series between the driver and the transmission line, and the terminal of the line is no longer connected to the termination resistor. This termination method is called series termination. Overshoot and ringing on longer lines can be controlled by series damping or series termination technology. Series damping is achieved by using a small resistor (generally 10~75Ω) in series with the output end of the drive gate. This damping method is suitable for use in conjunction with lines whose characteristic impedance is controlled (such as bottom board wiring, circuit boards without ground planes, and most winding wires).
When series termination is used, the sum of the value of the series resistor and the output impedance of the circuit (drive gate) is equal to the characteristic impedance of the transmission line. The series termination wiring has the disadvantages of only being able to use lumped loads at the terminal and having a long transmission delay time. However, this can be overcome by using redundant series termination transmission lines. - Non-terminated transmission line
If the line delay time is much shorter than the signal rise time, the transmission line can be used without series termination or parallel termination. If the round-trip delay (the time it takes for the signal to go back and forth once on the transmission line) of a non-terminated line is shorter than the rise time of the pulse signal, the kickback caused by the non-terminated line is about 15% of the logic swing. The maximum open line length is approximately:
Lmax＜tr/2tpd
Where: tr is the rise time
tpd is the transmission delay time per unit line length - Comparison of several termination methods
Both parallel termination and series termination have their own advantages. Which one to use, or both, depends on the designer’s preference and system requirements. The main advantages of parallel termination are fast system speed and complete signal transmission without distortion on the line. The load on the long line will neither affect the transmission delay time of the drive gate driving the long line nor its signal edge speed, but it will increase the transmission delay time of the signal along the long line. When driving a large fan-out, the load can be distributed along the line through branch short lines, instead of having to lump the load at the end of the line as in the series termination.
The series termination method enables the circuit to drive several parallel load lines. The delay time increment caused by the capacitive load of the series termination is about twice that of the corresponding parallel termination, while the short line slows down the edge speed and increases the delay time of the drive gate due to the capacitive load. However, the crosstalk of the series termination is smaller than that of the parallel termination. The main reason is that the signal amplitude transmitted along the series termination is only half of the logic swing, so The switching current is only half of the switching current of the parallel termination, and the signal energy is small, so the crosstalk is also small. - PCB board wiring technology
Whether to use a double-sided board or a multi-layer board when making a PCB depends on the maximum operating frequency, the complexity of the circuit system, and the requirements for assembly density. When the clock frequency exceeds 200MHZ, it is best to use a multi-layer board. If the operating frequency exceeds 350MHz, it is best to use a printed circuit board with polytetrafluoroethylene as the dielectric layer, because its high-frequency attenuation is smaller, the parasitic capacitance is smaller, the transmission speed is faster, and it saves power due to the larger Z0. The following principles are required for the routing of printed circuit boards
(1) All parallel signal lines should be spaced as far as possible to reduce crosstalk. If there are two signal lines that are close to each other, it is best to run a ground wire between the two lines, which can play a shielding role.
(2) When designing signal transmission lines, avoid sharp turns to prevent sudden changes in the characteristic impedance of the transmission line and reflection. Try to design it into a uniform arc line with a certain size.
The width of the printed line can be calculated according to the above-mentioned formula for calculating the characteristic impedance of the microstrip line and the stripline. The characteristic impedance of the microstrip line on the printed circuit board is generally between 50 and 120Ω. In order to obtain a large characteristic impedance, the line width must be made very narrow. But very thin lines are not easy to make. Considering various factors, it is generally more appropriate to choose an impedance value of about 68Ω, because choosing a characteristic impedance of 68Ω can achieve the best balance between delay time and power consumption. A 50Ω transmission line will consume more power; a larger impedance can certainly reduce power consumption, but it will increase the transmission delay time. The negative line capacitance will cause the transmission delay time to increase and the characteristic impedance to decrease. However, the intrinsic capacitance per unit length of a line segment with very low characteristic impedance is relatively large, so the transmission delay time and characteristic impedance are less affected by the load capacitance. An important feature of a transmission line with proper termination is that the branch short line should have little effect on the line delay time. When Z0 is 50Ω. The length of the branch short line must be limited to within 2.5cm. To avoid large ringing.
(4) For double-layer boards (or four-layer lines in a six-layer board), the lines on both sides of the circuit board should be perpendicular to each other to prevent mutual induction and crosstalk.
(5) If the printed circuit board is equipped with high-current devices, such as relays, indicator lights, speakers, etc., their ground wires should be separated and routed separately to reduce noise on the ground wires. The ground wires of these high-current devices should be connected to an independent ground bus on the plug-in board and the backplane, and these independent ground wires should also be connected to the ground point of the entire system.
(6) If there is a small signal amplifier on the board, the weak signal line before amplification should be kept away from the strong signal line, and the routing should be as short as possible. If possible, it should be shielded with a ground wire.
