Does anyone have a favorite solution for removing flux from PCB's in an ultrasonic cleaner? Question 2: has anyone used this specific product and did it work well?

https://www.digikey.com/en/products/detail/sra-soldering-products/UCS-35-PCB-1Q/10708340

The back story; I've got to manually build about 100 boards that have large through hole parts so they will be covered with flux and I'm looking for the least laborious way to get them clean.

 

My goto is still absolute ethanol plus about 10% to 25% acetone. I do not use ultrasonic. However, I have been experimenting with some of the lower Cellosolves (https://en.wikipedia.org/wiki/Glycol_ethers ) to which water (e.g., 10%, even up to 30% or more with some of the Cellosolves) can be added. So far the results are encouraging, but that project has been on the shelf since Spring. The reason for water is occasional salt-like residues. Not sure where they come from, maybe parts that have heavier oxidation on them.

I have noticed a difference between old flux (i.e., flux residues left over for a week or more on the board) and fresh new flux residue. The latter seems easier to remove.

Edit: Of course, if the board has clear plastics on it or other plastics that I suspect might be acetone-sensitive, I leave the acetone out. The acetone only makes the action a little quicker.