After you've reflowed solder in contact with a flux, you're always left with a certain amount of flux residue. There are no clear industry guidelines on how you refer to the residue, and new terminology is emerging all the time. If you leave it up to me, here is what I recommend : 
1/ "No clean" flux residues:
- Standard Residue: >40%
- Low Residue (LR): Between > 10% to 40%
- Ultralow Residue (ULR): Between >2% to 10%
- Near Zero Residue (NZR): Between 0 to 2%
Each % is given as the weight percent of flux residue after a real reflow process, and refers to the fraction of the raw flux, or flux component of a mixture (such as solder paste or metal-filled epoxy). Note that the exact amount of residue will vary with the reflow profile; the mass of flux or solder paste studied; and the rate of gas flow over the sample material, as well as secondary factors, such as the oxygen level in the reflow atmosphere.
Thermogravimetric analysis (TGA) is a pretty poor method for determining post-reflow residue levels. Results from the use of a platinum TGA sample cup with nitrogen flowing over it have been found in our testing to vary significantly with the mass of sample present, probably because the headspace in the cup acts as a "dead zone" for entrapment of vapor: TGA may therefore give artificially high % residue readings, compared to the results on a flat leadframe or other substrate.
From the viewpoint of a standard semcionductor assembl process, now consider the situation of a low-clearance direct chip attach "flip-chip" or package-on-package application, where the flux is essentially entrapped in a "cage" of I/O's, sandwiched between two flat diffusion barriers. As well as issues of flux residue, this also raises the question of how the electrical properties of the flux will be affected, if more of the solvent and other volatiles from the flux are trapped in the residue.
2/ "Water-soluble" (same principles apply for "Solvent cleanable") flux residues:
- Water-soluble: Residues can be truly dissolved in water to leave a transparent liquid: the color of the this rinse liquid is immaterial,
- Water-dispersible: Non-transparent rinse liquid with any hint of translucency or turbidity
I know that the differences here will be very dependent on rinse-water quality and temperature; chemistry of any cleaning agents; stage of bath-life and so on, but to my mind, if the rinsed liquid is not transparent, then the solids from the flux must be suspended as fine particulates. These particulates usually have refractive indices different from the bulk liquid: the result - turbidity. There may be a means of bath-life end-point determination by turbidity or dynamic light scattering (DLS) or a similar technique; possibly in combination with the standard refractive index measurement that is most commonly used.
In conclusion, note that ULR and NZR fluxes are showing increased usage in flip-chip applications, since these types of material interfere less with the curing of underfill polymers. NZR fluxes are becoming critical for copper-pillar bumping applications.
Just my thoughts - let me know what you think.
Cheers! Andy
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