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Talking about the basic knowledge of impedance PCB circuit board

First, the introduction of PCB circuit board impedance control concept

    1. To distinguish the resistance of direct current (DC), the resistance encountered by alternating current is called impedance (Z0), including resistance (R), inductive reactance (XC) and capacitive reactance (XL).

    2. Characteristic impedance is also called "characteristic impedance". Refers to a high frequency signal at a certain frequency, in the transmission signal line (that is, the copper wire of the circuit board we made), relative to a certain reference layer (also known as the shielding layer, the mapping layer or the reference layer). Or the resistance of electromagnetic waves during propagation is called characteristic impedance, which is actually a vector sum of electrical impedance, inductance, and capacitive reactance.

Second, the process factors affecting the impedance

    1. Due to etching, the impedance is greatly affected when the copper thickness is > 2 oz, and the impedance is generally not controlled.

    2. The layer blank without copper and wire in the design needs to be filled with the solidified sheet during production. When calculating the impedance, the thickness of the medium provided by the sheet supplier cannot be directly substituted, and the thickness of the blank place to be filled by the solidified sheet needs to be subtracted. This is one of the main reasons for the inconsistency between the calculated impedance and the pcb manufacturer's results.

Third, factors affecting characteristic impedance

    1. Dielectric constant of the medium, inversely proportional to the value of the characteristic impedance (Er)

    2. The dielectric thickness between the circuit layer and the ground plane (or outer layer) of the PCB circuit board is proportional to the characteristic impedance value (H). The following figure shows the conventional sheet parameters:

    3. Impedance line bottom width (lower end W1); line surface (upper end W2) width, inversely proportional to the characteristic impedance.

    4. Copper thickness, inversely proportional to the characteristic impedance value (T)

    5. The spacing between adjacent lines and lines is proportional to the characteristic impedance value (differential impedance) (S)

    6. The thickness of the base solder mask is inversely proportional to the impedance value (C)

Fourth, the significance of controlling the characteristic impedance of PCB circuit board

    1. Characteristic impedance is the core of solving signal integrity problems;

    2. PCB line safety not only acts as a current conduction in electronic products, but also acts as a signal transmission;

    3. When the transmission line is ≥ 1/3 of the rise time length, the signal will be reflected, and the characteristic impedance must be considered.

    4. The high frequency and high speed of electronic products require that the circuit performance provided by the PCB must ensure that the signal does not reflect during transmission, keeping the signal intact and not distorted;

    5. When an electronic device (such as a computer, communication switch, etc.) is operated, the signal sent by the driver (Device) needs to reach the receiving component (Receiver) through the signal line of the PCB circuit board. To ensure signal integrity, the characteristic impedance (Z0) of the signal line of the PCB board must be matched to the "electronic impedance" of the head and tail components;

Fifth, the calculation of PCB circuit impedance

    1. The calculation of impedance is relatively cumbersome, but we can summarize some empirical values to help improve computational efficiency. For the commonly used FR4, 50 ohm microstrip line, the line width is generally equal to twice the thickness of the medium; 50 ohm strip line, the line width is equal to one-half of the total thickness of the medium between the two planes, which can help us quickly lock the line width Range, note that the generally calculated line width is smaller than this value.

    2. In addition to improving the computational efficiency, we also need to improve the calculation accuracy. Do you often encounter inconsistencies between your own impedance and the board factory? Some people will say that this has anything to do with the board factory. But will there be a circuit board factory that can't adjust, let you relax the situation of impedance control? It is better to do a good job of the product or to do everything in your own hands.


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