From the perspective of transmission line theory, the resistance, inductance, leakage conductance (reciprocal of leakage resistance), and distributed capacitance of a wire relative to the reference layer (reference plane) cannot be ignored. They have an impact on the transmission speed and energy of signals in the wire; The signal level transmitted on the wire is not only a function of time t, but also a function of distance x; The current at each point on a wire with transmission characteristics is different.

As wires on a PCB, the line width of the same wire is usually uniform, and the dielectric and thickness between the wire and the relative reference layer are also basically the same. Therefore, PCB wires should be regarded as uniform transmission lines. The following figure (a) can be used for analysis. The entire wire can be regarded as n transmission lines of unit length, with corresponding equivalent circuits, as shown in figure (b).

There are many types of impedances, and their classification methods are also different. Impedance is generally classified according to the number of wire diameters and transmission methods:

Single ended impedance: composed of a line and a ground layer.

Differential impedance: composed of two lines and a geological layer.

A microstrip line is an impedance line that is parallel to the ground plane on one side, separated by a dielectric layer in the middle, and has a solder mask layer or nothing on the other side.

A strip line is an impedance line that separates the upper and lower sides of a line with a ground layer. This type of impedance line is called a strip impedance line, and compared to microstrip lines, signals are less susceptible to interference.

There are many types of impedance control, and the most commonly used impedance calculation software Polar-Si90 has over 8000 impedances. However, the most commonly used are the following four types and their two variants:

Surface Microstrip

Offset strip line

Edge coupled surface microstrip

Edge coupling offset stripline

The impedance value is influenced by PCB material thickness, material dielectric constant (Er) or relative dielectric constant (Dk), conductor width/spacing, conductor thickness, copper thickness, and signal transmission frequency, which are factors that PCB designers need to consider. As a PCB manufacturer, we believe there are 5 factors that can affect impedance values, which are:

Dielectric thickness

Conductor line width/spacing

dielectric constant

Conductor thickness

Solder mask thickness

The relationship between impedance value (Z0) and dielectric thickness (H): the thicker the dielectric, the larger Z0

The relationship between impedance value (Z0) and conductor width (W): the larger the line width, the smaller Z0

Impedance value (Z0) versus dielectric Er (Er): The larger Er, the smaller Z0

Impedance value (Z0) and conductor thickness (T): The thicker the conductor thickness, the smaller Z0

The relationship between impedance value (Z0) and solder mask thickness (T): The thicker the solder mask thickness, the smaller Z0

Before production, engineers from the PCB manufacturing company will review the Gerber files and recommend suitable materials and stacking to meet the customer's impedance requirements. Please refer to the following impedance matching techniques provided by our engineers for control.

Increasing the thickness of the medium can increase the impedance value, while reducing the thickness of the medium can decrease the impedance value. Engineering design, laminated board pressing control, and incoming material tolerance are key factors in controlling the thickness of the medium.

Increasing line width can reduce impedance, while reducing line width increases impedance. The line width is mainly controlled through etching and design.

Reducing the thickness of the conductor can increase the impedance value, while increasing the thickness of the circuit can reduce the impedance. The thickness of the wire can be controlled by graphic electroplating or selecting the corresponding thickness of the base copper foil.

Generally speaking, printing a solder mask on a PCB will reduce the impedance value: the single end will decrease by 2 ohms, and the difference will decrease by 8 ohms. Two solder mask layers will reduce the impedance by two times. Printing the solder mask layer three or more times should help reduce the impedance value.

When producing PCBs with controlled impedance, we need to ensure that the impedance value is the same as the customer's requirements. Therefore, we will test the impedance value before shipment.

Impedance testing is the process of using an oscilloscope to emit a pulse wave, receiving a reflected wave, and then comparing and analyzing two pulse waves to determine the impedance value based on the transmitted energy.

We will add impedance test pieces on each PCB board, and the impedance lines/traces will be the same as the PCB. It is produced under the same process conditions as printed circuit board production. Therefore, if the impedance value on the test piece meets the requirements, we can ensure that the impedance on the printed circuit board meets the requirements.

Time domain reflectometer is a special oscilloscope that can generate pulse waves internally and has the function of receiving and analyzing these pulses. We will use a TDR time-domain reflectometer to measure these impedance values.

If pursuing cost-effective FPC manufacturing services, choosing a Chinese manufacturer is the best choice. As a manufacturing powerhouse, China has abundant resources, cheap labor, and a large number of technical personnel. We recommend Haibo, a Chinese FPC assembly service provider with over 10 years of experience, who has performed outstandingly in various aspects

Efficient and reasonable quotation:There is an efficient quotation process to assist you in making decisions, providing reasonable prices and highly competitive quotes in China.
High quality and reliable products:Produce high standard printed circuit boards, manufacture and test according to specifications, international standards, and internal controls, and conduct pre production inspections to ensure robust processes.
Quick and on-time delivery:The assembly equipment is in good condition, with high productivity, minimal downtime, short delivery time, and also provides urgent services.
Timely and effective response:Always respond to customer needs, provide accurate information, answer questions through multiple channels, and offer one-stop services.
Strong technical strength:China's FPC manufacturing technology is leading, and Haibo has a complete SMT solution that covers assembly, inspection, and other capabilities. It also provides prototype production and customization services.