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PCB Tech

PCB impedance control【 pcba processing】
2021-10-04
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Author:Frank

PCB impedance control【 pcba processing】
As PCB signal switching speeds continue to increase, today's PCB designers 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 actual situations, it is necessary to control the trace impedance when the digital marginal 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 wires on the printed circuit board is an important indicator of circuit board design. Especially in the PCB design of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the wires is consistent with the characteristic impedance required by the device or signal, and whether they match. This involves two concepts: impedance control and impedance matching. This article focuses on the issues of impedance control and laminated design.

Impedance control

pcb

     Impedance control (eImpedance Controling), the conductors in the circuit board will transmit various signals. In order to increase the transmission rate, the frequency must be increased. The impedance value changes and the signal is distorted. 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 determined 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 actual situations, PCB transmission lines usually consist of a wire trace, one or more reference layers, and insulating materials. The trace and the board layer constitute the control impedance. PCB will often adopt a multi-layer structure, and 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

Configuration of traces and layers

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-shaped 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. The power or ground plane is a reference. As shown below:

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

Stripline:

The 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 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:

Dielectric constant Er of the insulating layer, trace width W1, W2 (trapezoid), trace thickness T and insulating layer thickness H.

Explanation for W1 and W2:

The calculated value must be within the red box. The rest 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 in the parameters, and select lossless calculation, as shown in the figure:

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

Substrate represents the substrate layer, that is, the dielectric layer, generally 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 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 conjunction, and CORE and CORE are bonded with PP.

10. Matters needing attention in PCB laminated design:

(1), warpage problem

The PCB laminate design should be symmetrical, that is, the dielectric thickness of each layer and the copper thickness of each layer are symmetrical. Take the six-layer board, the dielectric thickness of TOP-GND and BOTTOM-POWER are the same as the copper thickness, and GND-L2 is the same as that of BOTTOM-POWER. The dielectric thickness of L3-POWER is the same as the copper thickness. This will not warp during lamination.

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

(3) In the case of very high speed, you can add an extra ground layer to isolate the signal layer, but it is recommended not to isolate multiple power layers, which may cause unnecessary noise interference.

(4) The distribution of typical laminated design layers is shown in the following table:

(5) General principles of layer arrangement:
The bottom of the component surface (the second layer) is the ground plane, which provides a device shielding layer and a reference plane for the top layer wiring;
All signal layers are as close as possible to the ground plane;
Try to avoid two signal layers directly adjacent;
The main power supply is as close as possible to it correspondingly;
Take into account the symmetry of the laminated structure.
For the layer layout of the PCB motherboard, it is difficult for the existing motherboard to control the parallel long-distance wiring. For the board-level operating frequency above 50MHZ
(Refer to the situation below 50MHZ, and relax appropriately), the principle of arrangement is recommended:
The component surface and welding surface are a complete ground plane (shield);
No adjacent parallel wiring layers;
All signal layers are as close as possible to the ground plane;
The key signal is adjacent to the ground and does not cross the partition.