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76 SMT Magazine • May 2016 into a spray head where it's atomized with air. Nozzle-free spray heads are preferable because they are less likely to clog. Spray is delivered in a cone shape, in a variable band of materi- al onto the board. The narrower the band, the more likely flux will be directed into the plated holes. While it covers a lot of real estate, a wider band doesn't necessarily penetrate deeply into the through-hole to get good flux coating in the fillet. Another important factor for effective spray fluxing is the speed and position of the spray head. Fluxers with reciprocating heads provide a greater opportunity to direct flux to plated holes from a variety of angles, and electronical- ly driven heads are more apt to overcome issues with sticking due to flux residue and airborne contaminants than pneumatic systems, which are more susceptible to this problem. Finally, evaluate the fluxing system based on its construction attributes. A self-cleaning system will reduce downtime for maintenance. A robust machine will perform continuously without excessive maintenance attention. Micro-drop Jet Fluxer A micro-jet (or drop-jet) fluxing system sends an undiluted bead or bubble of flux di- rectly through the fillet, the through-hole and the solder ring of the circuit board in droplet form. A micro-jet system will generally use less flux than a spray system because it's only ap- plying flux precisely where it's needed. Most systems allow for a variety of dot patterns which are pulsed through the nozzle, ensuring accuracy. Micro-jet technology provides very fine con- trol of bead size and positioning, thus reducing liquid media waste and virtually eliminating post solder cleaning. An advanced micro-jet flux system is capable of depositing precise amounts of liquid flux with extremely high accuracy, achieving pinpoint through-hole penetration, with no wasted overspray. In the early years of micro-jet fluxing, de- signs were adapted from currently available ink- jet printer technologies, and often were not as well-suited to the PCB market as needed. How- ever, as the technology matured, dedicated de- signs have been developed that were (and are) better suited as integrated systems in the selec- tive soldering machine. Micro-jet technology, however, still comes with a couple drawbacks: 1. Programming is typically more compli- cated and time consuming; however, once pro- grammed, repeatability is excellent and the rec- ipe can be stored for producing the same boards again in the future. 2. Initial cost of a micro-jet system is higher than spray fluxing, but again, with its added efficiency, pinpoint accuracy and long-term reliability, this cost can often be made up very quickly in a production environment. The big- gest advantage: less flux, less cleaning. SElEcting a SElEctivE SoldERing SYStEM, PaRt 2 Figure 2: High pressure flux tank.