1. With the development of science and technology, especially advancements in integrated circuit materials, computing speed has significantly improved, leading integrated circuits toward higher density, smaller volume, and more compact designs.
2. These advancements have increased the high-frequency response requirements for today’s and future printed circuit boards.
3. The use of high-speed digital circuits necessitates precise impedance control, minimal distortion, low interference, and reduced crosstalk, as well as the elimination of electromagnetic interference (EMI).
4. PCB impedance design is becoming increasingly crucial in PCB design. As the initial phase of PCB manufacturing, it involves simulating impedance calculations and designing impedance matching structures.
5. Customers’ demands for precise impedance control have grown, leading to a higher number of impedance control requirements.
6. Efficient and accurate impedance design in PCB layout is a significant concern for pre-manufacturing personnel.
2. Main Types of Impedance and Influencing Factors
Impedance (Zo) Definition: Impedance (Zo) is defined as the total resistance encountered by alternating current flowing through a circuit at a specified frequency. For printed circuit boards, it refers to the total impedance of a specific circuit layer (signal layer) relative to its closest reference plane under high-frequency signals.
2.1 Types of Impedance:
(1) Characteristic Impedance: In electronic information products such as computers and wireless communications, the energy transmitted through the PCB circuit is typically a square wave signal (often referred to as a pulse signal) composed of voltage and time. The impedance encountered by this signal is known as the characteristic impedance.
(2) Differential Impedance: When two identical signal waveforms with opposite polarities are input at the driving end and transmitted through two differential lines, the impedance measured between these two lines is called the differential impedance (Zdiff). At the receiving end, the two differential signals are subtracted.
(3) Odd-Mode Impedance: The impedance of one of the two lines with respect to the ground is known as Zoo, and the impedance of both lines is identical in this mode.
(4) Even-Mode Impedance: This is the impedance (Zcom) when two identical signal waveforms with the same polarity are input at the driving end, and the two lines are connected together.
(5) Common-Mode Impedance: The impedance (Zoe) of one of the two lines with respect to the ground is the same for both lines and is usually greater than the odd-mode impedance.
Among these, characteristic and differential impedances are commonly encountered, while common-mode and odd-mode impedances are less frequent.
2.2 Factors Affecting PCB Impedance:
W — Line Width/Spacing: As the line width increases, the impedance decreases, while an increase in spacing between lines leads to an increase in impedance.
H — Insulation Thickness: As the insulation thickness increases, the impedance also increases.
T — Copper Thickness: An increase in copper thickness results in a decrease in impedance.
H1 — Solder Mask Thickness: As the thickness of the solder mask increases, the impedance decreases.
Er — Dielectric Constant: An increase in the dielectric constant (DK value) of the reference layer results in a decrease in impedance.
Undercut — W1-W Undercut: An increase in the undercut (the difference between W1 and W) leads to an increase in impedance.
2. These advancements have increased the high-frequency response requirements for today’s and future printed circuit boards.
3. The use of high-speed digital circuits necessitates precise impedance control, minimal distortion, low interference, and reduced crosstalk, as well as the elimination of electromagnetic interference (EMI).
4. PCB impedance design is becoming increasingly crucial in PCB design. As the initial phase of PCB manufacturing, it involves simulating impedance calculations and designing impedance matching structures.
5. Customers’ demands for precise impedance control have grown, leading to a higher number of impedance control requirements.
6. Efficient and accurate impedance design in PCB layout is a significant concern for pre-manufacturing personnel.
2. Main Types of Impedance and Influencing Factors
Impedance (Zo) Definition: Impedance (Zo) is defined as the total resistance encountered by alternating current flowing through a circuit at a specified frequency. For printed circuit boards, it refers to the total impedance of a specific circuit layer (signal layer) relative to its closest reference plane under high-frequency signals.
2.1 Types of Impedance:
(1) Characteristic Impedance: In electronic information products such as computers and wireless communications, the energy transmitted through the PCB circuit is typically a square wave signal (often referred to as a pulse signal) composed of voltage and time. The impedance encountered by this signal is known as the characteristic impedance.
(2) Differential Impedance: When two identical signal waveforms with opposite polarities are input at the driving end and transmitted through two differential lines, the impedance measured between these two lines is called the differential impedance (Zdiff). At the receiving end, the two differential signals are subtracted.
(3) Odd-Mode Impedance: The impedance of one of the two lines with respect to the ground is known as Zoo, and the impedance of both lines is identical in this mode.
(4) Even-Mode Impedance: This is the impedance (Zcom) when two identical signal waveforms with the same polarity are input at the driving end, and the two lines are connected together.
(5) Common-Mode Impedance: The impedance (Zoe) of one of the two lines with respect to the ground is the same for both lines and is usually greater than the odd-mode impedance.
Among these, characteristic and differential impedances are commonly encountered, while common-mode and odd-mode impedances are less frequent.
2.2 Factors Affecting PCB Impedance:
W — Line Width/Spacing: As the line width increases, the impedance decreases, while an increase in spacing between lines leads to an increase in impedance.
H — Insulation Thickness: As the insulation thickness increases, the impedance also increases.
T — Copper Thickness: An increase in copper thickness results in a decrease in impedance.
H1 — Solder Mask Thickness: As the thickness of the solder mask increases, the impedance decreases.
Er — Dielectric Constant: An increase in the dielectric constant (DK value) of the reference layer results in a decrease in impedance.
Undercut — W1-W Undercut: An increase in the undercut (the difference between W1 and W) leads to an increase in impedance.
