Posting here because all the threads asking for feedback on it are locked.
We have been extremely happy with the 4300 tacky flux for BGA attachment. Not only is it both washable and no-clean (i.e. wash-optional), it was also the only tacky flux that could consistently deal with a batch of marginal-ENIG-finish boards we got (which would otherwise have been scrapped; the vendor was scrapped regardless of course!).
It really does wash off using only water. Warm DI water in a cheap ultrasonic gives outstandingly clean results (we always use the ultrasonic for cleaning underneath BGAs, it's probably required in that case).
Interested in hearing anybody else's thoughts on this stuff.
Also, interested in knowing if any other vendor has a paste that is wash-optional (i.e. washable but wash not required). Is Amtech 4300 the only game in town?
We haven't switched to their paste yet since we have unusual requirements for the LGA components we use, and it took a long time to find a (non-washable) paste that worked reliably.
We have been using the Amtech line for over 12 years now with all of our PCB and BGA Rework. The NWS-4400 is mildly active and will give better results with Lead Free Solder. Note that the LF-4300 is considered a No-Clean. However it will show leakage at over 50 meg ohm measurements. So if you leave this on the PCB and use a high end meter like a Fluke. It can surprise you if you are not aware of this. Amtech has a great Type 5 Solder Paste if you are screening really fine pitch devices. We install our LGA's by solder bumping with solder balls and then installing with the Amtech Tacky Flux. This will result in void free solder joints. Check out our custom reballing fixture for high volume reballing and solder bunping.