With the continuous advancement of technology, especially in digital circuits and communication systems, the demand for higher signal frequencies and bandwidths is growing. As a result, signal integrity has become a critical factor affecting system performance and reliability. This refers to the quality of signal transmission in a circuit, ensuring that the signal is transmitted clearly and accurately to its destination without being affected by noise, interference, or crosstalk.
In printed circuit boards, vias are used to connect different layers of the circuit, but when the via length exceeds what is needed, it creates an unnecessary via stub, which can degrade signal integrity. To solve this issue, back drilling is widely used, which effectively eliminates the negative effects caused by via stubs.
If you’re not familiar with the PCB backdrilling process, this article will provide a comprehensive introduction to this technique, guiding you on how to apply it in projects from both design and manufacturing perspectives to ensure stable and reliable signal transmission. Let’s get started!
What is Back Drilling in PCB
When discussing back drilling, the concept of vias must be mentioned. Vias come in several types, and a through-hole vias is a type that runs through the entire PCB. Through-hole can be categorized into two types based on their functionality: PTH (Plated Through Hole) and NPTH (Non-Plated Through Hole).
The primary difference between these two types of vias is whether the hole wall is plated. Plating is done to create an electrical connection, ensuring that the electrical signals can flow between different layers. PTH vias provide a conductive path from one side of the PCB to the other and are typically used for electrical connections, while NPTH vias are used for mechanical purposes, typically not plated and not used for electrical connections, often found in mechanical support or other non-electrical functions.
A via stub is part of a PTH via, referring to the unremoved residual section of the via within the PCB. This section does not carry signals and, instead, can cause signal reflection, resulting in signal distortion, particularly in high-speed signal transmission, and potentially leading to signal integrity issues.
Back drilling refers to drilling a hole slightly larger than the PTH diameter on the back side of the PCB. The primary goal of back drilling is to remove the residual section of certain vias, especially those connecting high-speed signal layers. After backdrilling, the length of the via stub should be controlled to within 10 mils; otherwise, issues like signal reflection that impact signal integrity may still persist. Since these holes are backdrilled to a predetermined, controlled depth, backdrilling is also known as controlled-depth drilling.
Example of Back Drilled PCB
Let’s assume your multilayer PCB has a 12-layer stack-up and contains a PTH via intended solely for signal transmission between layers 1 and 3. However, the via stub formed between layers 3 and 12 may cause resonance and reflection at very high frequencies.
To resolve this issue, back drilling is required. Back drilling can remove the copper plating of the PTH via between layers 3 and 12, reducing the length of the via stub, thereby minimizing reflection and signal loss, as shown in the following diagram.
It is important to note that, in order to ensure the complete removal of excess copper from the via wall, the diameter of the back drilled hole should be at least 8 mils larger than the original hole diameter, and preferably 10 mils larger. Additionally, to prevent damage to traces and circuit layers near the back drilled hole, the minimum spacing between traces and layers should be 10 mils.
Understanding Back Drilling from a PCB Project Perspective
In addition to back drilling, there are other feasible solutions to mitigate the negative effects of via stubs. For example, laser drilling can be used to create smaller, more precise microvias; blind and buried vias establish direct electrical connections between specific layers; and adjusting the PCB stack-up to route signal traces closer to the end of the via stub can also effectively reduce the impact of via stubs. Each approach has its advantages and drawbacks, so designers need to carefully analyze the specific case and choose the most suitable solution.
Typically, for PCB with frequencies above 3 GHz, it is recommended to select other techniques rather than back drilling. In these cases, the PCB density is usually higher, and the design is more intricate. Strict impedance matching and signal path optimization techniques must be applied to minimize signal attenuation and reflection.
However, for PCB with frequencies ranging from 1 GHz to 3 GHz, back drilling is often the primary choice. In this frequency range, although the signal frequency has not yet reached the extreme high standards, the impact of via stubs on signal quality remains significant. Compared to other techniques, back drilling in PCB offers an effective, cost-efficient way to eliminate the negative effects of via stubs.
Back Drilled PCB Design Parameters
| Index | Type | Value min. |
|---|---|---|
| A | Backdrill Ø | 400µm |
| B | Plated-through via Ø | 200µm |
| C | Copper clearance | 150µm |
| D | Ø-Difference circumf. | 100µm |
| E | Backdrill depth | 200µm |
| F | Distance to connected layer | see below |
| G | Thickness target layer | see below |
| H | Thickness stub remainder (safety) | see below |
Ending
Back drilling is a critical technique in ensuring high-quality signal integrity in printed circuit boards, especially for high-speed and high-frequency applications. As the demand for faster, more reliable circuits continues to grow, addressing issues like via stubs becomes essential for maintaining the performance of modern PCB. While back drilling effectively reduces the negative impact of via stubs, it’s important to recognize that it is just one of several methods available for managing signal integrity.
By understanding the underlying causes of signal degradation due to via stubs and exploring alternatives like microvias, blind and buried vias, or optimizing the PCB stack-up, designers can make informed decisions on the most appropriate solution for their specific needs.
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