In recent years, micro-drills for holes smaller than 1 mm has been widely used across multiple fields. However, premature failure of micro-drills limits their development. This paper introduces three main base materials (high-speed steel, cemented carbide, polycrystalline diamond) and two core approaches to enhance material performance—grain refinement and tool coating. It analyzes failure mechanisms (tool wear and breakage), highlights the criticality and challenges of optimizing cutting edge and core groove structures, and proposes two balanced design requirements: chip evacuation vs. drill rigidity, and cutting resistance vs. tool wear. Additionally, it reviews innovative designs for cutting edges and spiral grooves, summarizing current issues and challenges in micro-drill development.

Figure 1 Typical applications of micro-drillsImage

Table 1 Comparison of micro-drills for different workpieces (drills <1.0 mm are defined as micro-drills; data provided by Xiazhi Technology Tools Co., Ltd.)Image

By synthesizing relevant literature, this review scientifically analyzes common issues such as tool wear and breakage in micro-drills. It explores three key dimensions: drill materials (base material grain refinement and coating technology), failure mechanisms (wear and breakage), and geometric structure improvements (cutting edge and core groove design). Finally, it summarizes the current status, problems, and challenges in micro-drill design.

Drill Materials

Base Materials

Base materials are critical to micro-drill performance, directly impacting tool life, precision, surface quality, and production costs. They require sufficient hardness, wear resistance, rigidity, toughness, and thermal conductivity—vital for heat dissipation in ultra-small tools, especially when drilling epoxy-containing PCBs.

The main base materials for micro-drills include:

• High-speed steel: Once widely used for its high toughness and low cost, but its poor wear resistance and low hardness have limited its application as micro-hole quality requirements rise.
• Cemented carbide (WC-Co): The current market mainstream, especially in PCB manufacturing, due to its high hardness, wear resistance, moderate bending strength, and cost-effectiveness. Most manufacturers use welding to combine WC-Co drill bits with high-speed steel shanks, balancing performance and cost.
• Polycrystalline diamond (PCD): Ideal for hard-brittle or carbon fiber composite materials, offering ultra-high hardness, wear resistance, thermal conductivity, and low friction. Compared to single-crystal diamond (SCD), PCD has better toughness and spallation resistance due to cobalt binders. However, its strong affinity with iron causes chemical wear when drilling steel, and its high processing difficulty and cost restrict use to high-end or hard-to-machine workpieces. Methods like peck drilling, continuous cooling, and ultrasonic/laser assistance improve deep drilling of brittle materials.

Figure 2 Mechanical property comparison of cemented carbide, high-speed steel, and polycrystalline diamondImage

Figure 3 Welding manufacturing of micro-drillsImage

Figure 4 Schematic of single-hole peck drillingImage

Grain refinement is an effective solution to the high failure rate of micro-drills. According to the Hall-Petch law, smaller, more uniform WC particles enhance hardness, strength, and wear resistance—mechanical properties improve significantly when WC particle size is <500 nm. Currently, mainstream ultra-fine cemented carbide has a grain size of 200 nm, with reported minima of 90 nm (still evolving toward smaller sizes). Inhibitors such as VC, Cr₃C₂, or nano-cerium dioxide prevent abnormal grain growth during sintering. Optimal comprehensive performance requires matching cobalt content with WC particle size; mismatches can drastically reduce wear resistance.

Coating Materials

Base material improvements alone cannot meet high-performance demands. Coating technology combines substrate toughness with surface hardness/wear resistance, extending tool life and improving hole quality. Coatings must be dense, smooth, and strongly adhesive to substrates; some also provide thermal insulation.

Figure 5 Functions of coatings on micro-drillsImage

Common coating methods include physical vapor deposition (PVD) and chemical vapor deposition (CVD). Key coatings for micro-drills:

• Crystalline diamond coatings (MCD, NCD): NCD coatings (average crystal size <100 nm, thickness 100–300 nm) offer a smooth, dense surface, overcoming MCD’s high roughness. MCD has stronger adhesion to cemented carbide, enhanced by substrate roughening or boron doping. Diamond coatings (50–100 GPa HRC, 700–1079 GPa Young’s modulus) provide superior wear resistance at a lower cost than diamond substrates.
• Diamond-like carbon (DLC): Similar to diamond in performance with low friction and cost, but limited adhesion to substrates leads to peeling.
• Transition metal carbon/nitride coatings (TiN, TiAlN): TiAlN outperforms TiN in hardness, wear resistance, oxidation resistance, and adhesion due to aluminum addition. Higher aluminum content promotes dense Al₂O₃ film formation, boosting oxidation resistance.

Figure 6 Performance comparison of MCD, NCD, DLC, and TiAlN coatingsImage

Figure 7 Microscopic morphologies of different coatings: a MCD (M1, M2, M3); b NCD (N1, N2, N3); c DLC (D1, D2, D3); d TiAlN (T1, T2, T3)Image

Figure 8 Flank wear evolution of micro-drills with different coatingsImage

Experiments show NCD and DLC coatings have smooth surfaces; NCD-PCB tribological pairs exhibit the lowest friction coefficient (0.35), making NCD the optimal coating for carbide micro-drills. Coating thickness significantly impacts performance: 3 μm NCD extends service life 5–7x vs. uncoated drills, while 1 μm TiAlN is optimal—excessive thickness causes size effect-induced degradation, reducing adhesion and dulling cutting edges.

Multi-layer coatings outperform single-layer alternatives: multi-layer diamond slows microcrack propagation; NCD/MCD composites combine strong adhesion and smoothness; Ti/TiN/TiCN/DLC coatings achieve 65N adhesion and friction coefficient <0.15, extending tool life 2.5x. Worn DLC layers are filled by underlying Ti/TiN/TiCN to prevent peeling.

Figure 9 Schematic of Ti/TiN/TiCN/DLC multi-layer coating: a Structure; b Before drilling; c After drillingImage

Drill Failure

Failure Types

Micro-drills have an aspect ratio exceeding 20:1 (still increasing) and much higher spindle speeds than conventional drills (up to 350,000 rpm for PCB drilling). Their core components—cutting edges (material removal) and chip grooves (chip evacuation)—face challenges including deep holes, limited evacuation space, and invisible cutting processes, leading to chip entanglement or clogging. Conditions are more complex when drilling composites like PCBs.

Main failure modes are cutting edge wear and drill breakage. Excessive wear increases cutting force, degrades quality, and may cause breakage. Due to the complexity of PCB composites, simulation models are often simplified to single metals or simple composites, reducing result reliability.

Figure 10 Problems and solutions for micro-drillsImage

Wear Mechanisms

The cutting edge of twist micro-drills (chisel edge + main/secondary cutting edges) is prone to wear. Multiple wear types occur simultaneously:

• PCB drilling: Abrasive wear dominates (glass fiber chips and epoxy fillers damage WC-Co); adhesive wear follows (softened epoxy forms block chips that adhere to drills). High temperatures may induce chemical reactions between epoxy byproducts and cobalt.
• Metal drilling: Higher temperatures amplify oxidative and diffusion wear; dominant wear types vary by material and drilling conditions (e.g., abrasive wear for 316L stainless steel, adhesive wear for dry-drilled AA2024 aluminum).
• PCD drills: Edge spallation is the primary wear mode.

Wear occurs mainly on cutting edges, chisel edges, side surfaces, and rake faces:

• Chisel edge wear increases friction; PCB drills generate >70% of total axial force from the chisel edge, making wear-induced force increases critical for low-rigidity micro-drills.
• Cutting edge wear enlarges edge radius—wear accelerates exponentially when the instantaneous radius reaches 6x the initial value, and a 11% radius increase may cause breakage.
• Rake face wear typically appears as crescent pits (adhesive + diffusion wear).

Figure 11 Scratches and resin adhesion on flanks: a Side view of micro-drill; b Magnified scratched area; c Chisel edge of new drill; d Chisel edge after 3,500 holesImage

Figure 12 Fracture morphology of main cutting edge: a Side view; b Magnified fracture areaImage

Fracture Mechanisms

Cutting forces (axial, radial, torque) cause micro-drill fracture, primarily via bending or torsion:

• Bending fracture: Structural rigidity is an order of magnitude lower than conventional drills. Trade-offs exist between chip groove space and rigidity; large aspect ratios and reduced outer diameter decrease rigidity, leading to radial deviation at high speeds. Poor self-centering on rough/inclined surfaces increases bending risk.
• Torsion fracture: Excessive torque stems from wear-induced inefficient material removal and poor chip evacuation. High temperatures soften epoxy, causing clogging and friction, forming a vicious cycle. Stress concentration at spiral groove roots (a weak point) triggers fracture, mitigable by gradual depth reduction or transition buffers.

Figure 13 Comparison of two micro-drills: a Small core thickness and groove width; b Large core thickness and groove widthImage

Figure 14 Comparison of normal and deviated drillingImage

Figure 15 Stress distribution of micro-drills under bending load over timeImage

Figure 16 Resin adhesion and chip cloggingImage

Figure 17 Stress distribution of micro-drills under torsional load over timeImage

Figure 18 Micro-drill morphology before and after fractureImage

Figure 19 Fracture morphology of micro-drills: a Front view; b Top view; c Magnified sectionImage

Drill Structure

Cutting Edges

Optimizing cutting edge parameters (rake angle, major cutting edge angle) is crucial for balancing cutting resistance and wear. Spiral micro-drills exhibit varying rake angles along the cutting edge (negative near the tip, increasing outward), causing dulling.

Key improvements:

• Rake angle optimization: Adjusting five cross-sectional rake angles to 19°, 21°, 23°, 25°, 27° reduces cutting temperature, stress, and pressure by over 40%.
• Spiral rake face grinding: Maintaining consistent positive rake angles reduces average axial force by 23.8% and torque by 13.2%.
• Thinned cutting edges: Enhances effective chip thickness, improving chip breaking and reducing axial force.
• Inner edge grinding: Creates positive rake angles, suppressing extrusion and lowering drilling temperature.

Figure 20 Conversion of 3D FEM model to 2D simulation Image

Figure 21 Cross-sectional schematic of main cutting edge Image

Figure 22 Simplified orthogonal cutting simulation model of micro-drills Image

Figure 23 Geometric views of flat-flank micro-drills Image

Figure 24 Comparison of standard and thinned-edge micro-drills: a Standard; b Thinned-edge Image

Figure 25 Micro-drill cutting edges before and after grinding Image

Cutting temperature is critical: PCB resin adheres to drills at 120°C and heavily deposits at 140°C. Higher temperatures when drilling metals degrade drill strength. Temperature rises with edge radius, and uneven heat distribution occurs at ultra-high speeds—insulating coatings mitigate this.

Figure 26 Temperature distribution of main cutting edges at different speeds: a 120k rpm; b 135k rpm Image

Core Grooves

Core grooves facilitate chip evacuation, with depth, shape, and helix angle directly impacting evacuation efficiency and drill rigidity. Key improvements:

• H-shaped cross-section: Enhances uniform force distribution and bending resistance.
• Moderate helix angle: Balances cutting sharpness and chip evacuation (smaller angles improve evacuation).
• Single-edge structure: Reduces grooves to boost rigidity, but compromises hole quality vs. double-edge designs.
• Asymmetric spiral grooves: Merges two grooves to improve rigidity and positioning accuracy.
• Variable core thickness/groove width: Increases from tip to root, balancing evacuation space and rigidity.
• Multi-groove designs: More grooves increase temperature/stress and manufacturing complexity.
• 

Figure 27 Cross-sectional comparison of micro-drills with identical core thickness: a Standard; b H-shaped Image

Figure 28 Helix angle and chip evacuation efficiency Image

Figure 29 Novel single-edge micro-drill Image

Figure 30 Alternative single-edge micro-drill tip shape Image

Figure 31 Novel micro-drill with asymmetric spiral grooves Image

Figure 32 3D geometry with variable core thickness and groove width: a Overall structure; b L=0.5 mm; c L=1.5 mm; d L=2.5 mm; e L=3.5 mm Image

Figure 33 Micro-drill models with different groove counts and simulation results: a Models; b Maximum cutting temperature; c Maximum cutting force; d Maximum Von Mises stress Image

Conclusions and Recommendations

This review focuses on micro-drill material design, failure analysis, and structural optimization, summarizing progress in grain refinement, coating technology, and structural innovation. Three key challenges remain:

1. Exploring optimal cobalt-WC particle size matching and coating thickness for cemented carbide;
2. Improving simulation accuracy for composite materials like PCBs;
3. Enhancing micro-drilling quality for high-hardness materials via hybrid traditional and non-traditional technologies.

This review serves as a reference for engineers, particularly in coating applications and structural innovation. Future research should prioritize manufacturing processes, advanced coatings, and hybrid machining to develop longer-lasting, higher-precision micro-drills. Additionally, manufacturing efficiency, hole quality, and non-twist micro-drills warrant further investigation.