1. Introduction to impedance control concept

In order to distinguish the resistance of direct current (DC), the resistance encountered by alternating current is called impedance (Z0), including resistance (R), inductive reactance (XC), and capacitive reactance (XL).

Special impedance is also called "special impedance". It means that at a certain frequency, in the transmission signal line (that is, the copper wire of the circuit board we manufacture), its height is relative to a certain reference layer (that is, the shielding layer, the alluding layer or the reference layer) The resistance experienced by high-frequency signals or electromagnetic waves in the process of wide spreading is called the special nature impedance, which is actually a vector of electrical impedance, inductive reactance, and capacitive reactance.

2. The significance of controlling the special impedance of *PCB*** PCB **not only plays the role of current conduction in electronic products, but also plays the role of signal transmission:

The high frequency and high speed of electronic products require that the performance of the circuit supplied by the PCB must ensure that the signal does not reflect during the transmission process and maintain the signal integrity and distortion;

The special nature impedance is the central place to solve the signal integrity problem;

When electronic facilities (such as computers, communication switches, etc.) are operating, the signal emitted by the driver must pass through the PCB signal line to the receiver. To ensure signal integrity, the special impedance (Z0) of the signal line of the PCB must be matched with the "electronic impedance" of the head and tail components;

When the transmission line is greater than or equal to 1/3 of the rising time length, the signal will be reflected, and the special impedance of the problem must be considered.

3. Factors affecting the impedance of a particular nature

The dielectric constant of the medium is inversely proportional to the resistance value of the particular property (Er). The following figure shows the common sense sheet material parameters:

The thickness of the medium between the circuit layer and the grounding layer (or outer layer) is directly proportional to the impedance value of the special property (H). The following figure shows the normal sheet material parameters:

Bottom width of impedance line (lower end W1): the width of the line surface (upper end W2), which is inversely proportional to the characteristic impedance.

Copper thickness, which is inversely proportional to the specific resistance value (T)

The distance between adjacent lines and lines is proportional to the special impedance value (differential impedance) (S)

The thickness of the base material solder mask is inversely proportional to the impedance value (C)

4. Process factors affecting ** PCB** impedance

Because of the corrosion, it has a great influence on the impedance when the copper thickness is 2oz, and there is generally no way to control the impedance.

The default layer blanks without copper and wires need to be replenished with a curing film during production. When calculating the impedance, it is not possible to directly substitute the thickness of the medium provided by the sheet supplier, and the need to subtract the curing film to supplement these empty spaces This is one of the main reasons why the impedance calculated by myself is not exactly the same as the final result of the manufacturer.

5. Calculation of ** PCB** impedance

The calculation of impedance is more complicated and trivial, but we can summarize some experience values to help increase the calculation rate. For the commonly used FR4, 50ohm microstrip line, the line width is usually equal to 2 times the thickness of the medium; for the 50ohm strip line, the line width is equal to half of the total thickness of the medium between the two simplest surfaces, which can help us quickly lock the line width Range, note that the line width calculated by ordinary calculation is smaller than this value.

In addition to increasing the calculation speed, we also need to increase the calculation precision. Do you often encounter that the impedance calculated by yourself is not exactly the same as the impedance calculated by the board manufacturer? Some people will say what this has to do with the board factory directly. But is there any situation where the board factory fails to adjust and makes you relax the impedance management control? To make a good product, it is better to have everything in your own hands.

The following items are for your reference:a, the line width is rather wide, not thin. What does this mean? Since we know that there are fine limits in the manufacturing process, there is no limit to width. If at that time the limit is encountered in order to adjust the impedance and fine-tune the line width, then it will be troublesome. Either increase the cost or relax the impedance management control. So in the calculation, the relative width means that the target impedance is slightly lower. For example, if the single-line impedance is 50ohm, we can calculate it to 49ohm. Try our best not to calculate it to 51ohm.

b. The group flashes a development direction. There may be multiple impedance management control purposes in our presets, so the group should be too large or too small, not 100 ohm too large and 90 ohm too small.

c. Consider the problem of residual copper rate and glue flow. When one or both sides of the prepreg are etched lines, the glue will fill the etched holes during the pressing process, so the thickness of the glue between the two layers will decrease. The smaller the residual copper rate, the more filling. The less is left. So if the thickness of the prepreg you need between the two layers is 5mil, you should choose a thicker prepreg based on the residual copper rate.

d. Specify the glass cloth and glue content. Engineers who have seen the sheet datasheet know that the dielectric coefficients of different glass cloths, prepregs or core boards with different glue content are different, even if the height is almost the same, it may be the difference between 3.5 and 4. This difference can induce a single-line impedance change of about 3ohm. The additional glass fiber effect is closely related to the window volume of the glass cloth. If you have a preset of 10Gbps or higher, and your laminate does not specify a material, the board factory uses a single 1080 material, it may be revealed Signal integrity issues come out.