Computer numerical control (CNC) machining is an essential part of taking your printed circuit board (PCB) design from prototype to production. CNC milling accurately fabricates complex 2.5D and 3D features on custom PCBs that cannot be achieved with traditional manufacturing means.
CNC machining removes unwanted copper from specified areas of the PCB through an automated, programmable process to shape the circuits as per the design files. This article provides a comprehensive overview of using CNC technology to machine your own PCB prototypes.
Benefits of CNC Machining PCBs
Here are some of the major advantages of using CNC milling for PCB prototyping:
- Creates functional prototypes from CAD models faster
- Enables fabrication of multi-layer PCBs
- Allows embedding active/passive components
- Saves time and cost compared to outsourced small-batch PCB production
- Achieves fine line width/spacing down to 125 μm
- Creates precision 2.5D contour milling not possible through etching
- Allows micro-vias and buried vias for dense interconnection
- Suitable for circuits with high-frequency, controlled impedance
By adopting CNC machining, PCB inventors can quickly implement and test ideas without relying on external factories.
CNC Machining Process Overview
The typical workflow for CNC milling PCBs involves:
1. Design – Creating the PCB layout in CAD software and generating CAM/Gerber files
2. Fixturing – Preparing the PCB blank and securing it onto the CNC machine bed
3. Probing – Work-piece coordinate system alignment to position the endmill over the board
4. Milling – Machining conductive traces by rastering the endmill per the toolpath
5. Deburring – Removing rough edges and burrs from milled conductors
6. Cleaning – Washing away milling debris and dust from the board
7. Visual Inspection – Checking for any shorts, opens or gouges
This basic process transforms the raw PCB blank into a finished board ready for component assembly and testing.
CNC Machines for PCB Prototyping
Various types of CNC machines like desktop routers, small-format mills, and mini-CNC machines are now available for machining custom printed circuit boards.
When selecting a CNC machine, key factors to consider are:
- Workspace envelope (max PCB size)
- Spindle speed and power
- Supported endmills (diameter, length)
- Drill precision and chip load
- Software integration and workflow
- Cost
| Machine | Description |
|---|---|
| Desktop CNC Routers | Compact and affordable machines for home workshops good for double-sided PCBs up to 200mm x 300mm size |
| Small Format Mills | Industrial grade machines with high spindle speeds and precision suited for faster, more intricate PCB milling |
| Micro-CNC Machines | Specialized systems for milling tiny circuits like sensors down to 25mm x 25mm size with small endmills |
| Laser CNC Machines | Uses a laser cutter instead of a router to ablate unwanted copper without physical contact |
Desktop CNC routers provide an economical starting point for prototyping digital circuits at home or in makerspaces.
PCB Design Rules for CNC Milling
To effectively machine your custom PCB board, the layout must be designed while keeping CNC constraints and capabilities in mind.
Follow these key PCB design rules when targeting CNC fabrication:
1. Allow adequate tool clearances: Provide at least twice the cutter diameter as spacing around pads and traces for tool access. Account for tool shoulders too.
2. Include fiducials: Add alignment markers to use optical probing for coordinate system referencing before milling.
3. Set appropriate track sizes: Minimum trace width and gap should exceed bit diameter. A 10 mil end mill yields 8 mil lines.
4. Limit trace angles: Avoid acute angles zones as endmills cannot machine those properly. Use 45°/90° traces.
5. Define board outline: Clearly indicate board contours to machine the outline profile cut. Panelize designs for better fixturing.
6. Minimize vias: Countersunk holes need extra machining time so use judiciously. Bury only critical signals needing layer transitions.
7. Split ground planes: Machine inability to clear large copper areas will require intelligent splits.
By designing upfront for manufacturability, efficient, optimized CNC programs can be generated to machine the boards.
PCB Materials for CNC Milling
The blank PCB substrate selected influences suitable milling parameters and quality of machined traces. Important considerations when choosing CNC milling PCB materials include:
Machinability: Softer, non-glass reinforced substrates like FR-4 machine faster with lower tool wear. Rogers RO4000 series bonds well but have higher abrasivity.
Dimensional Stability: Lower expansion coefficient substrates sustain precision geometries despite heating during milling better.
Bonding: Good copper foil adhesion prevents lifting or tearing during end mill passes while poor bonding raises chances of delamination.
Heat Tolerance: Glass transition temperatures above the heat generated during milling ensures material retain its strength. Ceramic-filled substrates withstand higher cutting temperatures.
Standard 1.6mm FR-4 blanks with 1oz copper foil offer a reasonable starting point for DIY CNC milling PCB prototypes.
PCB CNC Milling Tools
Carbide endmills are commonly used to remove unwanted copper when CNC machining PCBs. Various types of tools for specific applications include:
Rigid Endmills – Single piece solid carbide spiral flute tools for general purpose 2D contour milling of traces.
Ball End Mills – Carbide cutters with a ball tip to perform 3D contour machining for sculpted circuits.
V-Bits – ‘V’ shaped carbide mills used make trenches for rectangular milled traces.
Drills – High speed carbide micro-drills create holes for conductive thru-hole vias.
Diamond Cutters – Polycrystalline diamond tools prolong tool life when milling abrasive substrates like ceramic PCBs.
Proper feeds, speeds, stepovers and axial/radial depths based on tool geometries must be set for robust milling performance.
Fixturing Considerations for PCB CNC Milling
Even small PCB blanks can resonate and vibrate extensively during aggressive material removal if not properly secured.
Here are some guidelines for reliably fixturing PCBs to withstand milling forces without movement:
- Use a spoilboard/sacrificial layer between PCB and machine bed
- Low profile clamps distribute holding forces closest to milling zone
- Vacuum beds utilize negative pressure for forceful workholding
- Adhesives temporarily bond board directly to spoilboard
- Add stiffening ribs or perimeter supporting fixtures structures
- Employ self-indicating pressure sensitive films to map grip distribution
Since PCBs expand on heating up during milling, use slip sheets or boundary constraints to permit thermal expansion.
PCB CAM Processing
To translate the PCB layout into toolpaths executable on a CNC machine, CAM (computer-aided manufacturing) software is necessary.
CAM software allows:
- Importing Gerber, ODB++ and other standard PCB file formats
- Specifying machine, tools and stock model
- Setting pass depths, stepover parameters
- Defining toolpaths (traces, outlines, holes etc.)
- Simulating milling process visually
- Calculating accurate cycle time estimates
- Generating machine-ready G-code programs
- Adding fiducials markers for probing
With optimal CAM strategies and post-processing, the milled quality of even high density PCBs with 6 mil lines/spaces can approach conventional PCB etched limits.
PCB Probing and Datuming
To accurately align the CNC endmill over desired regions of the blank PCB, probing is necessary. This establishes a coordinate system to locate all toolpath positions correctly relative the board.
Workflow:
- PCB loaded and clamped to machine bed
- Camera centers board in view field
- Touch probe electrically contacts added fiducial points
- Software records exact coordinate values
- Completed board profile aligned to programmed paths
Fiducials as reference datums may be:
- Round pads ⌀ 0.5 to 1 mm
- Square pads 1 mm
- Drilled holes 1 mm diameter
- Slot or edge markers
Datuming compensates for variations in blank positioning and allows achieving reliable, repeatable milling quality.
PCB CNC Milling Feeds and Speeds
The cutting parameters specify the velocity (feed rate) at which the endmill machines the PCB traces as well as its spindle RPM. This impacts:
- Material removal rate
- Cutting forces and torque
- Tool wear
- Machined edge finish
- Process cycle time
General guidelines for initial PCB milling parameters:
Spindle RPM – 15,000 to 20,000 RPM
Feed Rates – 50 to 150 mm/min
Higher number of flutes, adept use of climb versus conventional milling and optimizing chiploads boosts performance. Rigid fixturing is vital for chatter-free milling.
PCB CNC Milling Strategies
Various toolpath strategies exist to clear copper regions when isolating PCB conductors. Key methods include:
Trace Isolation Milling
Thin parallel passes with a certain overlap percentage traces out individual conductors by removing material between them. 50% overlap is commonly used balance cycle time and surface finish. Wider traces may allow faster 75% stepover overlap passes.
Slotting
plunge cutting trenches on both sides to define rectangular milled traces. Requires two pass milling but prevents copper burrs compared to outline milling.
Area Clearance Strategies
Fitting path rasters, direction parallel passes or adaptive clearing patterns remove large copper zones efficiently while preventing tool overload.
Drilling for Vias
Peck drilling, chip breaking and regular retracts during hole boring helps chip evacuation and prevent clogging to achieve clean vertical via holes.
Proper path sorting, entry/exit strategies and optimal tool orientation further boosts milling performance.
Post Processing Steps after PCB CNC Milling
After machining the board, some secondary processing steps help improve milled PCB quality:
Deburring – Removing rough copper edges around traces manually or through automated abrasive flow deburring equipment.
PCB Washing – Thorough cleaning with pressurized water jet sprays or PCB immersion tanks removes swarf, debris and dust.
Nickel Plating – Electroless nickel coating prevents copper traces from oxidation and improves solderability.
Electrical Testing – Validating continuity between intended conductors and absence of any shorts with a multimeter probe.
Visual Inspection – Microscope checks at 30X to 60X magnification confirm channel integrity with no copper smearing or gouges.
These post processing steps enhance machined PCB quality to build fully functional boards.
Lean Manufacturing Aspects
Adopting lean manufacturing principles maximizes CNC machine utilization, optimizes changeover times and improves productivity when milling PCB prototype boards in low volumes.
Some lean aspects that can boost efficiency are:
- Standardized fixturing elements for quick loading
- Color-coded setup components
- Shadow boards to organize endmills by type
- Custom macros for common parameter sets
- Probe routines to expedite referencing
- Storage racks near machines for ready access
- Digital status indicators for free capacity
- Preventive maintenance during machine downtime
- Setup templates to minimize changeover time
With enhanced workflow management, CNC milling works well for fast fabrication of digitally designed PCB innovations.
Advantages over PCB Etching
While chemical etching can produce basic PCBs, CNC machining offers many advantages:
Precision – Endmills can create fine details with tighter tolerances and superior dimensional accuracy than etching.
Reliability – Milling gives positively isolated traces instead of undercuts prone tracks from over-etching.
Consistency – Automated machining ensures uniform channel widths across the board unlike etching.
Flexibility – No need to generate etching masks. Easy to rapidly rework designs by just updating toolpaths.
Functionality – Can fabricate high density boards impossible through etching like 8-12 layer multilayer boards with blind/buried vias.
As DIY hobbyists push circuit complexity, CNC fabrication provides better reliability and performance.
Limitations of PCB CNC Milling
Some inherent limitations still constrain achievable quality levels relative to commercial PCB production using lithography when CNC milling prototypes:
- Surface finish not as smooth as commercial PCBs
- Maximum millable trace density lower than masking/imaging limits
- Restricted to proto-run volumes given process throughput
- Face milling constrains layer alignment precision unlike precision lamination
- Higher initial setup costs for CNC systems compared to etching
- Still requires manual assembly of components unlike SMT lines
- More process expertise needed to generate toolpaths, set parameters etc.
But the ability to quickly iterate innovative board designs provides tremendous value despite these technology restrictions.
Safety Precautions
CNC milling PCBs poses some unique hazards requiring appropriate precautions:
- Rotating tools can violently throw loose debris when they snag so secure workpieces and nearby objects
- Always keep hands away from moving machine elements with awareness barriers around work zones
- Wear safety glasses to prevent eye injuries from chips and guard against particulate inhalation
- Fire risk exists as insightsulative swarf ignites so have extinguishers and avoid combustion sources nearby
- High pitch noises mandate using hearing protection plugs or noise cancelling headgear
- Allow hot milled PCBs to adequately cool down before handling to prevent finger burns
With training and adoption of safe working habits, CNC milling provides a very versatile PCB prototyping choice.
Conclusion
As PCB designs increase in complexity with shrinking geometries and rising interconnect densities, CNC milling delivers fabrication capabilities scaling beyond chemical etching limits for prototyping digital logic circuits and high frequency analog boards alike. With easy desktop solutions now available, electrical engineers can cost-effectively implement innovations using agile inhouse milling. By applying appropriate design principles and CAM strategies, even 0.006 inch trace channels are realizable. While certain accuracy and volume barriers remain relative to mature PCB production ecosystem, the quick turnaround enables by CNC milling delivers huge savings in development costs through rapid design revisions. With sufficient fixturing, process expertise and safety precautions, CNC technology constitutes the future of PCB prototyping for innovators.
Frequently Asked Questions
Here are some common queries regarding CNC PCB milling:
Q: What PCB materials can be machined well?
A: Standard FR-4 material with 1oz foil bonds nicely without cracking for small prototype runs. Rogers RO4000 series works too but requires reduced feeds. Ceramic PCBs require diamond coated endmills.
Q: What should be the minimum clearance between traces?
A: Its ideal to keep >= 2 times tool diameter as spacing through isolation paths are generated around traces so a 10 mil cutter needs >= 20 mil gaps.
Q: Can CNC mill complex boards like 12 layer multilayers?
A: Yes, with a precision production CNC machine allowing variable Z-depth machining, complex multilayer PCBs can be section milled layer-by-layer by adjusting milling depth per pass.
Q: What tolerances are achievable while milling PCBs?
A: With tight fixture schemes and dialing in appropriate parameters, machining accuracy of +/- 0.003″ is definitely feasible meeting typical PCB needs. Precision systems can further boost accuracy.
Q: How to prevent short circuits between traces after CNC milling?
A: Perform visual inspection at high magnification after milling to check for insulation smearing. Alternatively, apply immersion tin plating to fully coat exposed copper traces preventing inter-channel shorts.
