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DESCRIPTION
1. Technical FIeld
The present invention is concerned with providing a cleaning composition
which is especially suitable for removing solder flux and solder flux
residue from a substrate, particularly during the manufacture of
integrated circuit modules. In addition, the present invention is
concerned with cleaning compositions suitable for removing various cured
synthetic organic polymer compositions from a substrate and especially for
removing the top seal of an integrated circuit chip module, such as a
cured polyimide coating composition. Moreover, the present invention is
directed to the process of employing the cleaning composition.
2. Background Art
During the preparation of integrated circuit modules, input/output (I/O)
pins are inserted into the substrate. The input/output pins provide the
needed electrical connections to the integrated circuit chip or chips
which are subsequently connected to the substrate or carrier. When the
module is to be connected to a printed circuit board by a soldering
process, solder flux compositions have been applied to the pins. Flux
compositions are employed to remove oxides from the pins and to prevent
the pins from oxidizing when subjected to elevated temperatures for
soldering and thereby serving to maintain the electrical conductivity of
the pins. Once the solder is applied, any flux composition or residue
therefrom (e.g.--polymerized species) remaining on the pins and substrate
must be removed to provide as clean a substrate as possible. However, the
obtaining of a suitable composition to remove the flux or residue
therefrom is quite a problem. In particular, the composition must act as a
selective solvent or dispersant for the flux and/or flux residue but at
the same time must not adversely affect (e.g.--etch) the substrate, pins,
and other materials present on the substrate. For instance, the substrate
can include, at preselected areas, certain metals, such as copper and/or
chrome, which if removed by the cleaning composition would destroy the
necessary metallurgical pattern on the substrate and could render the
integrated circuit module unsuitable for its intended purpose.
Integrated circuit chips are attached to the integrated cuircuit substrate
or carrier by applying solder to preselected areas on the substrate which
is generally referred to in the art as the chip pad area. Such areas can
be defined, for instance, by providing pre-selected exposed areas of a
metal which will accept the solder such as copper. In addition, the flux
composition would normally be applied to the substrate to facilitate the
application of solder to the chip pad area. After solder is applied to the
chip pad area, any flux and/or flux residue must be removed prior to
attaching the integrated circuit chip so as to provide as clean a module
as possible.
Therefore, an object of the present invention is to provide a composition
capable of removing flux and/or residue therefrom without adversely
affecting the rest of the module.
A top seal coating can be applied to the integrated circuit module after
attachment of the integrated circuit chip(s) in order to protect the
backside of such which contains the integrated circuit chip(s) from
corrosion and/or electromigration. When there are multiple integrated
circuit chips on a substrate and/or when multiple substrates are
interconnected such as by stacking and whereby the top seal acts to
maintain the multiple substrates in interconnection, the mal-function of
one chip would require discarding the entire integrated circuit package
unless the top seal could be removed without destroying the rest of the
module.
Accordingly, an object of the present invention is to provide a composition
capable of removing the top seal without adversely affecting the rest of
the module and thereby making it possible to remove any defective chip
which may be present and then have it replaced, thereby making it possible
to save the remainder of the integrated circuit module and reusing the
same.
One type of top seal employed is from curing a composition containing as
the binder a polyimide polymer. The polyimides are actually cured polymers
from what are referred to as polyamides or polyamide-acids containing
imide groupings along the polymer chain by condensation of the amide form
with an acid group of the polymer chain. Such polymers as is well known in
the art are prepared by reacting at least one diamine with at least one
polycarboxylic acid and/or anhydride thereof and/or ester. Accordingly, an
object of the present invention is to provide a cleaning composition
capable of removing a cured polyimide composition from a substrate.
N-methyl-2-pyrrolidone is a well-known solvent and cleaning agent. In fact,
such has been employed in combination with other constituents as a
stripping or depotting composition. For instance, U.S. Pat. No. 3,673,099
to Corby et al suggests compositions containing N-methyl-2-pyrrolidone in
combination with a strong base, such as an alkali hydroxide and use
thereof to strip certain cured resins, such as silicones and polyvinyl
cinnamate, from substrates. In addition, U.S. Pat. No. 3,876,461 to
Flowers suggests immersing plastic encapsulated semiconductor devices in
N-methyl-2-pyrrolidone. U.S. Pat. No. 3,706,691 to Duffy suggests
dissolving certain encapsulating compositions such as polyamides,
polyesters, and certain polyurethanes which have been deteriorated due to
exposure to a hot moist atmosphere by employing a composition which
contains benzyltrimethylammonium hydroxide in combination with
N-methyl-2-pyrrolidone. However, none of the above patents suggest the
compositions required by the present invention which contain
N-methyl-2-pyrrolidone nor suggest that such compositions are capable of
removing flux and/or flux residue and when properly modified are capable
of removing cured top seal polymers such as cured polyimides.
U.S. Pat. No. 3,331,718 to Ruffing suggests the removal of polyamide/imide
films with strong caustic but does not suggest the use of compositions
containing the required ingredients as will be discussed hereinbelow.
U.S. Pat. No. 3,551,204 to Bolger et al suggests compositions for
dissolving polyurethane and epoxy resin systems which contain about 0.1 to
3.0 parts by weight of water, about 3 to 7 parts by weight of alkali metal
hydroxide and about 90 to 96.9 parts by weight of certain monohydric
alcohols or certain ethylene glycol monoalkyl ethers. This patent further
suggests the addition of very small quantities (e.g.--up to about 5 or
10%) of other solvents including methyl pyrrolidone (see column 3, lines
10-18).
DISCLOSURE OF INVENTION
The present invention is concerned with a cleaning composition which
contains at least about 50% by weight of N-methyl-2-pryyolidone and at
least about 10% by weight of a water miscible alkanolamine. In addition,
the present invention is concerned with a cleaning composition which
contains the above defined ingredients along with about 0.1 to about 10%
by weight of an alkali metal and/or alkaline earth metal base. The present
invention is also concerned with cleaning a substrate by contacting the
substrate with a composition of the type defined hereinabove.
BEST MODES FOR CARRYING OUT THE INVENTION
The compositions of the present invention contain N-methyl-2-pyrrolidone
and a water miscible alkanolamine. Such compositions are especially
suitable for removing solder flux and the residue therefrom from a
substrate such as input/output pins and a ceramic integrated circuit chip
carrier both prior to and after attachment of the integrated circuit
chips.
The alkanolamines employed according to the present invention are
preferably water soluble and must be miscible with the
N-methyl-2-pyrrolidone. In addition, such amines should be relatively high
boiling materials such as boiling above about 100.degree. C. and have high
flash points such as above about 150.degree. F. The alkanolamines can be
primary, secondary, or tertiary amines and are preferably monoamines, or
diamines, or triamines, and most preferably monoamines. The alkanol group
of the amine is preferably an unsubstituted monoalkanol and preferably
contains from one to four carbon atoms. Examples of some suitable
alkanolamines include monoethanolamine, diethanolamine, triethanolamine,
isopropanolamine, 2-amino-1-propanol, 3-amino-1-propanol, and
isobutanolamine. The preferred alkanolamines employed according to the
present invention are monoethanolamine, diethanolamine, and
triethanolamine. Mixtures of alkanolamines can be employed when desired.
The presence of the alkanolamine enhances the solvent action of the
composition. Although methyl pyrrolidone, especially at elevated
temperatures, can remove certain flux stains due to polymerization of the
flux upon exposure to the elevated temperatures experienced during
soldering, the removal process is at a slow rate and requires a
considerable amount of time. The presence of the alkanolamine
significantly reduces the time needed to effect removal. Also, the use of
the alkanolamine makes it possible to remove stains or residue from solder
flux which is not removable to any significant extent by methyl
pyrrolidone alone. Moreover, it is believed tha the alkanolamines employed
act as a wetting agent to facilitate washing of the cleaning composition
from the substrate after contact with the substrate.
The cleaning compositions of the present invention contain at least about
50% by weight of the N-methyl-2-pyrrolidone, preferably at least about 75%
by weight, and most preferably at least about 90% by weight of the
N-methyl-2-pyrrolidone. In addition, the compositions of the present
invention contain at least about 5% by weight of the alkanolamine and
preferably at least about 10% by weight of the alkanolamine.
The above relative amounts are based upon the total of
N-methyl-2-pyrrolidone and alkanolamine present in the composition.
Moreover, the compositions of the present invention should be
substantially (e.g.--less than about 5% by weight), if not completely,
free from water. For instance, the presence of more than about 5% by
weight of water prevents significant removal of flux and/or flux residue.
It is also possible to include other solvents in the composition provided
such are not present in such amounts which would adversely affect the
compositions to an undesirable extent. For instance, up to about 35% of
such hydrocarbon solvents as the aromatic hydrocarbons, including toluene
and xylene, and cycloaliphatics, such as cyclohexane; ketones, such as
cyclohexanone, and chlorinated and/or fluorinated hydrocarbons such as
perchloroethylene and the Freons, such as tetrafluoroethane can be used.
The compositions of the present invention can also include minor amounts
(e.g.--up to about 5% by weight) of surface active agents including
cationic, anionic and amphoteric surfactants. An example of a suitable
surfactant is sodium lauryl sulfate.
The substrates which are treated according to the present invention include
those substrates commonly used in the preparation of integrated circuit
modules or carriers such as ceramic substrates. A ceramic is a product or
material manufactured by the action of heat on earthly raw materials.
Preferred ceramic substrates include aluminum oxides, silicon oxides and
silicates, such as aluminum silicate. The solder flux or residue therefrom
which can be treated with the above defined cleaning compositions include
those flux compositions which are particularly suitable in the preparation
of integrated circuit modules and include those compositions wherein the
principal or primary flux material includes abietic acid, mixtures which
contain major amounts of abietic acid, such as rosin, shellac, the
polyalkylene glycols, the polyoxyalkylene ethers, or mixtures thereof.
The alkylene moieties of the glycols and ethers include ethylene,
propylene, and butylene or mixtures thereof, and most preferably ethylene.
Such glycols and ethers generally have molecular weights from about 200 to
about 6000. Some commercially available polyalkylene glycols and
polyoxyalkylene ethers include Carbowax 300--monostearate form
[HO(CH.sub.2 CH.sub.2 O).sub.5 CH.sub.2 CH.sub.2 OOC(CH.sub.2).sub.16
CH.sub.3 ]; Carbowax 400--monolaurate form [HO(CH.sub.2 CH.sub.2 O).sub.8
CH.sub.2 CH.sub.2 OOC (CH.sub.2).sub.10 CH.sub.3 ]; and Carbowax
400--monooleate form (C.sub.36 H.sub.70 O.sub.11). An example of a
polyoxyalkylene ether is one commercially available from Dow Chemical
Company under the designation P-15-200. P-15-200 according to the
manufacturer is a polyoxyalkylene ether containing methyl side chains and
terminal hydroxyl groups. The figure "200" following the "15" designates
the viscosity of the material in centistokes at 100.degree. F.
Mixtures of principal flux materials can be present in any solder flux
compositions. Moreover, the solder flux compositions may include other
ingredients which are present, such as plasticizers and auxiliary bonding
agents.
The above compositions of the present invention are also capable of
removing organic polymeric solder mask compositions from the treated
substrates. Such compositions can be obtained from mixtures containing a
film-forming polymer such as a liquid polyepoxide or a polyimide/amide; a
detackifier; and a solid high temperature resistant filler. Preferred
solder mask compositions of this type are disclosed in U.S. Pat.
Application Ser. No. 045,524, filed June 4, 1979, entitled "Screenable and
Strippable Solder Mask and Use Thereof" to Bakos et al, the disclosure of
which is incorporated herein by reference.
The preferred polyimide/amides in said solder mask compositions have
molecular weights from about 500 to about 2500, and the liquid epoxides
are preferably polyepoxides of epichlorohydrin and bisphenol A. In
addition, the detackifying component of such compositions includes liquid
silicone oils, terpenes, terpineols, drying oils and mixtures of such.
When the film-forming polymer is a liquid polyepoxide, the detackifier
includes a silicone oil.
The above compositions once heated but without the filler would not be
removable from the substrate by use of compositions containing only
N-methyl-2-pyrrolidone and alkanolamine. The filler employed in such
compositions generally has a particle size of about 0.1 to about 20
microns, and preferably about 0.1 to about 5 microns. Such filler must be
capable of withstanding the elevated temperatures from exposure to hot
solder and should withstand temperatures of at least about 300.degree. C.,
and preferably at least about 350.degree. C. The solder is generally
applied at temperatures between about 300.degree. and about 400.degree. C.
Examples of some suitable solid high-temperature resistant fillers include
ground glass, zinc oxide, silicon dioxide, alumina, diamond dust, and the
high-temperature resistant sands.
The ratio of the film-forming polymer of the polymer portion of the
composition relative to the detackifier in such compositions is generally
about 2:1 to about 1:3, and preferably about 1:1 to about 2:3. The amount
of filler employed relative to the film-forming polymer component of the
polymer portion of the composition is about 1:3 to about 6:1, and
preferably about 2:1 to about 6:1.
Substrates to be treated with one of the above compositions are generally
contacted with one of the above compositions while the composition is at a
temperature of about 35.degree. to about 100.degree. C., preferably about
50.degree. to about 90.degree. C., and most preferably about 60.degree. to
about 85.degree. C. The contact is most readily achieved by dipping the
substrates into a vessel which contains the composition. The contact time
is generally from about 10 seconds to about 30 minutes. The temperature of
the composition and contact time are inversely related. That is the higher
the temperature, the shorter the contact time needed to assure removal. A
contact time of about 2 minutes for a composition of about 90% by weight
N-methyl-2-pyrrolidone and about 10% by weight to triethanolamine at a
temperature of about 65.degree. C..+-.5.degree. C. is especially suitable
for a commercial operation. It has been observed that the above specific
composition failed to remove flux residue at temperatures below
35.degree. C. and between 35.degree. and about 40.degree. C. required
about 23 minutes to contact time for removal.
The contact can be carried out with some agitation of the composition, such
as by stirring or shaking to facilitate contact of the composition with
difficult to reach portions of the substrate. Also, the composition can be
recirculated to a heat exchanger in order to maintain and control the
temperature of the composition.
The substrate, after contact with the composition and removal therefrom,
can be rinsed with water to wash off any remaining cleaning composition
therefrom. It is believed that the amines which were present in the
cleaning composition act as a detergent during this stage of the process.
In addition, it may be desirable to employ a drying step such as by
blowing air across the substrate.
In order to remove cured top seal compositions, such as cured polyimide
compositions, from a substrate, it is necessary to modify the above
defined cleaning compositions by incorporating therein an alkali metal
base and/or alkaline earth metal base. Suitable alkali metal and/or
alkaline earth metal base materials include the basic compounds of the
alkali and alkaline earth metals, such as the hydroxides, oxides, and
alcoholates. Examples of some suitable metals include lithium, sodium,
potassium, calcium and magnesium. Examples of some specific basic
compounds are lithium hydroxide, sodium hydroxide, potassium hydroxide,
and calcium hydroxide. Mixtures of the basic materials can be employed
when desired. The base should be miscible with the methyl pyrrolidone and
alkanolamine combination. Generally, the alkali meal base and/or alkaline
earth metal base is employed in amounts from about 0.1 to about 10% by
weight of the total of the methyl pyrrolidone, alkanolamine and alkali
metal base and/or alkaline earth metal base in the composition. The
compositions which contain such basic materials generally have a pH of
above about 10 as tested with litmus paper.
It is quite surprising that the compositions of the present invention
remove cured polyimide without damaging the matallurgy on the substrate,
such as chrome lines, since chrome is considered soluble in alkaline and
generally alkaline compositions with pH of 10 or more etch chrome.
However, according to the present invention, the compositions do not etch
chrome even when exposed to the compositions for 24 hours at about
100.degree. C. The modified compositions in addition to removing the cured
top seal coatings will also remove the substances which are removed by the
compositions of the present invention which contain only
N-methyl-2-pyrrolidone and the alkanolamine.
These modified compositions are also capable of removing exposed organic
photoresist compositions such as those obtained from polyvinylcinnamate,
polyvinylacrylate, and polyalkylacrylates, and polyalkylmethacrylates,
such as polymethylmethacrylate (e.g.--Reston from Du Pont). The modified
compositions are also capable of removing dried coatings from compositions
employing such binders as the polyalkylacrylates and
polyalkylmethacrylates; and drying oils, such as the natural drying oils
including linseed oil, tung oil and the like.
In the compositions of the present invention which contain the alkali metal
and/or alkaline earth metal base materials, the alkanolamine also acts as
a stabilizing agent to prevent the alkali metal and/or alkaline earth
metal base from precipitating out.
The compositions containing the alkali metal and/or alkaline earth metal
base material are preferably prepared by dissolving such into the
alkanolamine by use of elevated temperatures such as about 125.degree. to
about 170.degree. C. Next the composition of the alkanolamine and alkali
metal and/or alkaline earth metal base is admixed with the methyl
pyrrolidone. This sequence tends to stabilize the composition and to
prevent precipitation of the alkali metal and/or alkaline earth metal base
material.
Typical polyimide top seal compositions contain an aromatic polyimide resin
such as AI-10 from AMOCO which is from pyromellitic dianhydride and
4,4'-diaminodiphenol ether, and a minor amount (e.g.--about 1 to 2% by
weight) of a silane such as
beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane,
gamma-aminopropyltriethoxysilane,
N-bis(beta-hydroxyethyl)-gamma-aminopropyltriethoxysilane,
N-beta(aminoethyl)-gamma-aminopropyl-trimethoxysilane and
gamma-glycidoxypropyltrimethoxysilane.
Some examples of aromatic polyimides can be found in U.S. Pat. No.
3,331,718 and U.S. Patent Application Ser. No. 045,524, referred to
hereinabove.
The top seal polyimide compositions do not contain a solid high temperature
resistant filler in such amounts as employed in the above described solder
mask compositions which would render the top seal removable from the
substrate with only the N-methyl-2-pyrrolidone and alkanolamine
combination.
The modified compositions which contain the alkali metal and/or alkaline
earth metal basic material when employed to remove a cured top seal, such
as cured polyimides, are generally employed at elevated temperatures, such
as about 100.degree. to about 200.degree. C., and preferably at about
115.degree. to about 150.degree. C. The time of contact to remove the
compositions will depend upon the thickness of the coating to be removed.
For instance, a coating of about 7.6 to about 25.4 microns thickness
generally requires about 5 to about 10 minutes removal at the above
temperatures; whereas, a coating of about 80 to about 150 microns
thickness generally requires about 1/2 to about 3 hours, and preferably
about 1 to about 11/2 hours of contact. After contact with the solvent
composition, the substrates can be washed with water and then dried by
blowing with hot or cold air.
When the above compositions are employed to remove the top seal which in
turn is desirable in order that a defective chip can be removed from the
substrate, the substrate is then exposed to high enough temperatures to
melt the solder which bonds the chips to the substrate. Then the chips can
be removed and replaced.
Although the above discussion is mainly concerned with the treatment of
integrated circuit modules, it is understood that the present invention is
applicable to treating substrates in general which necessitate the removal
of the above discussed types of materials from the substrates.
The following non-limiting examples are provided to further illustrate the
present invention.
EXAMPLE I
About 920 ml of N-methyl-2-pyrrolidone and about 80 ml of triethanolamine
are admixed to provide a cleaning composition. Into a vessel containing
the composition at about 85.degree. C. are dipped ceramic integrated
circuit modules having flux stains and residue thereof on and in the
substrate. The ceramic substrate is an aluminum dioxide substrate having
thereon chrome and copper metallurgy and containing gold coated
input/output pins. The flux stains or flux residue stains are from a flux
composition which contained rosin and isopropylalcohol as a solvent.
The substrates are dipped into the cleaning composition for about 2
minutes. Next the substrates are contacted with N-methyl-2-pyrrolidone for
about 1/2 minute to rinse off the solvent and are rinsed in water for
about 1/2 minute and then dried by blowing air across the modules.
The composition of the present invention removed all of the flux or flux
residue from the substrate and pins. Modules which were exposed too long
to the high temperature of the soldering process due to jamming of the
solder machine contained some carbonized residue which could not be
removed by the above composition nor any other known composition.
The copper and chrome metallurgy and the other parts of the substrate were
not attacked by the removal composition.
EXAMPLE II
About 49.25 grams of triethanolamine is heated to about 130.degree. C., and
about 0.15 grams of sodium hydroxide are added while the hot
triethanolamine is stirred. The sodium hydroxide added is in a powder form
and requires about one hour to go into solution. If pellets of sodium
hydroxide are employed, about 2 to about 3 hours are required to dissolve
it into the triethanolamine.
After a clear transparent solution of the sodium hydroxide and the
triethanolamine is obtained, about 50 grams of N-methyl-2-pyrrolidone are
added to the composition. Next, the temperature is raised to about
130.degree..+-.5.degree. C. Substrates of the type employed in Example I
which also contained an integrated circuit chip soldered to the substrate,
and a top seal of a cured polyimide containing composition obtained is
dipped into the heated solvent solution with stirring of the solution. The
contact is employed for about 12 minutes after which the substrate is
rinsed with water.
The cured top seal is removed completely from the substrate and without
adversely affecting any other parts of the module and/or chip.
EXAMPLE III
Example I is repeated except that the composition employed contained about
19.2 grams of triethanolamine, about 0.8 grams of sodium hydroxide, about
50 grams of N-methyl-2-pyrrolidone, and about 30 grams of cyclohexane. The
composition is heated to about 90.degree..+-.5.degree. C. for the contact
with the substrate. The above composition also removes various paint
compositions, such as polyalkylacrylates, and drying oil paint
compositions, and photoresists, such as polyvinylcinnamate. It completely
very rapidly removed the flux stains.
EXAMPLE IV
Example I is repeated except that the composition contains about 1 gram of
sodium hydroxide; about 23 grams of triethanolamine; about 50 grams of
N-methyl-2-pyrrolidone; and about 31 grams of tetrafluoroethane. The
composition is at about 125.degree. C. during contact with the substrate.
The results obtained are similar to those of Example 3.
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