PCB Back-Drill Defect Inspection

Controlled depth drilling, also known as back drilling or back drilling, is a technique used to remove unused portions or short sections of copper barrels from through-holes in printed circuit boards. It is mainly applied in industries with stringent requirements for high-frequency and high-speed signal transmission, such as high-performance servers, AI accelerator cards, GPU/CPU motherboards, and storage devices. With the advancement of 6G communications, AI computing chips, and L4/L5 autonomous driving, back drilling technology will evolve toward higher precision (e.g., laser back drilling) and more complex stack-up structures (HDI with more than 16 layers) to meet stricter signal integrity requirements.

                   

Reducing signal distortion: By eliminating excess copper stubs, signal reflection and distortion are reduced, improving signal quality.

Reducing EMI/EMR radiation: Reducing radiation interference from high-frequency signals improves the electromagnetic compatibility of the circuit.

Optimizing routing density: Allows more compact routing designs, supporting the installation of high-density BGA and other components.

Reducing bit error rate (BER): Lowering the bit error rate in signal transmission improves data transmission reliability.

Common defect types in PCB back-drilled holes

Inaccurate hole diameter: Holes that are too large or too small affect component mounting and electrical connections, potentially leading to signal reflection and crosstalk issues.

Rough hole wall: Due to drill bit wear, improper parameter settings, or substrate material quality issues, the hole wall surface is not smooth, affecting plating adhesion and electrical performance.

Residue inside the hole: Debris or impurities may be left during the drilling process, affecting hole cleanliness and reliability.

Drilled hole deviation: The drilled hole position does not match the design position, possibly caused by positioning accuracy issues or software setting errors.

Broken drill bit or incomplete penetration: Drill bit damage or insufficient drilling depth results in holes that do not fully penetrate the substrate.

Irregular hole shape: Shape deviations may lead to unstable signal transmission, affecting circuit performance.

Burrs: Excess metal fragments generated during drilling may affect subsequent processes and reliability.

Missed or plugged holes: Missed or plugged holes may occur during drilling, affecting interlayer connection quality.

These defects directly affect the performance and reliability of PCBs, so it is necessary to reduce defect occurrence by optimizing process parameters, improving equipment, and strengthening quality control.

        In the back drilling process, controlling hole deviation anddepth tolerance are key parameters for limiting drilling depth to ensure that drilling does not damage the useful copper, which should serve as conduction for other layers.

When high-speed signals propagate through copper barrels betweenPCB layers, they may become distorted. If the use of signal layers results in stubs, and the stubs are long, this distortion becomes significant.

These stubs can be removed after manufacturing by re-drilling with a slightly larger drill bit.

The hole structure and drilled hole deviation are shown in Figure 1.

Figure 1 Side view of PCB through-hole

The main function of a through-hole is conduction, so copper plating is required on the hole wall. However, the quality of the copper plating on the hole wall cannot be directly observed from the outside, nor is it easy to detect issues such as hole cracks.

In the past, the usual approach was to slice defective (NG) samples, destroying the sample to find defects and then carry out improvement work.

Today, 2D X-ray can first be used to preliminarily observe the overall copper plating condition and whether there is hole deviationwithout destroying the sample.

To further observe the copper plating condition of the entire hole wall in detail, 3D X-ray tomography scanning can be used for inspectionor algorithms can be used to synthesize multi-angle 2D images into 3D images 。

In 2D imaging at different angles, we can observe different manifestations of defects, as shown in Figures 2-1 and 2-2.

Figures 2-1, 2-2  2D imaging of the same defect at different angles

Under specific sample conditions, special 2D imaging can also reveal the complete defect state, enabling rapid assessment of defect type and severity.

This helps enterprises quickly complete the evaluation of back-drilled hole defects and process improvement, enhancing yield, as shown in Figure 3.

Figure 3  Rapid and clear identification of back-drilled copper stub defects

In industrial practice, to efficiently and accurately identify defect locations and reduce manual identification errors,

Beijing Top Unistar, based on machine vision recognition, quickly locates defect positions, with green indicating normal copper stubs and non-back-drilled areas, and red indicating defect locations, as shown in Figure 4.

Figure 4 Machine recognition of back-drilled hole defects