PCB News - How to make a good PCB circuit board

How to make a good PCB circuit board

We say that to make a PCB circuit board is to turn a designed schematic diagram into a real circuit board. Please don’t underestimate this process. In fact, there are many things that work in principle but are difficult to achieve in engineering, or others. Some people can't realize the things that can be realized. Therefore, it is not difficult to make a good 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 PCB circuit boards produced by different people have different As a result, how can we make a good PCB board? Based on our past experience, I would like to talk about my views on the following aspects:

1. Design goals must be clear

Receiving a design task, you must first clarify its design goals, whether it is an ordinary PCB board, a high-frequency circuit board, a small signal processing PCB circuit 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 methods must be used to deal with them to reduce the load.

When there are signal lines exceeding 40MHz on the PCB circuit board, special considerations should be made to these signal lines, such as crosstalk between lines. If the frequency is higher, there is a stricter limit on the length of the wiring. According to the network theory of distributed parameters, the interaction between the high-speed circuit and its wiring is a decisive factor and cannot be ignored in system design. As the gate transmission speed increases, the opposition on the signal lines will increase accordingly, and the crosstalk between adjacent signal lines will increase proportionally. Generally, the power consumption and heat dissipation of high-speed circuits are also very large, so high-speed PCBs are being made. Enough attention should be paid.

When there are millivolt or even microvolt weak signals on the PCB circuit board, these signal lines need special care. Small signals are too weak and 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 submerged by noise and cannot be extracted effectively.

The commissioning of the circuit board should also be considered in the design stage. The physical location of the test point, the isolation of the test point and other factors 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, you must have a good idea of the design goals for the design.

2. Understand the layout and wiring requirements of the functions of the electronic components used

We know that some special electronic components have special requirements in the layout and wiring, such as the analog signal amplifiers used by LOTI and APH. The analog signal amplifiers require stable power and small ripple. Keep the analog small signal part as far away from the power device as possible. On the OTI board, the small signal amplification part is also specially equipped with a shield to shield the stray electromagnetic interference. The GLINK chip used on the NTOI board uses ECL technology, which consumes a lot of power and generates heat. Special consideration must be given to the heat dissipation problem in the layout. If natural heat dissipation is used, the GLINK chip must be placed in a place with relatively smooth air circulation., And the heat radiated can not have a big impact on other chips. If the PCB circuit board is equipped with speakers or other high-power devices, it may cause serious pollution to the power supply. This point should also be paid enough attention.

Three, the consideration of the layout of electronic components

The first factor that must be considered in the layout of electronic components is electrical performance. Put the closely connected electronic components together as much as possible. Especially for some high-speed lines, the layout should be as short as possible, and the power signal should be as small as possible. The signal device should be separated. On the premise of meeting the circuit performance, the components must be placed neatly and beautifully, and easy to test. The mechanical size of the circuit board and the location of the socket must also be carefully considered.

The grounding and the transmission delay time on the interconnection line in the high-speed system 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 high-speed ECL circuits. Although the integrated circuit block itself is very fast, it is due to the use of ordinary interconnect lines on the backplane (the length of each 30cm line is about The delay of 2ns) increases the delay time, which can greatly reduce the speed of the system. Synchronous working parts such as shift registers and synchronous counters are best placed on the same plug-in board, because the clock signals to different plug-in boards The transmission delay time is not equal, which may cause the shift register to produce a major error. If it cannot be placed on one board, the length of the clock line connected from the common clock source to the plug-in boards must be equal in the place where synchronization is the key.

Four, consideration of wiring

With the completion of the design of OTNI and star optical fiber network, there will be more PCB circuit boards with high-speed signal lines above 100MHz that need to be designed in the future. Some basic concepts of high-speed lines will be introduced here.

Transmission line

Any "long" signal path on the printed circuit board can be regarded as a kind of transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the main reflections produced during the signal rise period will be submerged. Overshoot, recoil 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, the trace can be as long as meters without signal distortion. For faster logic circuits, especially ultra-high-speed ECL.

For integrated circuits, due to the increase in edge speed, if no other measures are taken, the length of the trace must be greatly shortened to maintain the integrity of the signal.

There are two ways to make high-speed circuits work on relatively long lines without serious waveform distortion. TTL adopts Schottky diode clamping method for fast falling edges, so that the overshoot is clamped to a diode voltage drop lower than the ground potential. At the level of “H”, this reduces the amplitude of the backlash. The slower rising edge allows 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 of the level "H" state, the kickback problem is not very prominent. For HCT series devices, if the Schottky diode clamp and series resistance termination method are combined, it will improve The effect will be more obvious.

When there is fan-out along the signal line, the TTL shaping method introduced above appears to be somewhat inadequate at a higher bit rate and a faster edge rate. Because there are reflected waves in the line, they will tend to be synthesized at a high bit rate, 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: the line impedance matching method. In this way, the reflection can be controlled and the integrity of the signal can be guaranteed.

Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not very necessary. For high-speed ECL devices with faster edge speeds, transmission lines are not always needed. But when using transmission lines, they have the advantages of predicting the connection delay and controlling reflection and oscillation through impedance matching.