How to consider impedance control and stack design during PCB design process?
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With the continuous growth of PCB signal switching speed, today’s PCB design manufacturers need to understand and control the impedance of PCB traces. Correspondingly to the shorter signal transmission time and higher clock speed of modern digital circuits, PCB traces are no longer simple connections, but transmission lines.
In practical situations, it is necessary to control the trace impedance when the digital marginal velocity exceeds 1ns or the analog frequency exceeds 300Mhz. One of the key parameters of PCB traces is their characteristic impedance (i.e. the ratio of voltage to current when waves are transmitted along the signal transmission line). The characteristic impedance of wires on printed circuit boards is an important indicator in circuit board design, especially in high-frequency circuit PCB design. It is necessary to consider whether the characteristic impedance of wires is consistent with the required characteristic impedance of devices or signals, and whether it matches. This involves two concepts: impedance control and impedance matching. This article focuses on the issues of impedance control and stack design.
Impedance control
Impedance Control (eImpedance Control) involves the transmission of various signals in the conductors of a circuit board. In order to improve its transmission rate, it is necessary to increase its frequency. If the circuit itself is affected by factors such as etching, stack thickness, wire width, etc., it will cause changes in impedance values and cause signal distortion. Therefore, the impedance value of conductors on high-speed circuit boards should be controlled within a certain range, which is called “impedance control”.
The impedance of the PCB trace will be determined by its inductive and capacitive inductance, resistance, and conductivity coefficient. The factors that affect the impedance of PCB wiring mainly include the width and thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the solder pad, the path of the ground wire, and the wiring around the wiring. The range of PCB impedance is 25 to 120 ohms.
In practice, PCB transmission lines typically consist of a wire trace, one or more reference layers, and insulation materials. The trace and board layer form the control impedance. PCBs will often adopt multi-layer structures, and control impedance can also be constructed in various ways. However, regardless of the method used, the impedance value will be determined by its physical structure and electronic characteristics of the insulation material:
Width and thickness of signal traces
The height of the core or pre filled material on both sides of the trace
Configuration of trace and board layers
Insulation constant of core and pre filled materials
There are two main types of PCB transmission lines: Microstrip and Stripline.
Microstrip is a strip conductor, which refers to a transmission line with only one Plane of reference. The top and side are exposed to air (coating layer can also be applied), which is located on the surface of the circuit board with the insulation constant Er, with the power supply or ground plane as the reference.
A stripline is a stripline placed between two Plane of reference, and the dielectric constant of the dielectric can be different. Specific microstrip