PCB milling uses a rotating endmill tool to cut away unwanted copper from a board to fabricate traces and isolation. Selecting the right milling bit is crucial for achieving good routing quality and productivity.
Key bit parameters like diameter, tip shape, coating, and material significantly impact performance. This article provides guidance on choosing optimal PCB milling bits based on board requirements and machine capabilities.
PCB Milling Process Overview
PCB milling employs a multi-axis mechanical platform to position a spinning endmill over a copper-clad board. The milling bit cuts away copper to fabricate isolation gaps and circuit conductors based on programmed toolpaths.
Key stages in the milling process:
- Clamping material blank onto machine bed
- Aligning board origin reference
- Loading milling bit into spindle
- Setting spindle speed and feed rates
- Importing isolation and routing toolpaths
- Executing toolpaths to cut board features
- Unloading finished board
Milling provides a flexible digital fabrication approach for rapid PCB prototyping without chemicals. The right bit helps balance quality and speed.
Key Factors in PCB Milling Bit Selection
Consider the following parameters when selecting a bit for PCB milling:
Bit Diameter:
- Smaller diameters allow finer details and cuts
- Larger diameters remove material faster with less tool stress
Tip Shape:
- Ball end vs flat end have different cutting characteristics
- End cutting vs center cutting impact edge finishes
Flute Geometry:
- Number of flutes affects removal rate and tool strength
- Flute length impacts composite cutting and chip clearance
Coating:
- Uncoated, TiN, TiCN etc. influence tool wear, friction, heat
- Insulating coatings prevent copper buildup and shorting
Material:
- Carbide, Cobalt, tool steel have different hardness and wear resistance
- Material must withstand high RPM without fracturing
Matching bit properties to the board material, layer stackup, line widths, tolerances and target milling time ensures optimized PCB routing.
Bit Diameter
Bit diameter is the most basic factor influencing the milling process. Diameter ranges from around 100 microns up to 6.35 mm (1/4″) or larger are commonly used for PCB milling.
Considerations for bit diameter:
- Finer lines/spaces – Smaller bits allow thinner traces and finer pitch. 100um sufficient for 0.5mm lines.
- Component areas – Small bits can cutout small chip packages and connectors.
- Drilling holes – Bit must be smaller than desired hole size.
- Large board areas – Larger bits mill board outlines faster. 3-6mm range useful.
- Layer material – Bits sized for substrate rather than just copper thickness.
- Machine limits – Spindle power determines max bit size possible.
- Tool deflection – Larger bits resist deflection with tool length.
- Tool life – Smaller bits wear faster with smaller cross-section.
Selecting bit diameter requires balancing these considerations for the particular board layout. 2mm, 1mm and 0.5mm are commonly used sizes.
Bit Tip Shape
Milling bits come in ball nose and flat/square end shapes. The tip shape impacts cutting performance and edge finish:
Ball Nose Bits
- Radius tip with round cutting edges
- Mainly cuts with tip surface not sides
- Leaves curved path with rounded corners
- Less prone to chipping edges
- Cuts smoother ramped sidewalls
- Ideal for 3D surfacing and contour milling
Flat/Square End Bits
- Cutting occurs both at tip and along sides
- Leaves flat bottoms with sharp corners
- Machines vertical walls with minimal taper
- Requires more rigid machine since side-cutting is harder
- Produces fine finish on flat surface areas
- Must avoid plunging into material
Ball nose tools are commonly used for isolation routing while flat endmills are suitable for trace fabrication. The toolpaths must suit the bit tip style.
Bit Coatings
Applying specialized coatings onto milling bits enhances certain properties:
Uncoated Bits
- Least expensive option
- No added performance benefits
- Prone to galling and adhesion of material
Titanium Nitride (TiN) Coated
- Thin gold-colored ceramic coating
- Reduces friction and heat generation
- Improves tool life 2X over uncoated carbide
- Resists buildup of material on cutting edges
Titanium Carbonitride (TiCN) Coated
- Gray ceramic coating
- Excels in high speed machining situations
- Tool life 3-5X over uncoated carbide
- Withstands higher cutting temperatures
Insulating Coatings
- Prevent conductive tool material shorting signals
- Required for milling multilayer boards
- Diamond or silicon nitride coatings commonly used
Coatings improve milling performance for longer tool life, higher feeds/speeds, and multilevel boards. The benefits merit the extra cost over uncoated tools.
Flute Geometry
The flutes of an endmill are the spiral cutting grooves running along the tool body to the tip. Flute characteristics impact milling behavior:
Number of Flutes
- 2 to 6 flutes common for PCB tools
- More flutes allow higher feed rates but require more rigid setup
- Fewer flutes provide easier swarf/debris clearance
Flute Length
- Longer flutes allow cutting with tool sidewall
- Helps dissipate heat over larger contact area
- Requires rigid machine and clamping to prevent deflection
Flute Pitch/Helix Angle
- Affects direction of chip flow and evacuation
- Variable helix improves shearing and chip clearance
Flute Profile
- Square end flutes suitable for slotting
- Radiused flutes for high feed contouring
Selecting suitable flute characteristics improves milling productivity and workpiece finish.
Bit Composition and Hardness
Carbide endmills are predominantly used for PCB milling due to the required hardness and wear resistance when machining copper.
Some important material properties:
- Hardness – Harder grade carbides better sustain cutting forces without excessive wear.
- Fracture Toughness – Carbide must resist chipping at high RPM speeds.
- Heat Tolerance – High heat is generated, requiring thermal shock resistance.
- Chemical Compatibility – Some carbide grades may react with copper.
- Insulating Properties – Tool material should not short circuit conductors when wet.
Cobalt-enriched micrograin tungsten carbide grades offer a good balance of hardness, toughness and conductivity. The carbide grade should match the application and parameters.
Tool Length
Longer endmills inherently deflect more than short mills due to the extended flutes. Deflection reduces routing dimensional accuracy.
Guidelines for tool length:
- Use shortest tool that fully cuts board thickness
- For thin boards, bits longer than material can be helpful
- Limit length to around 2-4X bit diameter
- Ensure stiff machine, fixtures, collets and toolholding
- Reduce ramping into material to limit tool loads
- Run slower feeds/speeds for long thin tools
- Perform test cuts to verify rigidity and precision
Keeping tool length to the minimum required reduces deflection for better milling precision.
Summary of Bit Selection Considerations
| Parameter | Key Factors |
|---|---|
| Diameter | Feature size, tool loads, machine power |
| Tip Shape | Contouring vs vertical walls, plunge requirements |
| Coating | Tool life, friction/heat reduction, insulation |
| Flute Geometry | Material removal, chip clearance, tool strength |
| Composition | Hardness, toughness, heat resistance, conductivity |
| Length | Deflection/rigidity, tool extension, precision |
Balance these interdependent factors to select tooling providing the optimum combination of capability, productivity and accuracy for the target board.
Milling Bit Suppliers
Many companies offer endmills suitable for PCB routing and isolation. Some notable carbide PCB tooling suppliers include:
- Harvey Tool – Wide range of tool diameters/geometries for PCB
- Lakeshore Carbide – Specializes in miniature endmills
- Datron – Tools designed for their own PCB milling machines
- Kyocera – Recognized for micro-tooling expertise
- Mitsubishi – Leading producer of carbide endmills
- OSG – Diverse micro-machining tool line
- Performance Micro Tool – Ultra small diameter tools
The range of bit parameters offered by these manufacturers facilitates dialing in a tooling solution tailored to the application requirements.
PCB Routing Toolpath Considerations
The toolpath strategy used for milling must suit the bit characteristics selected. Key factors:
Direction:
- Along trace axes vs diagonal gives different finish and edge quality.
Stepover:
- Lateral spacing between toolpath passes impacts material removal rate and surface finish.
Contouring:
- Manage transitions between straight and curved toolpaths to limit gouging.
Entry/Exit Moves:
- Control bit ramping into material for clean entry starts and exits.
Climb vs Conventional Milling:
- Climb milling loads bit less. Conventional clears debris better.
Tolerance:
- Tighter path tolerance for smaller tools and features.
Feeds/Speeds:
- Optimize for tool size, materials, coatings etc. Start conservative.
The toolpath strategy complements bit characteristics for optimal routing effectiveness.
Summary
Key takeaways on selecting PCB milling bits:
- Match bit diameter, tip shape, coating and material properties to board requirements.
- Smaller bits enable high resolution details but limit material removal rate.
- Ball nose bits excel at 3D contouring while flat endmills machine vertical walls cleanly.
- Coatings like TiCN boost tool life and performance significantly over uncoated tools.
- Optimal flute geometry improves swarf evacuation, tool strength and cutting capability.
- Rigid, precision fixturing is critical for thinner, longer tools prone to deflection.
- Toolpath strategy must suit bit geometry and parameters.
- Obtain endmills from reputable carbide tooling specialists.
Choosing the best PCB milling bit for the application results in reduced tool wear, improved surface finish, faster milling time, and greater dimensional accuracy.
Frequently Asked Questions
Q: What drill bits can be used for making holes in circuit boards?
For hole drilling, small diameter twist drills made from cobalt steel work well for clean results. Carbide drill bits are also an option but more costly. Uncoated 1/64″ to 1/32″ drills at high RPMs drill holes neatly in typical 0.062″ PCB thickness.
Q: What are the downsides of using a bit that is too small?
Using an overly small bit has some disadvantages:
- Requires slower feed rates to avoid tool failure
- Prone to rapid tool wear and breakage
- Deflection effects more significant
- Much longer milling time for bulk material removal
- Higher chatter and poorer surface finish
Avoid bits well under 50% of minimum feature sizes.
Q: What are some signs of a worn out milling bit?
Indications of tool wear:
- Visible rounding, chipping, fraying of cutting edges
- Increased cutting forces and torque
- Higher noise levels and vibration
- Decline in dimensional accuracy
- Deteriorating surface finish
- Uncut copper, burring, rough edges
- Smoke from higher friction
Replace or resharpen tool promptly when wear is detected.
Q: How should milling bits be stored between uses?
Recommended storage practices:
- Use bit holders or organized rack systems
- Avoid loose storage where bits can contact each other
- Prevent exposure to moisture and chemicals
- Protect cutting edges from damage
- Ensure stable temperature conditions
- Clean bits after use and apply anti-corrosion oil
- Check for any signs of corrosion periodically
Proper care preserves tool life.
Q: How can deflection from long thin bits be reduced during milling?
Strategies to minimize deflection:
- Shorten flute length where possible
- Increase tool diameter relative to stickout
- Reduce depth of ramping into material
- Use most rigid holding method – hydraulic, heat shrink
- Lower feedrates and spindle RPM
- Ensure setup, fixture, stock are completely secure
- Verify tool runout is minimal
- Upgrade to stiffer machine if needed
Eliminating all sources of play and flex dramatically improves results.
