PCB News - Ten why PCB design

1. When making PCB board, should ground wire be formed in closed and sum form in order to reduce interference?

When making PCB board, it is generally necessary to reduce the loop area in order to reduce interference. When laying ground wire, it is better not to cloth into closed form, but into branches, and to increase the area as much as possible.

2. If the emulator uses one power supply and the PCB board uses one power supply, should the ground of the two power supplies be connected together?

If you can use separate power supply, of course, because it is not easy to interfere with the power supply, but most of the equipment is specific requirements. Since the emulator and PCB use two power supplies, in my opinion, they should not be co-located.

3. "protection of the mechanism" is not the protection of the casing?

Yes. The chassis should be as tight as possible, with little or no conductive materials, and grounded as far as possible.

4. Do we also need to consider the ESD problem of the chip itself when choosing the chip?

Whether double - layer board or multi - layer board, should maximize the area of the ground. When choosing a chip, we should consider the ESD characteristics of the chip itself, which are generally mentioned in the instructions of the chip, and even the performance of the same chip from different manufacturers will be different. Pay more attention to the design, consider the overall point, make the performance of the circuit board will be certain to ensure. However, THE problem of ESD may still occur, so the protection of the mechanism is also very important to the protection of ESD.

5. A circuit is composed of several PCB boards, should they be common ground?

A circuit consists of several PCBS, most likely requiring common ground, because it is not practical to use several power supplies in a circuit. But if you have specific conditions, you can use different power sources with less interference.

6. How to avoid crosstalk in PCB design?

Changes of the signals (such as step) propagation along the transmission line from A to B, c-d will generate A transmission line coupling signal, once the end is the change of signal back to stable dc at ordinary times, there is no coupling signal, so the crosstalk occurs only in the process of signal jump, the faster and the change of signal along the (conversion), the greater the crosstalk. The coupled electromagnetic field in space can be extracted as a set of countless coupling capacitors and coupling inductors. The crosstalk signals generated by coupling capacitors can be divided into forward crosstalk and reverse crosstalk Sc in the victim network, and the two signals have the same polarity. The crosstalk signal produced by the coupled inductor is also divided into forward crosstalk and reverse crosstalk SL, which are of opposite polarity. The forward crosstalk and reverse crosstalk produced by the coupled inductor and capacitor exist at the same time and are almost equal in size. Thus, the forward crosstalk signals on the victim network cancel each other due to opposite polarity, while the reverse crosstalk signals have the same polarity and are enhanced superposition.

The modes of crosstalk analysis usually include default mode, three-state mode and worst-case mode analysis. The default mode is similar to the way we actually test crosstalk, that is, the offending network driver is driven by a flip signal, the victim network driver is left in its initial state (high or low), and then the crosstalk value is calculated. This method is effective for crosstalk analysis of unidirectional signals. Three-state mode means that the driver of the infringing network is driven by the reversal signal, and the three-state terminal of the affected network is set to the high-resistance state to detect crosstalk. This method is effective for bidirectional or complex topological networks. Worst-case analysis means that the driver of the victim network is left in its initial state and the emulator calculates the sum of crosstalk from all the default offending networks to each victim network. Generally, this method only analyzes individual critical networks, because there are too many combinations to be calculated and the simulation speed is slow.

7. How to check whether the PCB meets the design process requirements when leaving the factory?

Many PCB manufacturers have to go through the network connection test before PCB processing is finished and leaves the factory to ensure that all connections are correct. At the same time, more and more manufacturers are also using X-ray testing, to check for etching or laminating some faults. For finished boards after SMT processing, ICT test inspection is generally adopted, which requires the addition of ICT test points in PCB design. If there is a problem, it can also be ruled out by a special X-ray inspection equipment whether the processing cause of the fault.

8. Design a handheld product with LCD and metal shell. When testing ESD, it cannot pass ice-1000-4-2, CONTACT can only pass 1100V and AIR can only pass 6000V. During the ESD coupling test, the horizontal can only pass 3000V and the vertical can only pass 4000V. The CPU frequency is 33MHZ. Is there any way to pass the ESD test?

Handheld products are metal shell, ESD problems must be more obvious, LCD is also afraid there will be more adverse phenomenon. If there is no way to change the existing metal material, it is suggested to add anti-electric material inside the mechanism, strengthen the PCB ground, and try to make the LCD ground. Of course, how to operate depends on the specific situation.

9. Design a system containing DSP, PLD, from which aspects to consider ESD?

For general systems, the main consideration should be given to the parts that are in direct contact with the human body, and appropriate protection should be carried out on the circuits and mechanisms. How much IMPACT ESD can have on the system depends on the situation. Under dry environment, ESD phenomenon will be more serious, more sensitive and fine system, ESD impact will be relatively obvious. Although the impact of ESD is not obvious in large systems, more attention should be paid to the design to prevent it.

10. In a 12-layer PCB board, there are three power supply layer 2.2V, 3.3V, 5V, the three power supply in one layer, how to deal with the ground wire?

Generally speaking, three power supplies are done in three layers respectively, which is better for signal quality. Because it is unlikely that there will be signal segmentation across plane layers. Cross segmentation is a key factor affecting signal quality, which is often ignored by simulation software. For both the power layer and the formation, it is equivalent for the high frequency signal. In practice, in addition to considering the signal quality, the power plane coupling (using adjacent ground planes to reduce the AC impedance of the power plane), overlapping symmetry, are all factors that need to be considered.