Someone mentioned on Hackaday a while back of using Detergent 8 to remove flux from PCBs.  I looked the product up and found the product page here. There was a link to order a sample so I did. A little time later an 8 oz. sample bottle arrived.

I first tried a 3% solution on a PCB I had lying around, the before picture was a bit blurry so I didn't post the before and after of that. It got most of the flux off but there was still some stubborn traces of flux left. 

I decided to try it again, this time making a ~6% solution (instructions specify a 3-5% solution for manual soaking.) I measured it out in a graduated cylinder. 

The test subject was a PCB I soldered together in the morning (I didn't solder in the crystal as I didn't want it to be submerged.)

I poured the solution in a bowl, heated it in the microwave for about 20 seconds and then placed the PCB in the bowl.

I let it soak for about half an hour and then removed the board.  Just about all of the flux was removed!  I didn't even have to scrub the board.  I'm impressed!

The good news: it beats having to use isopropyl alcohol and having to brush the crap out of the board with a toothbrush. The bad news: It's not cheap. A one gallon bottle costs over $100 USD.  Fortunately, the 8 oz. sample bottle will last me quite a while.

Some tips to make things easier. Use a dropper the get the concentrate out of the bottle and put it in a dropper bottle. Don't try pouring directly out of the bottle like I did as some likes to run down the side of the bottle.