IMHO, A photo would be great... before you clean it up.
For mass production, it is very neccessary to discuss with supplier on these details, even specify the solder used or demand an extra cleaning process.
For small batch manufacturing, the factory uses hand soldering. As far as I have seen, there are 3 most popular flux:
- the yellow solid one, which would probably leave many messy to the soldering point, most difficult to clean (chemical solvent).
- Light yellow solid one, it leaves flimsy white residue after soldering, even your finger nail could remove them easily.
- Light yellow fluid-like, it leaves some oily residue, a tissue will remove it perfectly.
The third one is the most expenisve, but the final result is only on visual part. They all contain flux, cleaned away or left messy.