Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2000-06-30
2003-05-27
Carrillo, Sharidan (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S001000, C134S022100, C134S022140, C134S022190, C134S036000, C134S040000, C134S042000, C134S060000, C134S076000, C134S111000, C134S184000, C134S902000, C510S175000, C510S407000, C510S413000, C510S421000
Reexamination Certificate
active
06569252
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a new apparatus and method of semi-aqueous solvent cleaning of paste processing residue from substrates. More particularly, the invention encompasses an apparatus and a method that utilizes a semi-aqueous solvent cleaning method for cleaning of paste residue from screening objects comprising the use of at least one propylene-glycol alky-ether solvent.
BACKGROUND OF THE INVENTION
In the fabrication of multilayer ceramic (MLC) substrates for packaging semiconductor devices, conductive metal patterns are screened onto individual ceramic green sheets by printing, such as, extrusion printing, which uses metal mask, such as, Mo, Ni, or Cu, which are placed in contact with the green sheet, or by screen printing, which involves paste squeegee using mesh mask or emulsion mask. After screening, the green sheets are assembled and aligned, and laminated, followed by sintering operation to form multilayer ceramic substrate having internal metallurgy for mounting and interconnecting a plurality of integrated circuit semiconductor devices.
For advanced ground rule electronic packaging structures requiring to print closely spaced conductive metal patterns on a substrate, the screening masks used have highly dense fine dimension etched features. Under certain conditions such screening masks may have the problem of paste residue entrapment in the mask features in addition to a surface residue when the paste is screened to deposit conductive pattern on green sheets. This requires that the metal mask be cleaned after one or more screening passes to eliminate/minimize the possibility of defects in a subsequently screened pattern. Any defects in the screened paste pattern replicates into the final product causing yield loss.
Conductive pastes used in screening processes for delineation of wiring and/or via metallurgy, and I/O pad (input-output) patterns comprise metal particles dispersed in an organic binder and solvent vehicle along with wetting agents, dispersants/surfactants, plasticizers, and other additives, such as, thickening agents, antioxidants, and coloring agents which are all well known in the fabrication of electronic components.
Most commonly used conductive pastes in multilayer ceramic fabrication are based on molybdenum or tungsten metal powder dispersed in an organic polymer binder, such as, for example, ethyl cellulose, polymethyl methacrylate, and the like, or poly-hydrocarbon based thermoplastic resins dissolved in a high boiling solvent system.
Conductive pastes that are typically used for screening processes in multilayer ceramic comprise molybdenum, copper, tungsten, nickel, gold, tin, and the like. Because of the variety and complex chemical make-up of metal/polymer composite pastes, it is required that the mask cleaning medium and process selected be such that it provides complete and efficient cleaning of all types of pastes from screening masks and associated equipment, such as, extrusion heads or paste nozzles, etc.
Solder pastes commonly used in electronic assembly processes for interconnections constitute eutectic alloys, specifically, Pb—Sn, Bi—Sn, In—Sn, and related powder alloys dispersed in an organic carrier. Solder paste application onto substrates for component attachment is done by screen printing through a screen stencil, for example, stainless steel stencil, or by the dispense method. Various solder paste formulations constitute a eutectic alloy dispersed in an organic carrier, for example, a solvent, rheology modifier, fluxing agent, such as, rosin based flux. It is important to clean solder paste residue from metal mask stencils, especially in the case of ultra-fine pitch BGA (Ball Grid Array) attachment to assure defect-free printing on substrate metal pads and thus eliminate solder defects.
Traditionally used cleaning solvents for cleaning/degreasing purposes in many industrial and consumer product applications include aromatic hydrocarbons, chlorinated solvents, typically, 1,1,2-trichloroethylene, perchloroethylene, methylene chloride, and fluorochlorocarbons as CFC-113, and CFC-112. The chlorinated solvents were especially preferred because they are non-flammable, i.e., have no flash point, and are highly effective solvents.
Because of the environmental and health issues associated with the use halogenated hydrocarbon solvents in production processes, there has been major focus in the past decade to introduce environmentally safer/acceptable organic solvent alternatives, and water-based cleaning formulations. Recently, ultrasonic and spray cleaning equipment for water-based cleaning chemistry have become commercially available and several detergent compositions have been described in the patent literature for cleaning/degreasing purposes. These compositions are used, for example, for cleaning printed circuit board assemblies and other parts in device fabrication to remove soldering flux, oil/grease, and other organic residues invariably formed during bonding and assembly processes in microelectronics. These compositions are generally based on a combination of surfactants in water and/or alkaline detergent compositions, comprising alkali metal salts, such as, sodium metasilicate, sodium carbonate, tribasic sodium phosphate, sodium tripolyphosphate, and combinations thereof, as well as, highly alkaline solutions based on alkali metal salts, alkali metal hydroxides, and mixture thereof with alkanolamines. Aqueous cleaner formulations comprising this category of alkali metal carbonate and bicarbonate salts are utilized with an alkali metal silicate, as described in U.S. Pat. No. 5,234,506 (Winston), for the removal of solder flux, oils, waxes, and greasy substances, adhesives and other residues from electronic circuit assemblies, such as, printed circuit or printed wiring boards during their fabrication.
Alternate organic solvent cleaning methods are based on the use of low boiling alcohol, ketone, or ester solvents, for example, isopropyl alcohol (IPA), acetone, n-butylacetate, methyl ethyl ketone (MEK), and aromatic hydrocarbons, specifically xylene. These solvents have safety and environmental issues due to their low boiling points, high flanmmability, volatile emissions and VOC (Volatile Organic Compound) regulations, and thus are considered undesirable for production processes. Various aqueous cleaning alternatives designed for saponification mechanism, on the other hand, generally have a high pH (>11-13) detergent formulations. High pH aqueous cleaners have associated safety issues which require special handling and costly equipment designs to assure protection against any accidental contact exposure. With alkaline aqueous cleaners comprising alkali metal salts, for example, phosphates, silicates, hydroxides, and surface active agents, there is also an issue of wastewater management for compliance with the Clean Water Act regulations which require that all possible sources of aquatic toxicity and other environmentally undesirable constituents be removed from the effluent before discharge to surface waters. This requires special and costly wastewater treatments involving precipitation with complexing agents to form sludge that carries active ingredients of cleaning compositions, metals, and other dissolved constituents resulting in solids loading to the waste. To minimize waste and reduce cost with conservation of water in aqueous cleaning, there is currently significant industry direction toward environmentally suitable zero waste processes by devising methodology for reclamation, recycling, and reuse.
PURPOSES AND SUMMARY OF THE INVENTION
The invention is a novel method and an apparatus for semi-aqueous solvent cleaning of paste processing residue from substrates.
This invention basically is a semi-aqueous method of cleaning polymer/metal composite paste residue from screening masks/stencils and paste processing equipment in the production of multilayer ceramic substrates, organic chip carriers and cards, and of composite solder paste application in electronic circuit assembly. The
Humenik James N.
Lei Chon Cheong
Pomerantz Glenn A.
Sachdev Krishna G.
Carrillo Sharidan
International Business Machines - Corporation
Pepper Margaret A.
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