PCB problems and answers

1. What is the principle of properly selecting the point at which the PCB copy board and the outer shell are grounded?

The principle of selecting the PCB grounding point and the grounding point of the casing is to use the chassis ground to provide a low impedance path to the returning current and the path to control the return current. For example, the ground plane of the PCB copy board can be connected to the chassis ground by a fixing screw near the high-frequency device or the clock generator to minimize the entire current loop area and reduce electromagnetic radiation.

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?

3. Automatically generating test points through high-density PCB 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.

4. 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.

5. How many PCB copy boards form the system, and how should the ground wire between each copy board 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 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.

6. 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 board has reached GHz, and the number of layers is as much as 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 routing requirement, the need for blind/buried vias, mircrovias, and build-up craaft processes is also increasing. These design requirements are available to manufacturers in large quantities.

7. Two characteristic impedance equations that are often referred to:

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

Stripline Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} where H is the distance between the two reference planes and the trace is in the middle of the two reference planes . This formula must be applied with W/H<0.35 and T/H<0.25.

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

In the middle of the differential signal, the ground line cannot be added. Because the most important point of application of differential signals is the benefits of mutual coupling between differential signals, such as flux cancellation, noise immunity, and the like. If the ground wire is added in the middle, the coupling effect will be destroyed.

9. Does the rigid flex board pcb design require special design software and specifications? Where can I undertake this type of circuit board processing in China?

A flexible printed circuit can be designed using software that generally designs PCBs. The same is done in the Gerber format for FPC manufacturers. Because the manufacturing process is different from the general PCB, each manufacturer will have restrictions on the minimum line width, minimum line spacing, and minimum aperture depending on their manufacturing capabilities. In addition, some copper can be reinforced at the turning point of the flexible circuit board. As for the manufacturer of the product, the online "FPC" can be found as a keyword query.

10, the circuit board DEBUG should start from those aspects?

In the case of digital circuits, first determine three things in order:

A.Verify that all power supply values are within the design requirements. Some systems with multiple power supplies may require some specification of the order and speed of some power supplies.

B.Verify that all clock signal frequencies are working properly and that there are no non-monotonic problems on the edges of the signal.

C.Confirm that the reset signal meets the specifications. If these are normal, the chip should signal the first cycle. Then follow the system operation principle and bus protocol to debug.