I would recommend the no-clean process. The problem with a water soluble process for QFN and DFN devices, is that cleaning underneath is very difficult due to their low standoff.
The residue from a water soluble process is corrosive, and, if not completely removed, can cause a variety of defects and, ultimately, package failure.
Unfortunately, there is no hard and fast rule for when to use a no-clean vs. a water soluble process. It really depends on the components you are placing and what your process needs are.
For example, if you have a board where excessive levels of oxidation or contamination are present, you may want to select a water soluble process for its ability to aggressively strip oxides.
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These component types have very low stand off heights and will be very difficult to clean underneath. Water soluble flux residues are normally corrosive and must be removed therefore I would not recommend using this option.
No clean would seem to be your preferred route providing you are satisfied that it can stay on the board.
Principal Engineer - CMA Lab
Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.