Cleaning processes using hydrofluorocarbon and/or...

Details Cleaning processes using hydrofluorocarbon and/or... Cleaning processes using hydrofluorocarbon and/or...

2000-09-26

2003-07-08

Carrillo, Sharidan (Department: 1746)

Cleaning and liquid contact with solids

Processes

For metallic, siliceous, or calcareous basework, including...

C134S025400, C134S026000, C134S028000, C134S030000, C134S034000, C134S035000, C134S036000, C134S042000, C134S902000, C134S011000, C134S019000, C134S031000

Reexamination Certificate

active

06589355

ABSTRACT:

FIELD OF INVENTION
The present invention relates generally to the use of hydrofluorocarbon and hydrochlorofluorcarbon compounds in cleaning processes. More specifically, the present invention relates to a method of cleaning a wide variety of articles of manufacture, especially printed circuit boards, using hydrofluorocarbons and/or hydrochlorofluorocarbons in certain subcritical states and in supercritical states.
BACKGROUND
In the manufacture of printed circuit boards, surface-mount assemblies and other, similar, electronic assemblies, solder flux is commonly applied to the solderable surfaces of the assembly prior to soldering. Solder flux is usually a blend of rosin (abietic acid) and activators. The activators are typically ionic compounds such as amine hydrochlorides. The purpose of the flux is to remove metal oxides from the surface of metal leads in order to render them more solderable. During the soldering operation the flux is heated to very high temperatures (essentially the temperature of molten tin/lead solder) and is “baked” onto the assembly. This solder flux residue is potentially harmful to the assembly and must be removed. The ionic materials, when exposed to moisture, are particularly harmful as they become mobile and conductive.
In the recent past, chlorofluorocarbon (CFC) solvents were used to clean solder flux from articles of manufacture such as printed circuit boards. These solvents were very effective and economical. However, since the recognition that CFC's are harmful to the environment, many manufacturers have switched to aqueous cleaners. Such cleaners clean adequately but are disadvantageous because they require energy intensive processes and costly disposal after use.
Carbon dioxide, both in the liquid state and in the supercritical state, has been proposed as an alternative to aqueous cleaning. However, the use of carbon dioxide is disadvantageous for several reasons. One such disadvantage is that the use of carbon dioxide requires very high pressures, which necessitate the use of relatively expensive pressure equipment. Another disadvantage is that carbon dioxide is not effective in cleaning some polar substrates.
Hydrofluorocarbons (HFC's) are an environmentally safer alternative to CFC's but it has been reported in the art that they are ineffective cleaners. Leblanc, for example, notes that “though they [HFC's] contain no chlorine, their high vapor pressure and low solubility make them poor cleaners.” [“The Evolution of Cleaning Solvents, Precision Cleaning, May, 1997, p. 14.] This is supported by Kanegsberg who comments that HFC's, “while useful as carrier for other solvents and as rinsing agents, with a Kauri Butanol (KB) number of about 10, on their own have little or no solvency for most soils of interest”. [“Precision Cleaning Without Ozone Depleting Chemicals”, Chemistry & Industry, October, 1996, p. 788.]
Recognizing these and other drawbacks of the prior art, the present inventors have perceived a need for a new, efficient and more desirable method for cleaning articles of manufacture, such as printed circuit boards. These and other objects are achieved by the present invention as described below.
DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
The present invention is directed to a method of cleaning a wide variety of articles of manufacture, but especially printed circuit boards, using hydrofluorocarbon (“HFC”) compounds or hydrochlorofluorocarbon (“HCFC”) compounds, or mixtures of HFC and HCFC compounds. The present inventors have found unexpectedly that certain HFC's and HCFC's alone or in combination with small amounts of polar solvents dramatically outperform carbon dioxide in liquid and supercritical based cleaning processes. Since the HFC's and HCFC's have both lower vapor pressures and critical pressures than carbon dioxide, the required operating pressures for equipment needed for the present methods will be lower, and the equipment with which they are used will be less expensive. This is important since one of the barriers to the use of supercritical technology is the high cost of very high pressure equipment.
According to certain embodiments of the present invention, the methods of cleaning an article of manufacture containing contaminants on a surface thereof comprise the steps of (a) providing a hydrohalocarbon compound as a supercritical fluid; (b) contacting at least the contaminated surface of the article of manufacture with said supercritical hydrohalocarbon-containing fluid; and (c) removing substantially all of said supercritical hydrohalocarbon-containing fluid from said article of manufacture. As used herein, the term hydrohalocarbon refers to any compound that is an HFC or an HCFC.
It is believed that the methods of the present invention can be used to clean a wide variety of articles of manufacture including, for example, printed circuit boards, including circuit boards with surface mounted devices, and silicon wafers. For silicon wafers, the present invention can be used to effectively clean and remove cured and uncured photoresists, which are deposited on said wafers when patterns for the manufacture of computer chips are laid down thereon. Moreover, it is believed that the methods of the present invention are especially applicable to the cleaning of printed circuit boards.
As used herein, the term “supercritical fluid” refers generally to a fluid, comprising at least one HFC or HCFC compound, in its supercritical state and also refers to HFC or HCFC compounds which are in the liquid state and have a boiling point below about 16° C. at atmospheric pressure. The present inventors believe that such liquid fluids act in certain respects like fluids in their supercritical state for many embodiments of the present invention. Any fluid in a supercritical state comprising a single HFC or HCFC fluid or mixture of HFC and/or HCFC fluids, as well as any mixture of HFC or HCFC fluid(s) with other materials and/or additives will be a “supercritical fluid” for purposes of the present invention. Any liquid-state fluid, having a boiling point below about 16° C. at atmospheric pressure, comprising a single HFC or HCFC fluid or mixture of HFC and/or HCFC fluids, as well as any mixture of HFC or HCFC fluid(s) with other materials and/or additives will be a “supercritical fluid” for purposes of the present invention.
Any of a wide range of HFC's or HCFC's can be used in the method of the present invention to provide supercritical fluids. Suitable HFC's include, for example, difluoromethane (“R-
32
”), pentafluoroethane (“R-
125
”), isomers of tetrafluoroethane (“R-
134
and R-
134
a
”), trifluoromethane (“R-
23
”), trifluoroethane isomers (“R-
143
” and “R-
143
a
”), pentafluoroethane (“R-
125
”) and isomers of pentafluoropropane (for example, “R-
245
fa
”, “R-
245
ea
”, “R-
245
ca
”, and “R-
245
eb
”) and the like. Suitable HCFC's include chlorodifluoromethane (“R-
22
”), tetrafluorochloroethane (“R-
124
”) and the like. In certain preferred embodiments, the HFC used in the present invention is R-
23
or R-
32
. In other preferred embodiments, the HCFC used in the present invention is R-
22
or R-
124
.
The HFC and HCFC compounds of the present invention can be used alone or in admixture with one another. When mixtures are used, azeotropes or azeotrope-like or constant boiling mixtures containing the compounds are particularly preferred. For example, such mixtures include the azeotrope of R-
32
and R-
125
as described in U.S. Pat. No. 4,978,467, incorporated herein by reference.
Other materials can be added to the hydrohalocarbon-containing fluids to improve their cleaning ability. These materials include: straight chain, branched and cyclic alcohols having from about one to about eight carbons, such as methanol, ethanol, propanol and butanol; straight chain, branched and cyclic chlorinated alkanes such as chloroform, methylene chloride, chlorinated ethanes, propanes and butanes; oxygenate compounds such as aldehydes, ketone

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