1. **The characteristic impedance value in the PCB transmission line must match the electronic impedance of the Driver and Receiver**

In recent years, with the advancement and widespread application of IC integration, signal transmission frequencies and speeds have been continuously increasing. As a result, in the PCB traces, once the signal transmission (emission) reaches a certain threshold, it begins to be affected by the PCB itself. The trace’s inherent properties can lead to significant distortion or even complete loss of the transmitted signal. This highlights that what is being “carried” by the PCB trace is not just current, but the transmission of square waves or pulses of energy. The resistance encountered during the transmission of this “signal” is referred to as “impedance,” symbolized as Z0. Thus, it is not enough to simply address issues like “open,” “broken,” or “short-circuited” traces on the PCB; controlling the impedance of the traces is equally important.

2. **What is impedance?**

Impedance is a parameter used to characterize the behavior of electronic components. It refers to the total opposition a component presents to alternating current at a specific frequency.



**(B)** *Why do we need impedance control?*

The characteristic impedance of a PCB transmission line must match the electronic impedance of both the Driver and the Receiver. If they do not match, it can lead to signal reflection, attenuation, and delays in signal arrival time. In extreme cases, this mismatch may prevent the signal from being correctly interpreted or cause boot-up failures.

When a signal propagates along the PCB traces, several factors affect the “characteristic impedance,” including the trace’s cross-sectional area, the thickness of the insulating material between the trace and the ground layer, and the dielectric constant of the material. The primary factors affecting impedance are:

1. Trace width

2. Prepreg (PP) thickness

3. Dielectric constant (e.g., FR-4 = 4.3)

Other factors, such as solder mask thickness, undercutting, and copper thickness, also influence impedance. These variables affect the distribution of the magnetic field, which in turn alters the assembly resistance. To control this, it’s important to first understand the tolerance requirements for assembly resistance, then infer the maximum allowable tolerance for the manufacturing process, and calculate whether it is achievable using simulation software.

In PCB manufacturing, process control should focus on keeping material and wire diameter tolerances within 10%, and ensuring the interlayer thickness after lamination is also within 10%. By doing so, the design specifications can be met.

**(C)** *What is the Er value?*

The dielectric constant (Er), or relative permittivity, is a measure of the electrostatic energy that can be stored in an insulating material per unit volume for each unit of potential gradient. A higher dielectric constant indicates that more signal energy is stored in the material, which can degrade signal quality and reduce propagation speed. Materials like PTFE (Teflon), which have an Er value of 2.5, are typically used in high-performance applications where signal integrity is critical.

**(4)** *General Impedance Categories*

Impedance is typically classified into three types:

1. **Characteristic impedance** (also known simply as impedance).

For example, when designing a 4-layer PCB, if the impedance of the outer traces needs to be controlled, the software model used to calculate the required trace width for a given impedance is as follows.

**(5)** *Design Notes for Coupons*

1. The general impedance design includes a design layer (signal layer) and a reference ground layer (corresponding layer). For a 4-layer board without specific customer requirements, the typical layer stackup is:

– L1 (signal layer)

– L2 (ground layer, corresponding to L1)

– L3 (ground plane, corresponding to L2)

– L4 (signal layer).

2. Special care should be taken with impedance control for inner layers of the PCB. For example, when two consecutive signal layers are controlled by trace width, they may not have a dedicated ground layer or copper plane beneath them, which can affect impedance values.

3. For outer trace impedance, it is generally advisable to protect the traces with copper pour. The wider the copper pour, the better, and the spacing between the traces and the copper pour should be at least 10 mils.

4. When designing the inner layers of a PCB with impedance control, it is important to consider the upper and lower ground planes (corresponding layers) and whether they are covered with copper. Impedance calculation software and impedance strip design tools may vary in how they treat these factors.

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