PCB design requirements have been heard before in terms of signal integrity and impedance, but I still have a lot of do not understand this, and after each others discussed here would not know how to continue the conversation going on this issue. Happened to have on hand a little work recently have some needs in this area, we brought in took a little time to understand a bit seriously. Feel just learned that the tip of the iceberg, in which it learned of knowledge excerpt summary, the follow-up of new knowledge and then follow up.
This paper describes the PCB impedance control , information from the Internet, the source attached thereto.
Impedance and its influencing factors
Impedance control (eImpedace Controling), a conductor wiring board will transfer various signals, to increase its transmission rate must be increased frequencies of different factors, itself Ruoyin etching, lamination thickness, conductor line width, etc., will result in change in impedance value, so that the signal distortion. Therefore, in the high-speed circuit board conductor, the impedance value should be controlled within a certain range, the title "impedance control."
In practice, typically less than 1ns digital or analog signal edge frequency is necessary to control the impedance exceeds 300MHz. Characteristic impedance of the printed circuit board tracks is an important indicator board design, especially in the high frequency circuit PCB design must be considered characteristic or the characteristic impedance of the device and the desired signal conductor impedance is consistent, match.
Impedance of PCB trace will be determined by its inductive and capacitive inductance, resistance and conductance. Factors Affecting the impedance of PCB trace are: thickness, the thickness of the pad, the ground path permittivity, dielectric width of copper, the thickness of the copper, medium, go outside the alignment line or the like. PCB impedance range of 25 to 120 ohms [ 1 ]. Note: This article from the majority of Reference [ 1 ], to express my thanks to the original author.
In practice, PCB trace transmission line typically consists of a wire, and one or more reference layers composed of an insulating material. Traces and slabs constitute a control impedance. The PCB is often used multilayer structure and controlling the impedance may be constructed in various ways. However, no matter what manner, the impedance value will be its physical structure and electronic properties of insulating materials is determined:
- Width and thickness of the signal trace
- Or pre-filled core material at both sides of the trace height
- Configuring trace and plies
- Dielectric constant and pre-filled core material
It should be noted understood that the impedance control / matching between the signal layers mainly refers to the reference plane and the adjacent layer .
PCB transmission line and the laminate
PCB transmission lines are mainly two: a microstrip line (Microstrip) and the stripline (Stripline).
Briefly, top or bottom layer microstrip line is a transmission line. Stripline is disposed between two stripline reference plane, i.e., the inner layer wiring.
We mentioned in the text generally planar power plane or ground plane. Parameters needed Note that: in the actual manufacture of PCB, usually covered with a layer of green oil on the surface of the PCB, so in practice the impedance calculation is generally used with model calculations of green oil (described in detail later in this section) .
Microstrip and stripline difference of [ 2 ]
- A transmission unit length of the microstrip line delay time depends only on the dielectric constant irrespective of the line width or spacing.
- Because the microstrip line (a microstrip) side is FR4 (or other dielectric) is an air side (low dielectric constant) Thus fast, go to facilitate high speed requirements of signal (e.g. differential line, typically a high-speed signal, while the interference relatively strong).
- Stripline (Stripline) on both sides or bottom of the power supply, the impedance is easy to control, while the shield preferably, the signal speed is slower.
- Dielectric losses are typically the same conditions microstrip line (a microstrip) small (width), a large loss stripline (Stripline) a (thin line, via holes).
PCB transmission line described
in a lot of information, the mentioned cross-sectional PCB transmission line is a construction of trapezoid like (reasons for its formation is PCB plate making process from top to bottom formed by etching, so that a cross-section which presents a trapezoid).
As shown above, in which the line width set rendering software engineering, a length W1 of the transverse section of the edge, which is calculated W1 = W - A, on which the variables A, there is a dependent relationship lookup.
Wherein H OZ, 1 OZrepresents the unit thickness of PCB copper foil, a thickness of 1 OZ (ounce) at about 1.35 (1.4) mil, H OZ denotes a half (Half) ounces, i.e. 0.5 OZ.
As for why the use of OZ copper foil thickness in particular is not very clear, but where to find some of the information to be simply explained as follows [ 3 ].
In the PCB industry, 1 OZ mean weight 1 OZ uniform copper plated on one square feet (FT2) attains a thickness of the area. It is used to represent the weight of the average thickness of the copper foil. That is expressed by the formula, 1 OZ = 28.35 g / FT2.
Specifically, it can be said that the length and thickness of conversion method is as follows:
首先，我们知道铜的密度常数和相关单位换算公式如下：
铜的密度 ρ = 8.9g/cm3
1 mil ≈ 25.4 um
1 FT2 ≈ 929.0304 cm2
1 mil ≈ 25.4um
根据质量的计算公式 m＝ρ × V ＝ρ × S × t（厚度），知道铜箔的重量除以铜的密度和面积即为铜箔厚度！
从前文又知，1 OZ = t × 929.0304 cm2 × 8.9 g/cm3 = 28.35 g/cm2
所以，t = 28.35 ÷ 929.0304 ÷ 8.9 cm ≈ 0.0034287 cm ≈ **1.35mil**
由此可知，1 OZ铜箔的厚度约为 35 um或者 1.35 mil。PCB impedance calculation
Since the PCB impedance calculation is very complicated, too many factors to be considered, for easy initial software we use to calculate the impedance of this manual to find a suitable spot. In addition also due have not found how to use a formula to be calculated, and then the follow-up found to complement the article content.
PCB impedance calculation software download readers can find themselves in this recommendation two, Polar 's CITS25 and Si9000 . Which CITS25 enough lightweight interface is very simple, that the software unit CITS25 filled values should be noted for the mil. SI9000 more professional and one of the models also more units are also free to choose, more flexible.
Two about how to use the software to calculate impedance, these were not described in this article, if this aspect of reading needs, you can click here to jump to learn more about how to get started using it.
By impedance calculation, we can determine are: impedance magnitude, the layers and the thickness of the individual layers of the substrate . These are also the system board, and when we agreed upon vendor requirements.