I order this stuff at work:
https://www.amazon.com/Chip-Quik-SMD291NL-lead-free-applications/dp/B00HKJTJ6U/
It cleans off ok with IPA, or alternatively:
https://www.amazon.com/ChemtronicsES835B-Flux-Remover-Brush-fl-oz/dp/B00DK2AY1U