Semi-aqueous process

Versatile ultrasonic precision cleaning system for aqueous or semi-aqueous processes with all-in-one rinse and dry operations... 

Rae A, Dalton S, Bixenman M. Cleaning with a centrifugal semi-aqueous process. Electronic Packaging and Production. Aug. 2001. 

In order to quantify the effectiveness of the two main groups of solvent based cleaning agents that can be used for semi-aqueous processes, the following key aspects has to be taken into consideration ... 

Developed in the late 1980s, the semi-aqueous process was designed to use low-cost solvents, (terpenes and refined hydrocarbons ), and water, in order to replace the cleaning performance of CFC-113 ("Freon") and 1,1,1-trichIoroethane (TCA). Unfortunately for the developers, the initial applications have become nearly the only applications through 2013. 

Figure 3.1 Simple Process Flow Diagram of the Semi-aqueous Process...

There are many semi-aqueous processes, each slightly different and sponsored by a firm with commercial interest in solving a specific problem. 

This was a hard-earned lesson for developers of the semi-aqueous process. Unless soil materials are familiar in nature and quantity, laboratory tests are essential to commerdal success with the semFaqiteous process An emulsion is a mixture of two (or more) immiscible fluids. Emulsions are unstable, do not form spontaneously, and their formation is aided by addition of mechanical energy to the fluid mixture. Over a time span of minutes to hours, emulsions generally tend to revert to the stable state of the phases comprising them. 

Figure 3.3 Equipment Diagram for a Semi-aqueous Process Used to Clean Flux from PWBs...

Cleaning PWBs with the Semi-Aqueous Process... 

The semi-aqueous process, when used to dean flux (organic adds) and flux residues from PWBs necessarily becomes even more complex (Figure 3.3). 

The emulsion can become something else — a foam. If the spray nozzles are not properly aimed (and sized) so that they act directly on collections of liquid and not on part surfaces, foam can be generated. Foam is a fearsome enemy in all cleaning systems. Foam is air and chemicals. When foam breaks in tbe rinse vessel by the air being removed, the cleaning chemicals remain on part surfaces as chemical-rich residues. Usually, when foam is noticed within the semi-aqueous process, the operating machine must be shut down and the system flushed with fresh water — else soiled parts will continue to be produced. 

Note also how the version of the semi-aqueous process used to clean PWBs is parted in half where the air knife separates the cleaning operations from the rinsing operations. Upstream of this blow-off station, the solvent-water mixture (SA) is used for both immersion and spray cleaning downstream of the blow-off station, the RA is used to dilute dragout containing SA and dissolve non-volatile residues. 

Not shown in the above figures is any method by which parts are dried. This is because no specific method of drying is inherent to the semi-aqueous process. Yes, the liquid removed by drying is water. But it may be partially removed by impact with high-velocity air, nearly completely removed by evaporation, or almost totally removed by displacement with hydrophobic surfactants (Chapter 3.5.5). 

In some embodiments of the semi-aqueous process, the high-velodty air spray chamber is located after parts leave the immersion tank but before they enter the emulsion spray vessel. 

The point of Figure 3.8 isn t that there is soil in the rinse fluid (there will always be some). The simple point is that the concentration of it substantially increases during a modest time period due specifically to a tiny (and probably unmonitored) change in the feed pressure of the air blow-off system. Operators of semi-aqueous processes find it essential to manage the blow-off system with care and concern. 

But, the broader point of this subchapter is that the semi-aqueous process is metastable. Cleaning effectiveness can be excellent reproducibility and repeatability of that outcome can be acceptable, or unacceptable. 

Fortunately, process control technology is simple, mature, available , and organized to improve the reproducibility and repeatability of operations. With it, the semi-aqueous process can be rendered stable. There are two keys for success (1) identify all the appropriate aim points to validate correct process operation, and (2) always use appropriate process control technology to audit operation is on-aim. 

There is nothing uncommon about the equipment components used in the semi-aqueous process. They are the pumps, filters, tanks with cove comets, spray nozzles, etc., noted as "Best" in Chapter 7 " of Ref. 1. The image of Figure 3.9 shows one such apparatus in which that process is practiced. 

Said another way, semi-aqueous cleaning apparatus are not the most expensive facilities for cleaning parts, but they are certainly not inexpensive. Automation, necessary to practice the semi-aqueous process, is a nudal component which probably at least doubles the purchase price of new equipment. 

Facilities to practice the semi-aqueous process would not be purchased on the internet as a commodity machine, without specific laboratory testing. They would be purchased in a highly competitive environment because applications are highly customized. 

Key capabilities needed to practice the semi-aqueous process not normally present in the equipment found in aqueous cleaning Unes are to ... 

Treat rinse water in large volirmes because the semi-aqueous process is practiced in applications where large... 

The semi-aqueous process also forces the consideration of two management issues not associated with aqueous cleaning technology ... 

Semi-aqueous cleaners usually are formulated for very specific applications. No user considering cleaning with the semi-aqueous process should adopt it without a legitimate demonstration of operation of the solvent mixture/water decanter without the actual soil components present. Of secondary but significant importance is a demonstration of the immersion and spray cleaning operations with actual soil components. 

The real advantage of semi-aqueous processes is that they provide acceptable-to-excellent cleaning for heavy oils, greases, flux and flux residues, waxes, and tars — especially compared to aqueous cleaning technology. Yes, the cleaning quality with volatile halogenated solvents is at least equivalent. But those solvents raise concerns about SHE issues (Ref. 3, Chapter 20), and solvents used in the semi-aqueous process do not. 

The real disadvantage of the semi-aqueous processes is that it requires a superior level of willingness and capability to manage chemical processes on site. 

Treat with a second solvent, simply attach a pressure-capable tank to the work chamber to store, dispense, and recover the second solvent. With an enclosed machine, there is no reason for the semi-aqueous process as low-cost hydrocarbon and terpene-based SA solvents can be used, contained, and evaporated at any temperature. If in the unlikely event that the second solvent must be water, one should use construction materials of 315 stainless steel for protection against corrosion, and implement a desiccant dryer (Chapter 3.7.7). 

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