Everyone knows that to make a PCB board is to turn a 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 What others can achieve, others can't, so it is not difficult to make a PCB board, but it is not easy to do 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 the following aspects.
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 size is accurate. If there are medium load lines and long lines, certain measures must be used to reduce the load. The long lines must be driven. The focus is to prevent long-line reflections. When there are signal lines exceeding 40MHz, special consideration should be given to these signal lines; for example, crosstalk between lines and other issues, if the frequency is higher, there will be more stringent restrictions on the length of the wiring, according to the network theory of distributed parameters, The interaction between the high-speed circuit and its connection is a decisive factor, which 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 large. When doing high-speed PCBs Enough attention should be paid.
When there are millivolts or even microvolts of weak signals on the board, these signal lines need special care. Because the small signal is too weak, it is very susceptible to interference from other strong signals. Shielding measures are often necessary, otherwise, it will greatly reduce the signal-to-noise ratio so that the useful signal is submerged 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, the isolation of the test point, and other factors It 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 sub-modules. The design goals of the design must be made before the PCB board.
We know that some special components have special requirements in the layout and routing; for example, the analog signal amplifier used by LOTI and APH, the power supply requirements of the analog signal amplifier should be stable, the ripple and the small analog small signal part should be 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 from the GL I NK chip used on the NT OI board. The ECL process is used. The power consumption is high and the heat is severe. The heat dissipation problem must be carried out during the layout. Special considerations.
If natural heat dissipation is adopted, the GL I NK chip should be placed in a place where the air circulation is relatively smooth, and the heat radiated must not have a major impact on other chips; if the board is equipped with speakers or other high-power devices, it may be Enough attention should also be paid to the serious pollution caused by the power supply.
The first factor that must be considered in the layout of components is to put the components with close connections together as much as possible for electrical performance. Especially for some high-speed line layouts, it is necessary to make it as short as possible of power signals and small signals.
The components 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 and transmission delay time on interconnection lines in high-speed systems is also the first factors to be considered in system design. The transmission time on signal lines 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, there is a delay of about 2ns per 30cm line length with the ordinary interconnection line on the backplane; the increase of the delay time can greatly reduce the system speed., like a shift register. It’s better to put the synchronous working parts like the synchronous counter 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 a major error; Above, the length of the clock line from the common clock source to each plug-in board must be equal where synchronization is the key.
With the completion of the design of OT NI and the star optical fiber network, there will be more boards with high-speed signal lines above 100MHz that need to be designed. Here are several types of high-speed lines:
Whether to choose a double-sided board or a multi-layer board when making a PCB depends on the highest operating frequency, the complexity of the circuit system, and the requirements for assembly density. It is better to choose a multi-layer board when the clock frequency exceeds 200MHZ. If the operating frequency exceeds 350MHz, it is better to choose a printed circuit board with PTFE as the dielectric layer; because its high-frequency attenuation is smaller and the parasitic capacitance is smaller Some; the transmission speed should be faster. Also due to the large Z0 and save power consumption, there are the following principle requirements for printed circuit board wiring: