In recent years, with the improvement and application of IC integration, its signal transmission frequency and speed are higher and higher. Therefore, in the printed board conductor, when the signal transmission (transmission) reaches a certain value, it will be affected by the printed board conductor itself, resulting in serious or complete loss of transmission signal. This shows that the “thing” flowing through PCB wires is not current, but the transmission of square wave signals or pulses in energy. The resistance to the above “signal” transmission is also called “impedance”, and the representative symbol is Z0. Therefore, it is not enough to solve the problems of “on”, “off” and “short circuit” on PCB wires, but also to control the impedance of wires.

(a) what is impedance?

Impedance is a parameter used to evaluate the characteristics of electronic components. Impedance is defined as the total resistance of an element to alternating current at a given frequency.

(b) why impedance control?

Because the characteristic impedance value in PCB transmission line must match the electronic impedance of driver and receiver, otherwise it will cause reflection and attenuation of signal energy and delay of signal arrival time. In serious cases, it is impossible to judge independence and start-up. When the signal propagates in the circuit board, the factors affecting its “characteristic impedance” include the cross-sectional area of the circuit, the thickness of the insulating material between the circuit and the grounding layer, and its dielectric constant. The most influential parts of impedance are: 1. Line width, 2. PP thickness, 3. Dielectric value (FR-4 = 4.3), followed by anti welding thickness, side corrosion, copper thickness, etc. These will change the distribution of magnetic lines of force, and then change the variables of assembly reactance. First understand the requirements of assembly reactance tolerance value, and then deduce the maximum tolerance value of the process, and calculate whether it can be achieved by software. The key point of PCB process control is that the tolerance of material / wire diameter is within 10%, and the accuracy of interlayer thickness after pressing is within 10%, which can meet the design requirements.

(c) what is the ER value?

Generally, dielectric constant or relative permittivity is the electrostatic energy that can be stored per unit volume of insulating material under each unit potential gradient. If the dielectric constant is high, many signals are stored in the plate, resulting in poor signal and slow propagation rate. Generally, those with high signal quality requirements will be limited to PTF ((Teflon)) because its ER = 2.. 5.

（4） General impedance is divided into three categories:

1. Characteristic impedance.

If the customer controls the impedance of the outer linewidth for a 4-layer board, the software mode for calculating the outer linewidth impedance is as follows.

The design of the impedance strip is as follows:

For 6-layer board, if 1, 3, 4 and 6 layers are wiring layers, the impedance linewidth shall be controlled. Note that the impedance linewidth of inner layers L3 & L4 shall be opened and shall not be overlapped to avoid affecting the test of impedance value. The software mode for calculating the impedance of inner layer linewidth is as follows.

The design of the impedance strip is as follows:

If the customer has 6 layers of boards, and 1, 3 and 6 layers are wiring layers, the impedance linewidth must be controlled. The design of the impedance strip is as follows:

If the customer controls the line width of 4-layer plate and inner layer (2 and 3 layers), note that copper foil is laid on the upper and lower corresponding layers of the impedance line width, and the impedance line width of L2 & L3 shall be designed without overlap to avoid affecting the test of impedance value. The design of the impedance strip is as follows:

When making the inner layer impedance, the customer shall pay attention to the required group impedance linewidth, whether the upper and lower corresponding layers are covered with copper foil or not, and the upper and lower layers are covered with copper foil. The calculation software is the same as above. If one layer is not covered with copper foil and the other layer is covered with copper foil, the impedance calculation software mode is as follows.

The impedance strip is designed as follows:

For example, if the customer controls the impedance of the outer and inner linewidths of 4-layer plates, it should be noted that the impedance linewidth of the inner layer should be designed without overlapping to avoid affecting the test of impedance value. The design of the impedance strip is as follows:

2. Differential impedance.

The customer controls the impedance of the line width / spacing / line width of the outer layer of the 4-layer board. The impedance software mode for calculating the impedance line width / spacing / line width is as follows.

The design of the impedance strip is as follows:

If the customer has 6 layers of boards, and layers 1, 3, 4 and 6 are wiring layers, the impedance linewidth must be controlled. The software mode for calculating the inner linewidth impedance is as follows.

The design of the impedance strip is as follows:

3. Ranbus impedance.

The customer controls the impedance linewidth for a certain linewidth / spacing of the 4-layer outer layer, and the software mode for calculating the outer layer ranbus impedance is as follows.

The design of the impedance strip is as follows:

The customer controls the impedance linewidth for a certain linewidth / spacing of 6 layers (1,3,4,6). It should be noted that the inner impedance linewidth should be opened and cannot be overlapped to avoid affecting the test of impedance value. The calculation mode of inner ranbus impedance calculation software is as follows.

The design of the impedance strip is as follows:

（5） Coupon design considerations:

1. The design of general impedance includes design layer (circuit layer) and reference grounding layer (corresponding layer). If the customer has no specification limit, the specifications of 4-layer board are L1 (design layer) – L2 [L2 grounding layer (corresponding layer)], L3 [L3 grounding layer (corresponding layer) – L4 (design layer).

2. There is impedance control in the inner layer. It should be noted that if there is linewidth control for two consecutive layers, there is no barrier of other layers [grounding layer copper foil (corresponding layer)], the linewidth should be designed without overlapping, so as to avoid affecting the test of impedance value.

3. Generally, the impedance line width of the outer layer shall be protected by copper strip. The wider the copper strip width is, the better, and the spacing shall be more than 10 mil min.

4. The inner wiring layer has impedance control. Pay attention to whether the upper and lower grounding layers (corresponding layers) are covered with copper foil. The design of impedance calculation software and impedance strip are different.