- PCB Milling -

At FabLab Puebla, we are equipped with three different models of Mini-Mill. These compact yet powerful tools operate by translating digital designs into physical circuit boards. Here’s how they work: A high-speed rotary cutter is used to carve away unwanted copper from a copper-clad substrate, accurately creating pathways that form the circuit. Unlike larger CNC machines, these mini mills are optimized for the fine detail required in PCB work, using advanced software to control the milling process. This software directs the movement of the cutter based on the PCB design files, ensuring that each cut is precise and corresponds exactly to the circuit layout. The result is a cleanly milled PCB, ready for component soldering and testing for rapid prototyping.

Mini Mills work as a CNC, so their parts are easy to identify

1. Front Cover: This is the protective shield that covers the front part of the mini mill. It helps to safeguard the user from debris and provides a safety barrier between the milling process and the operator.
2. (Power) Button: This is the main power switch for the machine. It allows the user to turn the mini mill on or off.
3. Spindle Head: The spindle head houses the spindle motor and is the part of the machine that moves in various axes to mill the PCB material. Here’s where the tool will be mounted
4. Working Table: his is the flat surface where the PCB material is secured before the milling process begins. The table moves along different axes to position the material correctly under the spindle head for precise milling.

- Mini Mill Models -

Here are the three laser machine models we have in the fablab.

- Compatible Material List -

These machines can work with a wide array of materials, from soft fabrics like cotton and felt to hard substances like MDF, wood, and acrylic. The following tables provide a comprehensive guide for materials that the lasers can work with:

Material	Description	Common Uses
Modeling Wax	Soft material, easy to mill, and leaves no residue	Prototyping, practice
Chemical Wood	Dense and stable, good for fine details	High-quality models, mold making
Foam	Lightweight and easy to shape	Prototyping, architectural models
Acrylic	Hard plastic with a glossy finish	Signage, prototypes, decorative items
Polyacetal (Delrin)	Tough, with low friction and good wear properties	Gears, insulators, high-wear parts
Acrylonitrile Butadiene Styrene (ABS)	Strong, durable, and slightly flexible	Functional prototypes, cases, automotive parts
Printed Circuit Boards (PCB)	Copper-clad boards for electronic circuits	Electronic prototypes, custom PCBs
HDPE	High-density polyethylene, resistant to impact	Containers, plastic parts
Wood	Natural material, varies in hardness	Artistic pieces, furniture components
Polycarbonate	Tough, transparent plastic with high impact resistance	Protective gear, bulletproof glass, electronics housing

- Safety Guidelines -

Here are some safety guidelines for the mini mill machine

- Tooling -

For the Fab Academy we are going to be using the SRM-20 due to its highers precision as PCB milling requires repeatibilty and precision for fine details. Starting with the tooling we're using two tools. For engraving we're using a 15 degree V-bit and for cutting we're using a 0.8 mm diameter two flute end Mill.

There are a lot manufacturing capabilities in electronics Production as seen in this JLCPCB webpage. However when PCB Milling there's only a few we can can possibly control and test. This are the following:

• Drill Hole Size: This parameter is crucial because it determines the ability to place vias and mount components accurately. The size must be precise to ensure that components fit properly without causing stress or misalignment. Incorrect drill hole sizes can lead to poor electrical connections or even damage to the components during assembly.
• Material: The choice of substrate material affects the PCB's thermal, electrical, and mechanical properties. Different materials offer varying levels of conductivity, durability, and heat resistance, impacting the PCB's performance in specific applications. For example, FR4 is commonly used for its balance of cost and performance, while more specialized applications may require materials like Rogers for their superior high-frequency performance.
• Minimum Clearance: This parameter refers to the smallest distance allowed between any two conductive elements (e.g., traces, pads) on the PCB. Ensuring adequate clearance is essential to prevent electrical shorts and ensure reliable operation. It's influenced by the voltage levels the PCB will be exposed to, with higher voltages requiring greater clearances.
• Minimum Trace Width: This defines the narrowest width for any conductive trace on the PCB. It's important for ensuring that traces can carry the required current without overheating or breaking. The minimum trace width is determined based on the maximum current the trace needs to carry, factoring in the material's conductivity and the PCB's operating temperature.
• Number of Layers: This parameter indicates how many conductive layers the PCB has. Multilayer PCBs are used to increase the density of the components and routing complexity without enlarging the board size. The number of layers impacts the PCB's electrical performance, manufacturing complexity, and cost. It's crucial to balance the need for more layers with the cost implications and manufacturing capabilities.

Starting with Drill Hole size, our limit would be defined by our smallest end mill. So from our tooling we can know that it's 0.8 mm.

As for materials we have a few options this when milling PCBs we use copper clads and they are classifed by FR."FR" stands for Flame Retardant, a term used to describe materials that are designed to inhibit or resist the spread of fire. The most common ones are FR1 and FR4.

• FR-1: This is a paper-based laminate with a phenolic resin binder. FR-1 is typically cheaper than FR-4 and is used for single-layer or low-layer count PCBs in consumer electronic products where cost is a significant consideration. It has a lower thermal durability compared to FR-4, making it less suitable for high-temperature applications. FR-1 is also less mechanically robust than FR-4, which limits its use in more demanding environments.
• FR-4: This is the most commonly used material for PCBs. It consists of a woven fiberglass cloth with an epoxy resin binder that provides excellent strength, durability, and moisture resistance. FR-4 is suitable for multilayer PCBs and is used in a wide range of applications, from consumer electronics to automotive and aerospace industries. Its superior electrical insulation properties and thermal resistance make it ideal for high-performance and high-reliability electronic devices.

Ideally for our milling purposes we should use FR1 as it's a softer material and it's dust is les toxic. However in Mexico is really hard to get a stable supplier for the copper clads, and sometime even some vendors sell as FR1 but are actually FR4. So it will depend on supplies.

Minimum Clearance and Width

We are using Neil's CAM trace width file to get this parameters this file has on the top part a clearence from 1 mil up to 20 mil mil references thousands of an inch or .0254 mm for metric users (as a sidenote most electronics packages are more commonly found in mil). On the lower part is a test of track width going in the same range as the clearence.

To transform the image to .rml file which is the type our machine needs. We used MODS as it has a program already made for the SRM-20 Mill specifically for PCBS.

It looks kind of daunting at first but it's actually really easy to use

STEPS

- Results -

Here's a table with planned parameters

Machine	Tool: traces	Cut depth: traces	Cut speed (X, Y)	Spindle speed	Remarks
Modela SRM-20	0.4mm end mill	1.1936 mm	4mm/s	7000 RPM	Used the calculator of V-bits parameters, it went too deep and the clearence part even from 20 mil was lost
Modela SRM-20	0.1mm end mill	0.25 mm	4mm/s	7000 RPM	Looks better but it still is a bit too deep and it still loses a lo of material
Modela SRM-20	0.4mm end mill	0.1 mm	4mm/s	7000 RPM	Best looking so far, however ut has a lot of unclean edges, which we attribute to maybe a failing tool
Modela SRM-20	0.4mm end mill	0.004 inch	4mm/s	7000 RPM	Still looking good and with a lot of unclean edges, however it looks deeper, which could mean the bed is uneven
Modela SRM-20	0.4mm end mill	0.25 mm	4mm/s	7000 RPM	This one confirms the unevenes of the bed as from left to right you can see how it becomes whiter meaning it went less deep

There was a retest with a more even table, and a newer V-bit to see new results. In General the results look cleaner which means we were right about the tool. The first 4 tests are the same parameter of 0.4 mm width 0.1 mm depth butt it goes from 4 offsets till 1 offset. The next 2 are tests using a 0.3 mm width tool parameter and the final 2 uisng 0.2 mm width same depth in all the only ones we liked were the 0.4 mm width do that's the main parameter we'll be using

- IMPORTANT LINKS -