What is .DSN File? Specctra Autorouter Format Explained

If you’ve tried to use an external autorouter with KiCad, Altium, or other PCB design software, you’ve encountered the .DSN file format. This is the Specctra Design file—an industry-standard interchange format that lets you export your board to external autorouters and import the routing results back into your EDA tool.

Understanding the .DSN (Specctra) format is essential for anyone who wants to leverage powerful autorouting tools like FreeRouting, TopoR, or ELECTRA. This guide explains what Specctra .DSN files contain, how the autorouting workflow works, and how to use this format effectively across different PCB design platforms.

What is a Specctra .DSN File?

A Specctra .DSN file is a text-based design interchange format originally developed by Cooper & Chyan Technology (CCT) in 1989 for their shape-based PCB autorouter. When Cadence Design Systems acquired CCT in 1997, the format became integrated into Cadence’s Allegro PCB Router (formerly called Specctra).

The .DSN (Specctra) format describes a PCB design in terms that autorouters need: board outline, component placement, pad definitions, netlist connections, and design rules. Unlike native PCB formats that contain everything about a design, Specctra .DSN files focus specifically on the information required for automated trace routing.

Specctra .DSN File Identification

Important: .DSN File Types Are Different

The .DSN extension is used by two completely different file types in electronics design—and confusing them causes significant headaches.

.DSN File Type Comparison

When you receive a .DSN file, check its contents in a text editor. Specctra .DSN files are readable text starting with (pcb and containing S-expression syntax. OrCAD .DSN files are binary and unreadable—these are schematic files, not autorouter files.

History of the Specctra Format

The Specctra autorouter and its .DSN format have a significant history in PCB design automation.

Specctra Timeline

The .DSN (Specctra) format succeeded because it solved a real problem: how to let any PCB tool use any autorouter. Before this standardization, autorouters were tightly coupled to specific EDA software. The open nature of the text-based .DSN format enabled tool interoperability that benefits designers to this day.

Inside the Specctra .DSN File Format

Specctra .DSN files use S-expression syntax—nested parenthetical structures similar to Lisp programming language notation. This format is human-readable and relatively straightforward to parse programmatically.

.DSN File Structure Sections

Example .DSN File Header

(pcb /home/user/project/myboard.dsn  (parser    (string_quote “)    (space_in_quoted_tokens on)    (host_cad “KiCad’s Pcbnew”)    (host_version “7.0.0”)  )  (resolution um 10)  (unit um)  (structure    (layer F.Cu      (type signal)      (property        (index 0)      )    )    (layer B.Cu      (type signal)      (property        (index 1)      )    )    (boundary      (path pcb 0 0 0 100000 0 100000 80000 0 80000 0 0)    )    (via “Via[0-1]_800:400_um”)    (rule      (width 250)      (clearance 200)    )  )  …)

The structure section defines the board’s physical characteristics: copper layers, board outline (boundary), via definitions, and default design rules like trace width and clearance.

The Specctra Autorouting Workflow

Using Specctra .DSN files follows a well-defined workflow that separates PCB design from autorouting.

Standard DSN/SES Workflow

The key insight is that .DSN and .SES files work as a pair: the .DSN (Specctra) file describes what needs to be routed, and the .SES (Session) file contains the routing solution.

Related Specctra File Types

Software Supporting Specctra .DSN Format

The .DSN (Specctra) format has become an industry standard supported by most major PCB design tools and several autorouters.

EDA Tools with DSN Export/Import

Autorouters Using DSN/SES Format

Exporting .DSN Files from KiCad

KiCad is the most common free tool for working with Specctra .DSN files. Here’s the detailed process.

KiCad DSN Export Steps

Common Export Issues

Using FreeRouting with .DSN Files

FreeRouting is the most popular free autorouter that uses the Specctra .DSN format. It’s open-source, Java-based, and produces excellent routing results.

FreeRouting Workflow

FreeRouting Settings

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What is .DSN File? Specctra Autorouter Format Explained

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What is a .BRD File? Eagle PCB File Format Explained

What is a .DRC File? Design Rule Check Reports Explained

What is a .PCB File? Complete Protel/Altium PCB Format Guide

What is a .SES File? Complete Specctra Session File Guide for PCB Engineers

What is a BOM File? Bill of Materials Format for PCB Assembly

What is a CPL File? SMT Component Placement Guide

What is a Fabrication Drawing? PCB Fab Notes Explained

What is a Gerber Job File? Project Metadata Explained

What is a Netlist? PCB Connectivity Data Explained

What is a Pick and Place File? SMT Centroid Data Guide

What is a Stencil Gerber File? Complete Solder Paste Stencil Guide

What is an Aperture File? Gerber Tool Definition Guide

What is an Assembly Drawing? Complete PCB Assembly Documentation Guide

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What is PDF/A for PCB? Documentation Standards Guide

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What is VRML in PCB? 3D Visualization Format Guide

Importing .SES Files Back to Your EDA Tool

After autorouting, the .SES session file contains all the traces and vias. Importing this file transfers the routing back to your PCB design.

KiCad SES Import

Import Troubleshooting

.DSN File Best Practices

Working effectively with Specctra .DSN files requires attention to several details.

Pre-Export Checklist

Quality Tips

1. Clean up placement first: Autorouters work best with well-organized component placement
2. Pre-route critical nets: Route high-speed or sensitive traces manually before export
3. Set realistic rules: Overly tight clearances cause routing failures
4. Use fanout: Let the autorouter fan out BGA/QFN pads before general routing
5. Iterate: Multiple autoroute passes with different settings often improve results

Specctra .DSN vs Other PCB Interchange Formats

The .DSN (Specctra) format isn’t the only PCB interchange option, but it remains the standard for autorouting specifically.

PCB Interchange Format Comparison

Specctra .DSN remains dominant for autorouting because it was specifically designed for this purpose, unlike manufacturing formats that focus on fabrication data.

Useful Resources for Specctra .DSN Files

Software Downloads

Documentation

Community Resources

Frequently Asked Questions About .DSN Files

What’s the difference between Specctra .DSN and OrCAD .DSN files?

These are completely different file types sharing the same extension. Specctra .DSN files are ASCII text using S-expression syntax—they’re autorouter interchange files containing PCB layout data for routing. OrCAD .DSN files are binary schematic capture files containing circuit schematics. Open the file in a text editor: if you see readable parenthetical syntax starting with (pcb, it’s Specctra; if it’s binary garbage, it’s OrCAD. The tools that open them are different too—Specctra .DSN opens in FreeRouting or autorouters, while OrCAD .DSN opens in OrCAD Capture.

Why does my .DSN export fail with “duplicate reference IDs”?

This error means multiple components in your design have the same reference designator (like two parts both labeled “R1”). The Specctra .DSN format requires unique identifiers for every component. Return to your schematic, run annotation to assign unique designators, update your PCB, and try the export again. In KiCad, use Tools → Annotate Schematic to fix this automatically.

Can I edit .DSN files manually in a text editor?

Yes, since Specctra .DSN files are plain ASCII text, you can edit them directly. This is occasionally useful for fixing minor issues, adjusting design rules, or understanding the format. However, be careful—incorrect syntax will cause import failures. Always keep a backup before manual editing. The S-expression format requires matched parentheses, so one missing parenthesis breaks the entire file.

Why won’t FreeRouting import my .DSN file?

Common causes include: the file is actually an OrCAD schematic .DSN (not Specctra format), the file was corrupted during export, or there are unsupported features in the design. Check that your file opens in a text editor as readable S-expression syntax. Verify your EDA tool completed the export without errors. Try exporting a simpler test design first to confirm the workflow works. Also ensure you’re using a current version of FreeRouting, as older versions may have compatibility issues.

How do I route specific nets manually while using autorouter for the rest?

Route your critical nets manually in your EDA tool before exporting to .DSN (Specctra) format. The export will include these pre-routed traces in the .DSN file’s wiring section. When the autorouter processes the file, it treats existing traces as fixed and routes only the remaining unconnected nets. This is the standard approach for mixed manual/automatic routing—sensitive signals like clocks, differential pairs, and power get manual attention while bulk routing happens automatically.

Conclusion

The Specctra .DSN format has served as the PCB autorouting interchange standard for over three decades, enabling designers to use specialized routing tools regardless of their primary EDA software. Understanding this format opens access to powerful free tools like FreeRouting that can dramatically speed up PCB layout work.

The key workflow is straightforward: export your placed (but unrouted) design to .DSN, open it in an autorouter, run the routing algorithm, export the .SES session file, and import the results back. This round-trip process preserves your component placement while adding automated trace routing.

For most users, the .DSN (Specctra) format works transparently—you export, route, and import without needing to understand the file internals. But when problems occur, knowing that it’s a text-based S-expression format makes troubleshooting much easier. A quick look in a text editor reveals whether the export succeeded, and you can often spot issues like missing board outlines or duplicate components directly in the file.

As PCB designs grow more complex and autorouting algorithms continue improving, the Specctra interchange format remains relevant. Whether you’re routing a simple two-layer board or a complex multilayer design, the .DSN/.SES workflow provides a reliable path to automated trace routing.