As technology advances, PCB designs and components are becoming smaller, faster, and consequently more complex. Today, it is crucial to have a deep understanding of critical nets and traces, impedance, and how the board affects signal performance.

The era of simple interconnecting traces and conductors is over. Nowadays, circuit speeds are continuously increasing, and signals in the GHz range are becoming commonplace.Therefore, impedance control plays a key role in ensuring signal integrity and the performance of circuit boards.

What is impedance?

In a circuit containing resistors, inductors, and capacitors, the opposition to the flow of current is called impedance. Impedance is typically denoted by Z and is a complex number, with the real part referred to as resistance and the imaginary part as reactance. The opposition caused by a capacitor to alternating current (AC) is called capacitive reactance, while the opposition caused by an inductor to AC is called inductive reactance. The combined opposition caused by capacitors and inductors in a circuit to AC is collectively known as impedance.

What is impedance control?

In a circuit board, conductors carry various signals, and to increase their transmission speed, their frequency must be raised. Variations in etching, layer thickness, and conductor width on the board can cause changes in impedance, leading to signal distortion. Therefore, in high-speed circuit boards, the impedance of the conductors should be controlled within a specific range, a practice known as "impedance control."

PCB transmission lines that require impedance control include single-ended microstrip, single-ended stripline, microstrip differential pair, stripline differential pair, embedded microstrip, and coplanar transmission lines (both single-ended and differential).

Factors Affecting Impedance

When one parameter changes while assuming all other conditions remain constant, the factors affecting impedance are as follows:

     1. Trace Width: Trace width is inversely proportional to impedance. The narrower the trace, the higher the impedance; the wider the trace, the lower the impedance. To achieve effective impedance control, trace width must be controlled within a tolerance of +/-10%. Any gap in the signal trace can affect the overall test waveform, with isolated impedance spikes leading to an uneven waveform. Impedance traces must not have any gaps, and any gap should not exceed 10%. Trace width is primarily controlled through etching. To ensure the correct trace width, process compensation is applied to the engineering film based on etching undercut, photoplotting errors, and pattern transfer errors.

     2. Dielectric Thickness: Dielectric thickness is directly proportional to impedance. The thicker the dielectric, the higher the impedance; the thinner the dielectric, the lower the impedance. Different prepregs have varying resin content and thickness. The thickness after lamination is related to the flatness of the press and the lamination program. To achieve the producible dielectric layer thickness, the key factors are engineering design, lamination control, and the tolerance of incoming materials.

     3. Dielectric Constant: The dielectric constant is inversely proportional to impedance. A higher dielectric constant results in lower impedance, while a lower dielectric constant results in higher impedance. The dielectric constant is primarily controlled by the material. Different materials have different dielectric constants, depending on the resin used. For example, the dielectric constant of FR4 material ranges from 3.9 to 4.5 and decreases with increasing frequency, while PTFE (Teflon) material has a dielectric constant of 2.2 to 3.9. High signal transmission requires a high impedance value, which in turn requires a low dielectric constant.

     4. Copper Thickness: Copper thickness is inversely proportional to impedance. The thicker the copper, the lower the impedance; the thinner the copper, the higher the impedance. Trace thickness can be controlled through pattern plating or by selecting a substrate with the appropriate copper foil thickness.

     5. Solder Mask Thickness: Solder mask thickness is inversely proportional to impedance. Within a certain thickness range, the thicker the solder mask, the lower the impedance; the thinner the solder mask, the higher the impedance. Normally, printing a solder mask once can reduce the single-ended impedance by 2 ohms and the differential impedance by 8 ohms. Printing twice results in a reduction twice as large as printing once, but after printing three or more times, the impedance value does not change further.

PCBWay Impedance Design & Control

(only sampling 100Ω differential impedance)

     a. Design stackup. The min. dielectric thickness between layers should be 50um.

     b. Design circuit width and space width. According simulation calculation design circuit trace.

     the trace/space width for stackup 1# is 70/130um;

     the trace/space width for stackup 2# is 95/140um;

     the trace/space width for stackup 3# is 125/130um;

     the trace/space width for stackup 4# is 105/150um.

Welcome to use our impedance calculation tool. To accommodate more design types, we will upgrade it in the future.

Impedance Calculation>>