Understanding Impedance Matching for High-Speed PCB Designs

With the advancement of technology and the ever wider application of integrated circuits, the frequency and speed of electronic signal transmission have been increasing, making it essential for PCB conductors to provide high-performance transmission lines. These transmission lines are responsible for delivering signals from a source to the input of a receiver accurately and completely. This requirement emphasizes the need for impedance matching.

Electrical impedance, commonly represented as Z and measured in ohms (Ω), refers to the combined effect of resistance, inductance, and capacitance in AC circuits. The impedance of a specific circuit is not constant; its value is determined jointly by AC frequency, resistance (R), inductance (L), and capacitance (C), and therefore varies with changes in frequency.

What is Impedance Matching?

Impedance matching is a way to ensure compatibility between a signal source or transmission line and its load. It can be classified into low-frequency and high-frequency matching. In low-frequency circuits, where the wavelength is relatively long compared to the transmission line, reflections can be disregarded. However, in high-frequency circuits, with shorter wavelengths comparable to the length of the transmission line, reflected signals superimposed on the original signal can alter its shape and affect signal quality.

As shown in the diagram above, a signal is transmitted from the source A, passes through the intermediate transmission line, and enters the receiving end B. During this transmission process, parasitic resistances, capacitances, and inductances in the circuit hinder high-speed signal transmission. When the signal propagates between these elements and encounters inconsistent impedance, it may lead to signal reflection, resulting in signal distortion.

Impedance matching effectively reduces or eliminates high-frequency signal reflections. Commonly used impedance lines can be classified into the following four types:

Impedance Design Considerations

(1) Impedance-controlled lines can be designed on the outer layer (all four types mentioned above are outer layer impedances) or the inner layer.

(2) The magnitude of impedance values depends on the product design and chip type. In general, component manufacturers have preset impedance values for signal sources and receivers (e.g., SDIO: single-ended 50 ohms, USB: differential 90 ohms).

(3) Impedance-controlled lines must have a reference layer, typically using adjacent ground or power layers as reference (e.g., for top layer impedance, the reference layer is usually the second layer).

(4) The purpose of the reference layer is to provide a return path for the signal and act as electromagnetic shielding. Thus, the reference layer must be poured with solid copper.

(5) Factors influencing line impedance

Line width: Impedance is inversely proportional to line width; the narrower the line, the higher the impedance.

Dielectric constant: Impedance is inversely proportional to the dielectric constant; the lower the dielectric constant, the higher the impedance.

Solder mask thickness: Impedance is inversely proportional to the solder mask thickness; the thicker the solder mask, the lower the impedance.

Copper thickness: Impedance is inversely proportional to the copper thickness on the surface; the thinner the copper, the higher the impedance.

Line spacing: Impedance is directly proportional to the distance between impedance lines; the greater the spacing, the higher the impedance.

Dielectric layer thickness: Impedance is directly proportional to the dielectric layer thickness; the thicker the dielectric layer, the higher the impedance.

(6) Calculation method for impedance lines: It is recommended to use JLCPCB's "Impedance Calculator" (click here for direct access). Alternatively, you can download impedance calculation software (e.g., SI9000) and combine it with our lamination parameters for calculations.

(7) A quick note on "line width and spacing": Line width refers to the horizontal width of the line, the distance from one edge of the line to the other edge. Line spacing refers to the distance from the edge of one line (or the surrounding copper plane) to the edge of a different line.

Impedance-Control Ordering Instructions

For orders requiring impedance control, it is essential to provide your impedance requirements in the form of a table or diagram, along with the compressed PCB files.

Open JLCPCB's "Impedance Calculator" and input the impedance values while selecting the corresponding layer stack-up and other relevant parameters like board thickness. Design the corresponding line width and spacing in your engineering data.

Important reminder: For orders with "Impedance Control" selected as "Yes", our factory will control the impedance within a tolerance of ±10%. If you choose "No", we will not control the impedance, but we will ensure that the line width and spacing are within a tolerance of +/-20%. Impedance control is not yet available for double-sided boards.

Conclusion

Impedance matching is a critical aspect of high-speed PCB design, ensuring optimal signal transmission and preserving signal integrity. By carefully considering impedance values, line widths, spacing, dielectric properties, and reference layers, designers can effectively minimize signal reflections and distortion. Implementing impedance-controlled lines and utilizing tools like JLCPCB's Impedance Calculator can streamline the design process and help achieve the desired impedance values. With proper impedance matching techniques, designers can enhance the performance and reliability of high-speed PCBs, enabling seamless transmission of electronic signals in modern electronic systems.

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