“Water cleanable solder pastes have advantages and disadvantages when compared to their RMA and “No-Clean” kin.
1)Higher Activity – Knowing that the flux residues will be removed from the assembly, the solder paste manufacturer has a certain amount of liberty to include aggressive ingredients that are more effective at removing oxides from the surfaces that are being soldered. A “No-Clean” paste does not have this luxury because its residues are designed to be left on the assembly, without cleaning. Therefore the ingredients in a “No-Clean” solder paste must be relatively benign. “Dirty” (heavily oxidized) surfaces require a higher level of flux activity as compared to fresh, clean, relatively oxide-free surfaces.
Even among water soluble pastes there are different levels of activity. An industry standard used to classify solder pastes, IPC J-STD-004, breaks them down into these categories: ORL0, ORL1, ORM0, ORM1, ORH0 and ORH1. The “OR” at the beginning of this four character code identifies it as a water cleanable flux. Literally, it stands for “ORganic acid”; organic acids being a family of chemicals that can be included in fluxes to help them remove oxides from the surfaces being soldered. The third character in the four character code, L (low), M (moderate), or H (high), refers to level of flux activity, specifically the level of corrosiveness of the flux as observed when left on a copper metallized glass slide. Generally, the more corrosive the flux, the more effective it is at removing oxides. The last character, which is either the number 0 or 1, indicates the presence and/or level of halides in the solder paste flux. “0” indicates a halide free or low halide flux; whereas “1” indicates the “presence” of halides. Halides typically improve the fluxes ability to remove oxides. There is a legacy fear of halides in solder paste fluxes. The fear relates to a belief that a halide containing flux residue, left on the board or not entirely removed with cleaning, is more prone to corrosion. However, with adequate cleaning and removal of a solder paste flux residue, there should be little, if any, halides remaining on the assembly.
2) Board Cleanliness – Due to the simple fact that the solder paste flux residue will be removed with cleaning adds to the cosmetically “clean” appearance of the board as compared to solder pastes where the flux residue is left on the assembly. It also may improve the reliability of the assembly because flux residue corrosion and leakage current concerns are virtually eliminated with proper cleaning.
3) Moisture “Sensitive” – One of the most distinct disadvantages of water cleanable solder pastes is that they are sensitive to the level of humidity in the working environment. They can “extract” moisture from room air causing them to become soupy in humid environments. Water cleanable solder pastes are best suited to situations where the humidity is controlled and exposure to air is minimized.
4) Shelf Life – The shelf life of water cleanable solder pastes is often less than comparable “No Clean” or RMA formulations. As mentioned earlier, water cleanable solder paste fluxes are typically more active than the other varieties. In solder paste, powdered metal (solder alloy) is suspended in the flux. That means that the flux is direct contact with the powdered metal. The “active” ingredients in the flux can react with the powdered metal even while the product is sitting on the shelf in a sealed container at room temperature. Therefore, refrigerated storage of water cleanable solder pastes is necessary to insure the optimum shelf life and condition of the solder paste.” –Eric Bastow