Many sellers from China/Hong Kong offering on eBay a “solder flux” which is of very bad quality.
This junk is marked as
“150g High Intensity Professional Solder Soldering Paste Flux Gel für BGA SMD Lot”
or
“ZJ-18 80g Environmental Soldering Solder Flux Paste Grease Gel”
or
“Advanced Quality ZJ-18 Soldering Solder Flux Paste”
DON’T BUY THIS!
Almost zero efficiency! Grease based with very low percentage of rosin, leaves the board fat, negligible effect on solder spreading, no cleaning effect. Bad product, worthless!