The Key of PCB Manufacturing: PCB Impedance Control, Are You Really Doing It Right?

PCB impedance is usually used in the design of multilayer PCBs, high - frequency PCBs, and antenna PCBs. With the development of science and technology, the high - frequency response of printed circuit boards and the use of high - speed digital circuits have made impedance design more and more important in circuit board design. Customers' requirements for impedance control are getting higher and higher, and the number of impedance controls is also increasing. Today, let's discuss PCB impedance control.

I. What is PCB impedance control?

PCB impedance control refers to the resistance and inductance encountered when current flows through a wire or current path, including resistance (R), inductive reactance (XC), and capacitive reactance (XL).

The characteristic impedance of a PCB, also known as "characteristic impedance", refers to the resistance of high - frequency signals or electromagnetic waves of a PCB in a transmission signal line relative to a reference layer (i.e., shielding layer, projection layer, or reference layer) at a certain frequency. During the propagation process, it is called characteristic impedance. It is actually the sum of vectors such as resistance, inductance, and capacitance.

Factors that affect the characteristic impedance of a PCB include the dielectric constant of materials, the width and thickness of wires, the thickness of the dielectric, the thickness of the solder mask, and the thickness of the copper foil.

II. How to control PCB impedance in design?

1. Use empirical values to record the impedance lines that have been made before and directly apply them next time.

2. First, according to the conventional design, highlight the lines that need impedance on the PCB, and then provide the PCB factory with a screenshot. If the PCB factory wants to control it, it will modify the data according to the customer's needs, such as adjusting the line width and line spacing to achieve the required impedance.

3. At the beginning of the design, according to the PCB lamination parameters and the relevant data provided by the manufacturer (PCB board, dielectric constant, green oil, PP thickness, etc.), use Si9000 software to calculate the impedance, and then use the calculated parameters to trace the impedance lines. Finally, obtain the PCB data. At the same time, the PCB factory needs to control the impedance of the PCB according to these data.

Note: In the first point, if the PCB stack parameters change, the impedance of the PCB will also change.  In the second point, factory engineers often call to say that the impedance of your PCB cannot be measured because the line width and line spacing of the PCB you designed are too different and there is not enough space on the PCB to widen the line width and line spacing.  Obviously, the third option is the safest, otherwise the PCB factory will not be able to control the impedance of the PCB.

1. The most commonly used impedance calculation tool is the Si8000 field solver provided by Polar Company. This software contains various impedance modules. Staff can calculate impedance results by selecting specific modules and inputting relevant information (such as line width, line spacing, dielectric thickness, copper thickness, etc.). The number of impedance controls for a PCB can range from as few as 4 or 5 groups to dozens of groups. If you check the data one by one and then manually input relevant parameters for calculation, it will be very time-consuming and error-prone.

2. Orbotech's Inplan software can automate the design of PCB impedance, greatly improving pre-manufacturing efficiency.  The Inplan system can be connected to Si8000 and automatically calculate impedance based on the establishment of the following databases:

Firstly, build a complete data warehouse in Inplan and classify it according to different PCB manufacturers and models.  Internally, obtain data such as lamination thickness, substrate copper thickness, and PP glue content based on the actual process parameters of the PCB factory.

Then, establish rules for calculating impedance in Inplan. When calculating impedance, Inplan will automatically bring out relevant parameter values that affect impedance to calculate optimized impedance results.  No matter how many impedance groups there are, as long as you click a button, all results can be obtained within a few seconds.

3. In order to correctly test the impedance values in the circuit board, the key lies in the design of the impedance strips.  If the customer does not design the PCB impedance strips by themselves, we need to design the PCB impedance strips ourselves.  By testing the circuit board impedance values of the impedance strips, make the circuit board impedance meet the customer's requirements.  

Currently, the competition in the PCB industry is becoming increasingly fierce, and the delivery time of PCB samples is getting shorter and shorter.  PCB impedance design accounts for a large proportion in PCB manufacturing work.  How to shorten the PCB impedance manufacturing time and make the impedance match to meet the customer's requirements is an important issue that PCB manufacturing must consider.

PCB MASTER Company has rich experience in impedance simulation and the design of impedance strips and can design PCB impedance quickly and accurately to meet the needs of customers. If you have any questions, please feel free to consult us. We will serve you wholeheartedly!