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Fifteen Questions about PCB Design Skills
PCB News
Fifteen Questions about PCB Design Skills

Fifteen Questions about PCB Design Skills

2021-11-10
View:34
Author:Kavie

1. How to choose PCB board?

The choice of PCB board must strike a balance between meeting design requirements and mass production and cost
Design requirements include electrical and mechanical parts
Usually this material problem is more important when designing very high-speed PCB boards (frequency greater than GHz)
For example, the commonly used FR-4 material, the dielectric loss (dielectric loss) at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable.


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As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency

2. How to avoid high frequency interference?

The basic idea of avoiding high-frequency interference is to minimize the interference of high-frequency signal electromagnetic fields, which is the so-called crosstalk (Crosstalk)
Can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal
Also pay attention to the noise interference from the digital ground to the analog ground

3. How to solve the problem of signal integrity in high-speed design?

Signal integrity is basically a problem of impedance matching
The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load, the topology of the trace, etc.
The solution is to rely on the topology of termination and adjustment of the wiring

4. How is the differential wiring method realized?

There are two points to pay attention to in the layout of the differential pair. One is that the length of the two wires should be as long as possible, and the other is that the distance between the two wires (this distance is determined by the differential impedance) must be kept constant, that is, to keep parallel.
There are two parallel ways, one is that the two wires run on the same wiring layer (side-by-side), and the other is that the two wires run on two adjacent layers above and below (over-under).
Generally, the former has more side-by-side implementations

5. How to implement differential wiring for a clock signal line with only one output terminal?
To use differential wiring, it makes sense that the signal source and receiver are also differential signals.
Therefore, it is impossible to use differential wiring for a clock signal with only one output terminal.

6. Can a matching resistor be added between the differential line pairs at the receiving end?

The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance
This way the signal quality will be better

7. Why should the wiring of the differential pair be close and parallel?

The wiring method of the differential pair should be appropriately close and parallel
The so-called appropriate proximity is because this spacing will affect the value of differential impedance, which is an important parameter for designing differential pairs
The need for parallelism is also because of the consistency of the differential impedance
If the two lines are suddenly far and near, the differential impedance will be inconsistent, which will affect signal integrity and timing delay

8. How to deal with some theoretical conflicts in actual wiring
1. Basically, it is right to divide and isolate the analog/digital ground
It should be noted that the signal trace should not cross the divided place (moat) as much as possible, and the return current path of the power supply and signal should not be too large.
2. The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the loop gain and phase specifications. The oscillation specifications of this analog signal are easily disturbed. Even if ground guard traces are added, it may not be able to completely isolate the interference.
And too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit.
Therefore, the distance between the crystal oscillator and the chip must be as close as possible
3. It is true that there are many conflicts between high-speed wiring and EMI requirements
But the basic principle is that the resistance and capacitance or ferrite bead added by EMI cannot cause some electrical characteristics of the signal to fail to meet the specifications.
Therefore, it is best to use the skills of arranging traces and PCB stacking to solve or reduce EMI problems, such as high-speed signals through the inner layer
Finally, the resistance capacitor or ferrite bead method is used to reduce the damage to the signal

9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?

At present, most of the automatic routers of strong wiring software have set constraints to control the winding method and the number of vias.
The winding engine capabilities and constraint setting items of various EDA companies are sometimes far from each other.
For example, whether there are enough constraints to control the way of serpentine winding, whether it is possible to control the trace spacing of the differential pair, etc.
This will affect whether the routing method of the automatic routing can meet the designer's idea
In addition, the difficulty of manually adjusting the wiring is also absolutely related to the ability of the winding engine.
For example, the pushing ability of the trace, the pushing ability of the via, and even the pushing ability of the trace to the copper coating, etc.
Therefore, choosing a router with strong winding engine capability is the solution.
10. About test coupon

The test coupon is used to measure whether the characteristic impedance of the produced PCB board meets the design requirements with TDR (Time Domain Reflectometer)
Generally, the impedance to be controlled has two cases: a single line and a differential pair
Therefore, the line width and line spacing on the test coupon (when there is a differential pair) should be the same as the line to be controlled
The most important thing is the location of the grounding point during measurement
In order to reduce the inductance of the ground lead, the grounding place of the TDR probe is usually very close to the probe tip. Therefore, the distance and method between the signal measurement point and the ground point on the test coupon To match the probe used

11. In high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper coating of multiple signal layers be distributed on the ground and power supply?

Generally, most of the copper plating 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 applied copper will reduce the characteristic impedance of the trace a little
Also be careful not to affect the characteristic impedance of other layers, for example in the structure of the dual stripline

12. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line on the power plane?
Can the signal between the power supply and the ground plane be calculated using the stripline model?

Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be regarded as the reference plane
For example, a four-layer board: top layer-power layer-ground layer-bottom layer. At this time, the characteristic impedance model of the top layer is a microstrip line model with the power plane as the reference plane

13. Can test points be automatically generated by software on high-density printed boards under normal circumstances to meet the test requirements of mass production?

Generally, whether the test points automatically generated by the software meet the test requirements depends on whether the specifications for adding test points meet the requirements of the test equipment
In addition, if the wiring is too dense and the specifications for adding test points are strict, there may be no way to automatically add test points to each segment of the line. Of course, you need to manually fill in the places to be tested.


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

Whether it will affect the signal quality depends on the method of adding test points and how fast the signal is.
Basically, additional test points (do not use the existing perforation (via or DIP pin) as test points) may be added to the line or pulled out a short line from the line
The former is equivalent to adding a small capacitor on the line, the latter is an extra branch
Both of these conditions will affect the high-speed signal more or less, and the degree of the effect is related to the frequency speed of the signal and the edge rate of the signal.
The magnitude of the impact can be known through simulation
In principle, the smaller the test point, the better (of course, it must meet the requirements of the test tool) the shorter the branch, the better

15. Several PCBs form a system, how should the ground wires between the boards be connected?

When the signal or power supply between each PCB board is connected to each other, for example, if board A has a power supply or a signal is sent to board B, there must be an equal amount of current flowing from the ground back to board A (this is Kirchoff current law)
The current on this ground will find the place with the least impedance to flow back
Therefore, at each interface, whether it is power or signal interconnection, 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 part with a large current, and adjust the connection of the ground layer or ground wire to control the current flow (for example, make a low impedance somewhere so that most of the current flows from this Place to walk) to reduce the impact on other more sensitive signals