sch to png conversion is most likely related to export of various CAD/PCB schematics (.sch) files to PNG format, typically for the purpose of making preview picture of it, or some similar export. Most PCB software actually offer export to raster formats and PNG format is always listed.
Since .sch files are generic and might come from many different software, you might need to find one that supports your particular file type.
Updated: September 12, 2023