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March, 2014 Advanced Drop-Jet Fluxing for Selective Soldering By Alan Cable, President, ACE Production Technologies, Inc., Spokane Valley, WA
solids-content fluxes, and water-soluble, alcohol- based, or rosin-based fluxes. Just as the choice between various lead-free solder alloys is often made according to the service-life requirements of a given end product, selection of a particular type of liquid flux is typically based on the end-user application. Electronics contract manufacturers (CMs) often have little control over the flux chem- istry to be used and therefore must be equipped to work with virtually any type of liquid flux. No-clean fluxes are increasingly used with
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selective soldering equipment since the residues left after the solder operation are generally non-
any different flux chemistries are available when implementing selective soldering technology, including low pH fluxes, high-
conductive and noncorrosive, although they can leave a visible white residue. However, for many end-user applications no-clean fluxes require post- soldering cleaning to ensure that any residues are completely removed and will not adversely affect the electrical performance of a printed-circuit- board (PCB) assembly. Many no-clean fluxes contain active ingredi-
An advanced drop-jet flux dispenser is integrated side-by-side with a standard aerosol spray head.
ents such as mild organic acids that remain active from the time of application to a PCB until they are consumed by a reaction or volatized by heat. Most active ingredients within no-clean fluxes are burned off by direct contact with molten solder. Because of this, direct contact with the solder noz- zle of a selective soldering solder system is essen- tial to mitigate no-clean flux residues. Proper ther- mal processing is essential to ensure the level of any remaining flux residues is within acceptable limits or post-soldering cleaning will be required. Regardless of the type of selec-
tive soldering equipment used, three types of selective flux applica- tors are available: spray, ultrasonic, and drop-jet fluxers. The spray and ultrasonic fluxers apply atomized flux to a specific area. They are capable of high-speed flux deposi- tion, but apply a wide spray pattern which requires post-solder cleaning to remove flux residue outside the intended selective solder path. When using no-clean fluxes with selective soldering systems, the pre- ferred method for applying liquid flux is to use a drop-jet fluxer rather than an atomizing-type aerosol spray head or an ultrasonic spray fluxer. The drop-jet flux applicator applies small droplets of flux that remain within the path of the solder nozzle so that the no-clean residues are consumed by direct contact with the molten solder. This complete consumption of
no-clean flux residues when using selective soldering and a drop-jet flux applicator is a distinct advan- tage of selective soldering over wave soldering using aperture wave pal- lets since no-clean flux residues can- not become entrapped underneath wave pallets. While a drop-jet fluxer is more precise than the other types of fluxers, the choice of a drop-jet, aerosol, or ultrasonic flux applicator for selective soldering depends on the circumstances surrounding a particular soldering application.
Drop-Jet Technology Drop-jet technology provides
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complete control of droplet size and low consumption of the liquid media being applied. Most commercial drop-jet spray fluxers are essential- ly exchangeable dispensing valves originally designed for a wide range of applications, including automo- tive, electronics, medical, and solar panel manufacturing. These com- mercial drop-jet spray fluxers were originally developed for industrial coding and marking applications using mainly inks, primers, and ink-based dyes. Adapting them as liquid flux applicators for selective soldering has been problematic at best since many liquid fluxes are more thixotropic than marking inks. To overcome the limitations of
available commercial drop-jet spray fluxers, an advanced drop-jet flux dispenser has been developed where all engineering, design, and fabrica- tion was under the direct control of
See at APEX, Booth 1027 Continued on next page
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