How to calculate PCB impedance

  • In the design of a PCB multilayer board, we often encounter signals such as USB, HDMI, LVDS, DDR, and various antennas to be controlled by PCB impedance so as to ensure the stability of the machine and the test pass rate of various indicators.

    table of Contents:

    1. Introduction to Impedance Control Concept

    2. The significance of controlling PCB characteristic impedance

    3. Factors affecting the characteristic impedance

    4. Calculation of impedance

    1. Introduction to the impedance control concept

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

    Characteristic impedance is also called “characteristic impedance.” It means that at a certain frequency, in the transmission signal line (that is, the copper wire of the PCB multilayer board we made), relative to a certain reference layer (that is, the shielding layer, the shadow layer, or the reference layer), the high frequency The resistance of a signal or electromagnetic wave in the propagation process is called the characteristic impedance, which is actually a vector sum of electrical impedance, inductive reactance, and capacitive reactance.


    2. The significance of controlling the characteristics of PCB impedance

    PCB not only plays the role of current conduction in electronic products but also plays the role of signal transmission;

    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 the transmission process and keep the signal intact and undistorted; when the transmission line ≥ 1/3 of the rise time length, the signal will reflect. Consider the characteristic impedance.

    Characteristic impedance is the core of solving signal integrity problems;

    When electronic equipment (such as computers, communication switches, etc.) is operating, the signal sent by the driver must reach the receiver through the PCB signal line. In order to ensure signal integrity, the characteristic impedance (Z0) of the signal line of the PCB must be matched with the “electronic impedance” of the head and tail components;


    3. Factors affecting the characteristic impedance

    3.1 The dielectric constant of the medium is inversely proportional to the characteristic impedance (Er);

    3.2 The width of the bottom of the PCB impedance line (lower end W1) and the width of the line surface (upper-end W2) are inversely proportional to the characteristic PCB impedance;

    3.3 Copper thickness is inversely proportional to the characteristic impedance value (T);

    3.4 The thickness of the base material solder mask is inversely proportional to the impedance value (C);

    3.5 The thickness of the medium between the circuit layer and the ground layer (or outer layer) is proportional to the characteristic impedance (H);

    3.6 The distance between adjacent lines and lines is proportional to the characteristic impedance (differential impedance) (S).

    3.7 Process factors affecting the impedance

    Due to etching reasons, when the copper thickness is greater than 2oz, the impedance is greatly affected, and the impedance cannot be controlled generally. In the design, the layer blanks without copper and wires need to be filled with solidified film during production. When calculating the impedance, the thickness of the medium provided by the plate supplier cannot be directly substituted, but the thickness of the solidified film filling these blank spaces needs to be subtracted. This is one of the main reasons for the inconsistency between the impedance calculated by myself and the result of the manufacturer.


    4. Calculation of impedance

    The impedance calculation is relatively cumbersome, but we can summarize some experience values to help improve the calculation efficiency. For the commonly used FR4, 50ohm microstrip line, the line width is generally equal to 2 times the thickness of the dielectric; for the 50ohm stripline, the line width is equal to one-half of the total thickness of the dielectric between the two planes, which can help us quickly lock the line width Range, note that the calculated line width is generally smaller than this value.

    In addition to improving calculation efficiency, we also need to improve calculation accuracy. Do you often encounter inconsistencies between the PCB impedance calculated by yourself and that calculated by the board manufacturer? Some people will say what this has to do, let the PCB multilayer board factory adjust it directly. But is there a situation where the board factory can’t adjust it and let you relax impedance control? To make a good product, it is better to do everything in your own hands.

    So how do we control the impedance when designing?

    4.1 Use empirical values to record the impedance lines that have been done before, for example, the line width and line spacing and the thickness of the board and apply it directly next time.

    4.2. First, according to the conventional design, highlight the line that needs to be PCB impedance, and then take a screenshot to the PCB board factory. If the board factory controls it, the board factory will modify the data according to our required impedance, such as adjusting the line width and line spacing to achieve The PCB impedance we require.

    4.3 At the beginning of the design: We need the PCB multilayer board manufacturer to provide relevant information (sheet, dielectric constant, green oil, PP thickness, etc.) according to the stacking parameters, and we combine the Si9000 software to calculate the impedance and then use the calculation The parameters of the board are taken through the impedance line, and finally the board washing data is sent to the PCB factory, and the board factory is required to control the impedance. The advantage of this is that the board factory will not change our data in general, and the adjustment is also very small.

    It can be seen from the above that if the PCB stacking parameters change at the first point, the PCB impedance will also change, and it is easy to make mistakes if it continues to be applied. Second, it is often encountered that the line width and line spacing of the board manufacturer’s feedback design is too different and the PCB multilayer board.

    There is not enough space to widen the line width and line spacing, and impedance cannot be done. Obviously, the third solution is the safest, and there will be no PCB multilayer board.

    The phenomenon that the factory cannot control the impedance occurs.