How to consider impedance control and stackup design during PCB design?

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How to consider impedance control and stackup design during PCB design? [Copy link]

How to consider impedance control and stackup design during PCB design?

(The following text is reproduced from the Internet. Everyone is welcome to add and correct.)
As the switching speed of PCB signals continues to increase, today's PCB design 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 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 PCB traces 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 the circuit board design. Especially in the PCB design of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the conductor is consistent with the characteristic impedance required by the device or signal, and whether it matches. This involves two concepts: impedance control and impedance matching. This article focuses on the issues of impedance control and stacking design.

Impedance Control

Impedance Control (eImpedance Controlling), various signals will be transmitted in the conductors in the circuit board. In order to increase its transmission rate, its frequency must be increased. If the line itself is due to etching, stacking thickness, wire width and other factors, the impedance value will change, causing its signal distortion. 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 factors that affect the impedance of the PCB trace are mainly: 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, the trace around the trace, etc. The range of PCB impedance is 25 to 120 ohms.

In actual situations, PCB transmission lines are usually composed of a conductor trace, one or more reference layers and insulating materials. The traces and board layers constitute the controlled impedance. PCB will often adopt a multi-layer structure, and the controlled impedance can also be constructed in various ways. However, no matter which method is used, the impedance value will be determined by its physical structure and the electronic properties 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

the configuration of the trace and the board layer

the insulation constant of the core and pre-filled material. There are two main forms of

PCB transmission lines: microstrip and stripline.

Microstrip:

Microstrip is a strip conductor, which refers to a transmission line with a reference plane on only one side. The top and sides are exposed to the air (or coated), located on the surface of the circuit board with an insulation constant of Er, with the power or ground layer as the reference.


Stripline:

Stripline is a strip conductor placed between two reference planes, and the dielectric constant of the dielectric can be different. There are many specific microstrip lines and striplines, such as coated microstrip lines, which are all related to the specific PCB stacking structure.

The equation used to calculate the characteristic impedance requires complex mathematical calculations, usually using field solution methods, including boundary element analysis. Therefore, using the specialized impedance calculation software SI9000, all we need to do is control the parameters of the characteristic impedance: the

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

Notes on W1 and W2:

The calculated values must be within the red box. The same applies to the rest.

Below, SI9000 is used to calculate whether the impedance control requirements are met:

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

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

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

Substrate refers to the base layer, that is, the dielectric layer. Generally, FR-4 is used. 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 to set the copper thickness of the copper plating. The copper thickness determines the thickness of the trace.

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

PP (prepreg) is a dielectric material composed of glass fiber and epoxy resin. Core is actually a PP type dielectric, but it is covered with copper foil on both sides, while PP is not. When making multi-layer boards, CORE and PP are usually used together, and PP is used to bond the COREs.

10. Precautions in PCB stacking design:

(1) Warping problem The stacking design of

the PCB should be symmetrical, that is, the dielectric layer thickness and copper plating thickness of each layer should be symmetrical from top to bottom. For a six-layer board, the dielectric thickness and copper thickness of TOP-GND and BOTTOM-POWER are consistent, and the dielectric thickness and copper thickness of GND-L2 and L3-POWER are consistent. In this way, no warping will occur during lamination.

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

(3) In the case of very high speed, an extra ground layer can be added to isolate the signal layer, but it is recommended not to use multiple power layers for isolation, as this may cause unnecessary noise interference. (4)

General principles for layer arrangement: the

ground plane is below the component surface (the second layer), providing a device shielding layer and a reference plane for the top layer wiring;

all signal layers are as close to the ground plane as possible;

try to avoid two signal layers being directly adjacent to each other;

the main power supply is as close to it as possible; and the symmetry

of the laminate structure is taken into account.

For the layer arrangement of the motherboard, it is difficult for the existing motherboard to control parallel long-distance wiring. For board-level operating frequencies above 50MHZ

(the situation below 50MHZ can be referred to and appropriately relaxed), the recommended arrangement principles are:

the component surface and the welding surface are complete ground planes (shielded);

there are no adjacent parallel wiring layers;

all signal layers are as close to the ground plane as possible;

key signals are adjacent to the ground layer and do not cross the partition area.

soso

The author of the post is engaged in PCB work. Do you know what are the most commonly used PCB software?

led2015

That is, there are Altium Designer, KiCad, Eagle, OrCAD, PADS, Mentor Graphics

qzc飘曳

I am using AD, and the operation is very comfortable

soso

led2015 posted on 2023-6-26 20:24 That is to say, there are Altium Designer, KiCad, Eagle, OrCAD, PADS, Mentor Graphics

That’s quite a lot.

瑞兴诺pcb

soso posted on 2023-6-27 16:10 That’s really a lot.

They are all experts. I am still a rookie. I have just started working in the PCB industry. I can only use CAM350 to view files.

lkh747566933

They are all experts. I am still a rookie. I have just started working in the PCB industry. I can only use CAM350 to view files.

fengerbbz

When designing a PCB, it is very important to consider impedance control and stackup design, especially for high-frequency signals and high-speed transmission lines. Here are some key considerations: Signal integrity requirements: First, determine the signal integrity requirements involved in the design. Determine the required impedance control range based on factors such as the frequency range, rise time, and transmission distance of the signal. Material selection: Select PCB materials with the required properties, especially in high-frequency and high-speed applications. The dielectric constant and loss factor of different materials will affect the impedance value, so careful evaluation and selection of suitable materials are required. Layer thickness and layer stack: Reasonable layer thickness and layer stack design are crucial for impedance control. By adjusting the copper foil thickness and dielectric layer thickness of different layers, the required impedance matching can be achieved. Differential signal design: Impedance matching becomes more important in differential signal transmission. Ensure the impedance matching and consistency of differential gaps between differential signal pairs to reduce crosstalk and mismatch. Line width and line spacing: Design signal lines with appropriate line width and line spacing based on the required impedance value. Impedance calculation tools and design specifications can help determine the appropriate line width and line spacing. Ground plane and power plane: Reasonable layout and planning of ground plane and power plane can provide good signal reference plane and power distribution, thereby improving impedance control and signal integrity. Simulation and verification: Use professional SI/PI simulation tools to simulate and verify PCB design. Simulation can help predict and analyze signal integrity issues and optimize impedance matching and stackup design. Manufacturing and manufacturing process: Work with PCB manufacturers to understand their capabilities and manufacturing process requirements. Ensure that the design meets the requirements of the manufacturing process to ensure reliable impedance control and stackup manufacturing. In summary, impedance control and stackup design are important links that cannot be ignored in PCB design. Combined with appropriate materials, layer thickness, line width, line spacing and plane planning, as well as the application of simulation and verification tools, high-quality impedance control and signal integrity design can be achieved. With professional PCB design

一叶知秋霜

I'm a beginner, come here to get some experience points, thanks for sharing, let's make progress together, thank you la la la la

一叶知秋霜

This is a good piece of knowledge. I will also learn from it and improve myself.

lkh747566933

Good knowledge points, I will also learn and improve myself. I used to draw casually