What is controlled impedance in PCBs

Impedance (Z) is generally defined as the total resistance (impedance, capacitive reactance, inductive reactance) encountered by a device or circuit when providing current at a specific frequency. Impedance is the most important parameter in displaying the characteristics of electronic circuits, components and component materials. 

In the transmission signal line of electronic equipment, the resistance encountered when the high-frequency signal propagates in the transmission line is called characteristic impedance(Z0).

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    Why controlled impedance is needed in PCBs

    With the increasing requirements of electronic products in signal transmission, many printed circuit boards have the requirements of characteristic impedance. Especially in the field of communication. The following are the main factors.

    • The working frequency of electronic equipment is constantly increasing
    • High fidelity signal transmission
    • Development of integrated circuit IC and SMT technology
    • Embedded components on PCB
    • The electronic circuit needs to have the ability of anti-interference, anti-crosstalk and high stability.

    The theoretical basis of controlled impedance

    From the theory of transmission line, the resistance, inductance, leakage conductance (reciprocal of leakage resistance) and distributed capacitance of the wire relative to the reference layer (reference plane) can not be ignored. They have an impact on the speed of signal transmission in the wire and the energy of transmission; The signal level transmitted on the wire is not only a function of time t, but also a function of distance “x. The current at each point on the wire with transmission characteristics is different.
    As a wire on a PCB, the line width of the same wire is usually uniform, and the medium and thickness between the wire and the relative reference layer are basically the same. Therefore, the PCB wire should be considered as a uniform transmission line. It can be analyzed by using the following figure (a), The whole section of conductor can be regarded as n transmission lines of unit length, and there is corresponding equivalent circuit, as shown in Figure (b).

    The classification of controlled impedance

    There are many kinds of impedance and different classification methods. Normally impedance is divided by number of trace and transmission methods: 

    The classification of controlled impedance

    • Single-ended Impedance: it is composed of one line plus ground layer.
    • Differential impedance: it is composed of two lines plus ground layer.

    It is divided into categories according to the mode of transmission:

    Microstrip line is an impedance line with one side parallel to the ground layer, the middle separated by dielectric, and the other side with solder mask or nothing.
    Stripline is a kind of impedance line which is separated from the upper and lower sides of the line by the ground layer. This kind of impedance line is called strip impedance line. Compared with microstrip line, the signal is not easily disturbed.

    Most common type of controlled impedance in PCBs

    There are many types of controlled impedance. There are more than 90 types of impedance in the most commonly used impedance calculation software: Polar-Si8000. But the most commonly used are the following 4 types with 2 variants:
    • Surface Microstrip
    • Offset Stripline
    • Edge-Coupled Surface Mirostrip
    • Edge-Coupled Offset Stripline

    Main factors affecting impedance

    Impedance value are affected by PCB material thickness, material dielectric constant (Er) or relative permittivity (Dk), conductor width/space, conductor thickness copper thickness, and also signal transmission frequency which should be considered by PCB designer. From a PCB manufacturer, we think there are 5 factors which will affect the impedance value, they are:

    • Dielectric thickness
    • Conductor trace width/space
    • Dielectric Er
    • Conductor thickness
    • Solder mask thickness
    • Impedance value (Z0) to dielectric thickness(H): the thicker the dielectric, the bigger the Z0
    • Impedance value (Z0) to conductor width(W): the bigger the trace width, the smaller the Z0
    • Impedance value (Z0) to dielectric Er(Er): the bigger the Er, the smaller the Z0
    • Impedance value (Z0) to conductor thickness(T): the thicker the conductor thickness, the smaller the Z0
    • Impedance value (Z0) to solder mask thickness(T): the thicker the solder mask thickness, the smaller the Z0
    Factors affecting impedanceImpedance Value Influence
    Dielectric thickness
    48%
    Conductor width/space
    24%
    Dielectric Er
    16%
    Copper conductor thickness
    8%
    Solder mask thickness
    4%

    Different material have different content and thickness after pressing, and the thickness after pressing is related to the flatness of the press and the procedure of the pressing laminate

    For any kind of material used, the thickness of the dielectric that can be produced shall be obtained to facilitate design and calculation

    The thickness of the dielectric layer is the most important factor affecting the impedance value

     With dielectric thickness increase, the impedance value increases, and its thickness deviation should be controlled within 10% at least

    The dielectric constant of different PCB material is different

    The dielectric constant of FR4 is 3.8-4.7

    The dielectric constant of PTFE is 2.9-3.9

    The impedance value increases with the decrease of dielectric constant

     High impedance value is required to obtain high signal transmission, thus low dielectric constant material is needed.

    The thicker the copper, the smaller its impedance

    To obtain large impedance value, thin copper foil should be used

    The control of copper thickness is required to be uniform. So copper plane is added to the thin wire to balance the current (for the electrical process in the figure) to prevent the uneven copper thickness on the wire from affecting the impedance

     For the uneven distribution of copper on the top layer and bottom layer, it is necessary to carry out cross and reverse PCB panel to achieve the goal of same copper thickness on both layers

    Print solder mask on the PCB will reduce the impedance value

     Under normal circumstances, printing one time of solder mask can reduce the single end by 2 Ohms, and can reduce the differential by 8 Ohms. The reduction value of two times of printing is twice that of one time. When printing more than three times, the impedance value will not change.

    Impedance matching from PCB manufacturer

    Before production, engineers in PCB manufacturing company will review the Gerber files and advise suitable material and stack-up to meet customers’ impedance requirements. Please refer to the the following techniques in controlled impedance matching from our engineers.

    Increasing the thickness of the dielectric can increase the impedance value, and reducing the thickness of the dielectric can reduce the impedance value. Engineering design, laminate pressing control, and incoming material tolerance are the key to the control of the thickness of the Dielectric.

    Increasing the line width can reduce the impedance, and decreasing the line width can increase the impedance. The line width is mainly controlled by etching and design.

    Reducing the conductor thickness can increase the impedance value, and increasing the line thickness can reduce the impedance.The wire thickness can be controlled by graphic electroplating or selecting the corresponding thickness of the base material copper foil.

    Generally speaking, printing solder mask on PCB will reduce the impedance value: single end by 2 Ohms and the difference by 8 Ohms. 2 times of solder mask will reduce the impedance 2 times. 3 or more times of solder mask printing should be be helpful to reduce impedance value.

    How to test impedance value

    When PCBs are produced with controlled impedance, we need to make sure the impedance value are the same as customer’s requirement. So we will test the impedance value before shipping out.

    Impedance test principle

    The impedance test is to receive the reflected wave after the oscilloscope sends out a pulse wave, and then compare and analyze the two pulse waves to obtain the impedance value from the size of the transmitted energy.

    Impedance coupon should be added

    We will add impedance coupon on each panel of the PCB, with the same impedance line/trace as the PCB. It is made under the same process conditions as the production of printed circuit board. So, if the impedance value on the test coupon meet the requirement, then we can make sure the impedance on the printed circuit boards meet the requirement.

    TDR time domain reflector

    Time domain reflector is a special oscilloscope that can generate pulse wave internally, and has the function of receiving and analyzing this pulse. We will use TDR time domain reflector to these the impedance value.
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