U.S. patent application number 10/391279 was filed with the patent office on 2003-11-13 for photoimageable, aqueous acid soluble polyimide polymers.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Eitouni, Hany B., Mao, Guoping, Pocius, Alphonsus V., Scheibner, John B., Somasiri, Nanayakkara L.D., Stacey, Nicholas A., Viehbeck, Alfred.
Application Number | 20030211425 10/391279 |
Document ID | / |
Family ID | 24184472 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211425 |
Kind Code |
A1 |
Mao, Guoping ; et
al. |
November 13, 2003 |
Photoimageable, aqueous acid soluble polyimide polymers
Abstract
A photoimageable, aqueous acid soluble polyimide polymer
comprising an anhydride, including a substituted benzophenone
nucleus, a diamine reacted with the anhydride to form a
photosensitive polymer intermediate, and at least 60 Mole % of
solubilizing amine reacted with the photosensitive polymer
intermediate to form the photoimageable, aqueous acid soluble
polyimide polymer. An emulsion for electrophoretic deposition of a
coating of a photoimageable, aqueous acid soluble polyimide polymer
comprises a dispersed phase, including the photoimageable aqueous
acid soluble polyimide polymer, dissolved in an organic solvent and
a dispersion phase including a coalescence promoter and water. The
emulsion may be applied, by electrophoretic deposition, to a
conductive structure to provide a photoimageable coating on the
conductive structure. After exposing the coating to a pattern of
radiation for photocrosslinking exposed parts of the photoimageable
aqueous acid soluble polyimide polymer, an aqueous acid developer
solution removes unexposed photoimageable aqueous acid soluble
polyimide polymer to reveal a crosslinked polyimide polymer image
of the radiation pattern.
Inventors: |
Mao, Guoping; (Austin,
TX) ; Eitouni, Hany B.; (Katy, TX) ; Pocius,
Alphonsus V.; (St. Paul, MN) ; Scheibner, John
B.; (Austin, TX) ; Somasiri, Nanayakkara L.D.;
(Austin, TX) ; Stacey, Nicholas A.; (Austin,
TX) ; Viehbeck, Alfred; (Austin, TX) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
24184472 |
Appl. No.: |
10/391279 |
Filed: |
March 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10391279 |
Mar 18, 2003 |
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10038270 |
Jan 3, 2002 |
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6559245 |
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10038270 |
Jan 3, 2002 |
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09547390 |
Apr 11, 2000 |
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6379865 |
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Current U.S.
Class: |
430/311 ;
430/315; 430/329; 430/331 |
Current CPC
Class: |
G03F 7/0387 20130101;
C08G 73/1025 20130101; G03F 7/164 20130101; C08G 73/1007 20130101;
H05K 2203/135 20130101; Y10S 430/136 20130101; H05K 3/287
20130101 |
Class at
Publication: |
430/311 ;
430/329; 430/331; 430/315 |
International
Class: |
G03F 007/16; G03F
007/20; G03F 007/32 |
Claims
What is claimed is:
1. A method for imaging a photoimageable aqueous acid soluble
polyimide polymer applied to a conductive structure used for
connecting electrical and electronic components, said method
comprising the steps of: providing a conductive structure used for
connecting electrical and electronic components; applying a coating
to said conductive structure using an electrophoretic coating
technique, said coating comprising an anhydride including a
substituted benzophenone nucleus; a diamine reacted with said
anhydride to form a photosensitive polymer intermediate; and at
least 60 Mole % of a solubilizing amine reacted with said
photosensitive polymer intermediate; exposing said coating to a
pattern of radiation for photocrosslinking of exposed parts of said
photoimageable aqueous acid soluble polyimide polymer; and applying
an aqueous acid developer solution to remove unexposed
photoimageable aqueous acid soluble polyimide polymer to reveal a
crosslinked polyimide polymer image of the radiation pattern.
2. A method according to claim 1, wherein said aqueous acid
developer contains an acid selected from the group consisting of
acetic acid, ethoxyacetic acid, propionic acid, butyric acid,
lactic acid, glycolic acid, formic acid and succinic acid and
mixtures thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application claiming
priority from U.S. application Ser. No. 10/038,280, filed Jan. 3,
2002, now allowed, which is a divisional application of U.S.
application Ser. No. 09/547,390, filed Apr. 11, 2000, now issued as
U.S. Pat. No. 6,379,865 B1.
FIELD OF THE INVENTION
[0002] This invention relates to coating formulations and a method,
useful in microelectronics applications, for isolating and
protecting fine-pitch, electrically conducting circuit
interconnects, and related structures. More particularly the
invention provides coating materials for application to conductive
elements using an electrophoretic deposition technique. The
coatings provide protective, high resistivity, low dielectric
constant, negative image bearing layers after exposure to radiation
patterns of suitable wavelength, followed by development with mild
aqueous acid solutions.
BACKGROUND TO THE INVENTION
[0003] Modern society relies upon the trouble-free conveniences
provided by electrical and electronic devices. Since the earliest
recognition that useful devices could be developed by combining
electrical circuits, circuit combinations have become more complex,
and the resulting devices more sophisticated in their capabilities.
Effective circuit performance relies upon electrical current
isolation within a particular circuit with no possibility of
current leakage into a neighboring circuit. Any unintended current
transfer between circuits of a multi-circuit, multi-function
electrical device will ultimately cause an inconvenient malfunction
of the device.
[0004] Isolation or insulation of circuits from each other
represents an increasing challenge with the continuing emphasis on
more complex printed circuit designs and increased functionality
for electrical devices, especially miniature electronic devices.
Progress in electrical device design has caused a transition from
the interconnection of discrete electrical components, using
pre-insulated wiring structures, to interconnection, with modern
printed circuits, using conductive traces only microns wide.
Protection and isolation of such narrow traces, from each other,
demands materials that may be precisely placed over the elongate
current carrying traces while leaving tiny contact points exposed
for electrical connection to other circuits that form part of a
particular device. For a significant period of time it was possible
to essentially cover the printed circuit with a protective coating,
leaving voids in the coating corresponding to the needed points of
contact. More recently, however, the introduction of flexible
printed circuits and multi-layer printed circuits has led to the
need for coatings and processes capable of high precision in
protective cover formation and placement. High precision techniques
provide a cover-layer with essentially just sufficient insulation
to protect a conductive trace without straying into other portions
of a printed circuit substrate. Such coatings tend to be very thin
and subject to attack by, e.g. solvents, moisture, or other
potentially damaging environments. For this reason, precision
coating of printed circuits must provide both insulative and
environmental protection for electrical conductors.
[0005] A variety of coating methods exists for applying coatings,
covercoats and the like as protective, insulating coatings to
printed circuit patterns. The term covercoat refers to a dielectric
coating, over the printed circuit basestock, applied after the
conductive circuit pattern has been etched. The covercoat serves to
protect the copper conductors from moisture, contamination and
damage. Conventional coating methods include screen printing and
application of continuous layers by methods such as knife coating,
spin coating, extrusion coating, dip coating, curtain coating, and
spray coating. Application of continuous coatings covers not only
the leads but also the area in between the leads. This condition
has several disadvantages when found in intricately structured
printed circuits. For example, differences in expansion
coefficients between a continuous cover-coat and a flexible printed
circuit substrate may introduce stresses that cause the circuit to
adopt an inconvenient curl-set. Segmentation of a cover-coat, into
separate coated areas, is less likely to be subject to this
condition.
[0006] Selective deposition processes, such as electrophoretic
deposition, also known as "e-coat," may achieve coating separation
and precise positioning (details of this process may be found in
the "Handbook of Electropainting Technology" by W. Machu,
Electrochemical Publication Limited, 1978). Application of
electrophoretic deposition techniques began at least three decades
ago for painting automobiles and appliances. Electrophoretic
deposition involves precise distribution of a layer of charged
droplets over a conducting surface that represents an electrode of
an electrolytic cell operating under direct current potential.
Charged droplets migrate towards an oppositely charged electrode to
be deposited thereon. Droplet deposition and layer formation may
occur at either an anode or a cathode. Preferably the droplets are
positively charged for deposition on a cathodic surface. Cathodic
coatings do not suffer the oxidative corrosive processes associated
with anodic deposition. Also, electrophoretic deposition of
water-based compositions produces essentially void free and
substantially non-polluting coatings.
[0007] Compared to conventional coating processes, such as screen
printing, electrophoretic deposition selectively places a
protective layer only on conductive portions of the printed
circuit. Use of electrophoretic deposition should produce
individually encapsulated conductors, whereas conventional
techniques coat the entire printed circuit. Selective deposition
also offers other advantages, such as the production of lighter
weight circuits which is important for hard disk drive (HDD)
flexible circuits applications.
[0008] The use of electrophoretic deposition is known for coating
printed circuits with photoresists. U.S. Pat. Nos. U.S. Pat. No.
4,845,012; U.S. Pat. No. 5,055,164; U.S. Pat. No. 5,607,818; U.S.
Pat. No. 5,384,229; U.S. Pat. No. 5,959,859; and U.S. Pat. No.
5,439,774 contain reference to the technique. Other U.S. Pat. Nos.
U.S. Pat. No. 4,592,816 and U.S. Pat. No. 5,181,984 describe
epoxy/acrylate compositions for electrophoretic deposition of
solder mask/covercoat systems. Photoresist and solder mask
materials are typically photosensitive and developable to a
patterned polymer, covering selected (imaged) portions of the
printed circuit. This provides evidence of photoimageable coatings,
formed by electrophoretic deposition. Additionally, U.S. Pat. No.
4,832,808 teaches electrophoretic deposition of coatings of
piperazine-containing polyimides. However, such coatings possess
neither photosensitivity nor solubilization in aqueous acid
developers.
[0009] The effective use of electrophoretically deposited,
photoimageable coatings may depend upon the image resolution
attainable with such systems. Printed circuits of increasing
density require the use of photoresists of increasing image
resolution. Image resolution depends upon radiation scattering
within photosensitive layers and the variation of image
characteristics, i.e. resolution, related to developers and
development processes.
[0010] Polyimide-containing formulations provide potentially useful
materials for photoimageable coatings produced by electrophoretic
deposition. They also have the thermal and dielectric properties
suitable for protecting and insulating electrical current carrying
conductors. Image development of polyimide coatings, after exposure
to an image pattern, may involve non-aqueous, solvent-based
developers or aqueous-based developers. The use of solvent-based
development systems applies to photoimageable polyimides that may
use a benzophenone moiety as a built-in photo-crosslinker. U.S.
Pat. Nos. U.S. Pat. No. 4,629,685; U.S. Pat. No. 4,656,116; U.S.
Pat. No. 4,841,233; U.S. Pat. No. 4,914,182; U.S. Pat. No.
4,925,912; U.S. Pat. No. 5,501,941; U.S. Pat. No. 5,504,830; U.S.
Pat. No. 5,532,110; and U.S. Pat. No. 5,599,655; and European
Patent No. EP 0456463 A2 provide evidence of autosensitized
polyimides. As indicated previously, these materials need organic
solvents for image development. High volume use of solvent
developers, in production operations, may cause environmental
problems associated-with solvent pollution and disposal. Aqueous
developers provide a more environmentally friendly alternative to
organic solvent developers. Some alkaline aqueous developers
contain tetramethylammonium hydroxide as an agent for image
development of photoimageable polyimides derived from either
polyamic acid or phenolic derivatives. These precursors tend to
produce polyimides having residual reactivity, leading to copper
oxide formation, when deposited on copper, along with related
corrosion of metallic copper that could result in poor coated film
properties.
[0011] Considering the disadvantages of previously discussed,
solvent-based and alkaline aqueous image developers and the
benefits of selective coating deposition processes, there is a need
for electrophoretically deposited, photoimageable polyimide
coatings, soluble in non-polluting, preferably aqeous image
developers.
SUMMARY OF THE INVENTION
[0012] The present invention provides photoimageable polyimide
coatings applied from emulsion or solution formulations using
electrophoretic deposition techniques. Such coatings function as
image recording materials through exposure to a pattern of suitable
radiation. An image, formed in a coating according to the present
invention, may be revealed using an acidified aqueous developer. An
intended use of these photoimageable polyimides is the precise
placement of protective, electrically insulating coatings over
conductive parts of a printed circuit pattern, followed by
imagewise exposure and development to remove the coating from those
parts of the circuit that provide points of connection to other
circuits or electrical devices. Acidified aqueous developers offer
advantages over previously discussed solvent and aqueous alkaline
developers by preventing problems of copper corrosion and copper
oxide formation. The use of photoimageable, aqueous acid
developable polyimides distinguishes coating materials, according
to the present invention, from materials using less desirable types
of image developer.
[0013] More particularly the invention provides a photoimageable,
aqueous acid soluble polyimide polymer comprising an anhydride,
including a substituted benzophenone nucleus, a diamine reacted
with the anhydride to form a photosensitive polymer intermediate,
and at least 60 Mole % of solubilizing amine reacted with the
photosensitive polymer intermediate to form the photoimageable,
aqueous acid soluble polyimide polymer. An emulsion for
electrophoretic deposition of a coating of a photoimageable,
aqueous acid soluble polyimide polymer comprises a dispersed phase,
including the photoimageable aqueous acid soluble polyimide
polymer, dissolved in an organic solvent and a dispersion phase
including a coalescence promoter and water. The emulsion may be
applied, by electrophoretic deposition, to a conductive structure
to provide a photoimageable coating on the conductive structure. A
method for imaging a photoimageable aqueous acid soluble polyimide
polymer applied to a conductive structure, used for connecting
electrical or electronic components, comprises the steps of,
providing a conductive structure used for connecting electrical and
electronic components, and applying a coating to the conductive
structure using an electrophoretic coating technique. The coating
comprises an anhydride including a substituted benzophenone
nucleus, a diamine reacted with the anhydride to form a
photosensitive polymer intermediate, and at least 60 Mole % of a
solubilizing amine reacted with the photosensitive polymer
intermediate. Thereafter, exposing the coating to a pattern of
radiation for photocrosslinking exposed parts of the photoimageable
aqueous acid soluble polyimide polymer, and applying an aqueous
acid developer solution to remove unexposed photoimageable aqueous
acid soluble polyimide polymer to reveal a crosslinked polyimide
polymer image of the radiation pattern.
[0014] Electrophoretic deposition techniques allow relatively
precise placement of material on charged surfaces included in an
electrolytic cell, operated by direct current. The charged surfaces
could include suitably connected printed circuits to induce
material placement on individual metal traces of the circuitry.
Using electrophoretic deposition techniques, deposition of material
occurs predominantly on conductive surfaces. This facilitates the
coating of unsupported leads and relatively inaccessible portions
of a printed circuit such as conductive traces disposed within the
structure of a multilayer circuit. Traditional coating methods do
not provide desirable protection for such features. In addition,
precision coating via electrophoretic deposition techniques uses
less material than traditional coating methods thereby providing
beneficial cost savings and waste reduction. The selective placing
of electrophoretically deposited films provides an added advantage,
for coating flexible printed circuits, compared to blanketing
layers produced with conventional coating methods. Regardless of
differences in coefficient of thermal expansion, selectively
deposited coatings cannot exert a force to distort the general
shape of the flexible substrate material. Flexible circuits, coated
using electrophoretic deposition, are lighter and less likely to
exhibit cure-stress-induced curl after processing. Lower circuit
weight is important for certain applications, such as interconnects
for hard disk drives.
[0015] Definitions
[0016] For clarification, the following definitions provide the
meaning of terms that may be used throughout this
specification.
[0017] The term "covercoat" refers to a dielectric coating, over
the basestock, applied after the conductive pattern has been
etched. The basestock may be a conventional printed circuit
substrate, including flexible polyimide sheet, used as a support
for etched metal patterns, particularly those formed by etching
copper.
[0018] The term "current density" means the amount of current
flowing through a substrate, per unit area, perpendicular to the
direction of current flow.
[0019] The term "e-coat" is synonymous with electrophoretic
deposition and may refer herein to a coating, and technique for
electrophoretically depositing such a coating.
[0020] The terms "emulsion" and "solution" are used somewhat
interchangeably to refer to polyimide containing fluids that may be
understood as conventional emulsions except when suspended
particles become so small that the liquid is essentially clear with
little or no evidence of turbidity, i.e. its visual appearance is
that of a solution. When the "emulsion" used for electrophoretic
deposition appears to possess solution-like properties, it is
considered as a solution and is so described herein.
[0021] The term "unsupported lead" means a conductive trace or lead
that spans a void in a substrate or extends over the edge of a
substrate and thereby exists in an unsupported condition.
[0022] The term "mole % amine" as used herein is based upon the
original population of anhydride groups before reaction with a
diamine to form a photosensitive polyimde moiety. For example, 60
Mole % of solubilizing amine represents an amount equivalent to 60%
of the anhydride groups available in the anhydride starting
material.
[0023] The "polymer intermediate" refers to a reaction product, of
at least two monomers, that has the capability for further reaction
with other selected reactants. Anhydrides reacting with diamines,
as described herein, produce polymer intermediates for further
reaction with solubilizing amines.
[0024] The term "solubilizing amine" refers to materials containing
amine functionality that may react with polymer intermediates to
increase polymer solubility in solutions of aqueous acid.
[0025] The term "aqueous acid soluble polymer" refers to a polymer
that is at least partially soluble in aqueous acid solutions.
[0026] The term "aqueous acid developable polymer" refers to a
photoimageable, aqueous acid soluble polymer crosslinked by
exposure to suitable radiation so that crosslinked material no
longer dissolves in dilute aqueous acid. This allows dissolution of
unexposed material to leave an insoluble pattern of crosslinked
material corresponding to the pattern of radiation used for
exposure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 provides a perspective end view of a section of
printed circuit wherein conductive traces have a conventional
protective coating.
[0028] FIG. 2 provides a perspective end view of a section of
printed circuit wherein conductive traces have a protective coating
according to the present invention.
[0029] FIG. 3 provides a perspective end view of a section of
printed circuit wherein conductive traces have a protective coating
according to the present invention and the conductive traces
include an unsupported lead.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention provides modified photoimageable
polyimide materials, having solubility in mildly acidic aqueous
solutions. These materials are suitable as electrically insulating
protective layers for delicate, fragile conductive traces of the
type produced by etching copper layers to form printed
circuits.
[0031] Referring now to the drawings, wherein like parts have like
identifying numerals throughout the several views, FIG. 1 depicts a
section 10 from a printed circuit. The section 10 includes a
substrate 12 supporting a plurality of conductive traces 14. A
coating 16 protects the conductive traces 14 from environmental
contaminants and, at the same time, electrically insulates the
traces 14 from one another. The coating 16 covers, as a covercoat,
the surfaces of both the conductive traces 14 and the substrate 12
between the traces 14. A covercoat of this type results from the
use of conventional coating techniques, such as dipping, extrusion
coating or spray coating.
[0032] The section of printed circuit 20 of FIG. 2 differs from
that of FIG. 1 by illustrating a photoimageable coating 18 which
covers individual conductive traces 14 with an essentially uniform
layer of protective, insulating material according to the present
invention. The use of an electrophoretic deposition coating
technique limits the photoimageable coating 18 to the conductive
traces 14 leaving the substrate 12 between the traces 14
substantially free of coating material.
[0033] FIG. 3 shows a section 20' of printed circuit that includes
an unsupported lead 17 as an extension of one of the conductive
traces 14 that projects beyond the edge of the substrate 12. Such
unsupported leads 17, although fragile, are common in complex
printed circuits used for interconnection of electronic devices.
Protection and insulation of unsupported leads, is difficult to
accomplish using conventional coating techniques. The use of
electrophoretic deposition techniques simplifies the task, by
producing a uniform layer over the entire surface of the
unsupported lead portion 17 of one of the conductive traces 14.
[0034] An aqueous acid developable photoimageable polyimide,
according to the present invention, may be prepared by reacting a
suitable anhydride molecule, containing a substituted benzophenone
nucleus, with an aromatic diamine to form a photosensitive polymer
intermediate which becomes aqueous acid soluble upon reaction with
a solubilizing amine.
[0035] Suitable anhydrides are usually substituted benzophenone
dianhydrides and related structures including:
benzophenone-tetracarboxyl- ic dianhydride;
anthraquinone-tetracarboxylic dianhydride;
fluorenone-tetracarboxylic dianhydride;
thioxanthone-tetracarboxylic dianhydride. Mixed anhydrides may be
used with optional anhydrides providing up to 25% of the total
anhydride content. Optional anhydrides include:
biphenyl-tetracarboxylic dianhydride; 3,3'diphenylsulfone-tetrac-
arboxylic dianhydride; 4,4'oxydiphthalic anhydride;
2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane; and
bis-dicarboxyphenyl sulfide.
[0036] Suitable aromatic diamines include: tetramethylphenylene
diamine; 4,4'methylene-bis(2-methylaniline);
4,4'methylene-bis(2-ethylaniline);
4,4'methylene-bis(2-dimethylaniline);
4,4'methylene-bis(2-diethylaniline)- ; dimethylphenylenediamine;
trimethylphenylenediamine; 2,4-dimethyl-1,5-phenylenediamine; and
2,4,5-trimethyl-1,3-phenylenediami- ne. Diamine mixtures may
contain up to 25% of optional diamines including:
1,4-bis(4-aminophenoxy)benzene; 4,4'-oxydianiline;
2,2'-bis(4-aminophenyl) hexafluoropropane; 4,4'-methylenedianiline
and terminal difunctional amino-siloxanes.
[0037] The polyimide materials become image forming upon exposure
to patterns of radiation, usually ultraviolet radiation. The
solubility of these polyimides in dilute solutions of weak acid,
before exposure, provides a way to develop the image patterns.
Amine functionality incorporated into the polyimide structure
improves the solubility of the resulting polymers in aqueous
solutions of weak acids. The amine functionality should be present
at a molar concentration -preferably of about 60 Mole %, or
greater, to offer acid developability in 0.2-0.5% aqueous acetic
acid. Preferably the soluble polymer is a reaction product
containing at least about 60 Mole % to about 70 Mole % of an
effective amine.
[0038] Aqueous acid solubility of polymers according to the present
invention may also be a function of the molecular weight of the
product of the reaction of a polymer intermediate and a
solubilizing amine. Photoimageable aqueous acid soluble polymers
exist in the molecular weight range from about 20,000 to about
300,000 and preferably from about 50,000 to about 150,000 with a
molecular weight distribution (Mw/Mn) from about 1.1 to 3.5,
preferably from about 1.5 to 3.0.
[0039] Solubilizing amines suitable for imparting polyimide
solubility in aqueous acid solutions include: 1-methylpiperazine,
1,-(N,N-dimethylamino)-3-(N'-methylamino)propane
(N,N,N'-trimethyl-ethyle- nediamine), 2-(dimethylamino)morpholine,
3-(dimethylamino)piperidine,
1-methyl-4-(2-methylamino-ethyl)piperazine,
bis-2-dimethylaminoethyl(N,N,-
N',N',-tetramethyldiethylenetriamine), N,N-dimethylethylenediamine,
1-hydroxymethylpiperazine, 1-hydroxyethylpiperazine,
N,N-dimethylamino-ethanol, and 3-dimethylaminopropylamine, with
1-methylpiperazine being particularly preferred.
[0040] Aqueous acid solutions, suitable for developing the
polymeric compositions, may be selected from acetic acid,
ethoxyacetic acid, propionic acid, butyric acid, lactic acid,
glycolic acid, formic acid and succinic acid and mixtures
thereof.
[0041] The invention also includes a method for applying coatings
of photoimageable polyimide compositions, preferably by
electrophoretic deposition of material onto conductive surfaces.
Electrophoretic deposition occurs by migration of charged particles
or droplets present in the electrolyte of an electrochemical cell.
Application of a potential difference to the cell causes charged
particles to migrate to the electrode of opposite charge where
charge neutralization, on the conductive surface, causes material
to deposit and coat the electrode. Electrophoretic coating,
according to the present invention, requires the application of
direct current and coating deposition at the negatively charged
electrode, or cathode. Specifically, the polymer is deposited from
an emulsion or solution bath containing some amount of solvent
soluble polymer dispersed in an aqueous dispersion phase. As
discussed herein, electrophoretically depositable emulsions or
solutions include a polyimide composition dispersed in an
appropriate medium or carrier. The carrier comprises water and a
coalescing solvent. With current flowing between electrodes
immersed in the solution or emulsion bath, particles or droplets of
the dispersed phase begin preferential migration towards the
electrode of opposite charge.
[0042] Control of current density and voltage is important for
electrophoretic deposition coatings to promote adhesion and prevent
the formation of porous coatings. An appropriate current density is
in the range of about 0.6 mA/cm.sup.2 to about 5.0 mA/cm.sup.2.
Preferably the current density range is from about 0.8 mA/cm.sup.2
to about 1.5 nA/cm.sup.2. The applied potential difference or
voltage bias may be between about 10 volts and about 100 volts.
Preferably, the applied voltage is between about 20 volts and about
50 volts. For this range of voltages a suitable power supply is a
Hewlett Packard 6633A System DC Power Supply with a maximum voltage
of 50 volts and a maximum current of 2 amps. As used herein the
term "current density" means the amount of current flowing through
a substrate, per unit area, perpendicular to the direction of
current flow.
[0043] Coated film thickness and quality depends upon the e-coat
formulation, voltage, current density, and the duration of the
applied current. The electrophoretic deposition process should be
limited to a few minutes. Times from about 1 minute to about 3
minutes are acceptable, but longer coating times may result in
films possibly contaminated with acid residues sufficient to cause
corrosion at the electrode surface. Such attack could damage a
copper printed circuit trace functioning as an electrode for
electrophoretic deposition. Other factors that affect
electrophoretically deposited coating thickness include solid
content of the emulsion, emulsion particle size, conductivity of
the emulsion and pH.
[0044] Polyimide coating formulations, according to the present
invention, include solutions but preferably comprise oil in water
emulsions having an average dispersed phase particle size
preferably from about 0.002 .mu.m to about 20 .mu.m in diameter and
even more preferably less than about 2 .mu.m in diameter. Such
emulsions may be prepared by dissolving the polymer in an organic
solvent such as N-methylpyrrolidone, y-butyrolactone,
dimethylformamide or the like, adding an acid and a coalescing
solvent and adding water. Emulsion preparation involves the use of
a high-speed blender with acid neutralization for improved
stability. Suitable neutralizing acids include lactic or acetic
acid. Emulsions of the present invention exhibit shelf life and
effective performance for periods of at least eight months and
usually more than one year without agitation.
[0045] Electrophoretically deposited coatings of these emulsions
contain coalescence promoters which cause coating coalescence
immediately after the electrophoretic deposition process. The
process also provides excellent edge coverage that could result in
uniform coverage of connecting leads, both supported and
unsupported, used as current carriers in printed circuits and
related structures. Coalescence promoters include
n-butyl-cellosolve, propylene glycol monomethyl ether, propylene
glycol ethyl ether acetate, propylene glycol methyl ether,
propylene glycol n-butyl ether, propylene glycol n-propyl ether,
propylene glycol phenyl ether, dipropylene glycol, propylene
glycol, propylene carbonate, propylene glycol ethyl ether,
propylene glycol methyl ether acetate, ethylene glycol monomethyl
ether, butylene glycol, diethylene glycol, diethylene glycol ethyl
ether, ethoxyethanol, ethoxy ethanol acetate, ethylene glycol,
triethylene glycol, ethylene glycol diacetate, ethylene glycol
propyl ether, methoxy ethanol acetate, methoxy ethanol, phenoxy
ethanol, n-butanol, di(ethylene glycol)butyl ether, 2-ethyl
hexanol, acetophenone, toluene, propylene glycol methyl ether
acetate, and selected mixtures thereof, with n-butyl-cellosolve
being preferred.
[0046] After drying an electrophoretically deposited coating at
approximately 85.degree. C., an image may be formed by exposing the
coating to an pattern of ultraviolet radiation. The photoimaging
process occurs via crosslinking exposed areas of the polyimide
layer using a photo-mask between the coating and a broadband
ultraviolet lamp. Photoimaged, crosslinked polyimide no longer
dissolves in dilute aqueous acid solution. Image patterns,
corresponding to the photo-mask, may be developed by dipping the
imaged coating in a solution containing from about 0.1% to about
0.5% of acetic acid in water. This removes the unexposed,
non-crosslinked polyimide that is still soluble in aqueous acid.
After acid development, followed by rinsing with
tetramethylammonium hydroxide solution and deionized water, the
developed image, corresponding to the pattern of ultraviolet
radiation, may be fixed by curing at least about 300.degree. C.,
preferably at least about 350.degree. C., in a nitrogen filled
oven. The heating rate and dwell time at the image curing
temperature require relatively careful control. A positive nitrogen
pressure prevents oxidation of the imaged film during curing. The
use of a slow rate of heating, to the final curing temperature,
allows volatile products to escape before the polyimide cures
fully. A controlled heating rate also prevents foaming and film
delamination. Generally cured polyimide films, as covercoats on
flexible circuits, exhibit good adhesion (ASTM D3359), hardness of
about 3H pencil hardness (ASTM D3363), without cracking, during
bending at a radius of 0.3 mm, and less curl compared to
conventional epoxy acrylate covercoats.
[0047] Aqueous acetic acid development provided images with lines
of .+-.25 microns (.+-.1 mil) resolution. Electrophoretically
deposited coatings from about 1.0 .mu.m to about 15.0 .mu.m provide
this level of image definition. Preliminary work with spray
development suggests a reduction in the amount of time to develop
films thicker than 15.0 .mu.m. Spray development offers advantages
over dip development. The use of aqueous acid developers is
convenient and environmentally beneficial compared to solvent-based
developers which require disposal, after use, in compliance with
environmental regulations. Photoimageable polyimides, according to
the present invention, may be developed, after exposure, with a 2:1
volume ratio of N-methyl pyrrolidone:methyl alcohol, but this
mixture is much less environmentally compatible than the preferred
aqueous acid developer.
[0048] Electrophoretic deposition of photoimageable polyimides on
conductive surfaces should provide a relatively precise approach
for covering and protecting fragile leads, i.e. unsupported leads,
used for electrical interconnection in high density printed
circuits including flexible circuits. Photoimageable polyimide
materials, applied in this way, have potential application as
barrier coatings to provide abrasion resistance and electrically
insulating protective layers for product applications in areas such
as integrated circuit packaging (ICP), ink jet printers, hard disk
drives, medical and biomedical equipment and automotive
applications.
[0049] Experimental
[0050] Preparation of Photosensitive Polyimides
[0051] Dianhydride and diamine monomers were added to a
nitrogen-filled 500 ml flask. A quantity of
1-methyl-2-pyrrolidinone (NMP) was added to the flask with stirring
to produce a solution of monomers in NMP. The resulting viscous
solution was maintained under nitrogen with stirring for sixteen
hours. After cooling the solution to 0.degree. C., using an
ice-bath around the flask, a solution containing 1,3
dicyclohexylcarbodiimide in NMP was added dropwise. During the
addition, the color of the solution adopted a dark reddish color,
typical of polyisoimide polymers. Upon completion of this addition,
the flask and contents were allowed to warm up to room temperature
and stirring continued for a further extended period of
approximately fifteen hours. Upon addition of 1-methyl piperazine,
there was evidence of an exotherm and, within about fifteen
minutes, the solution changed color, from dark red to light
pink.
[0052] The contents of the flask were stirred under nitrogen for a
further six hours then filtered into approximately three liters of
an alcohol and water mixture containing 1 part of methyl alcohol to
2 parts of water. A pink colored solid formed as a precipitate in
the alcohol and water solution. This was isolated by filtration and
dried under vacuum at room temperature.
[0053] This method was used for preparation of Examples 1 and 2
shown in Table 1.
EXAMPLES
[0054]
1TABLE 1 Photoimageable Polyimide Compositions Example 1 Example 2
BTDA 10.58 g (32.8 mmol) 33.22 g (103.0 mmol) MBDMA 13.13 g (51.6
mmol) TMPDA 5.39 g (32.8 mmol) 8.45 g (51.4 mmol) NMP 150 ml 300 ml
DDC 17.5 g (84.8 mmol) 51.0 g (248 mmol) in 100 ml NMP in 150 ml
NMP Piperazine 6.7 ml (60 mmol) 20.6 ml (185 mmol) Mn 92,000
258,000 Mw/Mn 1.83 1.60 Key to materials: BTDA is
3,3',4,4'-benzophenone tetracarboxylic dianhydride MBDMA is
4,4'-methylene-bis(2,6-dimethylaniline) TMPDA is
2,3,5,6-tetramethylphenylene diamine NMP is anhydrous
1-methyl-2-pyrrolidinone DCC is 1,3-dicyclohexylcarbodiimide
Piperazine is 1-methyl piperazine
[0055] Emulsion Preparation
[0056] A quantity of photoimageable polyimide, dissolved in
1-methyl-2-pyrrolidinone (NMP) was stirred at 1000 rpm, using a
Dispermat.RTM. FE Laboratory Dissolver from VMA-GETZMANN. After
dropwise addition of lactic acid and butyl cellosolve, the stirring
speed was increased to 5000 rpm. Water was added dropwise to the
rapidly stirring solution until the emulsion phase inverted from
water in solvent to solvent in water. An additional quantity of
water was added and the emulsion was kept stirring for 2-3 minutes.
Finally, the fluid composition was filtered through a 1.0 .mu.m
filter into a 4 oz. wide-mouth jar for storage.
[0057] Cathodic Electrophoretic Deposition of Photoimageable
Coatings on Copper
[0058] Emulsion samples were placed in the reservoir of an
electrochemical cell having a cathode, in the form of a copper
layer on a flexible polyimide substrate, and a platinized anode
separated from the cathode by a fixed distance. A Hewlett Packard,
6633A DC power supply was connected to the electrodes. The maximum
voltage bias and current density settings were selected. Polyimide
deposition, under the selected conditions, continued for about two
minutes before the cathode was removed from the emulsion to be
rinsed with deionized (DI) water, from a spray bottle, then
immersed in dilute, 0.1N tetramethylammonium hydroxide (TMAH)
solution for about two seconds. After a final rinse with deionized
water, the sample was dried in a conventional oven controlled at
85.degree. C. for 15 minutes. The deposition process produced a
smooth polymer film on the surface of the cathode.
2TABLE 2 Coating Formulations and Conditions for Electrophoretic
Deposition Coating Formulation 1 Coating Formulation 2 Polyimide
Example 1 - 3.5 g Example 2 - 4.66 g NMP 14 g 18.6 g 85% solution
of Lactic 0.92 ml 0.62 ml acid Butyl cellosolve 10 ml 13.8 ml Water
65 ml 78.4 ml PH 4.46 5.80 Conductivity (.mu.S) 1330 780 Electrode
separation 3.0 cm 4.0 cm Voltage bias 50 V 20 V Current density 1
mA/cm.sup.2 1 mA/cm.sup.2 Time of deposition 2 minutes 2 minutes
Drying time at 85.degree. C. 15 minutes 10 minutes
[0059] Image Formation and Development
[0060] A copper substrate, coated with a 6 .mu.m thick layer of the
polymer of Example 1, was exposed, through a mask, to a wide
spectrum ultraviolet radiation source (available from Hybrid
Technology Group--Model #LS66-1OX-220/254 UV Lamp). The intensity
of exposure was approximately 30 mW/cm.sup.2 for about 40 seconds
(1200 mJ/cm.sup.2).
[0061] The exposed layer was developed in a 0.1% solution of
aqueous acetic acid for about 30-40 seconds followed by rinsing
with water, tetramethylammonium hydroxide (TMAH) solution, and a
second water rinse before drying and baking the developed coating.
The baking or thermal curing of imaged samples took place in a
nitrogen filled oven according to a thermal profile that included 2
hours at 260.degree. C., then 0.5 hour at 300.degree. C. and a
final bake step of 5 minutes at 350.degree. C. Image resolution of
about 25 .mu.m (1 mil) was observed following this process.
[0062] Using essentially the same process, under the following
conditions, a coating of Example 2 gave an image coating 6 .mu.m
thick.
[0063] Imaging conditions: UV exposed with a mask at 1000
mJ/cm.sup.2 dose and developed with 0.2% acetic acid for about 60
seconds seconds followed by rinsing with water, tetramethylammonium
hydroxide (TMAH) solution, and a second water rinse before drying
and baking the developed coating.
[0064] Final cure: 3.degree. C./min. ramp to 240.degree. C. (2
hrs); 1.degree. C /min ramp to 300.degree. C. (1 hr.) 1.degree.
C./min. to 350.degree. C. (15 min.).
[0065] The solubility of modified polyimides, in aqueous acid
solutions, may be varied by adjusting the concentration of the
reactive amine. Investigation of the relationship between amine
content and acid solubility using 1-methyl piperazine indicates the
need for a high piperazine content (>60%) for sufficient
solubility in aqueous acid solution. Modified polyimides containing
less than 60% 1-methyl piperazine have less solubility in aqueous
acids making such solutions less effective as image developers.
3TABLE 3 Acid solubility data of Polyimides Modified with 1-methyl
piperazine* Mol. % Solubility in 0.5% Acetic Solubility in 40%
piperazine group Acid at room temperature. Acetic Acid at
65.degree. C. 35% No No 55% No No 60% Yes Yes 67% Yes Yes *As in
Example 1 - polymer Mn of about 90,000, Mw/Mn of about 1.8
[0066] Studies of emulsions, prepared with the polyimide of Example
1, show changes in pH and conductivity with increasing addition of
butyl cellosolve. As shown in Table 4, conductivity decreases and
pH appears to increase slightly with butyl cellosolve addition.
Conductivity was measured using a Coming PS-17 conductivity meter.
The pH was measured using a Coming pH-30 Sensor calibrated with
Ricca Chemical Company pH 4 and 7 buffer solutions.
4TABLE 4 Variation of pH and Conductivity of Emulsions. Sample - 1
Sample - 2 Sample - 3 Sample - 4 Polyimide of Example 1 3.5 g 3.5 g
3.5 g 3.5 g Lactic Acid (85%) 0.92 mL 0.92 mL 0.92 mL 0.92 mL NMP
14 g 14 g 14 g 14 g Water 70 mL 67.5 mL 65 mL 65 mL Butyl
Cellosolve 0 2.5 mL 5 mL 10 mL PH 4.30 4.34 4.39 4.46 Conductivity
(.mu.S) 1860 1720 1550 1330 Coating Appearance A B B C Key -
Coating Appearance A = Foamed coating Coating Appearance B =
Somewhat foamed coating Coating Appearance C = Fully coalesced
coating
[0067] Emulsions shown in Table 4 appear almost transparent
suggesting an emulsion particle size less than the wavelength of
visible light (0.4-0.8 .mu.m). The particle size of emulsions of
this invention may be below 0.1 .mu.m (100 nm).
[0068] A photosensitive, aqueous acid soluble polyimide polymer and
related coatings have been described according to the present
invention. It will be appreciated by those of skill in the art
that, in light of the present disclosure, changes may be made to
the embodiments disclosed herein without departing from the spirit
and scope of the invention.
* * * * *