PCB Milling Guide: DFM, Tolerances, and Cost Optimization

1. Table of Contents
2. 1. What is PCB Milling and Why It Matters?
3. 2. Milling vs. V-Scoring: Choosing the Right Profiling Method
4. 3. The Mechanical Layer: Vital Data for PCB Manufacturing
5. 4. Essential Design Rules for Milling (DFM Guide 2025)
6. 5. Handling Internal Cutouts and Slots
7. 6. Cost Implications: How Milling Affects Price
8. 7. FAQ: Common Questions About PCB Routing
9. 8. Conclusion: Ensuring Precision with NextPCB

1. What is PCB Milling and Why It Matters?

In the world of PCB manufacturing, precision defines performance. PCB milling (also known as CNC routing) is the mechanical process of removing specific areas of the copper-clad laminate to form the board's outline, internal cutouts, and slots. Unlike chemical etching which defines the traces, milling defines the physical shape of your printed circuit board.

For hardware engineers and procurement managers, understanding the milling process is critical. A poorly defined mechanical layer can lead to dimensional errors, assembly failures during PCBA (PCB Assembly), or costly production delays. In 2025, with electronic devices becoming smaller and enclosures becoming tighter, the tolerance requirements for PCB milling have tightened significantly—often requiring accuracy within ±0.1mm.

2. Milling vs. V-Scoring: Choosing the Right Profiling Method

When finalizing your design for production, you generally have two options for separating individual boards from the manufacturing panel: Milling (Routing) or V-Scoring.

V-Scoring (V-Cut)

V-scoring involves cutting a "V" shaped groove on the top and bottom of the PCB, leaving a thin web of material remaining. This is ideal for:

• Low cost PCB projects with square or rectangular shapes.
• Mass production where material utilization needs to be maximized (zero spacing between boards).
• Rigid boards that will be manually separated or de-paneled by machine after assembly.

PCB Milling (CNC Routing)

Milling uses a rotating cutting tool (router bit) to cut through the board completely. This is the standard choice for:

• Complex shapes (circles, curves, L-shapes).
• Boards requiring high dimensional accuracy.
• Projects with internal cutouts or slots.
• Quick turn PCB prototypes where setup time for V-score blades isn't efficient.

Pro Tip: For high-volume turnkey PCB orders, NextPCB often recommends a combination: milling for the intricate features and V-scoring for the straight panel edges to optimize both cost and stability.

3. The Mechanical Layer: Vital Data for PCB Manufacturing

The success of the milling process depends entirely on the data provided in your Gerber files. The "Mechanical Layer" (often GKO, GM1, or Outline layer) acts as the blueprint for the CNC machine.

Critical Data Requirements

To ensure your board is manufactured correctly by NextPCB's engineering team, your mechanical layer must contain:

1. Precise Outline: A continuous, watertight closed loop defining the board edge.
2. 1:1 Scale: Never scale your data. Always output Gerbers at a 1:1 ratio.
3. Dimensions: While not readable by the machine, dimension lines (in mm or inches) help CAM engineers verify the scale.
4. Internal Features: Clearly defined paths for slots, cutouts, and non-plated holes.

The "Center Line" Rule

Unlike etching data where linewidth matters, milling data is interpreted as a path. Most PCB manufacturing CAM systems interpret the line in your mechanical layer as the center of the router bit's path. However, professional fabricators like NextPCB will automatically apply "tool compensation" to cut along the outside of your line to ensure the final board dimensions match your design exactly.

4. Essential Design Rules for Milling

To avoid "Engineering Questions" (EQ) delays and ensure a smooth transition to PCB assembly, follow these DFM (Design for Manufacturing) guidelines:

Standard Tool Sizes

The standard routing bit diameter in the industry is 2.0mm (approx. 79 mil).

• Implication: The minimum internal corner radius will be 1.0mm.
• Design Tip: If you need a tighter corner, you must specify a smaller tool (e.g., 1.6mm, 1.2mm, or 0.8mm). Note that smaller bits are more fragile and may slightly increase processing time/cost.

Copper Clearance (Pull-back)

Keep copper features (traces, pads, planes) away from the milling path. Cutting through copper can cause:

• Burrs: Rough edges that can cause shorts in the chassis.
• Delamination: Physical stress tearing the copper foil.

Recommended Clearance: ≥ 0.25mm (10 mil) from the board edge or cutout edge.

5. Handling Internal Cutouts and Slots

Internal milling is often used for mounting holes, ventilation, or high-voltage isolation (creepage).

The "Dog Bone" Issue

Since a round router bit cannot cut a perfect 90° square corner, creating a rectangular slot results in rounded corners. If you are inserting a rectangular component (like a USB connector or a fuse holder) into the slot, it might not fit.

Solution: Use "Dog Bone" corners (drilling over-sized holes in the corners) to create clearance for the component's sharp edges.

Plated vs. Non-Plated Slots

Clearly distinguish between Plated Through Slots (PTS) and Non-Plated Slots (NPTS) in your mechanical layer. PTS requires milling before the plating process, while NPTS is done after. Ambiguity here is a top cause of scrapped boards.

6. Cost Implications: How Milling Affects Price

While standard outline milling is included in the base price, complex milling can influence the cost of your low cost PCB strategy.

• Total Milling Length: Excessive routing (like elaborate internal cutouts) increases machine time.
• Small Tooling: Requiring a router bit smaller than 0.8mm slows down the feed rate to prevent bit breakage.
• Breakaway Tabs: Using "mouse bites" (perforated holes) instead of full milling can sometimes reduce cost and improve panel rigidity for PCBA.

At NextPCB, we optimize the routing path during our CAM review to provide the most competitive pricing without sacrificing quality.

7. FAQ: Common Questions About PCB Routing

Q1: Can I get a perfectly sharp 90-degree internal corner?

A: Not with standard milling. The only way to achieve a true 90° internal corner is to use a secondary process like laser cutting or punch pressing (for high volumes), but "Dog Bone" reliefs are the standard engineering solution.

Q2: What is the minimum slot width NextPCB can mill?

A: We can mill slots as narrow as 0.8mm (using a 0.8mm bit). For slots narrower than this, we recommend treating them as a series of overlapping drill holes (nibbling) or reviewing the design.

Q3: Does milling affect my Turnkey PCB Assembly quote?

A: Indirectly, yes. The panelization method (tab-route vs. V-score) affects how the PCBA rails hold the board. Proper milling design ensures the boards don't flex or break away prematurely during the SMT placement process.

8. Conclusion: Ensuring Precision with NextPCB

PCB milling is more than just cutting a shape; it is the foundation of mechanical integration for your product. Whether you are building a simple prototype or a complex high-frequency aerospace board, the accuracy of the mechanical layer is paramount.

Why Partner with NextPCB?

• Advanced CAM Engineering: We verify your milling data against 2026 IPC standards before production begins.
• Turnkey Capabilities: From quick turn PCB fabrication to final PCB assembly, we handle the entire mechanical and electrical compatibility check.
• Reliability: Certified to ISO9001 and IATF16949 standards. >> View Certifications at NextPCB

Ready to bring your design to life? Upload your Gerber files today for an instant DFM check and quote.

About the Author

Sylvia Zhang

Sylvia joined NextPCB two years ago and has already become the go-to partner for clients who need more than just boards. By orchestrating supply-chain resources and refining every step from prototype to mass production, she has repeatedly delivered measurable cost savings and zero-defect launches. Consistency is her hallmark: every client, every order, receives the same uncompromising quality and responsive service.

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