PCB Tech - How to make the PCB circuit board very well

To make a PCB board is to turn a well-designed PCB schematic 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 others can achieve Some people can't realize the same thing, so it is not difficult to make a PCB board, but it is not easy to make a PCB board well.

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 PCBs produced by different people have different results., So 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. Make clear design goals

Receiving a design task, we must first clarify its design goals, 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 measures must be used to reduce the load, and the long line must be strengthened to drive, and the focus is to prevent long line reflections.

When there are signal lines exceeding 40MHz on the 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-level weak signals on the board, these signal lines need special attention. Small signals are too weak and are very susceptible to interference from other strong signals. Shielding measures are often necessary, otherwise they will Greatly reduce the signal-to-noise ratio. As a result, the useful signal is submerged by noise and cannot be extracted effectively.

The commissioning of the 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 relevant 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 requirements for layout and routing of the functions of the components used

We know that some special components have special requirements in the layout and routing, such as the analog signal amplifier used by LOTI and APH. The analog signal amplifier requires a stable power supply 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 amplifying part is also specially equipped with a shielding cover 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 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 component layout

The first factor that must be considered in the layout of components is electrical performance. Put closely connected components together as much as possible, especially for some high-speed lines, make them as short as possible when laying out power signals and small signal devices. To 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 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 amount of 2ns) increases the delay time, which 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 clocks on different plug-in boards The signal 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 from the common clock source to each plug-in board must be equal where synchronization is the key.

Fourth, the consideration of wiring

With the completion of the design of OTNI and the star optical fiber network, there will be more 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 PCB can be regarded as a PCB 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.