Declines in PCB Signal Pathways
In the world of electronics, the efficiency of a Printed Circuit Board (PCB) transmission line is crucial for maintaining signal integrity. One of the key factors affecting the efficiency is conductor loss, which we will delve into in this article.
Conductor loss in a PCB transmission line is primarily influenced by several factors. The physical dimensions of the copper trace, such as width and thickness, play a significant role. Increasing these dimensions generally reduces resistance and thus conductor loss [4][5].
The surface roughness of the copper foil or via barrels is another critical factor. Rough surfaces can cause additional signal scattering and skin effect losses, particularly at high frequencies. Smoother surfaces help minimize these losses [1].
The material properties, including the conductivity of the copper and the dielectric environment, also impact conductor loss. Thermal properties are significant since heat affects resistance [2][3]. At higher frequencies, current tends to flow on the conductor surface (skin effect), effectively reducing the conductor cross-section and increasing resistance, thus increasing conductor loss [4].
The proximity to ground planes, trace spacing, and adjacent structures can affect the effective resistance and inductance of the trace, thereby influencing conductor loss and signal integrity [5]. Elevated temperatures increase copper resistivity, leading to higher conductor losses. Thermal cycling can also cause microfractures affecting long-term reliability [2].
It's worth noting that the return current is not uniformly distributed over the plane, with most of the current concentrated on a strip of width three times the width of the signal trace and just underneath the signal trace. If the frequency is larger than the skin depth, it causes additional conductor losses [6].
Skin depth, a measure of surface roughness of PCB foils, determines the depth of the conductor involved in the signal current. At very low frequencies, the entire conductor participates in the signal current. However, at higher frequencies, only a certain depth called the skin depth of the conductor is involved [7]. Beyond 15 MHz, the signal current travels only in-depth less than 0.7 mils and decreases as the frequency increases [8].
The signal trace resistance in a PCB transmission line can be calculated using the formula: Rsig = (ρ / (W * δ)), where ρ is the resistivity of the copper, W is the width of the trace, and δ is the skin depth [9]. At high frequencies (if f is in GHz, W and T in mils), conductor loss is proportional to the square root of the frequency [10].
The total insertion loss in a PCB transmission line is the sum of conductor loss (alfa C) and dielectric loss (alfa d) [11]. Experimentally, losses in the case of VLF foil are lower than in the case of HTE foil [12].
In a circuit board, the 'copper conductor - dielectric interface' is roughened into a teeth-like structure to increase the peel strength of conductors on the circuit board [13].
In conclusion, understanding and managing conductor loss is essential for optimising the performance of PCB transmission lines. Factors such as physical dimensions, surface roughness, material properties, frequency-related effects, temperature, and PCB stackup/layout all play crucial roles in determining conductor loss. By considering these factors, designers can create more efficient and reliable PCBs.
Controlled impedance technology is often employed to maintain signal integrity in PCB transmission lines, as it accounts for various factors influencing conductor loss such as physical dimensions, surface roughness, material properties, and frequency-related effects. The importance of managing conductor loss for optimizing PCB performance cannot be overstated, considering its role in factors like physical dimensions, surface roughness, material properties, frequency-related effects, temperature, and PCB stackup/layout.
