Printed Circuit Board Milling

PCB milling involves the removal of copper from the PCB laminate to redevelop the structure, pad, and signal traces according to the layout design. In other words, you have to remove the PCB material through rotary cutters.

What Is Milling?

The milling machine helps in drilling, cutting, and boring materials in an array. So, the technique of removing material through the tool axis is what we call milling. The milling machines have a variety, and they are used in PCB and other industries, including aerospace and auto parts.

You need a machine to perform all milling functions to develop a prototype PCB involving vias and through-hole plating. The milling machine needs an AC, an outlet, as well as a vacuum.

Tools Required For PCB Milling

A milling machine is also known as CNC. You need several tools for PCB milling as only the right tools can get you good results. So, let’s discuss all these things in the below section.

Milling Machines Bits

The milling bits include engraving bits, and drilling bits, and end mill.

The bit is a vital part of a milling machine, so it should be fast enough to perform its function. A fine engraving bit can mill even little traces, and the bit having a thickness of 0.1 mm is ideal for PCB milling. It can carry out 8 to 10 tasks of medium level.

The drilling bit helps to drill the mounts and vias for through-hole components. You have to manually change the bit for each hole, but choosing a bit of size 0.8 to 1mm can drill multiple holes

The end mill is a bit to cut out the board from a large PC Board layer or clad, and it should be a minimum of 1 to 2 mm to perform well.

PCB Clad

The PCB clad has to be ready for milling, as the surface should be cut smoothly through a CNC machine. A single-sided PCB works fast in this case, whereas the double-sided PCB also involves good milling.

We will simplify here that PCB is a printed circuit board having a laminated material with copper layers. So, you have to remove copper traces from it with a milling machine.

Additional Tools
Other than the above tools, you also need some others, such as;
• You need a two-sided tape to help bind the PCB clad to your cutting board where you want to do milling.
• Then, you need a brush with hard metal bristles to finish the surface after milling. Lots of dust gathers on the board that you have to brush off.
• Sandpaper to clean the fine burrs that remain there even after brushing.
• In the end, you need alcohol for final finishing. Like, take a soft brush then soak it with alcohol and rub off the milled PCB.
All these additional tools help perform PCB milling perfectly. Even it can be a DIY process if you follow it the right way.
Preparation of a Mechanical Setup
The cutting depth should remain the same in this case. The V-bits are suitable for giving you a wide-cut throughout the board, providing precise engraving. Milling cannot be perfect without a right bit as it can result in thin traces.
Manufacturers do it through trial and error by using different cutting depths. In the case of V-bits, a size of 0.8 mm to 10 mm is suitable for an even depth.
Auto-Levelling Process
Then you have to prepare for auto-leveling through a software setup that also includes probing before you go for milling. You have to solder a wire to the PCB’s clad corner, binding it to an even piece of wood. It needs lots of taping, including with the two-sided tape.
After taping, both wood and milling bit are secured, then you have to attach the crocodile clips to the wire, finishing the probing successfully. The main purpose is to level the PCB to avoid milling errors as a Printed Circuit Board is often uneven in terms of height.
Using the software

It involves different steps, such as

1. Preparation of CAD Files

Initially, you need to prepare the CAD files, using the AutoCAD software. The three Gerber files, including the upper, lower, and drill are essential. You must follow all guidelines before proceeding with the design to enhance the milling process. Like, you should consider the following factors,

Selecting the right footprints: You have to select the footprints carefully because tiny footprints involve time traces that can affect milling. So, an expert designer should perform this task.

Take care of traces clearance: Your work will get affected if you don’t leave a proper clearance for the traces. The clearance from one trace to another should be double the bit width. Like, if the CNC bit is 0.1 mm, you should leave 0.2 mm of clearance.

Width of traces: The width of a trace should be larger than the bit width. As a thumb rule, double the bit width, making the trace width more than it. Like, if the bit is 0.1 mm, the trace width should be at least 0.3 mm on the safer side.

Leaving a pour clearance: A proper pouring clearance helps you to get a ground plane without much effort. There should be a minimum of 0.3 to 0.4 clearance from pour to trace. It also helps to reflow the PCB without using a solder mask.
All these steps help you to produce precise milling, however, professional milling involves minimum restrictions in this case.
2. Using The Flatcam
You need to turn the GERBER files to GCODE for a CNC to run it, and you can do it through FlatCam. This software helps to design a PCB’s 2D, and you can import the GERBER files to create different paths for multiple tasks, like from cutout to isolation, and milling, and other paths.
You only have to add some measurements regarding the drilling depth, and size of the bits. In the case of a single-sided PCB, you have to create the 3 GCODE files, including the drill, the upper layer, and the cutout.
3. Autoleveller
The process of auto-leveling is the last software task, where you process the GCODE file having a top layer. You need the software named Autoleveller that helps in probe spacing which is essential for a prob’s resolution.
After creating this file, you can replace the previous GCODE file having a top layer. After this, your board is ready for milling.
Now Comes Milling
• Now you have three files ready, including the top layer file along with probing, the second is the drill file, and the third is the milling file. You can start by milling the top layer file.
• This process involves 3 Axes and placement of the spindle that is a few millimeters on the PCB corner. Keeping the 3 axes zero. However, don’t lose the position of the X-Y planes during different tasks.
• You should place the probing circuit correctly, as a wrong position can affect milling. Then, after probing, the mill will start.
• After milling, the process of drilling comes does not involve a perfect re-leveling. You only have to put the drill bit flush against the PCB. This process involves drilling all holes.
• Finally, you have to cut the board from the PCB clad.
Board Finishing
In the end, you have to remove the dust with the brush as explained above. Then you can use the sandpaper for a final finish, and wipe off the surface with alcohol. Now, you can solder the components.
Even, some traces of the copper residue are still there, but they can’t affect the PCB function due to enough distance between the pours and traces.
You can do the same kind of milling for double-sided PCBs, but it involves a lot of practice to handle two boards.
Important Tips To Use A PCB Milling Machine
• Follow proper design rules to create perfect GERBER files to import them. Your design should have correct dimensions and clearances for better results.
• You must know how various end mills can cut through the PCB material. Such knowledge can help you correct your design in case of an issue.
• In the case of isolation cuts, you have to route signals from one point to another, including the ground and power. Understanding everything will help you create copper pours.

Frequently Asked Questions

Can I Make My PCB?

Yes, but you must know how to use the right software for PCB design. Different software are available, including the AcadCam

What Is The Process Of PCB Etching?

Manufacturers use chemicals to remove material from the PCB surface, called etching.

Can I Design A PCB Online?

Yes, many PCB fabrication companies offer online services for a custom design of the PCB. You can create the GERBER files through available software and use them for custom designing.

Is PCB Milling Expensive?

It depends on if you do it yourself or get the PCB milling from a company. The most expensive tool is the CNC or milling machine.
PNC is providing the turnkey solution for your PCB and SMT Assembly requirements. Interested to know more? Just write us at sales@pnconline.com.

Written by Sam Sangani

Sam Sangani is the President & CEO of PNC Inc., a Nutley, NJ based Printed Circuit Board manufacturer. Sam graduated from L. D. Engineering College with a BS Degree in Mechanical Engineering. He also continued his education and graduated from Steven’s Institute of Technology where he acquired a Master’s degree in Computer Science.

After completion of his BS, Sam worked as a QC Manager, for Xerox, Romania and London. He was responsible for the Quality Control of Cable and Wire Harness imports from Romania. After completing his Master’s Degree, he worked as a Senior Programmer with IBM, Tucson, Arizona. Sam was responsible for leading the Mainframe System Programming Team.

In 1997, Sam acquired PNC INC., a Nutley, NJ based PC Board fabrication Shop. From 1997-2013, Sam has made tremendous improvements and changes within PNC INC., as he added many new Products and Technologies in PNC’s portfolio. With his proven track record and leadership, PNC has never had an unprofitable year and has continued its growth yearly since 1997.

His current responsibilities are Strategic Planning, Corporate Management, New Business Ventures, Sales & Marketing, Trade Shows, Professional Services and leading productive teams to achieve peak potential. He has also utilized Lean Management techniques which have built a foundation for PNC’s high-paced growth. Sam also enjoys real-estate investing, web design & SEO, trading stocks, options, futures and Forex markets.