PCB Impedance Control

2018/12/29 15:39:05

As PCB signal switching speeds continue, today's PCB designers need to understand and control the impedance of PCB traces. Corresponding to the shorter signal transmission times and higher clock rates of modern digital circuits, PCB traces are no longer simple connections, but transmission lines.


In the actual case, it is necessary to control the trace impedance when the digital marginal speed is higher than 1 ns or the analog frequency exceeds 300 Mhz. One of the key parameters of a PCB trace is its characteristic impedance (ie, the ratio of voltage to current as the wave travels along the signal transmission line).


The characteristic impedance of the conductor on the printed circuit board is an important indicator of the board design. Especially in the PCB design of the high-frequency circuit, it must be considered whether the characteristic impedance of the conductor and the characteristic impedance required by the device or signal are consistent or not.


This involves two concepts: impedance control and impedance matching. This article focuses on the issues of impedance control and stack design.


Impedance control, the transmission of various signals in the conductors of the circuit board, in order to increase its transmission rate must increase its frequency, the line itself due to etching, laminate thickness, wire width and other factors, will Causes the impedance to vary and the signal is distorted. Therefore, the conductor on the high-speed circuit board, the impedance value should be controlled within a certain range, called "impedance control."


The impedance of the PCB trace will be determined by its inductive and capacitive inductance, resistance and conductance. The factors that affect the impedance of the PCB trace are: the width of the copper wire, the thickness of the copper wire, the dielectric constant of the dielectric, the thickness of the dielectric, the thickness of the pad, the path of the ground wire, and the traces around the trace. The PCB impedance ranges from 25 to 120 ohms.



In practical situations, a PCB transmission line usually consists of a wire trace, one or more reference layers, and an insulating material. Traces and slabs form the control impedance. PCBs will often be multi-layered and the control impedance can be built in a variety of ways.


There are two main forms of PCB transmission lines: Microstrip and Stripline.

The microstrip line is a strip conductor, which refers to a transmission line with a reference plane on one side. The top and sides are exposed to the air (also coated with a coating) on the surface of the insulation constant Er board. The power or ground plane is a reference.


The stripline is a strip conductor placed between two reference planes, as shown in the following figure, the dielectric constants of the dielectrics represented by H1 and H2 may be different.


The above two examples are only a typical example of microstrip lines and strip lines. There are many kinds of microstrip lines and strip lines, such as laminated microstrip lines, which are related to the laminated structure of a specific PCB.


The equations used to calculate the characteristic impedance require complex mathematical calculations, usually using field solving methods, including boundary element analysis, so using the special impedance calculation software SI9000, all we need to do is to control the parameters of the characteristic impedance.


The dielectric constant Er of the insulating layer, the trace widths W1, W2 (trapezoidal), the trace thickness T, and the thickness H of the insulating layer.

The characteristic impedance is inversely proportional to the width of the transmission line. The wider the width, the lower the impedance, and vice versa.


In the design requirements of some boards, when the thickness of the layer is limited, in order to achieve better impedance control, it is very important to use a good laminate design. From the actual calculations, the following conclusions can be drawn:


1. Each signal layer must have a reference plane adjacent to ensure its impedance and signal quality.


2. Each power plane must have a complete ground plane adjacent, so that the performance of the power supply can be better guaranteed.


Through software calculations, it is found that changing the spacing of differential pairs has a greater impact on impedance control, but another problem involved here is the coupling problem of differential pairs.


The main purpose of differential pair coupling is to enhance the anti-interference ability to the outside world and suppress EMI. The coupling is divided into a tight coupling method (ie, the differential line spacing is less than or equal to the line width) and a loose coupling method.




If you can ensure that all the surrounding traces are far away from the differential pair (for example, more than 3 times the line width), the differential traces do not need to ensure tight coupling. The most important thing is to ensure that the trace lengths are equal. (See the explanation of the differential trace on Johnson's Signal Integrity website, which asks his layout engineer to move the differential line farther away so that it can be wound around).


However, most of the multi-layer high-speed PCB boards have very tight wiring space, and it is impossible to isolate the differential traces from other traces. Therefore, it is appropriate to maintain tight coupling to increase the anti-interference ability.


Tight coupling is not a necessary condition for differential traces, but when the space is not enough, the tight coupling of the traces can enhance the anti-interference ability of the differential traces. Therefore, for the impedance control problem of the differential pair, how to adjust each parameter needs to comprehensively consider the above factors and select the best. In general, the pitch and line width of the differential pair are not easily adjusted.


The underside of the component surface (the second layer) is the ground plane, providing the device shielding layer and providing a reference plane for the top layer wiring.

All signal layers are as close as possible to the ground plane,

Try to avoid direct proximity of the two signal layers,

The main power source is adjacent to it as far as possible,

Consider the symmetry of the laminated structure.


For the layer layout of the mother board, the existing motherboard is difficult to control parallel long distance wiring, and the board operating frequency is above 50 MHz.


For cases below 50MHZ, refer to the appropriate relaxation, the recommended arrangement principle,


1, The component surface and the welding surface are complete ground planes.

2, No adjacent parallel wiring layers.

3, All signal layers are as close as possible to the ground plane.

4, The key signals are adjacent to the formation and do not span the partition.

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Antonio Semeraro

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