PCB board design step
date：2018-11-13 10:02:00
PCB board design step
1. About the test coupon.
The test coupon is used to measure the characteristic impedance of the PCB produced by TDR (Time Domain Reflectometer) to meet the design requirements. Generally, the impedance to be controlled has a single line and a differential pair. Therefore, the trace line width and line spacing (with differential pairs) on the test coupon should be the same as the line to be controlled. The most important thing is the position of the grounding point when measuring. In order to reduce the inductance of the ground lead, the TDR probe is usually grounded very close to the probe tip, so the distance and way of measuring the signal's point to the ground point on the test coupon To match the probe used.
2. In high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper of multiple signal layers be distributed on the ground and the power supply?
Generally, most of the copper in the blank area is grounded. Just pay attention to the distance between the copper and the signal line when applying copper next to the high-speed signal line, because the copper applied will reduce the characteristic impedance of the trace. Also be careful not to affect the characteristic impedance of its layer, such as in the structure of a dual stripline.
12. Is it possible to calculate the characteristic impedance using the microstrip line model on the signal line above the power plane? Can the signal between the power supply and the ground plane be calculated using the stripline model?
Yes, both the power plane and the ground plane must be considered as reference planes when calculating the characteristic impedance. For example, a four-layer board: top layer - power layer - ground layer - bottom layer, then the model of the top trace characteristic impedance is a microstrip line model with the power plane as a reference plane.
13. Automatically generate test points through high-density printed boards through software. Under normal circumstances, can it meet the test requirements for mass production?
Whether the general software automatically generates test points to meet the test requirements must see whether the specifications of the test points meet the requirements of the test equipment. In addition, if the wiring is too dense and the specification of the test points is strict, there may be no way to automatically add test points to each line. Of course, it is necessary to manually fill in the places to be tested.
14. Does adding test points affect the quality of high-speed signals?
As for whether it will affect the signal quality, it depends on how the test points are added and how fast the signal is. Basically add test points (without vias or DIP pins as test points) may be added online or pulled a short line from the line. The former is equivalent to adding a small capacitor on the line, while the latter is a branch. Both of these conditions will affect the high-speed signal more or less, and the degree of influence is related to the frequency speed of the signal and the edge rate of the signal. The size of the impact can be seen through simulation. In principle, the smaller the test point, the better (and of course the requirements of the test tool). The shorter the branch, the better.
15. How many PCB components are formed, and how should the ground wire between the boards be connected?
When the signal or power supply between the PCB boards is connected, for example, the A board has power or signal sent to the B board, there must be an equal amount of current flowing from the ground to the A board (this is the Kirchoff current law). The current on this formation will flow back where the impedance is the least. Therefore, at each interface where the power source or signal is connected to each other, the number of pins allocated to the ground layer should not be too small to reduce the impedance, which can reduce the noise on the ground layer. In addition, you can also analyze the entire current loop, especially the larger current, adjust the ground or ground connection to control the current travel (for example, make a low impedance somewhere, let most of the current from this Place to reduce the impact on other sensitive signals.
16. Can you introduce some foreign technical books and materials on high-speed PCB design?
Applications for high-speed digital circuits today include communications networks and computers. In terms of communication networks, the working frequency of the PCB has reached GHz, and the number of layers is as much as I know by 40 layers. Computer-related applications are also due to advances in chips. Whether it is a general PC or a server, the maximum operating frequency on the board has reached 400MHz (such as Rambus). In response to this high-speed, high-density wiring requirement, the demand for blind/buried vias, mircrovias, and build-up process processes is also increasing. These design requirements are available to manufacturers in large quantities.
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