PCB Blog - How to make a good PCB board

Everyone knows that making a PCB board is to turn a designed schematic into a real PCB board. Please don’t underestimate this process. There are many things that work in principle but are difficult to achieve in engineering, or It is something 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 do a good job of a 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 board production is particularly important. The same principle design, the same components, and the PCB boards produced by different people have different characteristics. The result, so how can we make a good PCB board? Based on our past experience, we would like to share our views on the following aspects:

1. Be clear about your design goals

When receiving a design task, first of all, it is necessary to clarify the design goal, whether it is an ordinary PCB board, a high-frequency PCB board, a small-signal processing PCB board, or a PCB board with both high-frequency and small-signal processing. If it is an ordinary PCB board, As long as the layout and wiring are reasonable and tidy, and the mechanical dimensions are accurate, if there are medium load lines and long lines, certain means must be used to reduce the load. When there are more than 40MHz signal lines on the board, special consideration should be given to these signal lines, 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 wiring is a decisive factor, which cannot be ignored in system design. With the increase of the gate transmission speed, 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 large. When making high-speed PCBs Sufficient attention should be paid to the board. When there are weak signals of millivolt level or even microvolt level on the board, special care is needed for these signal lines. Because the small signal is too weak, it is very easy to be interfered by other strong signals, and shielding measures are often necessary. Greatly reduces the signal-to-noise ratio. As a result, the useful signal is overwhelmed by noise and cannot be extracted effectively. The commissioning of the board should also be considered in the design stage. Factors such as the physical location of the test point and the isolation of the test point cannot be ignored, because some small signals and high-frequency signals cannot be directly added to the probe for measurement. In addition, other related factors should be considered, such as the number of layers of the board, the package shape of the components used, and the mechanical strength of the board. Before making a PCB board, it is necessary to know the design goals of the design.

2. Understand the layout and routing requirements of 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 in LOTI and APH. The analog signal amplifiers require stable power supply and small ripple. The analog small signal part should be kept away from the power device as much as possible. On the OTI board, the small signal amplifying part is also specially equipped with a shielding cover to shield the stray electromagnetic interference. The GLINK chip used on the NTOI board adopts the ECL process, which consumes a lot of power and generates heat. Special consideration must be given to the heat dissipation problem during the layout. If natural heat dissipation is used, the GLINK chip must be placed in a place where the air circulation is relatively smooth. , and the heat dissipated can not have a great impact on other chips. If the board is equipped with speakers or other high-power devices, it may cause serious pollution to the power supply, which should also be paid enough attention.

3. Component layout considerations

One of the first factors to be considered in the layout of components is the electrical performance. The components that are closely related to the wiring should be placed together as much as possible. Especially for some high-speed lines, the layout should be as short as possible. Power signal and small signal devices to separate. On the premise of satisfying the circuit performance, it is also necessary to consider that the components are placed neatly and beautifully, which is convenient for testing. The mechanical size of the board and the location of the socket also need to be carefully considered. Grounding and propagation delay times on interconnects in high-speed systems are also the first considerations in system design. The transmission time on the signal line has a great influence on the overall system speed, especially for high-speed ECL circuits. Although the speed of the integrated circuit block itself is very high, due to the use of ordinary interconnecting lines on the backplane (about 30 cm in length per line) 2ns delay) will increase the delay time, which can greatly reduce the system speed. Synchronous working components such as shift registers and synchronous counters are placed on the same plug-in board, because the transmission delay time of the clock signals to different plug-in boards is not equal, which may cause the master error of the shift register. board, where synchronization is critical, the length of the clock lines from the common clock source to each board must be equal.

4. Considerations for wiring

With the completion of the design of OTNI and star fiber network, there will be more boards with high-speed signal lines above 100MHz to be designed in the future. Some basic concepts of high-speed lines will be introduced here. Any "long" signal path on a printed circuit board can be considered a transmission line. If the propagation delay time of the line is much shorter than the signal rise time, any reflections produced during the signal rise will be drowned out. Overshoot, kickback and ringing are no longer present. For most of the current MOS circuits, since the ratio of rise time to line transmission delay time is much larger, traces can be measured in meters without signal distortion. And for faster logic circuits, especially ultra-high speed. For integrated circuits, due to the increase in edge speed, if no other measures are taken, the length of traces 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 to one diode drop below ground potential. This reduces the magnitude of the subsequent kickback, the slower rising edge allows overshoot, but it is attenuated by the relatively high output impedance (50-80Ω) of the circuit in the level "H" state . In addition, due to the high immunity of the level "H" state, the recoil problem is not very prominent. For HCT series devices, if the Schottky diode clamping and series resistance termination methods are used, the improvement will be improved. effect will be more obvious. At higher bit rates and faster edge rates, the TTL shaping methods described above are somewhat inadequate when there is fan-out along the signal line. Because of the reflected waves in the line, they will tend to combine at high bit rates, causing severe signal distortion and reduced immunity to interference. Therefore, in order to solve the reflection problem, another method is usually used in the ECL system: the line impedance matching method. In this way reflections can be controlled and signal integrity guaranteed. Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not very necessary. Transmission lines are also not always required for high-speed ECL devices with faster edge speeds. But when using transmission lines, they have the advantage of being able to predict wire delays and control reflections and oscillations through impedance matching. There are five basic factors that determine whether to use a transmission line. They are: (1) Edge rate of system signal, (2) Wiring distance (3) Capacitive load (how much fan-out), (4) Resistive load (line termination method); (5) Allowable Percent kickback and overshoot (reduction in AC immunity).

5. Several types of transmission lines

1) Coaxial cable and twisted pair: They are often used for system-to-system connections. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and twisted pair is usually 110Ω.

2) The microstrip line on the printed board, the microstrip line is a strip conductor (signal line). Isolated from the ground plane with a dielectric. If the thickness, width, and distance from the ground plane of the line are controllable, its characteristic impedance is also controllable. The propagation delay time per unit length of microstrip line depends only on the dielectric constant and has nothing to do with the line width or spacing.

3) Striplines in printed boards

A stripline is a copper stripline 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 propagation delay time per unit length of stripline is related to the width or spacing of the line. is irrelevant; only depends on the relative permittivity of the medium used.

Terminate the transmission line: Terminate the receiving end of a line with a resistance equal to the characteristic impedance of the line, then the transmission line is called a parallel-terminated connection. It is mainly used for obtaining electrical performance, including driving distributed loads. Sometimes in order to save power consumption, a 104 capacitor is connected in series with the terminating resistor to form an AC termination circuit, which can effectively reduce the DC loss. A resistor is connected in series between the driver and the transmission line, and the end of the line is no longer connected to the terminating resistor. This termination method is called series termination. Overshoot and ringing on longer lines can be controlled with series damping or series termination techniques. Series damping is achieved by using a small resistor (usually 10 to 75Ω) in series with the output of the drive gate. This damping method is suitable for use with wires whose characteristic impedance is controlled (such as backplane wiring, circuit boards without ground planes, and most wire-wraps, etc.). The value of the series resistor when terminated in series is related to the circuit (drive gate) output impedance. The sum is equal to the characteristic impedance of the transmission line. The series-terminated line has the disadvantage that only lumped loads can be used at the termination and a longer propagation delay time. However, this can be overcome by using redundant series-terminated transmission lines. Parallel-terminated lines Both of them have their own advantages, which one or both of them are used depends on the designer's hobby and the requirements of the system. Complete and distortion-free. The load on the long line will neither affect the transmission delay time of the drive gate that drives the long line, nor will it affect its signal edge speed, but will increase the transmission delay time of the signal along the long line. When driving a large fan When out, the load can be distributed along the line through the branch short line, instead of the terminal that must aggregate the load on the line as in the series termination. The series termination method makes the circuit have the ability to drive several parallel load lines. The delay time increment caused by the load is about twice as large as that of the corresponding parallel-terminated line, while the short line slows down the edge speed and increases the drive gate delay time due to the capacitive load. However, the crosstalk of the series-terminated line is higher than that of the parallel-terminated line The main reason is that the amplitude of the signal transmitted along the series-terminated wiring is only one-half of the logic swing, so the switch current is only half of the parallel-terminated switch current, and the signal energy is small. The crosstalk is also small on PCB board.