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104 SMT Magazine • January 2017 important to understand if the flux you are us- ing is a help or hindrance to your selective sol- dering process. Low-solids/no-clean fluxes in general, have less active chemistry and are more challenging to solder with than rosin or water soluble. Some fluxes are made to overcome particular issues— issues you may not have; while others may be weak in an area that is an issue for you. Fre- quently the type of products you manufacture can impact the flux that is best for your process. Or, simply the variety of products you manufac- ture can influence your choice of flux—and it may even require using different fluxes for dif- ferent products. One issue is that many of the available fluxes being used in selective soldering were originally intended for wave soldering. Re- gardless of the product or application, the wave soldering process was relatively the same across the industr y and easier to adapt for these different fluxes. Only recently have flux manufacturers started producing flux - es specifically for selective soldering, recog- nizing that it is a distinctly different process than wave soldering. However, among these various flux options, the rosin/resin containing, alcohol-based, low- solids/no-clean fluxes are usually the best op- tion for the selective soldering process. They work well across various surface finishes, have a relatively wide process window, handle a wid- er range of time at high temperature, work with leaded and lead-free solders, and burn-off well, generally leaving safer residues.† Understanding the Flux Types There are three key attributes for a flux that determine the flux categories. These attributes also govern whether you need to clean your boards after soldering. However, the level of ac- ceptability is not necessarily universal and de- pends on the requirements of the product. • Activity • Solids content • Material type With that in mind the three basic flux types can be simply distinguished in this way: • Low-solids/no-clean fluxes (2-8% solids content) – Solvent based – with or without rosin/ resin – Water based (VOC-free) – no rosin/resin (rare exceptions) – Low to medium activity – "Short life" (in process) – May or may not require cleaning • Rosin fluxes – Full/high-solids rosins with 15-45% – Solvent based – Can be low activity, but normally medium to high activity – "Long life" (in process) – Typically, always cleaned • Water soluble fluxes – Generally, high solids (11-35%) – Solvent based (occasionally water-based) – Always highly active – Very "long life" (in process) – Always cleaned Activity and solids content are usually the two key attributes that determine whether the product will require cleaning of the flux residues after soldering. And from a material type and solids content perspective the fluxes break-down further, as shown in the tree diagram below. The table below breaks down fluxes as hav- ing a high solids content or low solids content, whether it contains rosin or not, and whether it is water based or alcohol based (solvent based). You will also notice four-character designator starting with RO, RE, OR, etc. The IPC institut- ed this designator system to more clearly classi- fy, or identify, the activity of fluxes as low, me- dium or high, based on tests outlined in IPC-J- STD-004. All flux manufacturers perform these tests and identify each flux with the appropriate designator. How the designator applies is illus- trated in the table below. Flux manufacturers include this designator on the technical datasheet for every (recent) flux. However, fluxes are rarely referred to by these designators alone. Most fluxes are still re- ferred to as Low-solids/no-clean, rosin and wa- ter soluble fluxes. The designators alone do not tell you exactly what type of flux you are deal- ing with. For example, ROL1 could either be full CHOOSING THE CORRECT FLUX —ADVANTAGES/DISADVANTAGES