What is the SLDASM file format?
The .sldasm file is a SOLIDWORKS Assembly file created by Dassault Systèmes' SOLIDWORKS CAD software. Rather than storing geometry directly, an assembly file acts as an organizer that references individual .sldprt part files (and nested sub-assemblies) and defines how they fit together through mate constraints - geometric relationships such as coincident faces, concentric axes, and fixed distances that lock components into position.
The file uses Microsoft's OLE2 Compound File Binary container, which means its first eight bytes (D0 CF 11 E0 A1 B1 1A E1) are identical to those of legacy .doc or .xls files. The format is proprietary and undocumented; the internal stream structure changes between SOLIDWORKS versions, so files saved in a newer release cannot be opened in an older one.
Assembly files support multiple named configurations, each of which can suppress or swap components and alter mate values - useful for product families or design variants. A Bill of Materials table can be embedded to list components, quantities, and custom properties. For very large assemblies, components can be loaded in lightweight or Large Design Review mode to reduce memory consumption.
To share a design without requiring a SOLIDWORKS license, the assembly can be published to the lightweight .easm format, or exported to exchange formats such as STEP or IGES. The SOLIDWORKS Pack and Go utility bundles the .sldasm file with every referenced part, sub-assembly, and drawing into a single folder or ZIP archive for safe transfer.
Security & safety
RISK: LOWSLDASM is a passive CAD data file - geometry, references and metadata, not executable code, so opening one in a viewer is safe. The practical cautions are non-malware: an assembly references external part files, so a .sldasm received alone may open with missing components; a file saved in a newer SOLIDWORKS may not open in an older one; and exporting to mesh formats (STL/OBJ) discards editability. Treat shared assemblies as intellectual property and get files from sources you trust.
Format details
in a nutshellPrograms that open SLDASM files
Windows3 apps
SolidWorks
Paid The native editor. File > Open the .sldasm (keep its referenced parts alongside) to view and fully edit the assembly. 
eDrawings Viewer
Free Free official Dassault viewer. Open the .sldasm to rotate, explode, measure and print - no SOLIDWORKS needed. 
Autodesk Fusion 360
Freemium Upload/insert the .sldasm; Fusion imports SOLIDWORKS assemblies for viewing, editing or export. 
macOS2 apps
eDrawings Viewer
Free Free Dassault viewer for Mac; open the .sldasm to view/measure (SOLIDWORKS itself is Windows-only).
Autodesk Fusion 360
Freemium Mac CAD app that imports SOLIDWORKS assemblies; insert the .sldasm to view and re-export. 
Android1 app
eDrawings web / online CAD viewers
Free Upload the .sldasm to a browser viewer to rotate and inspect without installing anything. Technical details
deep spec| Container format | OLE2 Compound File Binary (Microsoft Compound Document Format) |
| File signature (hex) | D0 CF 11 E0 A1 B1 1A E1 at byte offset 0 - shared with all OLE2 documents; the format is distinguished by internal SOLIDWORKS-specific streams and the .sldasm extension, not by magic bytes alone |
| MIME type | application/octet-stream |
| Encoding | Proprietary binary; internal stream structure is undocumented and version-dependent, not human-readable |
| Component references | Stores relative and absolute paths to referenced .sldprt part files and nested .sldasm sub-assemblies; unresolved references prevent the assembly from fully loading |
| Mate constraints | Records geometric mate relationships (coincident, concentric, parallel, distance, angle, etc.) that position and constrain components relative to each other |
| Configurations | Supports multiple named configurations per file; each can independently suppress or replace components, change mate dimensions, and define unique display states |
| Assembly features | Supports assembly-level features - holes, cuts, welds, and component patterns - that span across multiple component bodies |
| Bill of Materials | Embeds BOM tables listing components, quantities, part numbers, and custom properties; drives drawing BOMs and export cut lists |
| Lightweight and LDR mode | Components can be loaded as lightweight or in Large Design Review (LDR) mode to reduce RAM use for assemblies containing thousands of parts |
| Motion and simulation data | Can contain SOLIDWORKS Motion study data and references to SOLIDWORKS Simulation (FEA) setups stored alongside the assembly |
| Version compatibility | Files saved in a newer SOLIDWORKS version cannot be opened in older releases without using Save As to an earlier format version |
| Export interoperability | Exportable to neutral exchange formats including STEP, IGES, STL, 3MF, DXF, and PDF through built-in SOLIDWORKS translators |
| eDrawings publish | Can be published to the lightweight .easm format for viewing, measuring, and marking up without a SOLIDWORKS license |
| Pack and Go | SOLIDWORKS Pack and Go utility collects the .sldasm file together with all referenced parts, sub-assemblies, and drawings into a folder or ZIP archive for complete project transfer |
| Released | 1995 (SOLIDWORKS 95) |
| Specification | www.solidworks.com |
SLDASM conversions
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