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| Electronics |
| Applying solder flux to printed circuit assemblies and other electronic components. |
| Coating silicon wafers with photoresist and photoresist developer as part of the photolithography process. |
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Other applications include the deposition of thin film coatings (e.g. a CVD-like process developed by Cornell University) used in conjunction with the fabrication of semiconductors and high-temperature superconductors, and the electrostatic coating of illumination devices. The following descriptions of these applications are intended to give you a brief overview of some of the possibilities for ultrasonic spray technology in this area. Detailed information is available. Solder Fluxing The application of liquid flux to printed circuit board assemblies, component leads, ball grid arrays, etc. is an essential part of many electronics assembly processes. Flux performs two important functions in the soldering process. It both rids the surfaces to be soldered of oxides and other contaminants, and creates surface tension conditions conducive to proper solder flow. Wave soldering, and more recently, techniques related to soldering surface mount devices are universally employed to produce electronic assemblies. Using spray as a method to apply flux is relatively new (circa 1990). We will get to the reasons that spray fluxing has achieved a dominant position in the soldering process after we explain what has transpired in the industry over the past few years. Historically, in the wave soldering process, flux has been applied using a technique known as foam fluxing. A bath of liquid flux, containing foaming agents, is aerated by an air stone, which causes a foam head to form on the surface of the bath. The printed circuit assembly is transported through the foam such that the underside becomes covered by a layer of flux. The board then passes over the solder wave to form the completed assembly. This technique has been used for many years with great success. However, over the past five years, a major upheaval in the way printed circuit assemblies are manufactured has occurred, brought about the world-wide ban on chloroflurocarbons (CFCs) as cleaning agents. Traditional fluxes consist of a blend of rosin and other solids, high boiling temperature solvents, saponifiers, and isopropyl alcohol. The solids content can be as high as 30%. After the soldering process is performed, the completed assembly is left with a significant amount of spent flux residue that must be removed by cleaning with an appropriate solvent, namely CFCs, in order to produce a reliable end-product. The phasing-out of CFCs as cleaning agents, the absence of viable cleaning alternatives, and the general tendency of industry to reduce or eliminate organic solvents from manufacturing, motivated the electronics industry to search for alternatives. The principal outcome of this search for alternatives was the development of fluxes that could meet industry performance standards without the use of CFC cleaning. Three such fluxes have gained prominence:
Spray fluxing has gained a dominant position in the industry over recent years because the traditional foam fluxing method is not well-suited for applying these types of fluxes. The principal reason for this is that, in each instance, the amount of flux applied to an assembly must be carefully controlled. Foam fluxers are incapable of providing such control, whereas by using a spray process to coat the assembly, a precise quantity of flux can be uniformly applied over the entire surface. The other major reason that spray fluxing is viewed favorably is an economic one. Foam fluxers apply an excessive amount of flux. Spray fluxers can be adjusted to provide the proper amount, which may be as much as 60% less. In addition, a foam fluxer operates using an open bath of flux, so that for alcohol-based fluxes, the bath must be constantly replenished with alcohol to replace that which has evaporated. Spray fluxers operate from closed flux reservoirs, thereby eliminating this problem. Overall, savings up to 80% in operating costs are routinely reported in installations that have switched from foam to spray fluxing. There are many more technical issues that are not addressed here relative to the pros and cons of spray fluxing technology. More information is available. There are other electronics processes that find ultrasonic nozzles ideal for flux application. Some were mentioned at the top of this page. Examples include:
Photolithographic Coatings The semiconductor industry has used ultrasonic spray nozzles for many years to apply a variety of chemical coatings to silicon and gallium arsenide wafer substrates, in spin-coating processes. The principal uses have been in the photolithographic area, for the application of both photoresist and photoresist developer. The earliest use, dating back to the mid-1980s, was in the deposition of developer. The spray processes that were created by various semiconductor and track manufacturers led to significant reductions in developer consumption (up to 70%) and improved critical dimension (CD) control. More recently, an ultrasonic nozzle spray process for applying photoresist to wafers has emerged as a promising method for dramatically reducing the consumption of these very expensive chemicals. The process was conceived by Sono-Tek, and has been supported under contract by SEMATECH, the organization formed by the American semiconductor industry and the federal government to foster the growth of semiconductor technology in the U.S. It has been demonstrated that it is possible to spray coat a 200 mm diameter wafer, coupled with spin-coating, using approximately 1.1 ml of photoresist, with the same across-the-wafer uniformity as can be achieved by conventional, non-spray dispense methods. The cost savings that can be realized are dramatic. Conventional photoresist dispense techniques generally require 3 to 4 times the amount of material used by ultrasonic spray deposition. The soft spray produced by ultrasonic nozzles eliminates any potential problems with overspray and consequent contamination issues. The capability to vary the dispense rate as the nozzle traverses from the edge of the wafer to its center, while the wafer is turning at a low spin speed, assures relatively uniform initial distribution of the material prior to spin-coating. Together, these two attributes make the ultrasonic spray process practical. Other Applications Many other applications for ultrasonic spray technology exist in the electronics area. The one that is briefly described here relates to the deposition of thin films on various substrates. The Material Sciences department at Cornell University, with support from Sono-Tek, has developed a process to deposit thin films, as thin as an atomic monolayer, using ultrasonic nozzles. This development has been portrayed by the investigators as an alternative to chemical vapor deposition (CVD). The claim made by the developers is that the production of thin films by this method can be accomplished with relatively simple and inexpensive equipment compared with available CVD systems, without sacrificing the quality of the coating. The ability to introduce reactants into a reaction chamber in liquid form, as opposed to a vapor, offers new possibilities in film preparation. Although the process is only starting to be commercialized, it appears promising and could offer many opportunities in this rapidly expanding field. |
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