February, 2017
www.us-
tech.com Choosing the Right Flux for Selective Soldering Continued from previous page
clean with some rosin. A flux labeled ORM0 could be an alcohol-based no- clean, a water-based no-clean (VOC- free), a standard water-soluble, or a water-based water-soluble. The most common types of low-
solids/no-clean fluxes available are ROL1/0 and REL1/0, while the most common VOC-free/low-solids/no-clean flux types available fit in the ORL1/0 category. The designator systems allows
the user to make a more informed decision about the activity and suit- ability of a particular flux and under- stand whether or not post-soldering cleaning is required. Full-rosin and water-soluble fluxes both have to be cleaned after soldering due to their high amount of solids and residues, and because of the high activity of their residues. Be cautious, not only about
water-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 temper- ature can prove to be detrimental to equipment. Also, because of some clever chemistry, some fluxes are actually more active than would be indicated by their IPC classification, which is something that only the manufacturer knows. It is always best to consult the manufacturer about flux characteristics before using an unfamiliar product.
Flux 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 and completely. Usually the more activity, the better the soldering and the process window. Full-solids rosin and water-soluble fluxes are well- known for excellent soldering per- formance, largely due to their activi- ty and endurance throughout the sol- dering process. Low-solids/no-clean fluxes do
not remove oxides as well or as com- pletely as water-soluble and full- rosin fluxes. They do not have nearly the chemistry to last as long through- out the soldering process, and as such, they generally have a dramati- cally smaller process window. Water-Soluble Flux. Water-soluble fluxes are excellent for soldering, and provide the best soldering possible. They have a great amount of activity that readily cleans the metals to be soldered and almost never burn off during the process. However, these chemistries are very aggressive, cor- rosive, persistent, and will continue to react after soldering. They are usually classified as ORH or INH and must be cleaned from soldered circuit boards thoroughly. Any remaining ionic contamination could easily result in a disastrous field fail- ure. Boards coming out of the wash should be tested for ionic contamina- tion, usually by an ionograph or omegameter. The detrimental effects of corro-
sion can even happen in the manu- facturing facility if residues are not cleaned off in a timely manner. Other potential failures can come in the form of dendrite growth, which are metallic, hair-like growths that develop between short, adjacent con- ductive paths. To be clear, the wash-
See at APEX, Booth 1904
ing systems for water-soluble fluxes have proven to be completely effec- tive for decades, but are expensive to operate and take up valuable floor space. And, the aggressive chemistry of water-soluble fluxes requires cor- rosive-resistant fluxers, which are more difficult to keep clean. Rosin Flux. Full-rosin fluxes also provide excellent soldering with the ability to clean the soldered metals and to last throughout the process. But, they do not have the same propensity as water-soluble to cor- rode and damage products. Instead, the rosin has the benefit of acting as a protective barrier during and after soldering to entrap ionic residues, preventing them from being mobile and reacting in a destructive way.
Dendrite growth between conductive paths. Continued on page 68
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