With the rapid development of today’s science and technology, micro-hole machining technology has become a key technology in many fields of industry.
Miniaturized equipment demands smaller, more precise, and faster micro-holes, making their study essential.
Micro-hole processing classification
At present, the diameter of the hole is usually less than 0 ∙ 3mm called micro-hole diameter 0 ∙ 3 ~ 1mm hole called small hole.
This paper reviews micro-hole machining methods and their applications, including drilling, stamping, EDM, ECM, ultrasonic, laser, and beam techniques.
Researchers divide micro-hole processing methods into mechanical and special processing (Table 1).
Micro-hole machining employs drilling and methods like EDM, laser, and ultrasound, chosen by material, precision, roughness, and hole size.
Machining is cheaper and more versatile; special processing is costly, so machining remains preferred when suitable.
Mechanical Processing
Drilling of microvias
Traditional machining removes material with a tool to shape parts and is widely used.
Microvia machining commonly uses drilling as a machining method.
Drilling provides high productivity, no conductivity limits, and precise small-hole machining.
Micro-hole machining relies on drilling, widely used in electronics and precision instruments.
But the drilling of small holes still have the following problems:
- As the drill bit diameter decreases, manufacturers find it increasingly difficult to make it.
Smaller drill diameters reduce stiffness and strength, increasing breakage risk from forces or vibrations.
- Chip space is small, especially in deep holes, making chip discharge difficult and damaging the drill.
- High drill bit temperature from poor heat dissipation shortens its service life.
- Deep small-hole drilling demands speeds over 10,000 r/min and minimal spindle error.
- Processing high hardness materials is more difficult.
Technical issues in small-hole drilling can be improved through machinery, technology, and tools.
Swiss Fisher and French Forest-line high-speed motors and spindles boost micro-hole drill speeds to 180,000 r/min.
New spindles and feed mechanisms enable smaller, more efficient micro-hole drills.
Air-floating and magnetic bearings further improve spindle rotation accuracy.
For example, the precision spindle produced by NSK in Japan has a rotary accuracy of 2μm.
Some use dynamic pressure on the drill to boost drilling efficiency and speed.
Drill sleeves, guide holes, and step drilling prevent breakage and improve life and accuracy.
Optimizing cutting speed, feed, and cooling is a key current research focus.
Tool advances improved micro-drills, enabling 30 μm diameters and 60 μm depth (Yoshitomo Electric).
The company also produces a ●0∙2mm carbide drill with a maximum machining depth of 10d=2mm.
Punching small holes
Compared with drilling, punching small holes is more suitable for processing a large number of small holes on the plate.
Punching microvias is characterized by:
High productivity. In mass production, its production cost is much lower than drilling.
Long life of the die wear slow processing out of the hole size stability.
When hole diameter is too small for a given plate thickness, limited die strength and rigidity prevent punching.
Punching uses a cylindrical punch and a matching die to press and cut a through-hole in a plate under external force.
But when the size of the punch and the negative die is very small when the two tools of the alignment problem becomes very urgent.
A micro shaft is shaped by WEDG for alignment, used in micro EDM to machine the die, then reshaped into a punch.
Punch and die are machined together without repositioning, ensuring accurate centerline alignment for plate punching.
Because the punching head can shift, a high-precision LM guide with sensors is needed for position detection and correction.
At present, the punching of small holes can process small holes with a diameter of 25μm.
Research on micro-hole punching focuses on enhancing die strength, stiffness, guidance, and protection.
Lapping and abrasive flow finishing of small holes
Flow machining can finish micro holes regardless of material hardness, including special shapes.
Grinding has many years of history and is widely used for hole finishing.
Abrasive flow machining polishes surfaces by grinding with pressurized viscous abrasives in reciprocating flow.
This method removes burrs, polishes various holes, and eliminates hardened layers from other processes.
This method can significantly improve the machining accuracy and surface quality surface roughness Ra up to 5nm.
Abrasive flow machining finishes micro-holes axially, reducing fluid resistance, making it ideal for micro-fluidic devices.
Operators usually use this method in combination with other micro-hole machining methods such as electrolytic machining.
Special processing
Composite materials are removed using one or more energy types in this processing method.
Special machining of microscopic holes can be divided into two categories:
One class uses solid tools like EDM, electrolysis, and ultrasonic processing, where solid tools serve as electrodes or tools.
Another type uses high-energy beams like lasers, electrons, or ions for precise, tool-free micro-processing.
Electrical Discharge Machining
EDM is a method of machining that utilizes electrical energy directly.
The process uses spark pulses between tool and workpiece to etch metal via localized high heat.
EDM micro-hole machining includes two methods: perforation and high-speed EDM.
They and other processing methods compared to micro-hole processing has certain advantages:
- It can process any conductive material regardless of its strength or hardness.
- The method can process sloping surfaces of blind holes, deep holes, slanted holes, and shaped holes.
- With minimal cutting force, tools need less strength and stiffness for processing micro-holes ≤ 10 μm.
Drilling with EDM encounters issues, mainly poor hole accuracy and surface quality.
Only conductive materials can be processed; non-conductive insulators like PCBs cannot.
Additionally, electrode preparation is difficult and processing efficiency is low, limiting its widespread use.
Researchers focus on three main directions in EDM micro-hole machining.
- Yamazaki et al. developed self-shaping electrodes that improve preparation and enable shaped hole processing.
Takahata et al. in Japan used LIGA technology to mass-produce microfabricated electrodes.
This technology enables processing of 20μm copper electrode arrays and 30–32μm micro-hole arrays.
Some boost EDM speed 10× using hollow rotating electrodes and high-pressure fluid, achieving depth ratios over 100.
- Researchers combine methods to create more efficient processing techniques.
Such as the formation of ultrasonic EDM technology combined with ultrasonic energy;
HungSungLiu combined EDM with high-frequency vibration grinding, cutting hole roughness from 2.12 μm to 0.85 μm.
Ultrasonic EDM technology improves efficiency and quality in shaped hole machining.
- Research on specialized EDM machines.
Developed countries have applied micro-fine EDM technology in industry and commerce.
Companies like Matsushita Seiki, Charmilles, and McWilliams offer mature products.
Matsushita Seiki’s equipment can stably process 5μm microholes.
China’s Harbin Institute and Nanjing University advanced micro-EDM; Harbin made 8 μm holes.
Ultrasonic processing
Ultrasonic processing uses a vibrating tool to impact the workpiece surface, crushing abrasives to feed and machine small holes.
The following characteristics describe ultrasonic processing:
- Suitable for hard, brittle, non-conductive materials like glass, ceramics, quartz, graphite, gemstones, and diamond.
- Can be processed shaped hole processing accuracy of 1μm surface roughness of Ra0 ∙ 03 ~ 0 ∙ 10μm.
Local abrasive impact minimizes cutting force, heat, and stress, avoiding deformation and damage.
- Minimum hole diameter up to ● 10μm depth to diameter ratio of 10 ~ 20.
- Machine structure is simple and low cost.
Disadvantages:
- Micro-ultrasonic tools are hard to prepare, wear easily, affecting hole accuracy.
- In the processing of certain metal materials, the processing speed is very low.
- Micro-ultrasonic processing has lower amplitude, power, and speed than conventional ultrasonic.
Ultrasonic processing precisely machines small holes in hard, brittle non-metals like glass, diamond molds, and shaped holes.
Combining ultrasonic machining with other methods creates new processing techniques.
For example, the ultrasonic EDM technology introduced earlier.
Combining ultrasonic vibration with micro-hole drilling forms a promising research direction.
Research on efficient, practical ultrasonic machining equipment is also a key development focus.
The University of Tokyo developed micro tools solving clamping and rotation issues, enabling 5 μm ceramic holes.
DMG’s ULTRASONIC machine delivers 48,000 pulses/s, achieving Ra0.2 μm surface and 5× productivity.
Its flexible program enables ultrasonic machining and milling of superhard materials like ceramics, glass, and carbide.
Beijing University of Aeronautics and Astronautics developed an ultrasonic drilling machine for small holes suitable for production.
Laser processing
Laser drilling mainly relies on photothermal ablation and photochemical ablation.
Photothermal ablation uses high-energy laser to quickly heat material, causing melting and evaporation to form pores.
Photochemical ablation uses UV lasers to break and remove organic molecules, forming small holes.
Laser processing of small holes are characterized by:
- Material hardness doesn’t affect punching; most materials can be punched quickly with minimal heat-affected zones.
- High processing capacity enables hundreds of holes per second and easy automated operation.
- Focused laser beams can process micro-holes larger than 1 μm with depth ratios exceeding 50.
- This non-contact process suits thin, elastic, and hard-to-reach parts.
- No tool wear, material pollution, or noise—making it eco-friendly.
Costlier than EDM, ultrasonic, and electrolytic methods, the equipment produces rougher, less round, flare-prone holes.
Output power and focusing limit the processing to small holes in thin plates.
The high cost of laser punching equipment limits its application.
To address laser processing issues, many companies develop specialized laser processing centers.
DMG DML laser offers 10–20kW power, Ra1μm roughness, 25mm³/min removal, and optical-controlled vertical sidewalls.
This laser processing center can process micro-holes with a minimum diameter of 5μm and a depth up to 20mm.
Researchers consider laser microvia machining a maturing and most promising technology.
Tech-advanced countries like the US, Japan, and Germany increasingly adopt laser micro-hole processing.
Electrolytic processing
Electrolytic machining uses electrochemical anodic dissolution to process metal dimensions.
Micro-hole machining uses the workpiece as the anode and a tubular or rod-shaped cathode matching the hole’s cross-section.
Rotate the cathode at a certain speed to improve machining accuracy when machining round holes.
Electrolytic machining of micro-holes is characterized by the following:
- It can process any conductive material regardless of strength, hardness, melting point, or thermal properties.
- Compared with the EDM high productivity and low surface roughness value of up to Ra0∙2 ~ 0∙8μm.
- No residual stress and deformation on the machined surface, no burr and no fringe on the orifice.
SeHyun Ahn et al. in Korea produced high-quality 8 μm and 12 μm holes in stainless steel using nanosecond pulses.
Small-hole cathode making is complex, less accurate, and corrosion limits equipment more than other methods.
Electron Beam Processing
E-beam processing uses focused high-energy electrons under vacuum to melt, vaporize, and remove material instantly.
As the electron beam can be focused to a very small spot, so it can be used to play small holes.
Its processing characteristics:
- High strength, high hardness, high toughness, high melting point of metal or non-metallic materials.
- Electron beams can focus to 0.1 μm or below 0.01 μm (3 nm).
- With power density of 10⁶–10⁹ W/cm², material evaporates instantly without causing stress or deformation.
- Strong processing capacity and high productivity can make 50 holes of ●0∙4mm per second on the steel plate with thickness of 2∙5mm.
- Machines process shaped, tapered, and curved holes with various curvatures.
- Vacuum processing prevents oxidation, making it ideal for oxidizable and ultra-pure semiconductor materials.
Its high cost and need for specialized vacuum equipment limit production use.
Electron beam processing is essential for microelectronics, enabling holes smaller than 3 μm and showing great promise.
Lon Beam Machining
Ion beam processing ejects surface atoms for nanoscale layer-by-layer removal.
The characteristics of ion beam processing are as follows:
- Argon ions in beam etching remove atomic layers per second, enabling ultra-high precision microfabrication.
- Controlling beam density and energy enables focused, scanned creation of round and shaped holes.
It can process both through and blind holes, with precise control over blind hole depth.
- Ion beam processing in vacuum prevents oxidation, ideal for oxidizable and high-purity semiconductors.
- With low stress and high quality, it suits workpieces with low rigidity.
It requires a complete set of specialized equipment and an expensive vacuum system.
Manufacturers mainly use ion beam micropore processing in semiconductor integrated circuit manufacturing.
Composite processing
Composite processing combines two or more methods to create new techniques.
It combines multiple processing advantages, making composite processing a current research trend.
Common composites include micro drilling-ultrasonic, electrolysis-grinding, ultrasonic-EDM, and laser methods.
These methods combine two processes, such as ultrasound-EDM and electrolytic-EDM.
The method uses laser or EDM drilling followed by EDM or ultrasonic finishing to improve accuracy and surface quality.
Conclusion
Rapid micro-hole tech development demands constant processing improvements.
Deeper micro-hole study yields better tools, advanced drilling, and cheaper, accurate processing.
Each micro-hole method has unique features and applications.
Researchers are increasing and improving methods to meet the demand for small holes.
Each method must improve machining accuracy and microfineness.
Composite processing boosts speed, accuracy, and surface quality, balancing precision and productivity.
FAQ:
Micro-hole machining refers to manufacturing processes that create holes smaller than 1 mm in diameter—commonly under 0.3 mm. These techniques are vital for miniaturized devices in fields such as aerospace, electronics, medical equipment, and microfluidics, where high precision and fine features are required.
Micro-hole machining methods fall into two main categories:
Mechanical processing, such as drilling and punching.
Special processing, including EDM, ECM, laser, ultrasonic, electron beam, and ion beam machining.
The method chosen depends on factors like material type, hole size, surface roughness, and production volume.
Challenges include tool breakage due to reduced stiffness, chip removal difficulties, overheating, and wear. Solutions include high-speed spindles (up to 180,000 r/min), dynamic tool alignment systems, guide sleeves, advanced coatings, and micro drills as small as 30 μm in diameter.
Micro-hole punching is more efficient for mass production on thin plates, offering lower costs and better dimensional consistency. It uses precisely aligned dies and punches but is limited by material thickness and tool rigidity when holes are extremely small.
Abrasive flow machining uses a pressurized, abrasive-laden fluid to polish internal surfaces and micro-holes. It removes burrs and improves surface finish to Ra values as low as 5 nm. It’s especially useful for finishing hard-to-reach internal geometries in microfluidic and biomedical components.
EDM uses spark discharges between an electrode and the workpiece to erode material. It’s ideal for conductive materials regardless of hardness and can produce complex holes down to 10 μm. Advancements include high-speed EDM and ultrasonic-assisted EDM for deeper and more precise holes.
Ultrasonic machining excels at producing micro-holes in brittle, hard, and non-conductive materials like glass and ceramics. It uses tool vibrations and abrasives for low-stress, low-heat cutting. It can achieve 10 μm holes with excellent surface finish (Ra 0.03–0.1 μm) but has slower speeds for metals.
Laser drilling enables non-contact, high-speed micro-hole machining on most materials, unaffected by hardness. It can produce 5 μm holes with depth ratios over 50. While costly, lasers offer unmatched precision and automation potential—ideal for electronics, medical, and aerospace components.
Electron beam machining creates ultra-small holes (<3 μm) with minimal stress, ideal for semiconductors. Ion beam machining removes atomic layers for ultra-precise, nanoscale holes, often used in integrated circuits. Both require vacuum systems and are suited for high-tech applications.
Composite machining combines techniques (e.g., EDM with ultrasonic, or laser with ECM) to enhance accuracy, speed, and surface quality. These hybrid methods address the limitations of individual processes, making them ideal for complex, high-precision micro-hole applications.
