PCB Backdrilling

What is PCB Backdrilling?

PCB backdrilling, also known as controlled-depth drilling, involves the removal of stubs in multi-layer PCBs to create plated through-holes. The purpose of backdrilling is to facilitate the flow of signals between different layers of the circuit board without interference from unnecessary stubs.

For a clearer explanation of the backdrilling process, let’s consider an example. Imagine a 12-layer PCB with plated through-holes connecting the first and twelfth layers. The goal is to connect only the first layer to the ninth layer while keeping the tenth to twelfth layers unconnected. However, the unconnected layers create “stubs” that can potentially disrupt signal paths, leading to signal integrity issues. Backdrilling involves drilling out these short stubs from the backside of the PCB to improve signal transmission.

So, when is backdrilling used? It is generally recommended to consider this technique when signals on the PCB board operate at rates of ≥1 Gbps. However, designing high-speed interconnects is a complex system engineering task that should also take into account factors like the chip’s driving capabilities and the length of the interconnect links. Therefore, performing system-level interconnect simulations is the most reliable way to determine the need for backdrilling.

Advantages and Disadvantages of Backdrilling

Advantages of PCB Backdrilling

• Reduces Signal Attenuation: Backdrilling helps reduce signal attenuation, ensuring stronger and more reliable signals. Additionally, this technique minimizes the impact of stubs on impedance matching, which in turn reduces EMI/EMC radiation.
• Prevents Signal Distortion: Backdrilling is an effective method for preventing signal distortion issues. It is well-known that via stubs can introduce deterministic jitter, possibly caused by signal crosstalk, electromagnetic interference, and noise. By eliminating these stubs, backdrilling helps eliminate sources of deterministic jitter, improving signal quality and preventing signal distortion.
• Minimizes Crosstalk Between Vias: Backdrilling holes contribute to minimizing crosstalk between plated through-holes.
• Reduces Deterministic Jitter: Implementing backdrilling can reduce deterministic jitter in signals, which can lead to an overall decrease in the bit error rate (BER) of the signals.
• Decreases Excitation of Resonance Modes: Backdrilling helps reduce the excitation of resonance modes.
• Minimizes the Use of Buried and Blind Vias: It simplifies PCB manufacturing by reducing the need for buried and blind vias.
• Minimal Impact on Design and Layout: Backdrilling has minimal impact on the overall design and layout.
• Expands Channel Bandwidth: It allows for the expansion of channel bandwidth.
• Lower Cost Compared to Sequential Lamination: Backdrilling can be more cost-effective compared to sequential lamination.

Disadvantages of PCB Backdrilling

• Frequency Limitation: Backdrilling is suitable for signal frequencies in the range of 1 GHz to 3 GHz and may not be viable for high-frequency boards with blind vias. Special techniques are needed to prevent any lateral traces and planes on the backside of the board from being damaged during backdrilling.

PCB Backdrilling Process

• Drilling Holes in the PCB: Holes are drilled in the PCB to create plated through-holes connecting multiple layers of the circuit board.
• Sealing Positioning Holes with Dry Film: Positioning holes are sealed with dry film before electroplating the holes to create conductive pathways.
• Copper Plating of Holes: Copper is plated onto the holes to create the conductive paths.
• Outer Layer Graphics Creation: After creating the outer layer graphics, graphic electroplating is performed on the PCB. Proper treatment of the sealed positioning holes is crucial before this process.
• Backdrilling Execution: During the initial drilling process, the positioning holes used for alignment in the backdrilling process are used, and the plated holes from this process are backdrilled.
• Cleaning the PCB: After backdrilling, it is necessary to clean the PCB to remove any residual drill debris.
• Inspection of the PCB: The PCB is inspected to verify the accuracy of the backdrilling process and to ensure enhanced signal integrity.

PCB Backdrilling Design Tips

To ensure proper backdrilling, it is essential to provide the PCB manufacturer with separate output files containing the backdrilling layers and detailed specifications of which layers require corresponding backdrilling. The diameter of backdrilled holes should be at least 0.2mm larger than the diameter of the first hole, with a distance of 0.35mm for the first drill and 0.2mm for the backdrill from the traces. During PCB layer stack-up design, consideration should be given to the dielectric thickness to avoid drilling into traces that should not be drilled into. If specific layers require drilling (e.g., “large” layers), the dielectric thickness between non-drilled and adjacent layers should be at least 0.2mm.

Additionally, optimizing the backdrilling process involves minimizing the number of blind and buried vias and avoiding them in less critical areas. Placing vias in less critical areas and maintaining minimum distances between backdrill holes and signal traces also helps prevent signal reflections and other issues. Keeping backdrilled hole diameters small is crucial to avoid damaging lateral traces and planes on the backside of the board. Furthermore, considering backdrilling during the initial design phase helps ensure necessary steps are taken to optimize signal integrity and prevent issues during the manufacturing process.

Challenges in the Backdrilling Process

• Control of Backdrilling Depth: Controlling the depth of backdrilling is crucial for precise machining of blind vias. The backdrilling depth tolerance is primarily influenced by the accuracy of the backdrilling equipment and the tolerance of dielectric thickness. However, external factors such as drill bit resistance, drill tip angle, contact effects between the cover plate and the measuring unit, and board flexing can also affect backdrilling precision. During the production process, selecting the right drilling material and method is crucial for achieving optimal results and controlling backdrilling accuracy. By carefully controlling the depth of backdrilling, design engineers can ensure high-quality signal transmission and prevent signal integrity issues.
• Control of Backdrilling Precision: Precise control of backdrilling is essential for quality control of the PCB in subsequent processes. Backdrilling involves secondary drilling based on the diameter of the primary hole, and the precision of this secondary drilling is critical. Several factors, including board expansion and contraction, equipment accuracy, and drilling methods, can influence the precision of overlapping secondary drills. Therefore, it is important to ensure accurate control of the backdrilling process to minimize errors and ensure optimal signal transmission and integrity.

Application of PCB Backdrilling

PCB (Printed Circuit Board) back drilling is primarily used in high-speed and high-frequency electronic applications to improve signal integrity and prevent signal distortion. Here are some specific applications where PCB back drilling is commonly employed:

• Telecommunications Equipment: PCBs used in telecommunications infrastructure, such as base stations and networking equipment, often require high-speed signal transmission. Back drilling helps ensure reliable signal paths and reduces signal degradation, which is crucial for maintaining communication network performance.
• Data Servers and Data Centers: Servers and data centers process vast amounts of data at high speeds. To maintain data integrity and prevent signal interference, back drilling is applied to PCBs within server equipment and data center infrastructure.
• Consumer Electronics: High-end consumer electronics, like smartphones, tablets, and high-definition displays, may utilize PCB back drilling to enhance signal quality and minimize signal distortion, especially in devices with demanding video and data transfer requirements.
• Medical Devices: Medical equipment, including imaging devices, diagnostic tools, and patient monitoring systems, often require precise and reliable signal transmission. PCB back drilling is used to ensure the accuracy of data transfer and reduce the risk of signal degradation.
• Aerospace and Defense: In aerospace and defense applications, PCBs are subjected to extreme conditions and demanding performance requirements. Back drilling helps maintain signal integrity in radar systems, avionics, satellite communication equipment, and military hardware.
• Automotive Electronics: Advanced automotive systems, such as autonomous driving and vehicle-to-vehicle communication, rely on high-speed data exchange between electronic components. PCB back drilling can be employed to minimize signal distortion and ensure safe and reliable communication within vehicles.
• Industrial Automation: Industrial control systems and automation equipment often use PCBs with back drilling to support high-speed communication between sensors, controllers, and actuators, ensuring precise and efficient industrial processes.
• Test and Measurement Instruments: Instruments used for scientific research, laboratory testing, and electronic measurement require precise signal transmission. PCB back drilling helps maintain signal integrity in these instruments, contributing to accurate measurements and analysis.
• Networking Equipment: Routers, switches, and other networking equipment depend on efficient data transmission. Back drilling is applied to PCBs in networking devices to reduce signal loss and improve network performance.
• High-Performance Computing: Supercomputers and high-performance computing clusters rely on PCBs with back drilling to achieve fast data transfer rates and minimal signal interference, enabling complex simulations and scientific research.

In these applications, the use of PCB back drilling helps ensure that high-speed signals travel through the PCB with minimal attenuation, signal distortion, or interference, ultimately enhancing the overall performance and reliability of electronic systems.