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106 SMT Magazine • January 2017 Since low-solids/no-clean fluxes are the more common flux used in selective soldering, we have correlated them to the IPC designators in the below table. The point of the designator system, is so that a user can make a more informed decision about the activity and suitability of their flux and un- derstand whether post soldering cleaning is re- quired (or at least investigated). Full rosins and water soluble fluxes both have to be cleaned af- ter soldering because of the high amount of sol- ids and residues, and/or because of high activity of their residues. NOTE: It is best to consult with the manu- facturer of the flux to discuss its performance and behavior. Some fluxes can have tendencies perform better, or worse, in certain aspects even though they appear virtually the same "on pa- per," which only the manufacturer or rep will know. CAUTION: Be cautious, not only about wa- ter soluble fluxes, but about any flux with low pH. In a number of cases, the pH on a datasheet can be misleading, as all fluxes are designed to become more active as heat is applied. But low pH at room temperature can prove to be det- rimental to equipment. Also, because of some clever chemistry, some fluxes are actually more active than they appear with IPC classification, which only the manufacturer knows. It is al- ways best to consult the manufacturer about its characteristics prior to using an unfamiliar flux. About the Fluxes: The Pros and Cons One of most important aspects of flux is its activity and ability to form a good solder joint— wetting the lead, the hole and the land quick- ly, completely and leaving a strong solder joint. Usually the more activity, the better the solder- ing and the bigger/better the process window. Full-solids rosin and, particularly water soluble fluxes, are well known for excellent soldering performance, largely due to their activity and abundance of chemistry that give it great en- durance throughout the soldering process. Low-solids no-clean fluxes do not remove oxides as well, or as completely as water solu- ble and full rosin fluxes. And they do not have nearly the chemistry to last as long throughout the soldering process. And as such, they gen- erally have a dramatically smaller process win- dow. Water Soluble Flux Water soluble fluxes are excellent for sol- dering, and provide the best soldering possible. They have a great amount of activity that read- ily cleans the metals to be soldered, and virtual- ly never burn off during the soldering process. However, these chemistries are generally very aggressive, corrosive, persistent and will con- tinue to react after soldering. They are virtually always classified as ORH, or INH, and must be cleaned from soldered circuit boards thorough- ly by a machine wash process, and one that has to be monitored very closely. Any remaining CHOOSING THE CORRECT FLUX —ADVANTAGES/DISADVANTAGES Table 2: Low-solids/no-clean fluxes correlated to IPC designators.