The calculation of impedance is relatively tedious, but we can summarize some empirical values to help improve the calculation efficiency. For commonly used FR4, 50ohm microstrip lines, the line width is generally equal to twice the thickness of the medium; 50ohm striplines, the line width is equal to one-half of the total thickness of the medium between the two planes, which can help us quickly lock the line width Range, note that the calculated line width is generally smaller than this value.

In addition to improving calculation efficiency, we also need to improve calculation accuracy. Do you often encounter inconsistencies between the impedance calculated by yourself and the board manufacturer? Some people will say that it has nothing to do with it, let the board factory adjust it directly. But will there be a situation that the board factory can't adjust and let you relax the impedance control? To make a product or everything is better at your own control.

The following points for consideration when designing the stack calculation impedance are for your reference:

1. The line width is preferably wider than thin. What does it mean? Because we know that there are fine limits in the process, there is no limit to width. If the line width is narrowed and the limit is reached in order to adjust the impedance, then it will be troublesome. Either increase the cost or relax the impedance control. So relatively wide in the calculation means that the target impedance is slightly lower. For example, the single-line impedance is 50ohm, we can calculate it to 49ohm, and try not to calculate it to 51ohm.

2, showing a trend overall. There may be multiple impedance control targets in our design, so the overall is too large or too small, not 100ohm is too large, 90ohm is too small.

3. Consider the residual copper rate and the amount of glue flowing. When one or both sides of the prepreg are etched lines, the glue will fill the gaps in the etch during the lamination process. In this way, the glue thickness time between the two layers will be reduced. The smaller the residual copper rate, the more you fill, and the more the remaining less. So if the thickness of the prepreg you need between the two layers is 5mil, choose a slightly thicker prepreg according to the copper residual rate.

4. Specify the glass cloth and rubber content. Engineers who have read the sheet datasheet know that the dielectric constants of different glass cloths and different prepregs or core boards with different rubber contents are different. Even the same height may be the difference between 3.5 and 4. This difference can cause a single line The impedance changes around 3ohm. In addition, the glass fiber effect is closely related to the size of the glass window opening. If you are designing at 10Gbps or higher speed and your stack does not specify the material, the board factory uses a single sheet of 1080 material, then signal integrity may occur. problem.

Of course, the calculation of the residual copper rate and the amount of glue is not accurate. The dielectric constant of the new material is sometimes inconsistent with the nominal. Some glass cloth board factories do not prepare materials. What to do? The best way is to let the board factory design a stack according to our requirements at the beginning of the design, so that they can get the ideal and achievable stack at most a few round trips.

Last time I talked about some "arts of trade-offs" between impedance calculation and process. It is mainly to achieve the purpose of impedance control, while also ensuring the convenience of process and reducing the processing cost as much as possible.

In order to suppress common impedance interference, the following measures can be adopted:

(1) One point ground

Make the several ground points of the same-level unit circuit as concentrated as possible to avoid the AC signals of other circuits from entering this stage, or the AC signals of this stage from entering other circuits. It is suitable for low-frequency circuits with a signal operating frequency less than 1MHZ. If the operating frequency is between 1 and 10MHz and one point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength. In short, one point grounding eliminates common impedance interference of the ground wire The basic principle.

(2) Multi-point grounding nearby

A large number of common ground wires are distributed on the edge of the PCB and present a semi-closed loop (anti-magnetic field interference). The circuits at all levels are grounded nearby to prevent the ground wire from being too long. Suitable for high-frequency circuits with a signal operating frequency greater than 10 MHz.

(3) Bus grounding

The busbar is made of silver-plated copper foil, and the ground wires of all integrated circuits on the PCB are connected to the busbar. The bus bar has the low-impedance characteristic of the bar-shaped symmetrical transmission line. In high-speed circuits, it can increase the signal transmission speed and reduce interference.

(4) Large area grounding

In the high-frequency circuit, all unused areas on the PCB are set as ground wires to reduce the inductive reactance in the ground wire, thereby weakening the high-frequency signals generated on the ground wire and shielding the electric field interference.

(5) Bold ground wire

If the ground wire is very thin, the ground potential will change with the change of the current, causing the timing level of the electronic equipment to be unstable and the anti-noise performance to deteriorate, and its width should be at least greater than 3mm.

(6) The ground of D / A (digital / analog) circuit is separated

The ground wires of the two circuits are independent, and then connected to the ground wire of the power terminal to suppress them from interfering with each other.

Calculation formula of resistance：

Z = R + i (ωL–1 / (ωC))
Note: The load is a complex of three types of resistance, inductive reactance and capacitive capacitive reactance, which are collectively called “impedance” after being compounded. The mathematical formula is: impedance Z = R + i (ωL–1 / (ωC)) . Where R is the resistance, ωL is the inductive reactance, and 1 / (ωC) is the capacitive reactance.
(1) If (ωL–1 / ωC)> 0, it is called “inductive load”;
(2) Conversely, if (ωL–1 / ωC) <0 is called “capacitive load”.

The calculation of impedance usually requires professional calculation software, and requires a complex series of up to more than 10 parameters, such as board thickness, copper thickness, PP thickness, PP power saving constant, soldering power saving constant, and soldering resistance on the line. Thickness, thickness of solder resist on the substrate, etc.

For a specific circuit, the impedance is not constant, but changes with frequency. In a series circuit of resistance, inductance and capacitance, the impedance of the circuit is generally greater than the resistance. That is, the impedance is reduced to a minimum. In a parallel circuit of an inductor and a capacitor, the impedance increases to a maximum value at resonance, which is the opposite of a series circuit.