Signal Integrity and Interconnects : PCB design Tutorial

The Printed Circuit Board (PCB) serves a crucial role in electronic devices by facilitating electrical connections between various components. This intricate web of connections is collectively known as connectivity. However, not all interconnects are created equal; some maintain signal integrity seamlessly, while others introduce noise that can degrade signals. In the previous PCB Technologies design tutorials we explained Soldermask Layer, Silk Screen, Thermal Relief Vias and PCB surface finishes. In this tutorial, we delve into the world of signal integrity and interconnects to understand the factors that make them transparent or non-transparent.

Understanding Transparent Interconnects

1. Transparent Interconnects: When the electrical properties of interconnects do not impact signal quality, we term them as transparent. These interconnects maintain the integrity of signals without introducing noise that could affect the product's performance.

2. Design for Connectivity (DFC): Achieving transparency involves adhering to Design for Connectivity (DFC) principles. This ensures that all nodes and terminals are correctly connected, and terminals that shouldn't be electrically linked remain separate.

3. Net List: In PCB design, terminals or pins connected by wires form a net. The collection of all nets on the circuit board, along with terminal connections, constitutes the net list—a database description of the PCB.

4. Solderless Breadboard Test: A product working on a solderless breadboard with haphazard wire connections suggests transparent interconnects. If it functions in this setup, it is likely to work on a PCB without specific signal integrity considerations, as long as connectivity is maintained.

5. Example of Transparent Interconnects:

In cases where interconnects are transparent, the layout of wires and part placement may introduce noise, but it remains within acceptable levels that won't impact the product. See example below.

Transparent Interconnects

Factors Influencing Signal Integrity

1. Rise Time and Noise: Many electrical noise issues scale inversely with the rise time of the signal and linearly with the length of the interconnects. Shorter rise times result in more noise, while longer interconnects contribute to increased noise levels.

2. Sources of Noise: Noise in interconnects often stems from factors like dI/dt or dV/dt, with loop inductance and capacitance between signal interconnects being critical contributors.

3. Transparent Interconnects' Appearance: Interconnects may appear transparent based on the rise time of signals and the lengths used in a product. However, this transparency can change with different rise times, making shorter interconnects generally more transparent.

4. Practical Example:

A practical example illustrates how the length of interconnects can impact noise. In this case, a transistor switching a current creates a voltage drop, with shorter interconnects resulting in a smaller noise dip compared to longer interconnects.


5. Rise Time Considerations: Interconnects that were transparent with one rise time might not be as transparent with a shorter rise time. At a fixed rise time, shorter interconnects tend to exhibit greater transparency.

In conclusion, achieving signal integrity in PCB design involves understanding the transparency of interconnects. Designing with connectivity in mind and considering factors like rise time and interconnect length are essential for minimizing noise and ensuring optimal product performance. As we navigate the intricacies of PCB design, embracing these principles will contribute to the creation of robust and reliable electronic devices.

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