PCB Technical - How does PCB control impedance?

How does PCB control impedance?
With the continuous increase of PCB signal switching speed, today's PCB planning manufacturers need to understand and control the impedance of PCB traces. Corresponding to the shorter signal transmission time and higher clock rate of modern digital circuits, PCB traces are no longer simple connections, but transmission lines.

In practice, it is necessary to manipulate the trace impedance when the digital edge speed is higher than 1ns or the analog frequency exceeds 300Mhz. One of the key parameters of a PCB trace is its characteristic impedance (that is, the ratio of voltage to current when the wave is transmitted along the signal transmission line). The characteristic impedance of the conductor on the printed circuit board is an important indicator of circuit board planning, especially in the PCB planning of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the conductor and the characteristic impedance required by the equipment or signal are common and whether they match . This involves two concepts: impedance manipulation and impedance matching. The main points of this article are to comment on the issues of impedance manipulation and stack planning.

Impedance control

Impedance control (eImpedance Controling), the conductors in the circuit board will transmit various signals. In order to increase its transmission rate, it is necessary to increase its frequency. Changes in impedance will result in distortion of the signal. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called "impedance control".

The impedance of the PCB trace will be confirmed by its inductive and capacitive inductance, resistance and conductivity. The main factors that affect the impedance of PCB traces are: the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path of the ground wire, and the wiring around the wire. The range of PCB impedance is 25 to 120 ohms.

In practice, the PCB transmission line is generally composed of a wire trace, one or more reference layers and insulating materials. The trace and the board layer constitute the control impedance. The PCB will often choose a multilayer structure, and the control impedance can also be constructed in various ways. However, no matter what method is used, the impedance value will be determined by its physical structure and the electrical characteristics of the insulating material:

The width and thickness of the signal trace

The height of the core or pre-filled material on both sides of the trace

Trace and layer configuration

Insulation constant of core and pre-filled material

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

Microstrip:

A microstrip line is a ribbon wire, which refers to a transmission line with a reference plane on only one side. The top and sides are exposed to the air (coating layer can also be applied), and it is located on the surface of the insulation constant Er circuit board. Power or ground plane for reference. As shown below:

Note: In practical PCB manufacturing, the board factory usually coats the surface of the PCB with a layer of green oil. Therefore, in the practical impedance calculation, the model shown in the figure below is generally used for the calculation of the surface microstrip line:

Stripline:

Strip line is a strip wire placed between two reference planes. As shown in the figure below, the dielectric constants of the dielectrics represented by H1 and H2 can be different.

The above two examples are just a typical demonstration of microstrip lines and strip lines. There are many types of specific microstrip lines and strip lines, such as coated microstrip lines, which are related to the specific PCB laminate structure.

The equation used to calculate the characteristic impedance requires messy mathematical calculations. Generally, the field solution method is used, which includes the analysis of the gap element. Therefore, using the special impedance calculation software SI9000, what we need to do is to control the parameters of the characteristic impedance:

The dielectric constant Er of the insulating layer, the wiring width W1, W2 (trapezoid), the wiring thickness T and the insulating layer thickness H.

Explanation on W1 and W2:

The calculated value must be within the red box. Other conditions can be deduced by analogy.

The following uses SI9000 to calculate whether the impedance control requirements are met:

First, calculate the single-ended impedance control of the DDR data line:

TOP layer: the copper thickness is 0.5OZ, the trace width is 5MIL, the distance from the reference plane is 3.8MIL, and the dielectric constant is 4.2. Select the model, substitute the parameters, and select the lossless calculation, as shown in the figure:

Coating indicates the coating layer. If there is no coating layer, fill 0 in thickness and 1 (air) in dielectric (dielectric constant).

Substrate indicates that the substrate layer, that is, the dielectric layer, generally chooses FR-4, the thickness is calculated by impedance calculation software, and the dielectric constant is 4.2 (when the frequency is less than 1GHz).

Click on the Weight(oz) item, you can set the copper thickness of the copper paving, and the copper thickness determines the thickness of the trace.

9. The concept of Prepreg/Core of the insulating layer:

PP (prepreg) is a kind of dielectric material, composed of glass fiber and epoxy resin. Core is actually a PP type medium, but it is covered with copper foil on both sides, while PP does not. When making multilayer boards, usually CORE and PP is used in cooperation, CORE and CORE are bonded with PP.

10. Matters needing attention in PCB stack-up planning:

(1), warpage problem

The PCB laminate plan should be symmetrical, that is, the dielectric thickness of each layer and the copper thickness are symmetrical. Take the six-layer board, the dielectric thickness and copper thickness of TOP-GND and BOTTOM-POWER are the same, GND-L2 Same with L3-POWER's dielectric thickness and copper thickness. In this way, no warpage occurs during lamination.

(2) The signal layer should be tightly coupled with the nearby reference plane (that is, the dielectric thickness between the signal layer and the nearby copper layer should be small); the power copper and ground copper should be tightly coupled.

(3) Under very high-speed conditions, it is possible to join the redundant ground layer to block the signal layer, but it is recommended not to block multiple power layers, which may cause unnecessary noise interference.

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