PCB Technical - PCB design methods and skills 5

76. For PCB with frequency above 30M, use automatic wiring or manual wiring when wiring; are the software functions of wiring the same?
Whether the high-speed signal is based on the rising edge of the signal rather than the absolute frequency or speed. Automatic or manual wiring depends on the support of the software wiring function. Some wiring may be better than automatic wiring manually, but for some wiring, such as checking distribution lines, bus delay compensation wiring, the effect and efficiency of automatic wiring will be much higher than manual wiring. Generally, the PCB substrate is mainly composed of a mixture of resin and glass cloth. Due to the different proportions, the dielectric constant and thickness are different. Generally, the higher the resin content, the smaller the dielectric constant, the thinner it can be. For specific parameters, please consult the PCB manufacturer. In addition, with the emergence of new processes, there are also PCB boards of some special materials that are provided for such as ultra-thick backplanes or low-loss RF boards.

77. In PCB boards, the ground wire is usually divided into protective ground and signal ground; power ground is divided into digital ground and analog ground. Why is the ground wire divided?
The purpose of dividing the ground is mainly for EMC considerations, and it is worried that the noise on the digital part of the power supply and the ground will interfere with other signals, especially analog signals through the conduction path. As for the division of signal and protective ground, it is because the consideration of ESD static discharge in EMC is similar to the role of lightning rod grounding in our lives. No matter how you divide it, there is only one land in the end. It's just that the noise emission method is different.

78. Is it necessary to add ground wire shields on both sides when fabricating the clock?
Whether to add a shielded ground wire or not depends on the crosstalk/EMI situation on the board, and if the shielded ground wire is not handled well, it may make the situation worse.

79. What are the corresponding countermeasures when distributing clock lines of different frequencies?
For the wiring of the clock line, it is best to perform signal integrity analysis, formulate corresponding wiring rules, and perform wiring according to these rules.

80. When the PCB single-layer board is manually wired, should it be placed on the top layer or the bottom layer?
If the device is placed on the top layer, the bottom layer is routed.

81. How to indicate the jumper wire when the PCB single-layer board is manually wired?
Jumper wire is a special device in PCB design. There are only two pads, and the distance can be fixed-length or variable-length. It can be added as needed during manual wiring. There will be a direct connection on the board, and it will also appear in the bill of materials.

82. Assuming a 4-layer board, the middle two layers are VCC and GND, the wiring goes from top to bottom, and the return path from BOTTOM SIDE to TOP SIDE is VIA or POWER through this signal?
There is no clear statement about the return path of the signal on the via. It is generally believed that the return signal will return from the nearest ground or power-connected via. Generally, EDA tools treat vias as an RLC network with fixed lumped parameters during simulation. In fact, it takes a worst-case estimate.

83. "Conduct signal integrity analysis, formulate corresponding wiring rules, and perform wiring according to these rules." How do you understand this sentence?
Pre-simulation analysis can obtain a series of layout and routing strategies to achieve signal integrity. Usually these strategies will be transformed into some physical rules to constrain the layout and routing of the PCB. The usual rules include topology rules, length rules, impedance rules, parallel spacing and parallel length rules and so on. PCB tools can complete routing under these constraints. Of course, the effect of the completion needs to be verified after post-simulation.
In addition, the ICX provided by Mentor supports interconnection synthesis, wiring and simulation at the same time to achieve one pass.

84. How to choose PCB software?
Choose PCB software according to your own needs. There are many advanced softwares available in the market. The key is to see if it suits your design capabilities, design scale and design constraints. The knife is fast and easy to use, too fast will hurt your hands. Find an EDA manufacturer, please go over and make a product introduction, everyone sits down and talks, whether you buy or not, you will be rewarded.

85. How should we understand the concepts of broken copper and floating copper?
From the perspective of PCB processing, copper foil with an area smaller than a certain unit area is generally called broken copper. These copper foils with a too small area will cause problems due to etching errors during processing. From an electrical point of view, the copper foil that is not connected to any DC network is called floating copper. The floating copper will produce an antenna effect due to the influence of the surrounding signals. The floating copper may be broken copper, or it may be a large area of copper foil.

86. Are near-end crosstalk and far-end crosstalk related to the frequency of the signal and the rise time of the signal? Will it change as they change? If there is a relationship, can there be a formula to explain the relationship between them?
It should be said that the crosstalk caused by the offending network to the victim network is related to the signal change edge. The faster the change, the greater the crosstalk caused (V=L*di/dt). The influence of crosstalk on the judgment of the digital signal on the victim network is related to the signal frequency. The faster the frequency, the greater the influence. For details, please refer to the relevant links:

87. How to draw and bind IC in PROTEL?

Specifically, the mechanical layer is used to draw the bonding drawing in the PCB, and the IC substrate liner is determined to be connected to vccgndfloat according to the IC SPEC. The mechanical layer print bonding drawing can be used.

88. When using PROTEL to draw the schematic diagram, the net list generated during the board production is always wrong, and the PCB board cannot be automatically generated. What is the reason?
You can manually edit the generated netlist according to the schematic diagram, and you can automatically route after passing the check. The board surface of automatic layout and wiring with board-making software is not very ideal. The netlist error may be that the component package in the schematic is not specified; it may also be that the library of the layout circuit board does not contain all the component packages in the specified schematic. Do not use automatic wiring if it is a single-sided board, but can use automatic wiring for a double-sided board. It can also be used for power supply and important signal lines manually, and others are automatic.
If it is a cleaning problem, use a special electrical contact cleaner to clean, or use an eraser for writing to clean the PCB. Also consider 1. Whether the gold finger is too thin, whether the pad does not match the socket; 2. Whether the socket has pine perfume or impurities; 3. Whether the quality of the socket is *.

89. The connection between PCB and PCB is usually achieved by plugging in gold-plated or silver "fingers". What if the contact between the "fingers" and the socket is not good?
If it is a cleaning problem, use a special electrical contact cleaner to clean, or use an eraser for writing to clean the PCB. Also consider 1. Whether the gold finger is too thin, whether the pad does not match the socket; 2. Whether the socket has pine perfume or impurities; 3. Whether the quality of the socket is *.

96. What effect does the pad have on high-speed signals?
A very good question. The impact of the pad on the high-speed signal is similar to the impact of the package of the device on the device. In a detailed analysis, after the signal comes out of the IC, it passes through the bonding wires, pins, package shells, pads, and solder to the transmission line. All joints in this process will affect the quality of the signal. But in actual analysis, it is difficult to give the specific parameters of the pad, solder and pin. Therefore, the package parameters in the IBIS model are generally used to summarize them. Of course, such analysis can be received at lower frequencies, and it is not accurate enough for higher frequency signals and higher-precision simulations. A current trend is to use IBIS's VI and V-T curves to describe buffer characteristics, and to use SPICE models to describe package parameters. Of course, in IC design, there are also signal integrity issues, and the impact of these factors on signal quality is also considered in package selection and pin allocation.

97. After the automatic floating copper, the floating copper will fill the blank space according to the position of the device on the board and the wiring layout, but this will form a lot of sharp corners and burrs less than or equal to 90 degrees (such as each pin of a multi-pin chip). There will be a lot of relative sharp corner floating copper), it will be discharged during the high voltage test, and it can’t pass the high voltage test. I don’t know if there is anything other than the automatic floating copper and manual correction to remove these sharp corners and burrs. Method.
The sharp-angled floating copper problem in automatic floating copper is indeed a very troublesome problem. In addition to the discharge problem you mentioned, the problem of acid drop accumulation during processing will also cause processing problems. Since 2000, mentor has supported dynamic copper foil edge repair functions in both WG and EN, and also supports dynamic copper pour, which can automatically solve the problems you mentioned. Please see the animated demonstration. (If there is a problem with opening directly, please right-click and select "Open in New Window", or select "Save Target As" to download the file to the local hard disk and open it.)

98. Do you need to pay attention to the distribution and wiring of power supply in PCB wiring as well as grounding. What kind of problems will it bring if you don't pay attention? Will it increase interference?
If the power supply is treated as a plane layer, the method should be similar to that of the ground layer. Of course, in order to reduce the common mode radiation of the power supply, it is recommended to shrink the height of the power layer from the ground layer by 20 times. If wiring, it is recommended to use a tree structure to avoid power loop problems. Power closed loop will cause large common mode radiation.

99. Should star wiring be used for address lines? If star wiring is used, can the Vtt terminal resistor be placed at the connection point of the star or at the end of a branch of the star?
Whether to use star wiring for the address line depends on whether the delay between the terminals meets the setup and hold time of the system, as well as the difficulty of wiring. The reason for the star topology is to ensure that the delay and reflection of each branch are consistent. Therefore, terminal parallel matching is used in star connection. Generally, matching is added to all terminals, and matching is added to only one branch, which cannot meet such requirements.

100. If you want to reduce the board area as much as possible, but plan to paste the front and back like a memory stick, can you?
Positive and negative PCB design, as long as your welding process is no problem, of course.

101. If there are only four DDR memory affixed to the motherboard, and the clock is required to reach 150Mhz, what are the specific requirements for wiring?
The 150Mhz clock wiring requires that the length of the transmission line be minimized and the influence of the transmission line on the signal is reduced. If you still cannot meet the requirements, simulate it to see if matching, topology, impedance control and other strategies are effective.

102. What is the relationship between the line width and the size of the vias on the PCB board and the size of the current passing through?
Answer: The thickness of the copper foil of a general PCB is 1 ounce. If it is about 1.4 mils, the maximum current allowed by the roughly 1 mil line width is 1A. The via hole is more complicated. In addition to the size of the via pad, it is also related to the thickness of the hole wall sinking copper after electroplating during the processing.