Impedance matching and reduction of transmission line loss are important indicators of high-speed printed circuit boards (PCB). As an important part of the PCB, the solder mask has a greater impact on the impedance and loss of the outer transmission line. For high-speed PCB design and manufacturing, it is of great significance to understand the influence of the solder mask on the impedance and loss and how to reduce the influence of the solder mask on the electrical performance of the PCB. This article compares and analyzes the changes in PCB electrical properties before and after the solder mask and solder mask are covered, and focuses on the influence of conventional solder mask ink on the impedance and loss of the outer transmission line, which can provide reference data for high-speed PCB design and impedance matching design. The dielectric constant of our solder mask ink is generally 4.0, and the loss factor is 0.02-0.04.
1 Introduction
With the development of science and technology, electronic products are constantly developing in the direction of high density, multi-function, high frequency and high speed of signal transmission. At present, the signal transmission frequency of the backbone network has reached 100Gbit/s, and the corresponding single-channel transmission rate is as high as 10Gbit. /s or even 25Gbit/s, and high-speed signal transmission maintains a rapid development trend. The high-speed development of signal transmission makes signal integrity problems such as reflection and crosstalk more likely to occur during signal transmission, and the faster the transmission rate, the greater the signal loss. How to reduce signal loss during transmission and ensure signal integrity It is a huge challenge in the development of high-speed PCB.
2. Main types of impedance and influencing factors
Impedance (Zo) definition: The total resistance to alternating current flowing through it at a known frequency is called impedance (Zo). For printed circuit boards, it refers to the total impedance of a certain circuit layer (signal layer) to its closest reference plane under high-frequency signals.
2.1 Impedance type:
(1) Characteristic impedance. In electronic information products such as computers and wireless communications, the energy transmitted in the PCB circuit is a square wave signal (called pulse) composed of voltage and time. The resistance encountered is called the characteristic impedance.
(2) Two identical signal waveforms with opposite polarities are input to the differential impedance driving end, which are respectively transmitted by two differential lines, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance Zdiff between the two wires.
(3) The impedance value of one line to the ground of the two lines of odd mode impedance is the same as the impedance value of the two lines.
(4) Impedance Zcom when two identical signal waveforms with the same polarity are input to the even mode impedance drive terminal, and the two wires are connected together.
(5) Common mode impedance The impedance Zoe of one line to the ground of the two lines is the same, and usually larger than the odd mode impedance.
Among them, characteristic and differential are common impedances, and common mode and odd mode are rare.
2.2 Factors affecting impedance:
W—the line width/line width between lines increases and the impedance decreases, and the distance increases the impedance increases;
H—Insulation thickness increases and impedance increases;
T——The copper thickness increases and the impedance decreases;
H1—The thickness of green oil increases and the impedance decreases;
Er—The DK value of the dielectric constant reference layer increases, and the impedance decreases;
Undercut—W1-Wundercut increases, and the impedance becomes larger.
3. Impedance calculation automation
Today, the most commonly used impedance calculation tool in our industry is the Si8000 Field Solver provided by Polar. Si8000 is a brand-new boundary element method field effect calculator software based on the easy-to-use user interface of the early Polar impedance design system we are familiar with. . This software contains various impedance modules. The personnel can calculate impedance results by selecting specific modules, inputting line width, line spacing, interlayer thickness, copper thickness, Er value and other related data. A PCB impedance control number can be as few as 4 or 5 groups, and as many as dozens of groups. The control line width, interlayer thickness, copper thickness, etc. of each group are different. If you check the data one by one, then manually enter the relevant parameter calculations. Very time-consuming and error-prone.
Next, we will introduce the parameters of the Si8000 software for impedance design automatically
Impedance matching diagram
Impedance matching diagram
Impedance matching diagram
Impedance matching diagram
Impedance matching diagram
4 Conclusion:
For the outer layer single-ended line and the outer layer check line, the larger the design impedance value, the more the impedance decreases after the solder mask ink is printed.
1.1 After the single-ended microstrip line with a design value of 60Ω is covered with solder mask, the impedance drops by 4.3Ω
1.2 The impedance of a single-ended microstrip line with a design value of 50Ω is reduced by 2.8Ω after being covered with solder mask ink
1.3 The impedance of a single-ended microstrip line with a design value of 40Ω is reduced by 2.0Ω after being covered with solder mask ink
2.1 After the differential line with a design value of 110Ω is covered with solder mask ink, the impedance drops by 10.1Ω
2.2 The impedance of the differential line with a design value of 100Ω is reduced by 8.5Ω after covering the solder mask ink
2.3 After the differential line with a design value of 80Ω is covered with solder mask, the impedance drops by 5.0Ω
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