What is PCB Milling or Routing?

Introduction

PCB milling, also known as PCB routing, is a process used to manufacture printed circuit boards (PCBs). It involves using a specialized mechanical milling machine, called a PCB router or CNC mill, to cut the desired traces and patterns into the copper layers of a PCB blank.

Milling creates physical traces and pads on the PCB by removing unwanted copper from the board's substrate. It provides a faster and more cost-effective method for prototyping and producing custom PCBs compared to traditional etching processes. Understanding the PCB milling process is key for any electronics hobbyist or engineer involved in designing and fabricating custom circuit boards.

How PCB Milling Works

PCB milling employs a rotating cutting tool, called an endmill, to remove material and create the required conductive tracks, traces, and holes in the copper layer(s) of the PCB blank.

The PCB Milling Machine

PCB routers use a spindle motor to spin the endmill at high speeds, often between 10,000-100,000 RPM. The PCB blank is fixed to a movable bed that allows precise positioning in the X and Y axes. By combining the high-speed endmill with the movable bed, complex patterns can be traced into the copper layer of the PCB.

The milling depth is also controllable to allow the isolation of different copper layers in multilayer PCBs. Many PCB mills also incorporate a drill to produce rounded holes and vias in the PCB substrate.

Most hobbyist and low-cost PCB mills are manually operated, while more expensive industrial mills can run unattended after the milling paths are programmed.

The Milling Tool: Endmills

The most important tool for PCB milling is the endmill bit. Endmills come in a range of sizes and shapes and use extremely hard materials like carbide or diamond. Typical endmill sizes range from 0.1mm to 3.175mm in diameter.

The smaller the endmill, the finer traces and details can be milled. Larger endmills are used for removing large areas of copper or for traces that need to handle high currents. The most common endmill types include:

• Ball nose - Rounded tip allows milling of curved surfaces and holes
• Tapered - Conical shape for milling complex shapes and traces
• Square end - Flat end for fine traces and slots
• V-shaped - Sharp V tip allows sharper corners

Using the proper endmill for each milling operation is key in optimizing the PCB milling process. Most machines use automatic tool changers to switch between different endmills.

The Milling Process

The basic PCB milling process involves the following steps:

1. Design - The PCB layout is designed in CAD software and converted to CAM/G-code files for the milling machine.
2. Setup - The desired tools are loaded into the machine and the PCB blank is fixed to the bed using clamps or vacuum hold-downs.
3. Copper clearing - A large endmill first clears away unwanted copper from the board surface.
4. Drilling - Holes and vias are produced with a smaller endmill.
5. Tracing - The final routing paths are traced into the copper layer(s) with a small endmill sized for the trace widths.
6. Finish - The milled PCB is unloaded from the machine for any secondary finishing operations.

The G-code program guides the endmill over the PCB to automate the milling operations with extreme precision. Multiple passes are made with smaller tools to achieve the fine trace sizes and clearances required on a circuit board.

Advantages of PCB Milling

Compared to traditional PCB fabrication methods like etching, PCB milling provides some unique benefits:

• Rapid prototyping - Mill a single PCB in under an hour. No need for submitting designs to a fab or waiting weeks for production.
• Design flexibility - Make changes and mill a new version anytime.
• Low startup cost - An affordable PCB mill and small accessories are all that's needed. No chemical etching or cleanroom.
• On demand production - Mill only the quantity needed right when required.
• Complex board shapes - Mill intricate outlines and curved boards.
• Simplified multilayer -Align layers precisely by milling or using a drill press.

The fast turnaround and flexibility of milling makes it the ideal choice for prototyping new PCB designs during the development process. Milling is also cost-effective for producing short runs of customized boards that don't require mass production.

Limitations and Considerations

While PCB milling has many benefits, it also comes with some limitations to consider:

• Lower trace precision - Typical mills cannot match the tight tolerances of commercial fabrication. Minimum trace/space is around 8 mils (0.2mm) for most hobbyist machines.
• One-off production - Each PCB must be individually milled. Time and cost increases significantly for larger quantities.
• Limited board size - Typical mills max out around 12"x12" (30x30cm).
• Exposed traces - No soldermask coating can be applied during milling. Traces are unprotected unless coated manually.
• More design work - Additional effort is needed to create manufacturing files from the PCB layout.
• Copper waste - A large amount of copper is machined away and discarded during milling.

For very high density designs, tight tolerances, or larger production volumes, sending designs to a professional PCB fab may still be preferable to milling. The cost crossover point depends greatly on the complexity and size of each design.

Applications of PCB Milling

Some of the most common applications where PCB milling excels include:

Prototyping

During development, milling allows rapid iterating and testing of new PCB layouts. Design flaws can be identified faster and multiple revisions can be fabricated in a short timeframe.

Custom Circuits

For one-off projects, it avoids the setup costs of custom fabrication. Small production runs of niche boards can also be cost-effective with milling.

Repair and Rework

Making minor changes, repairs, or trace fixes on existing boards is easy with milling. Damaged traces can be bypassed or replaced with new milled traces.

Education

In academic lab settings, milling enables hands-on PCB fabrication. Students can see the entire process from design to manufacturing.

Unique Shapes

Milling can produce curved, circular, and arbitrarily shaped boards that would be difficult and expensive to produce otherwise.

Getting Started with PCB Milling

Interested in trying PCB milling? Here are some things you'll need:

PCB Milling Machine

Select a small desktop router or mini-CNC mill suited for PCB work. Many hobbyist models are available starting under $1000. Look for simple designs with an XY movable bed.

Endmills and Tools

Carbide endmills in sizes from 0.1-3.0mm should cover most basic milling operations. V-bits, ballnose, and drills will provide more options.

PCB Holder

A method of tightly securing PCB blanks to the mill bed like clamps, vises, or vacuum holding.

CAM Software

To convert PCB designs into toolpaths for the milling machine. Common options are Fusion 360, VCarve, and FlatCAM.

Safety Gear

Eye protection, dust collection, and fume extraction is highly recommended when milling circuit boards.

Start with some single sided design and toolpath practice before moving onto more complex multilayer milling. Patience and careful setup is key to achieving good results.

Conclusion

PCB milling brings rapid in-house prototyping of custom printed circuit boards within reach of students, hobbyists, and engineers. By using a desktop router to remove copper from a PCB blank, it's an extremely flexible fabrication technique. While milling has limitations on precision and scale, it empowers innovators to quickly iterate their designs and produce short runs of unique PCBs cost-effectively. With the right approach, PCB milling can accelerate the development process and allow customization that is difficult through other fabrication means.

Frequently Asked Questions

Here are some common questions about PCB milling:

What materials can be milled?

The most common PCB materials like FR-4, Rogers, and flex PCB materials can be routed. Some exotic substrates may be too hard on cutting tools.

What software is required?

CAM software is needed to convert PCB designs to toolpaths for the milling machine. Common options include Fusion 360, VCarve, FlatCAM, and KiCad.

Can multilayer boards be milled?

Yes, multilayer boards can be fabricated by repeated alignment and milling of each internal copper layer. Alignment precision is critical.

What thickness of copper can be milled?

Up to 2 oz (70 um) copper can usually be milled with smaller endmills. Thicker copper requires more milling passes and time.

How accurate can PCB traces and spaces be?

The best mills can achieve trace/space down to 8 mils (0.2mm). Precision is limited compared to commercial fabrication.