PCB design tips 100, teach you to master PCB design easily (11-18)

Generally, copper deposition in the blank area is mostly grounded. Only when copper is applied to the high-speed signal line, pay attention to the distance between copper and the signal line, because the copper applied will reduce the characteristic impedance of the line. Also be careful not to affect the characteristic impedance of its layer, for example, in the structure of dual stripline.

12. Can we use the microstrip line model to calculate the characteristic impedance of the signal plane above the power plane? Can the signal between the power supply and ground plane be calculated by the stripline model?

Yes, when calculating the characteristic impedance, the power plane and ground plane must be considered as reference planes. For example, the four laminate: the top layer power layer - ground bottom, the top line characteristic impedance model is the microstrip line model based on the power plane as the reference plane.

13. Can the test points automatically generated by software on high-density printed boards meet the test requirements of mass production in general?

Whether the general software automatically generates test points to meet the test needs depends on whether the specifications of the test points are compatible with the requirements of the testing machines. In addition, if the line is too close and the specification of the test point is strict, it may not be able to automatically add test points to each line. Of course, it is necessary to manually complement the places to be tested.

14. Will adding test points affect the quality of high-speed signals?

As for whether the signal quality will be affected, it depends on how the test point is added and how fast the signal is. Basically, the additional test points (without the via or DIP pin) as the test point may be added to the line or a small line from the line. The former is equivalent to adding a very small capacitance online, while the latter is more than one branch. These two cases will affect the speed signal more or less. The degree of the impact is related to the frequency and speed of the signal and edge rate. The impact size can be learned through simulation. In principle, the smaller the test point is, the better. (of course, the test equipment needs). The shorter the branch, the better.

15. Several PCB systems, how should the ground wire between each plate be connected?

When each PCB board is connected to a signal or a power supply, for example, the A board has power or signal to the B board, there will be an equal amount of current from the formation flow back to the A board (this is Kirchoff current law). The current on this stratum will find the least impedance to flow back.  Therefore, the number of pins allocated to the stratum is not too small to reduce the impedance, so that the noise in the formation can be reduced in every interface that is connected to each other, regardless of the power or signal. In addition, we can also analyze the whole current loop, especially the larger part of the current, adjust the connection of the ground or ground line to control the way of the current (for example, to make a low impedance somewhere, let most of the current go from this place), and reduce the impact on other more sensitive signals.

16. Can you introduce some foreign technical books and materials about high-speed PCB design?

Nowadays, high-speed digital circuits are applied in related fields, such as communication networks and computers. In terms of communication network, the working frequency of the PCB board has reached GHz, and the number of stacked layers has reached 40 levels. Computer-related applications also because of the progress of the chip, whether it is the general PC or the server (Server), the maximum working frequency on the board has reached 400MHz (such as Rambus). In response to this high-speed and high-density line demand, the demand for blind hole drilling (blind/buried vias), microbial and build-up process technology is becoming more and more. These design requirements are all produced by a manufacturer. The following are a few good technical books:

• <1>.Howard W. Johnson, "High-Speed Digital Design - A Handbook of Black Magic";
• <2>.Stephen H. Hall, "High-Speed Digital System Design";
• <3>.Brian Yang, "Digital Signal Integrity";
• <4>.Google**rook, "Integrity Issues and Printed Circuit Board Design".

17. Two commonly referred characteristic impedance formulas:

Microstrip line (microstrip) Z={87/[sqrt (Er+1.41)]}ln[5.98H/ (0.8W+T)] in which W is line width, T is the copper thickness of the walking line, H is the distance to the reference plane, and Er is the dielectric constant of the PCB plate material (dielectric constant). This formula must be applied in the case of 0.1< (W/H) <2.0 and 1< (Er) <15.

The stripline (stripline) Z=[60/sqrt (Er)]ln{4H/[0.67 PI (T+0.8W)], where H is the distance of the two reference planes and the line is located in the middle of the two reference planes. This formula must be applied in the case of W/H<0.35 and T/H<0.25.

18. Can the ground line be added in the middle of the differential signal line?

In the middle of the differential signal, the ground line can not be added. One of the most important points of the application of differential signals is to use the benefits of coupling (flux cancellation, noise immunity) and other benefits. If the ground wire is placed in the middle, the coupling effect will be destroyed.