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Advanced Drop-Jet Fluxing for Selective Soldering
An advanced drop-jet flux dispenser is integrated side-by-side with a standard aerosol spray head.

Many different flux chemistries are available when implementing selective soldering technology, including low pH fluxes, high-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 chemistry to be used and therefore must be equipped to work with virtually any type of liquid flux.

No-clean fluxes are increasingly used with selective soldering equipment since the residues left after the solder operation are generally nonconductive 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 ingredients 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 nozzle of a selective soldering solder system is essential to mitigate no-clean flux residues. Proper thermal 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 selective soldering equipment used, three types of selective flux applicators 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 deposition, 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 preferred 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 advantage of selective soldering over wave soldering using aperture wave pallets since no-clean flux residues cannot 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 complete control of droplet size and low consumption of the liquid media being applied. Most commercial drop-jet spray fluxers are essentially exchangeable dispensing valves originally designed for a wide range of applications, including automotive, electronics, medical, and solar panel manufacturing. These commercial 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.
The internal jetting cartridge has been removed from this advanced drop-jet flux applicator for easy cleaning.

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 fabrication was under the direct control of a major selective soldering equipment manufacturer, rather than attempting to assemble a solution by purchasing off-the-shelf components. The advanced dispenser can be integrated side-by-side with a standard aerosol spray head for dual flux dispensing capability. This is often needed when both mass flux dispensing and precision flux dispensing are required in a selective soldering process. The advanced drop-jet flux dispenser is capable of depositing microscopic amounts of liquid flux with higher accuracy than commercial drop-jet fluxers and provides precise on-demand application of flux with no overspray for true no-clean processing.

Flux Chemistry Compatibility
While many commercial drop-jet spray fluxers are presented as capable of handling almost any type of flux, some of these units have incompatibility problems with flux types other than alcohol-based fluxes. Since many lead-free fluxes contain a small amount of rosin or synthetic resin to enhance their sustainability when exposed to the higher elevated processing temperatures encountered when using lead-free solder alloys, this can be a concern.

The advanced drop-jet flux dispenser has rugged yet simple construction. It outperforms all other applicators by handling a wide range of flux chemistries including water-soluble fluxes, rosin-based fluxes, low pH fluxes (down to pH2.0), and fluxes with solids content as high as 33 percent. The advanced drop-jet flux dispenser and commercial drop-jet spray fluxers are compatible with alcohol-based fluxes, but that is where the similarity between the two approaches ends. The advanced drop-jet flux dispenser is also compatible with low-pH, water-soluble, high-solids-content, and rosin-based fluxes, while commercial drop-jet spray fluxers are not (often having incompatibility problems with both water-soluble fluxes and fluxes with solids content greater than 7 percent). In some instances, the warranty for certain commercial drop-jet spray fluxers may be voided if they are used with high-solids-content fluxes, water-soluble fluxes, or rosin-based fluxes.

The advanced drop-jet flux dispenser is readily maintainable and serviceable while commercial drop-jet spray fluxers are not. Most commercial drop-jet flux heads are sealed units and are therefore non-repairable whenever they become clogged or contaminated with high-solids-content liquid flux or rosin-based fluxes. Since they are sealed units, this generally requires that the entire drop-jet fluxing head be replaced in the field which is costly and disruptive to production. The advanced drop-jet flux dispenser, with its modular design, is fully maintainable. The internal jetting cartridge can be easily removed for cleaning without complex or expensive tools.

The advanced drop-jet flux dispenser is available with a standard 130µm orifice diameter; other custom orifice diameters are available. The standard orifice is designed to control and dispense a single drop of flux or a continuous pattern less than 2mm wide, ideal for single through-hole terminations close to SMT components. This is ideal for microscopic deposition to the smallest selective soldering sites without leaving flux residues. The continuous deposition is ideal for fluxing header or connector pins. The operating system integrated with the drop-jet flux dispenser electronic control module provides the necessary on/off and drop frequency control. While the standard orifice of the advanced drop-jet flux dispenser produces a 2mm flux drop size, the practical flux drop size is adjustable from 2mm to 4mm and the practical deposition distance from the soldering site can be varied between 4mm and 55mm.

The advanced drop-jet flux dispenser is designed to deliver a higher velocity of flux droplets than commercial drop-jet flux heads, resulting in improved through-hole penetration. This is a critical factor since optimal through-hole penetration of liquid flux is essential to achieving complete barrel fill of a plated-through-hole (PTH), and results in improved solder joint quality.

Contact: ACE Production Technologies, Inc., 3010 North First St., Spokane Valley, WA 99216 509-924-4898 fax: 509-533-1299 E-mail: Web:  

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