Technical Exploration of HLC PCB Drilling with Aspect Ratio ≥30×——Taking Cusp HLC Drill Bits as an Example
March 20, 2026
Driven by applications such as AI Servers and High-Performance Computing (HPC), PCB structures are evolving toward higher layer counts (High Layer Count, HLC) and thicker boards, presenting unprecedented challenges to mechanical drilling processes. Among these, the high aspect ratio (Aspect Ratio ≥30) formed by continuous miniaturization of hole diameters and increased board thickness has become a key factor limiting the mass production stability of HLC PCBs. Starting from the perspectives of material mechanics and cutting mechanics, this paper combines engineering formulas and structural schematics to explore the core technical points of drilling with an aspect ratio of 30× or higher, and illustrates them with the design ideas of Cusp Technology's HLC drill bits as a practical case.
1. Technical Background of HLC PCB and High Aspect Ratio Drilling
In HLC PCBs with 40–60 layers or more and a board thickness of 6.0–8.5 mm, even if the hole diameter remains at the micron level, the drilling aspect ratio quickly enters the range of 30× or higher.
At this point, the drilling behavior can be regarded as a force problem of a slender tool in a deep hole, and its stability is no longer dominated by the cutting speed, but determined by the structural rigidity and force distribution.
2. Why Address the Bending Stiffness Issue in Drilling with Aspect Ratio ≥30×
2.1 Mechanical Concept of Bending Deflection
In deep hole drilling, the drill bit can be approximately regarded as a cantilever beam fixed at one end and subjected to a lateral force at the other. Its bending deflection δ can be expressed by the following formula:
δ ∝ E⋅I F⋅L 3
Where:
- (F): Lateral force generated during the cutting process
- (L): Effective cantilever length (approximately equal to the hole depth)
- (E): Elastic modulus of tungsten carbide material
- (I): Second moment of area (highly related to the web thickness)
When the aspect ratio increases to 30× or higher, the L 3 term expands dramatically, making even a small lateral force likely to cause significant deflection, thereby leading to hole deviation and misalignment.
2.2 Design Response of HLC Drill Bits to Bending Stiffness
The design strategy of Cusp HLC drill bits is not simply to thicken the overall size, but:
- Increase the equivalent web thickness in the key deep hole section
- Cooperate with a progressive web taper to improve the second moment of area I in the deep hole section without sacrificing the sharpness of the front-end cutting edge
This design can effectively suppress the lateral deflection under the condition of hole depth ≥30×.
3. Torque Behavior and Load Stability in High Aspect Ratio Drilling
3.1 Torque Sources and Deep Hole Amplification Effect
During the drilling process, the total torque T borne by the drill bit can be regarded as the combined result of material shear resistance and frictional resistance:
T ≈ ∫(τ ⋅ r dA) + T friction
As the hole depth increases, the following effects are simultaneously amplified:
- Contact friction between the flute and the hole wall
- Pushing resistance of chips in the deep hole
- Local heat accumulation leading to an increase in the friction coefficient
From the perspective of cutting mechanics, the drilling torque can be conceptually decomposed into three main sources:
T total = T cut + T chip + T friction
Where:
- : Additional resistance generated by the pushing of chips against the flute travel
- : Frictional torque between the outer diameter of the drill bit and the hole wall (highly related to the hole depth)
In short holes or low aspect ratio drilling, the proportions of T chip and T friction are relatively limited; however, under deep hole conditions of 30× or higher, the latter two will rapidly amplify and become highly unstable.
If the torque distribution is uneven along the hole depth, it is easy to form instantaneous torque spikes in the deep hole section, causing micro-cracks in the cutting edge and delamination of the board layers.
3.2 Design Ideas of HLC Drill Bits for Torque Stability
To reduce torque fluctuations at high aspect ratios, Cusp HLC drill bits adopt the following design strategies:
- Redistribute the edge geometry to make the material removal rate smoother along the hole depth
- Design deep hole guide flutes to reduce the additional torque caused by chip clogging and pushing
- Match with a low-friction, high-adhesion coating to suppress frictional heat and torque accumulation in the deep hole section
Its goal is not to eliminate the average torque, but to avoid unforeseen torque spikes.
4. Coupling Relationship Between Chip Removal Behavior and Deep Hole Stability
Under the condition of aspect ratio ≥30×, poor chip removal will simultaneously amplify:
- Lateral force (affecting bending deflection)
- Frictional torque (affecting hole wall quality and hole position accuracy)
Therefore, Cusp HLC drill bits optimize the flute angle and deburring chamfer to enable continuous chip discharge along a specific direction, reducing the uncertainty of structural stress and forming another link in the stable drill bit design.
5. Conclusion
In the HLC PCB manufacturing process, drilling with an aspect ratio of 30× or higher is essentially an engineering problem coupling structural mechanics and cutting behavior. Taking Cusp HLC drill bits as an example, this paper shows that their design does not pursue a single performance indicator, but through three means: structural rigidity enhancement, torque smoothing, and chip removal stabilization, enables the drill bit to maintain mass production performance close to that of short holes under deep hole conditions, providing key support for the reliability of mass production with aspect ratio ≥30×.
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