PCB BACK DRILLING PROCESS (Importance Challenges Advantage)

I. What is PCB backdrilling?

One of the challenges faced in PCB design and manufacturing is how to protect signal integrity. Backdrilling, also known as controlled depth drilling, is used to remove the conductive via stub remnants inside PCB through-holes. As part of the via, the stub can cause serious signal integrity issues in high-speed designs.

Additionally, the via stub can cause the signal to reflect back from the stub end, disrupting the original signal. In other words, if the stub is too long, the distortion will be severe. To solve this problem, backdrilling is performed, where a slightly larger drill bit is used to re-drill the hole and remove most of the via stub.

II. How does PCB backdrilling overcome signal integrity issues?

By using a slightly larger drill bit, the remaining via stub is removed after the plated through-hole is manufactured. The hole is backdrilled to a predetermined and controlled depth, which is close to but does not reach the last layer used by the hole.

The ideal remaining stub should be less than 10 mils. The backdrill hole diameter is larger than the plated through-hole diameter. Usually, the drill bit diameter is 8 to 10 mils larger than the original drill bit. This is because the clearance between the traces and planes must be large enough to avoid accidentally drilling through the traces and planes adjacent to the backdrill.

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III. Why is back drilling necessary?

Backdrilling can be used to shorten the via stub, which can interfere with high-speed signals. Typically, backdrilling results in via stubs that are less than 10 mils long, reducing the ability of the copper barrel to produce signal reflections.

For example, in a 10-layer PCB, a through-hole was drilled from layer 1 to layer 10. But your design only requires signals from layer 1 to layer 3. The via will have a stub after layer 3, and the unused portion can cause high-frequency reflections and resonance.

Therefore, backdrilling is needed to remove the excess plated copper after layer 3. Additionally, the backdrill must be larger than the original PTH size, otherwise, not all the unnecessary copper can be removed.

The actual back drilling process is as follows:

1. In the figure, you can see a PCB with via stubs extending beyond the signal path.

2. Use a slightly larger drill bit to perform the back drilling.

3. Reduce the via stubs by backdrilling.

IV. When to use backdrilling?

It is generally recommended to consider adding backdrilling when the PCB circuit traces have signals with a speed of ≥1 Gbps, but designing high-speed interconnects is a complex system engineering task that also requires considering factors such as chip drive capability and interconnect length. Therefore, system interconnect simulation is the most reliable way to determine whether backdrilling is necessary.

Common characteristics of PCB backdrilling:

• Typically on the backside, which is a rigid board
• Generally used for 8 layers or more
• Board thickness greater than 2.5 mm
• Minimum keepout dimension of 0.3 mm
• Backdrill diameter is 0.2 mm larger than the via
• Backdrill depth tolerance of +/- 0.05 mm

V. PCB Back Drilling Process Flow

• Seal the positioning holes with a dry film before plating.
• Use copper plating to create conductive paths.
• Create outer layer patterns on the plated PCB.
• After the outer layer pattern is completed, perform pattern plating on the PCB. Prior to this process, it is important to seal the positioning holes with a dry film.
• Back drilling utilizes the positioning holes from the initial drilling process, using the drill bit to back drill the plated holes that require this process.
• After back drilling, the board needs to be cleaned to remove any residual drill debris that may exist from the back drilling.
• Inspect the circuit board to verify the back drilling process was executed accurately and the signal integrity has been enhanced.

Example of Back Drilling:

Assume in a 12-layer stack there is a through-hole from Layer 1 to Layer 12, but the via is only applicable for signals from Layer 1 to Layer 3. Therefore, a via stub is created after Layer 3 and Layer 12, which will generate resonance and reflections at very high frequencies, reducing the signal's resonant frequency. Therefore, back drilling is performed after Layer 3 and Layer 12 to remove the plated copper, reducing the short stub length. The back drilling should be larger than the original size to remove the unnecessary copper.

VI. How Much Residual Stub Length Can Be Retained?

During back drilling, you must calculate how much residual stub length can be retained without affecting the PCB performance. This decision factor depends on several other factors, including signal integrity and the actual manufacturing process.

Typically, reducing the maximum residual stub length and increasing the via size used for back drilling will increase the manufacturing cost. This decision will depend on several interrelated factors, including the required signal, and the table below details the signal loss corresponding to the residual stub length.

VII. Challenges in Back Drilling Process

Back Drilling Depth Control

Back drilling utilizes the depth control function of the drill bit to machine blind holes, and its tolerance is mainly affected by the precision of the back drilling equipment and the tolerance of the dielectric thickness. Additionally, its accuracy is easily affected by external factors, such as drill bit resistance, drill tip angle, the contact effect between the cover plate and the measurement unit, and board warpage. Therefore, in production, it is necessary to select suitable drilling materials and methods, and control their precision, to achieve the best results.

Back Drilling Accuracy Control

Back drilling is based on a second drilling operation after the first drilling, and the accuracy of the second drilling is critical. Board expansion and contraction, equipment precision, drilling methods, etc. can all affect the accuracy of the second drilling overlap, which is crucial for the quality control of subsequent PCB processes.

VIII. Advantages and Disadvantages of PCB Back Drilling

Advantages

• Back drilling helps reduce signal attenuation, ensuring stronger and more reliable signals. Additionally, this technique helps minimize the impact of stubs on impedance matching, thereby reducing EMI/EMC radiation.
• Back drilling is also an effective method to prevent signal distortion problems. It is well known that via stubs can cause deterministic jitter, which may be caused by signal crosstalk, EMI, and noise. By removing these stubs, back drilling can help eliminate the source of deterministic jitter, improve signal quality, and prevent signal distortion issues.
• Back drilling holes help maximize the isolation between vias.
• By implementing back drilling, the deterministic jitter in the signal can be reduced, thereby lowering the overall bit error rate (BER) of the signal.
• Reduce the excitation of resonant modes.
• Minimize the use of buried and blind vias to simplify PCB production.
• Minimal impact on design and layout.
• Extend channel bandwidth;
• Lower cost compared to sequential lamination

Disadvantages

One disadvantage of back drilling is that it is only suitable for frequency ranges between 1GHz to 3GHz and high-frequency boards without viable blind vias. Additionally, special techniques must be used to prevent any damage to the traces and planes on the back side of the holes.

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References:

1. https://www.protoexpress.com/blog/back-drilling-pcb-design-and-manufacturing/

2. https://www.altium.com/documentation/altium-designer/controlled-depth-drilling-or-back-drilling-ad?version=18.1

3. https://www.orientdisplay.com/knowledge-base/pcb-basics/pcb-back-drilling/

4. Shi, H., Zhu, T. & Chen, Z. Back-drilling of high-speed printed circuit boards: a review. Int J Adv Manuf Technol 121, 1483–1499 (2022). https://doi.org/10.1007/s00170-022-09476-7

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