How to Generate NC Drill Files in Altium Designer: Complete PCB Engineer’s Guide

After years of working with Altium Designer across dozens of PCB projects, I’ve learned that NC drill file generation is one of those critical steps that separates smooth manufacturing runs from costly delays. The drill file might seem straightforward compared to complex Gerber configurations, but getting it wrong means holes that don’t align with pads — and that’s a board headed straight for the scrap bin.

This guide walks you through everything you need to know about generating NC drill files in Altium Designer, from basic setup to handling advanced scenarios like blind and buried vias.

Understanding NC Drill Files in PCB Manufacturing

NC drill files (Numerical Control drill files) contain the precise coordinate data and tool specifications that CNC drilling machines need to place every hole in your PCB. While Gerber files define your copper traces, solder masks, and silkscreen, the NC drill file tells the manufacturer exactly where to drill and what size bit to use.

The standard format for NC drill files is Excellon, which has become the de facto industry standard. Altium Designer exports drill data in Excellon 2 format, which virtually every PCB fabricator accepts.

What NC Drill Files Contain

Data Element	Description	Manufacturing Purpose
Tool Table	List of drill bit diameters	Tool selection and sequencing
Coordinates	X,Y positions for each hole	CNC machine positioning
Tool Changes	Commands to switch drill bits	Automated tool management
Hole Attributes	Plated vs. non-plated designation	Process routing

Why NC Drill Files Must Match Your Gerber Files

This point cannot be overstated: your NC drill files must use identical format settings as your Gerber files. If your Gerber files use 2:4 format in inches with leading zero suppression, your drill files need exactly the same configuration.

Mismatched settings cause the most common drill file problems:

• Drill holes offset from pad centers
• Scaled drill patterns (holes appear twice as large or half the size)
• Holes appearing in completely wrong locations

I’ve seen experienced engineers waste hours troubleshooting “faulty” drill files when the actual problem was a simple mismatch between Gerber and drill export settings.

Step-by-Step Guide: Generating NC Drill Files in Altium Designer

Step 1: Access the NC Drill Setup Dialog

With your PCB document open, navigate to:

File > Fabrication Outputs > NC Drill Files

This opens the NC Drill Setup dialog where you’ll configure all export parameters. Alternatively, you can access this through an OutputJob file for more repeatable, production-ready workflows.

Step 2: Configure NC Drill Format Settings

The format settings determine how coordinate data is written to your drill file.

Units Selection:

Option	When to Use
Inches	North American manufacturers, imperial designs
Millimeters	European manufacturers, metric designs

Format (Resolution):

Format	Resolution	Use Case
2:3	1 mil (0.001″)	Legacy systems only
2:4	0.1 mil (0.0001″)	Standard for most designs
2:5	0.01 mil (0.00001″)	High-precision designs

For most PCB designs, 2:4 format in inches or 3:3 format in millimeters provides sufficient resolution. Only use higher resolutions if your design has holes placed on grids finer than 1 mil — and verify your manufacturer supports that precision.

Step 3: Configure Zero Suppression Settings

Zero suppression reduces file size by removing unnecessary zeros from coordinates. Three options exist:

Setting	Effect	Recommendation
Keep leading and trailing zeroes	Full coordinate precision	Safest option, highest compatibility
Suppress leading zeroes	Removes zeros at start	Common choice, match to Gerber
Suppress trailing zeroes	Removes zeros at end	Less common, can cause issues

Pro Tip: If you’re experiencing drill alignment problems, try the “Keep leading and trailing zeroes” option. It eliminates a common source of coordinate interpretation errors between different CAM systems.

Step 4: Set Coordinate Origin Reference

Choose how coordinates are referenced:

• Reference to absolute origin — Uses the design’s absolute origin point
• Reference to relative origin — Uses the relative origin marker

Critical: This setting must match your Gerber file origin setting. If your Gerbers use absolute origin, your drill files must too. Mismatched origins cause the entire drill pattern to shift relative to copper features.

Step 5: Configure Advanced Options

Several additional options affect your output:

Optimize change location commands: Reduces file size by optimizing tool path sequences. Generally safe to enable.

Generate separate NC Drill files for plated & non-plated holes: Creates distinct files for PTH (Plated Through-Hole) and NPTH (Non-Plated Through-Hole) features. Many manufacturers prefer or require this separation.

Use drilled slot command (G85): Enables slot creation using overlapping drilled holes. Enable if your design contains slots.

Generate Board Edge Rout Paths: Creates a separate NC rout file defining the board outline. Useful for complex board shapes or internal cutouts.

Generate EIA Binary Drill File (.DRL): Creates a binary format file in addition to the text-based Excellon file. Note: Some manufacturers have reported issues with .DRL files, so consider whether you actually need this output.

Step 6: Generate the Output

Click OK to generate your NC drill files. Altium Designer creates the output files in your designated project output folder, typically under “Project Outputs for [ProjectName]” or within a “NC Drill Output” subfolder if you’ve enabled separate folders for each output type.

Understanding NC Drill File Output

After generation, you’ll typically see several files depending on your design complexity:

Standard Two-Layer Board Output

File	Extension	Content
Main Drill File	.TXT or .DRL	All through-hole drill data
Drill Tool Report	.DRR	Summary of tools and hole counts

Multilayer Board with Blind/Buried Vias

For complex layer stackups, Altium generates separate drill files for each layer pair:

File Extension	Layer Pair	Description
.TXT	Through-hole	Full stack penetration
.TX1	Top to Layer 2	Blind via (top side)
.TX2	Layer 3 to Bottom	Blind via (bottom side)
.TX3	Layer 2 to Layer 3	Buried via

The specific extensions depend on your layer stack configuration. Always include documentation explaining which file corresponds to which layer pair.

Handling Blind and Buried Vias

Modern HDI (High-Density Interconnect) designs frequently use blind and buried vias to increase routing density. Altium Designer automatically creates separate drill files for each via type based on your layer stack definition.

Configuring Via Types for Proper Drill Output

Before generating drill files for complex designs:

1. Open the Layer Stack Manager (Design > Layer Stack Manager)
2. Switch to the Via Types tab
3. Verify each via type has correct start and end layers defined
4. Ensure drill pairs are properly configured

Each unique layer pair in your via definitions generates a separate NC drill file with a unique extension.

µVia (Microvia) Considerations

Microvias require laser drilling rather than mechanical drilling. Altium handles this by creating separate drill files for each µVia layer pair, which your manufacturer will process on appropriate laser drilling equipment.

Using OutputJob Files for Repeatable NC Drill Generation

For production environments, I strongly recommend using OutputJob files (.OutJob) rather than generating outputs directly from the PCB menu.

Benefits of OutputJob Configuration

• Consistency: Same settings applied every revision
• Documentation: Settings saved with the project
• Automation: Generate all outputs with single click
• Version Control: Track output configuration changes

Setting Up NC Drill in an OutputJob

1. Create new OutputJob: File > New > Output Job File
2. In Fabrication Outputs, click Add New Fabrication Output
3. Select NC Drill Files
4. Double-click to configure settings
5. Assign to an output container (folder or PDF)

Settings configured in the OutputJob are stored separately from direct PCB export settings, allowing different configurations for different purposes.

Verifying Your NC Drill Files

Never send drill files to manufacturing without verification. Visual confirmation catches problems that automated checks miss.

Using Altium’s CAM Editor

Altium Designer includes CAMtastic, a built-in CAM editor for verifying manufacturing outputs:

1. After generating outputs, the CAM document may open automatically
2. If not, import your drill file: File > Import > Drill
3. Overlay with your Gerber files to verify alignment
4. Check that all holes center properly on pads

External Verification Tools

Tool	Platform	Cost	Notes
Gerbv	Windows/Linux	Free	Open source, reliable
ViewMate	Windows	Free version	Professional quality viewer
GC-Prevue	Windows	Free	Industry standard
Reference Gerber Viewer	Web	Free	Ucamco’s official viewer

Verification Checklist

Before submitting to manufacturing:

1. Load drill file and all Gerber layers in viewer
2. Verify holes align with pad centers on all layers
3. Check that no holes appear outside the board outline
4. Confirm plated and non-plated holes are in correct files
5. Verify slot positions and dimensions
6. Review the drill tool report for expected hole sizes

Common NC Drill File Problems and Solutions

Problem 1: Drill Holes Offset from Pads

Symptoms: Holes appear shifted relative to copper pads when viewing files together

Causes and Solutions:

Cause	Solution
Origin mismatch	Match coordinate origin setting between Gerber and drill
Unit mismatch	Ensure both use same units (inches or mm)
Format mismatch	Use identical format settings (e.g., 2:4)

Problem 2: Drill Pattern Scaled Incorrectly

Symptoms: Entire drill pattern appears too large or too small

Causes and Solutions:

Cause	Solution
Unit interpretation error	Verify units in both export and import
Wrong format setting	Match format between Gerber and drill

Problem 3: Missing Holes in Output

Symptoms: Some holes don’t appear in the generated drill file

Causes and Solutions:

Cause	Solution
Hole on wrong layer pair	Check via layer assignments
Hole size zero	Verify pad/via properties
Filter excluding holes	Check generation options

Problem 4: Manufacturer Rejects .DRL File

Symptoms: Fabricator cannot process the binary drill file

Solution: Some manufacturers prefer the text-based .TXT Excellon file over the binary .DRL format. Disable the “Generate EIA Binary Drill File” option and provide only the .TXT file.

Problem 5: Blind/Buried Via Files Not Generated

Symptoms: Complex via structures missing from output

Solution: Verify Layer Stack Manager has proper drill pairs defined for each via type. Check that “Generate separate NC Drill files for VIA features” is enabled if using IPC 4761 via types.

Best Practices for NC Drill File Management

Documentation Standards

Always include clear documentation with your drill files:

README.txt or Fabrication Notes:- List all drill files with layer pair explanations- Specify format settings (units, format, zero suppression)- Note any special requirements (slots, back drilling, etc.)- Include stack-up drawing reference

File Naming Conventions

Use descriptive names that identify the drill file purpose:

File Name	Description
ProjectName_PTH.TXT	Plated through-holes
ProjectName_NPTH.TXT	Non-plated holes
ProjectName_BlindTop.TXT	Top-side blind vias
ProjectName_Buried.TXT	Buried vias

Version Control

Include drill files in your version control system alongside design files. Track changes to drill output settings in your OutputJob files.

Useful Resources for Altium Designer Users

Official Documentation

Resource	URL	Description
Altium Documentation	altium.com/documentation	Comprehensive official docs
Altium Resources	resources.altium.com	Tutorials and guides
Altium Forum	forum.live.altium.com	Community support

Drill File Viewers

Tool	Access	Notes
Gerbv	gerbv.sourceforge.net	Free, open source
ViewMate	pentalogix.com	Free viewer version
GC-Prevue	graphicode.com	Industry standard

Manufacturer Guidelines

Always check your specific manufacturer’s documentation for:

• Preferred file formats and naming conventions
• Minimum hole sizes and tolerances
• Slot and routing capabilities
• Blind/buried via support

Frequently Asked Questions

What format should I use for NC drill files?

For most applications, use 2:4 format in inches or 3:3 format in millimeters. The critical requirement is matching your Gerber file format exactly. Verify your manufacturer supports your chosen resolution before using higher precision formats like 2:5.

Should I generate separate files for plated and non-plated holes?

Yes, enabling “Generate separate NC Drill files for plated & non-plated holes” is recommended. Many manufacturers require this separation because PTH and NPTH features go through different process steps. Even manufacturers who don’t require it can easily merge the files.

Why are my drill holes offset from the pads?

Drill-to-pad offset is almost always caused by mismatched settings between your Gerber and NC drill exports. Check that units, format, zero suppression, and coordinate origin settings are identical in both configurations. If problems persist, try using “Keep leading and trailing zeroes” option.

Do I need to generate the binary .DRL file?

In most cases, no. The text-based Excellon .TXT file is universally accepted and easier for manufacturers to process. Some fabricators have reported issues with binary .DRL files, so unless your manufacturer specifically requests it, you can safely disable this option.

How do I handle blind and buried vias in drill files?

Altium Designer automatically generates separate drill files for each layer pair when your design includes blind or buried vias. Ensure your Layer Stack Manager has correct drill pairs defined, and include clear documentation explaining which file corresponds to which layer span.

Altium Designer NC Drill vs. Other CAD Tools

Having worked with multiple PCB design platforms, I can offer perspective on how Altium’s NC drill generation compares.

Drill Export Comparison Across Tools

Feature	Altium Designer	KiCad	Eagle	OrCAD
Excellon Format	Yes	Yes	Yes	Yes
Auto Layer Pair Detection	Yes	Partial	No	Yes
Separate PTH/NPTH	Option	Manual	Manual	Option
OutputJob Integration	Yes	No	No	Partial
CAM Editor Included	Yes	GerbView	Limited	Yes
Blind/Buried Via Support	Excellent	Good	Limited	Good

Altium’s Advantages for Drill File Generation

Altium Designer excels in several areas for NC drill output:

Integrated Layer Stack Management: The Layer Stack Manager and Via Types definition work seamlessly with drill file generation. You define your via structures once, and Altium automatically creates the appropriate drill files for each layer pair.

OutputJob Automation: The ability to configure all fabrication outputs in a single OutputJob file and generate everything with one click streamlines production workflows significantly.

CAMtastic Integration: The built-in CAM editor allows immediate verification of generated outputs without switching to external tools.

Advanced Topics: Back Drilling and Controlled Depth

For high-speed designs, back drilling (also called Controlled Depth Drilling or CDD) removes unused via stubs that can cause signal integrity issues.

Back Drilling in Altium Designer

Altium supports back drilling through the Layer Stack Manager:

1. Define back drill spans in your layer stack
2. Enable back drilling for specific via types
3. Configure back drill depth and tolerance

The NC drill output automatically generates separate files for back drilling operations, including:

• Primary drill file for initial through-holes
• Back drill file with larger diameter and controlled depth specifications

Configuring Back Drill Parameters

Parameter	Description	Typical Value
Back Drill Diameter	Larger than signal via	0.3-0.5mm larger
Depth Tolerance	Acceptable depth variation	±0.1mm
Stub Length	Remaining via barrel	0.2-0.3mm

Always coordinate back drilling requirements with your manufacturer, as this process requires precise depth control and may not be available from all fabricators.

Slots and Routed Features in NC Drill Files

Slots present unique challenges in drill file generation because they’re not simple circular holes.

Slot Generation Options

Altium provides the G85 drilled slot command option for creating slots:

With G85 Enabled: Slots are defined using the G85 command, which tells the CNC machine to drill overlapping holes from start point to end point.

With G85 Disabled: Slots may be output as rout commands requiring a different manufacturing process.

Slot Best Practices

Consideration	Recommendation
Minimum slot width	Match to available drill sizes
End radius	Equal to half the slot width
Documentation	Include slot dimensions in fab notes

Check with your manufacturer regarding their slot capabilities before finalizing your design.

Integrating NC Drill Generation into Your Workflow

Pre-Generation Checklist

Before generating NC drill files, verify:

1. Layer stack is complete — All layers defined with correct materials and thicknesses
2. Via types are configured — Each via type has proper start/end layers
3. Design rules pass — No undrilled holes or via errors
4. Origin is set — Board origin placed at consistent location

Post-Generation Verification

After generating outputs:

1. Open in CAM viewer — Verify all holes are present
2. Check alignment — Overlay with copper layers
3. Review tool report — Confirm expected drill sizes
4. Verify file count — Ensure all layer pairs have files

Manufacturing Package Organization

Organize your fabrication package consistently:

Fabrication_Rev01/├── Gerber/│   ├── TopCopper.GTL│   ├── BottomCopper.GBL│   └── …├── Drill/│   ├── ProjectName_PTH.TXT│   ├── ProjectName_NPTH.TXT│   └── DrillReport.DRR├── Assembly/│   └── …└── README.txt

Conclusion: Ensuring Accurate NC Drill File Generation

Generating NC drill files in Altium Designer is straightforward once you understand the relationship between format settings and manufacturing requirements. The most important principle is consistency — your drill file settings must match your Gerber file settings exactly.

Key takeaways for reliable drill file generation:

1. Match all format settings between Gerber and NC drill exports
2. Use consistent origin references (absolute or relative)
3. Separate PTH and NPTH into distinct files
4. Verify outputs visually before sending to manufacturing
5. Include clear documentation with your fabrication package
6. Use OutputJob files for repeatable, production-ready outputs

Taking the extra time to configure and verify your NC drill files pays dividends in manufacturing success. A properly formatted drill file ensures that every via connects the right layers and every mounting hole lands exactly where your design intended.