Everyone knows that it is a PCB board that turns the designed schematic into a real PCB board. Please don't underestimate the process. There are many things that work well in the project but it is difficult to achieve in the project, or Something else that others can achieve can't be realized, so it is not difficult to say that it is a PCB board, but it is not an easy task to make a PCB board.
The two major difficulties in the field of microelectronics are the processing of high-frequency signals and weak signals. In this respect, the level of PCB production is particularly important. The same principle design, the same components, different 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 own views on the following aspects:
One: To clearly define the design goal to receive a design task, first of all to clarify its design goals, is the ordinary PCB board, high-frequency PCB board, small signal processing PCB board or PCB board with high frequency and small signal processing, if It is an ordinary PCB board. As long as the layout and wiring are reasonable and tidy, the mechanical dimensions can be accurate. If there are medium load lines and long lines, it must be treated by certain means to reduce the load. The long line should be strengthened. The key is to prevent long lines. reflection.
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 is a stricter limit on the length of the wiring. According to the network theory of the distributed parameters, the interaction between the high-speed circuit and its connection is a decisive factor, which cannot be ignored in the system design. As the transmission speed of the gate increases, the opposition on the signal line will increase accordingly, and the crosstalk between adjacent signal lines will increase proportionally. Generally, the power consumption and heat dissipation of the high-speed circuit are also large. Should be given enough attention.
When there are weak signals of millivolts or even microvolts on the board, special attention should be paid to these signal lines. Because the small signals are too weak, they are very susceptible to interference from other strong signals. Shielding measures are often necessary, otherwise Greatly reduce the signal to noise ratio. As a result, the useful signal is overwhelmed by noise and cannot be extracted efficiently.
The commissioning of the board should also be considered in the design stage. The physical location of the test points and the isolation of the test points 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, and the mechanical strength of the board. Before making a PCB board, it is important to make a design goal for the design.
two. Understand the function of the components used. The layout and wiring requirements We know that some special components have special requirements in place and route, such as the analog signal amplifier used by LOTI and APH. The analog signal amplifier requires stable power supply and small ripple. The analog small signal portion should be as far away as possible from the power device. On the OTI board, the small signal amplification section is also specially equipped with a shield to shield stray electromagnetic interference. The Glink chip used on the NTOI board adopts the ECL process, which consumes a lot of power and heat. The heat dissipation problem must be specially considered in 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 that is dissipated cannot make a big impact on other chips. If a horn or other high-powered device is installed on the board, it may cause serious pollution to the power supply. This should also be paid enough attention.
III. Consideration of component layout One of the first factors to consider in the layout of components is electrical performance. The components with close connection are put together as much as possible. Especially for some high-speed lines, the layout should be as short as possible. The power signal and the small signal device are to be separated. Under the premise of satisfying the performance of the circuit, it is also necessary to consider that the components are placed neatly and beautifully, and it is easy to test. The mechanical size of the board and the position of the socket need to be carefully considered.
Grounding and transmission delay times on high-speed systems are also the first considerations in system design. The transmission time on the signal line has a great influence on the total system speed, especially for high-speed ECL circuits. Although the speed of the integrated circuit block itself is very high, due to the common interconnection line on the bottom plate (about 30cm line length) 2ns delay amount) brings an increase in delay time, which can greatly reduce the system speed. Like the shift register, the synchronous counter is preferably placed on the same board because of the clock on different boards. Signal transmission delay times are not equal, which may cause the shift register owner to be wrong. If it cannot be placed on one board, the length of the clock line connected from the common clock source to each board must be equal where synchronization is critical.
Fourth, the consideration of wiring With 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. Here are some basic concepts of high-speed lines.
1. Any "long" signal path on the transmission line printed circuit board can be considered as a transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the reflection of the main generator during the signal rise will be overwhelmed. Overshoot, kickback, and ringing are no longer present. For most current MOS circuits, since the rise time has a much larger ratio of line propagation delay time, the trace can be longer in meters without signal distortion. For faster logic circuits, especially ultra-fast ECL
For integrated circuits, the length of the traces must be greatly shortened to maintain signal integrity due to the increased edge speed.
There are two ways to make high-speed circuits work on relatively long lines without severe waveform distortion. TTL uses a Schottky diode clamp method for fast falling edges, so that the overshoot is clamped to a diode drop below ground. At the level, this reduces the backlash amplitude, and the slower rising edge allows for overshoot, but it is attenuated by the relatively high output impedance (50-80 Ω) of the circuit at level "H". . In addition, due to the high level of immunity of the level "H" state, the backlash problem is not very prominent. For the HCT series devices, if the Schottky diode clamp and the series resistor termination method are combined, the improvement is improved. The effect will be more obvious.
When there is a fan-out along the signal line, the TTL shaping method described above appears to be somewhat inadequate at higher bit rates and faster edge rates. Because there are reflected waves in the line, they tend to be synthesized at high bit rates, causing severe 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. In this way, the reflection is controlled and the integrity of the signal is guaranteed.
Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not very desirable. For high speed ECL devices with faster edge speeds, transmission lines are not always required. However, when transmission lines are used, they have the advantage of predicting the connection delay and controlling the reflection and oscillation by impedance matching. 1
There are five basic factors that determine whether or not to use a transmission line. They are: (1) the edge rate of the system signal, (2) the connection distance, (3) the capacitive load (how much fanout), (4) the resistive load (the termination of the line); (5) allow The percentage of recoil and overshoot (the degree of reduction in AC immunity).
2. Several types of transmission lines
(1) Coaxial cable and twisted pair: they are often used in the connection between the system and the system. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and the twisted pair is usually 110Ω.
(2) The microstrip line on the printed board is a strip conductor (signal line) separated from the ground plane by a dielectric. If the thickness, width, and distance from the ground plane are controllable, its characteristic impedance is also controllable.
The propagation delay time of the microstrip line per unit length depends only on the dielectric constant and is independent of the width or spacing of the lines.
(3) The ribbon strip line in the printed board is a copper strip line placed between the dielectric layers between the 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, the characteristic impedance of the line is also controllable. Similarly, the transmission delay time of the strip line per unit length and the width of the line Or the spacing is irrelevant; it depends only on the relative dielectric constant of the medium used.
3. The terminating transmission line is terminated at the receiving end of one line with a resistor having the same characteristic impedance as the line, and the transmission line is said to be a parallel terminal. It is primarily intended for the best electrical performance, including driving distributed loads.
Sometimes in order to save power consumption, a 104 capacitor is connected in series with 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 serial termination. Overshoot and ringing on longer lines can be controlled by series damping or series termination techniques. Series damping is achieved with a small resistor (typically 10 to 75 Ω) in series with the output of the drive gate. This damping method is suitable. Used in conjunction with lines whose characteristic impedance is controlled (such as backplane wiring, ground plane-free boards, and most wiring).
The sum of the value of the series resistance and the output impedance of the circuit (drive gate) in series termination is equal to the characteristic impedance of the transmission line. The series connection terminal has the disadvantage of only using the lumped load at the terminal and the transmission delay time is longer. However, this This can be overcome by using redundant series termination of the transmission line.
4. 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 two-way delay of a non-terminal connection (signal on the transmission line The time of one time is shorter than the rise time of the pulse signal, so the backlash due to non-termination is about 15% of the logic swing. The maximum open route length is approximately:
Lmax
Where: tr is the rise time
Tpd is the transmission delay time per unit line length
5. Comparison of several termination methods The parallel terminal wiring and the series terminal wiring have their own advantages. Which one is used or two is used depends on the designer's preference and system requirements.
The main advantage of the parallel terminal wiring is that the system speed is fast and the signal is transmitted intact on the line without distortion. The load on the long line does not affect the transmission delay time of the drive gate driving the long line, nor does it affect its signal edge speed, but it will increase the transmission delay time of the signal along the long line. When driving a large fanout, the load can be distributed along the branch stubs, rather than having to concentrate the load on the terminal as in series termination.
The series termination method allows the circuit to have the ability to drive several parallel load lines. The delay in the series terminal due to the capacitive load is approximately double the delay of the corresponding parallel terminal, while the short line is the edge due to the capacitive load. The speed is slower and the drive gate delay time is increased. However, the crosstalk of the series terminal wiring is smaller than that of the parallel terminal wiring. The main reason is that the signal amplitude transmitted along the serial terminal wiring is only one-half of the logic swing. The switching current is also only half of the switching current connected in parallel, and the signal energy crosstalk is small.
5. PCB board wiring technology When making PCBs, whether to use double-panel or multi-layer boards depends on the highest operating frequency and the complexity of the circuit system and the requirements for assembly density. Multilayer boards are preferred when the clock frequency exceeds 200 MHz. If the operating frequency exceeds 350MHz, it is better to use a printed circuit board with Teflon as the dielectric layer, because its high frequency attenuation is smaller, the parasitic capacitance is smaller, the transmission speed is faster, and Z0 is better. Large and power-saving, the following principles are required for the trace of printed circuit boards
(1) Try to leave a large gap between all parallel signal lines to reduce crosstalk. If there are two signal lines that are close together, it is best to take a ground line between the two lines, which can be used as a shielding function.
(2) When designing the signal transmission line, avoid sharp turns, in order to prevent reflection of the characteristic impedance of the transmission line, and try to design a uniform circular arc with a certain size.
(3) The width of the printed line can be calculated according to the characteristic impedance calculation formula of the above microstrip line and strip line, and the characteristic impedance of the microstrip line on the printed circuit board is generally between 50 and 120 Ω. To get 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 appropriate to select an impedance value of about 68 Ω, because the characteristic impedance of 68 Ω is selected to achieve an optimum balance between delay time and power consumption. A 50Ω transmission line will consume more power; a larger impedance will reduce the power consumption, but will increase the transmission delay time. Since the negative line capacitance causes an increase in the transmission delay time and a decrease in the characteristic impedance. However, the intrinsic capacitance per unit length of the line segment with a low characteristic impedance is relatively large, so the transmission delay time and the characteristic impedance are less affected by the load capacitance. An important feature of a properly terminated transmission line is that branching short lines should have no effect on line delay time. When Z0 is 50Ω. The length of the branch short line must be limited to 2.5 cm to avoid large ringing.
(4) For double-panel (or four-layer wire in a six-layer board), the lines on both sides of the board should be perpendicular to each other to prevent mutual crosstalk from being induced.
(5) If large current devices such as relays, indicator lights, and horns are installed on the printed circuit board, their ground lines should be separated separately to reduce the noise on the ground line. The ground wire of these high current devices should be Connected to a separate ground bus on the board and backplane, and these separate ground lines 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 far away from the strong signal line, and the trace should be as short as possible, and if possible, shield it with a ground line.
The two major difficulties in the field of microelectronics are the processing of high-frequency signals and weak signals. In this respect, the level of PCB production is particularly important. The same principle design, the same components, different 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 own views on the following aspects:
One: To clearly define the design goal to receive a design task, first of all to clarify its design goals, is the ordinary PCB board, high-frequency PCB board, small signal processing PCB board or PCB board with high frequency and small signal processing, if It is an ordinary PCB board. As long as the layout and wiring are reasonable and tidy, the mechanical dimensions can be accurate. If there are medium load lines and long lines, it must be treated by certain means to reduce the load. The long line should be strengthened. The key is to prevent long lines. reflection.
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 is a stricter limit on the length of the wiring. According to the network theory of the distributed parameters, the interaction between the high-speed circuit and its connection is a decisive factor, which cannot be ignored in the system design. As the transmission speed of the gate increases, the opposition on the signal line will increase accordingly, and the crosstalk between adjacent signal lines will increase proportionally. Generally, the power consumption and heat dissipation of the high-speed circuit are also large. Should be given enough attention.
When there are weak signals of millivolts or even microvolts on the board, special attention should be paid to these signal lines. Because the small signals are too weak, they are very susceptible to interference from other strong signals. Shielding measures are often necessary, otherwise Greatly reduce the signal to noise ratio. As a result, the useful signal is overwhelmed by noise and cannot be extracted efficiently.
The commissioning of the board should also be considered in the design stage. The physical location of the test points and the isolation of the test points 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, and the mechanical strength of the board. Before making a PCB board, it is important to make a design goal for the design.
two. Understand the function of the components used. The layout and wiring requirements We know that some special components have special requirements in place and route, such as the analog signal amplifier used by LOTI and APH. The analog signal amplifier requires stable power supply and small ripple. The analog small signal portion should be as far away as possible from the power device. On the OTI board, the small signal amplification section is also specially equipped with a shield to shield stray electromagnetic interference. The Glink chip used on the NTOI board adopts the ECL process, which consumes a lot of power and heat. The heat dissipation problem must be specially considered in 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 that is dissipated cannot make a big impact on other chips. If a horn or other high-powered device is installed on the board, it may cause serious pollution to the power supply. This should also be paid enough attention.
III. Consideration of component layout One of the first factors to consider in the layout of components is electrical performance. The components with close connection are put together as much as possible. Especially for some high-speed lines, the layout should be as short as possible. The power signal and the small signal device are to be separated. Under the premise of satisfying the performance of the circuit, it is also necessary to consider that the components are placed neatly and beautifully, and it is easy to test. The mechanical size of the board and the position of the socket need to be carefully considered.
Grounding and transmission delay times on high-speed systems are also the first considerations in system design. The transmission time on the signal line has a great influence on the total system speed, especially for high-speed ECL circuits. Although the speed of the integrated circuit block itself is very high, due to the common interconnection line on the bottom plate (about 30cm line length) 2ns delay amount) brings an increase in delay time, which can greatly reduce the system speed. Like the shift register, the synchronous counter is preferably placed on the same board because of the clock on different boards. Signal transmission delay times are not equal, which may cause the shift register owner to be wrong. If it cannot be placed on one board, the length of the clock line connected from the common clock source to each board must be equal where synchronization is critical.
Fourth, the consideration of wiring With 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. Here are some basic concepts of high-speed lines.
1. Any "long" signal path on the transmission line printed circuit board can be considered as a transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the reflection of the main generator during the signal rise will be overwhelmed. Overshoot, kickback, and ringing are no longer present. For most current MOS circuits, since the rise time has a much larger ratio of line propagation delay time, the trace can be longer in meters without signal distortion. For faster logic circuits, especially ultra-fast ECL
For integrated circuits, the length of the traces must be greatly shortened to maintain signal integrity due to the increased edge speed.
There are two ways to make high-speed circuits work on relatively long lines without severe waveform distortion. TTL uses a Schottky diode clamp method for fast falling edges, so that the overshoot is clamped to a diode drop below ground. At the level, this reduces the backlash amplitude, and the slower rising edge allows for overshoot, but it is attenuated by the relatively high output impedance (50-80 Ω) of the circuit at level "H". . In addition, due to the high level of immunity of the level "H" state, the backlash problem is not very prominent. For the HCT series devices, if the Schottky diode clamp and the series resistor termination method are combined, the improvement is improved. The effect will be more obvious.
When there is a fan-out along the signal line, the TTL shaping method described above appears to be somewhat inadequate at higher bit rates and faster edge rates. Because there are reflected waves in the line, they tend to be synthesized at high bit rates, causing severe 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. In this way, the reflection is controlled and the integrity of the signal is guaranteed.
Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not very desirable. For high speed ECL devices with faster edge speeds, transmission lines are not always required. However, when transmission lines are used, they have the advantage of predicting the connection delay and controlling the reflection and oscillation by impedance matching. 1
There are five basic factors that determine whether or not to use a transmission line. They are: (1) the edge rate of the system signal, (2) the connection distance, (3) the capacitive load (how much fanout), (4) the resistive load (the termination of the line); (5) allow The percentage of recoil and overshoot (the degree of reduction in AC immunity).
2. Several types of transmission lines
(1) Coaxial cable and twisted pair: they are often used in the connection between the system and the system. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and the twisted pair is usually 110Ω.
(2) The microstrip line on the printed board is a strip conductor (signal line) separated from the ground plane by a dielectric. If the thickness, width, and distance from the ground plane are controllable, its characteristic impedance is also controllable.
The propagation delay time of the microstrip line per unit length depends only on the dielectric constant and is independent of the width or spacing of the lines.
(3) The ribbon strip line in the printed board is a copper strip line placed between the dielectric layers between the 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, the characteristic impedance of the line is also controllable. Similarly, the transmission delay time of the strip line per unit length and the width of the line Or the spacing is irrelevant; it depends only on the relative dielectric constant of the medium used.
3. The terminating transmission line is terminated at the receiving end of one line with a resistor having the same characteristic impedance as the line, and the transmission line is said to be a parallel terminal. It is primarily intended for the best electrical performance, including driving distributed loads.
Sometimes in order to save power consumption, a 104 capacitor is connected in series with 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 serial termination. Overshoot and ringing on longer lines can be controlled by series damping or series termination techniques. Series damping is achieved with a small resistor (typically 10 to 75 Ω) in series with the output of the drive gate. This damping method is suitable. Used in conjunction with lines whose characteristic impedance is controlled (such as backplane wiring, ground plane-free boards, and most wiring).
The sum of the value of the series resistance and the output impedance of the circuit (drive gate) in series termination is equal to the characteristic impedance of the transmission line. The series connection terminal has the disadvantage of only using the lumped load at the terminal and the transmission delay time is longer. However, this This can be overcome by using redundant series termination of the transmission line.
4. 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 two-way delay of a non-terminal connection (signal on the transmission line The time of one time is shorter than the rise time of the pulse signal, so the backlash due to non-termination is about 15% of the logic swing. The maximum open route length is approximately:
Lmax
Where: tr is the rise time
Tpd is the transmission delay time per unit line length
5. Comparison of several termination methods The parallel terminal wiring and the series terminal wiring have their own advantages. Which one is used or two is used depends on the designer's preference and system requirements.
The main advantage of the parallel terminal wiring is that the system speed is fast and the signal is transmitted intact on the line without distortion. The load on the long line does not affect the transmission delay time of the drive gate driving the long line, nor does it affect its signal edge speed, but it will increase the transmission delay time of the signal along the long line. When driving a large fanout, the load can be distributed along the branch stubs, rather than having to concentrate the load on the terminal as in series termination.
The series termination method allows the circuit to have the ability to drive several parallel load lines. The delay in the series terminal due to the capacitive load is approximately double the delay of the corresponding parallel terminal, while the short line is the edge due to the capacitive load. The speed is slower and the drive gate delay time is increased. However, the crosstalk of the series terminal wiring is smaller than that of the parallel terminal wiring. The main reason is that the signal amplitude transmitted along the serial terminal wiring is only one-half of the logic swing. The switching current is also only half of the switching current connected in parallel, and the signal energy crosstalk is small.
5. PCB board wiring technology When making PCBs, whether to use double-panel or multi-layer boards depends on the highest operating frequency and the complexity of the circuit system and the requirements for assembly density. Multilayer boards are preferred when the clock frequency exceeds 200 MHz. If the operating frequency exceeds 350MHz, it is better to use a printed circuit board with Teflon as the dielectric layer, because its high frequency attenuation is smaller, the parasitic capacitance is smaller, the transmission speed is faster, and Z0 is better. Large and power-saving, the following principles are required for the trace of printed circuit boards
(1) Try to leave a large gap between all parallel signal lines to reduce crosstalk. If there are two signal lines that are close together, it is best to take a ground line between the two lines, which can be used as a shielding function.
(2) When designing the signal transmission line, avoid sharp turns, in order to prevent reflection of the characteristic impedance of the transmission line, and try to design a uniform circular arc with a certain size.
(3) The width of the printed line can be calculated according to the characteristic impedance calculation formula of the above microstrip line and strip line, and the characteristic impedance of the microstrip line on the printed circuit board is generally between 50 and 120 Ω. To get 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 appropriate to select an impedance value of about 68 Ω, because the characteristic impedance of 68 Ω is selected to achieve an optimum balance between delay time and power consumption. A 50Ω transmission line will consume more power; a larger impedance will reduce the power consumption, but will increase the transmission delay time. Since the negative line capacitance causes an increase in the transmission delay time and a decrease in the characteristic impedance. However, the intrinsic capacitance per unit length of the line segment with a low characteristic impedance is relatively large, so the transmission delay time and the characteristic impedance are less affected by the load capacitance. An important feature of a properly terminated transmission line is that branching short lines should have no effect on line delay time. When Z0 is 50Ω. The length of the branch short line must be limited to 2.5 cm to avoid large ringing.
(4) For double-panel (or four-layer wire in a six-layer board), the lines on both sides of the board should be perpendicular to each other to prevent mutual crosstalk from being induced.
(5) If large current devices such as relays, indicator lights, and horns are installed on the printed circuit board, their ground lines should be separated separately to reduce the noise on the ground line. The ground wire of these high current devices should be Connected to a separate ground bus on the board and backplane, and these separate ground lines 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 far away from the strong signal line, and the trace should be as short as possible, and if possible, shield it with a ground line.
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