Most free alternatives are "blind" tracers. They follow the dark pixels blindly, resulting in vector files that are composed of thousands of microscopic jagged lines—a "spaghetti model" that crashes CAD software.
Scan2CAD’s secret sauce is pre-processing . It cleans the image before conversion. GIMP can do this better than any free tool. scan2cad free alternative
If you are looking to convert scans to CAD without breaking the bank, you don't need to compromise on quality. Here are the best that deliver professional results. Most free alternatives are "blind" tracers
on using the Trace Bitmap feature in Inkscape for your specific file type? scan2cad free alternative
The most reliable free route is combining (for conversion) with FreeCAD or LibreCAD (for engineering). Step 1: Vectorize with Inkscape