How to make a high-quality PCB board
Page Introduction
Classification | PCB board |
Keyword | pcb design issue, pcb board, pcb,pcb design tutorial |
Article core | How to make a high quality PCB board?It is not difficult to do a good piece of PCB board, but it is not easy to do a high-quality PCB board. |
Catalogs | I.Guidance |
II.Make clear the design goals | |
III.Understanding the requirements of the functions of the components used in the layout and wiring | |
IV.Consideration of component layout | |
V.Consideration of wiring | |
a. Transmission line | |
1. the basic factors that determine whether the transmission line is used: | |
2.Several types of transmission lines | |
3. End connected transmission line | |
4. Non end line transmission line | |
VI.Relevant information about "How to make a high-quality PCB board" |
Text
I.Guidance
It is not difficult to do a good piece of PCB board, but it is not easy to do a good PCB board.We said that do a PCB board is let the PCB design principle diagram to become a real PCB circuit board,Please don’t look down on this pprocess,there are a lot of things work principle in engineering is difficult to achieve, or others to achieve things that others can not, so good the board of PCB is not difficult, but to make a good PCB board is not an easy thing.
Two major difficulties of microelectronics processing in high frequency signal and weak signal, the PCB production level is particularly important, the principle of the same design, the same components, different people made PCB have different results, so how to make a good PCB board? According to our previous experience, we would like to talk about our views on the following aspects:
II.Make clear the design goals
To accept a design task, we must first clear the goal of the design, PCB board, PCB board, ordinary high frequency small signal processing PCB board or both high frequency and small signal processing board PCB, if PCB is in general, as long as the layout reasonable and tidy, mechanical dimensions can be accurate if, in the load line and the line will use some methods for processing, reducing the load, to strengthen the long-term drive, the focus is to prevent long-term reflection.
When there are more than 40MHz signal lines on the board, special consideration should be given to these lines, such as inter line crosstalk. If the frequency is higher, the length of the wiring will be more strictly limited. According to the network theory of distributed parameter, the interaction between high-speed circuit and its connection is the decisive factor, which can not be ignored in system design. With the increase of gate transmission speed, the opposition on signal line will increase correspondingly, and the crosstalk between adjacent signal lines will increase directly. Usually, the power consumption and thermal dissipation of high-speed circuit are also very large. Enough attention should be paid to high-speed PCB.
When there is a weak signal mv level even microvolt board, the signal line will need special care, because of the small signal is too weak, very easy to be interfered by other strong signal shielding measures, it is often necessary, otherwise it will greatly reduce the signal-to-noise ratio. So that the useful signal is inundated by noise and can not be effectively extracted.
The adjustment and measurement of boards should also be considered in the design stage. The physical location of test points and isolation of test points should not be ignored, because some small signals and high-frequency signals can not be directly measured by adding probes.
In addition, some other related factors, such as the number of plate layers, the package shape of the components and the mechanical strength of the board, are also considered. Before making the PCB board, you have to make a good number of goals for the design.
III.Understanding the requirements of the functions of the components used in the layout and wiring
We know that some special components have special requirements in layout and wiring, such as analog signal amplifier used by LOTI and APH, analog signal amplifier requires stable power supply and small ripple. The simulation of small signal parts should be far away from the power device. On the OTI board, the small signal amplification section is also specially added with a shield to shield the stray electromagnetic interference. GLINK chip NTOI board with ECL is used in the process of large power consumption, serious heat, cooling problems must be in the layout must be special consideration, if using natural cooling to GLINK chip on the air circulation is smooth, and scattered out of the heat also does not form a big impact on the other chip. If the board is equipped with a horn or other high-power device, it is possible to pay enough attention to the possibility of serious pollution to the power supply.
IV.Consideration of component layout
The first thing to consider is the electrical performance of components. The most important part is to connect closely connected components. Especially for some high-speed lines, we need to make them as short as possible, and separate power signals from small signal devices. Under the premise of satisfying the performance of the circuit, it is also necessary to consider the neat placement, beautiful appearance and easy testing. The mechanical dimensions and the location of the socket must be seriously considered.
The transmission delay time of the grounding and interconnect lines in the high speed system is also the first factor to be considered in the design of the system. The transmission time of the signal line of great influence on the speed of the system in general, especially for the high speed ECL circuit, while the integrated circuit block itself speed is very high, but due to the floor with ordinary interconnects (delay of each 30cm length about 2ns) increase the delay time, the system speed is greatly reduced as the shift register, synchronous synchronous counter the best working parts on the same piece of board, because the transmission clock signal on board the delay time is not the same, may make the shift register produce error, if it can not be placed on a piece of board, the synchronization is the key place, from the public when the clock source is connected to the clock line board length must be equal.
V.Consideration of wiring
With the completion of the design of OTNI and star shaped optical fiber network, there will be more than 100MHz plates with high speed signal line to be designed. Here are some basic concepts of high-speed line.
a. Transmission line
Any "long" signal path on a printed circuit board can be regarded as a transmission line. If the transmission delay of the line is much shorter than that of the signal rise time, the reflection of the host produced during the rise of the signal will be submerged. Showed no overshoot, recoil and ringing, most of the MOS circuit, the rise time delay time of transmission line are much larger than the long line, so go in meters without signal distortion. For the fast speed logic circuit, especially the super high speed ECL.
In an integrated circuit, the length of the line must be greatly shortened to maintain the integrity of the signal, due to the increase of the speed of the edge, if there is no other measure.
There are two ways to make high-speed circuit in a relatively long line of work and no serious distortion, fast TTL on the falling edge using Schottky diode clamp method, the overshoot is clamped at a level lower than the ground potential of the diode drop, which reduces the anti pulse amplitude behind the slow rise the edge allows the overshoot, but it is relatively high output impedance in the circuit level "H" under the condition (50~80) attenuation. In addition, due to the large degree of immunity of the "H" state, the recoil problem is not very prominent. For HCT series devices, if the combination of Schottky diode clamping and series resistance termination method is combined, the improvement effect will be more obvious.
When there is a fan out along the signal line, the TTL shaping method presented above is somewhat inadequate at a higher bit rate and faster edge rate. Because there are reflected waves in the line, they will tend to be synthesized at high rate, which can cause serious distortion of the signal and reduce the ability to resist interference. Therefore, in order to solve the reflection problem, another method is usually used in the ECL system: the line impedance matching method. By this method, the reflection can be controlled and the integrity of the signal is guaranteed.
Strictly speaking, the transmission line is not very necessary for conventional TTL and CMOS devices with slower edge speed. For high-speed ECL devices with faster edge speed, transmission lines are not always needed. But when transmission lines are used, they have the advantages of predicting connection delay and controlling reflection and oscillation by impedance matching.
1. the basic factors that determine whether the transmission line is used:
- (1) system signal edge rate,
- (2) the connection distance
- (3) capacitive load (fanout number),
- (4) a resistive load (line termination methods);
- (5) allow the recoil and overshoot percentage (decrease 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 a coaxial cable is usually 50 omega and 75 Omega, and the twisted pair is usually 110 Omega.
- (2) the microstrip line on the printed board: the microstrip line is a band guide (signal line). The microstrip line is separated from the ground plane with a dielectric barrier. If the line thickness, width and distance from the ground plane are controllable, the characteristic impedance of the line can also be controlled. The characteristic impedance of the microstrip line Z0 is:
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| The characteristic impedance of the microstrip line Z0--How to make a high-quality PCB board |
- (3) A strip line in a printed board: a strip line is a copper strip in the middle of a dielectric between two layers of conductive plane. If the thickness and width of the line, the dielectric constant of the dielectric, and the distance between the two conducting planes are controllable, the characteristic impedance of the line is also controllable, and the characteristic impedance of the stripline is as the above picture.
 |
| the characteristic impedance of the stripline--How to make a high-quality PCB board |
3. End connected transmission line
The transmission line is called a parallel terminal connection at the receiving end of a line with a resistance end that is equal to the line characteristic impedance. It is mainly used to obtain the best electrical performance, including the drive distribution load.
In order to save the power consumption, a 104 capacitor is connected to the end connection to form an AC end connection circuit, which can effectively reduce the DC loss.
A resistance is connected between the drive and the transmission line, and the terminal of the line is no longer connected with the end resistance. This end connection is called the series end connection. A long line of overshoot and ringing or available series damping series termination technology to control. Series damping is the use of a small resistor and gate drive output series (typically 10~75 ohms) to achieve. This method is suitable for damping associated with characteristic impedance to control line (such as floor wiring, circuit board no ground plane and most winding wiring.
Series termination series value and circuit resistance (gate drive) and the output impedance equal to the characteristic impedance of the transmission line. Series terminal there can only use the lumped load and transmission delay time long shortcoming in the terminal. However, this can be overcome by using the method of excess series terminated transmission line.
4. Non end line transmission line
If the line delay time is much shorter than the signal rise time, can be used in transmission line without series termination or parallel termination case, if a non terminal of the round-trip delay (the signal from a transmission line in time) than the pulse signal rise time is short, so the non termination caused by the recoil is about logic swing 15%. The maximum opening length is approximately as follows:
Lmax < tr/2tpd
In the form: TR is up time and TPD as the transmission delay time of the unit line length
5. Comparison of several end connection methods
The parallel connection end connection and the series terminal connection have each advantage, which one or two kinds are used, depending on the designer's hobby and the system requirement. The main advantages of parallel terminal connection are fast system speed and complete and undistorted transmission of signal on line. The load on the long line will not affect the transmission delay time of the drive gate driving the long line, but will not affect the edge speed of the signal, but it will increase the transmission delay time of the signal along the long line. When driving large fan out, the load can be distributed along the branch short line, rather than the terminal of the load set that must be collected online as in the series end connection.
Series termination method makes the circuit driving several parallel load line capacity series terminal due to capacitive load caused by the delay time increment than corresponding parallel end wiring doubled in size, while the short term due to the capacitive load edge speed slowed down and driving gate delay time increases, but the smaller end crosstalk series wiring than parallel end wiring, the main reason is the signal amplitude series terminal transmission is only 1/2 of the logic swing, so the switch current is only half of the current switch parallel termination, crosstalk is small small signal energy.
When doing PCB, it is the choice of double-sided or multi-layer board, which depends on the maximum working frequency and the complexity of the circuit system and the requirements for the density of the assembly. It is best to choose a multilayer board when the clock frequency is more than 200MHZ. If the work frequency is more than 350MHz, the best selection of PTFE as printed circuit board dielectric layer, because of the high frequency attenuation of its smaller and smaller parasitic capacitance, the transmission speed is faster, but also because of Z0 large province power, following the original requirements on printed circuit board wiring:
- (1) there is a large interval between all parallel lines to reduce crosstalk. If there are two signal lines that are close to each other, it is best to take a grounding wire between the two lines so that it can play a shielding role.
- (2) when designing signal transmission lines, avoid sharp corners, so as to prevent the reflection of the characteristic impedance of transmission lines, and try to design uniform circular arcs with a certain size.
- (3) the width of printed circuit can be calculated based on the formula of characteristic impedance of the above microstrip line and stripline. The characteristic impedance of microstrip line on printed circuit board is usually between 50~120 ohms. To get large characteristic impedance, the line width must be very narrow. But the fine lines are not easy to make.
- (4) Considering all kinds of factors, it is generally appropriate to choose the impedance value of around 68 ohms, because choosing 68 ohma characteristic impedance can achieve the best balance between delay time and power consumption. A 50 - Omega transmission line will consume more power; larger impedance can reduce the consumption of power, but the delay time of transmission will be abomination. Due to the negative line capacitance, the delay time of transmission and the reduction of characteristic impedance will be caused. But the intrinsic capacitance of the unit length of the line segment with very low characteristic impedance is larger, so the delay time and the characteristic impedance are less affected by the load capacitance.One of the important features of a transmission line with appropriate end connection is that the branch short line should have no effect on the line delay time. When Z0 is 50 Omega. The length of the branch and short line must be limited to within 2.5cm to avoid large ringing.
- (5) the two sides of the circuit board should be perpendicular to each other to prevent the mutual induced crosstalk between the two sides of the plate (or the four layers in the six laminate).
- (6) if the printed circuit board is equipped with large current devices, such as relays, lights, speakers, their best to ground separately, in order to reduce the noise on the ground, the ground current of the device should be connected to a separate bus board and back up, and these independent the ground should be connected with the grounding of the whole system.
- (7) if there is a small signal amplifier on the board, the weak signal line before amplification is far away from the strong signal line, and the line should be as short as possible. If possible, it will also be shielded by the earth wire.
VI.Relevant information about "How to make a high-quality PCB board"
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