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 theoretical basis of controlled impedance
The classification of controlled impedance
The classification of controlled impedance
It is divided into categories according to the mode of transmission:
Most common type of controlled impedance in PCBs
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:
|Factors affecting impedance||Impedance Value Influence|
Copper conductor thickness
Solder mask thickness
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
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 addedWe 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 reflectorTime 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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