Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CREATING CIRCUIT ASSEMBLIES
FIELD OF THE INVENTION
[oool] The present invention relates to methods for fabricating electrical
circuit
assemblies.
BACKGROUND OF THE INVENTION
[0002] Electrical components, for example, resistors, transistors, and
capacitors, are commonly mounted on circuit panel structures such as printed
circuit boards. Circuit panels ordinarily include a generally flat sheet of
dielectric material with electrical conductors disposed on a major, flat
surface
of the sheet, or on both major surfaces. The conductors are commonly
formed from metallic materials such as copper and serve to interconnect the
electrical components mounted to the board. Where the conductors are
disposed on both major surfaces of the panel, the panel may have via
conductors extending through holes (or "through vias") in the dielectric layer
so as to interconnect the conductors on opposite surfaces. Multi-layer circuit
panel assemblies have been made heretofore which incorporate multiple
stacked circuit panels with additional layers of dielectric materials
separating
the conductors on mutually facing surfaces of adjacent panels in the stack.
These multi-layer assemblies ordinarily incorporate interconnections
extending between the conductors on the various circuit panels in the stack as
necessary to provide the required electrical interconnections.
[0003] In microelectronic circuit packages, circuits and units are prepared in
packaging levels of increasing scale. Generally, the smallest scale packaging
levels are typically semiconductor chips housing multiple microcircuits and/or
other components. Such chips are usually made from ceramics, silicon, and
the like. lntermediate package levels (i.e., "chip carriers") comprising multi-
layer substrates may have attached thereto a plurality of small-scale chips
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housing many microelectronic circuits. Likewise, these intermediate package
levels themselves can be attached to larger scale circuit cards, motherboards,
and the like. The intermediate package levels serve several purposes in the
overall circuit assembly including structural support, transitional
integration of
the smaller scale microcircuits and circuits to larger scale boards, and the
dissipation of heat from the circuit assembly. Substrates used in conventional
intermediate package levels have included a variety of materiais, for example,
ceramic, fiberglass reinforced polyepoxides, and polyimides.
[0004] The aforementioned substrates, while offering sufficient rigidity to
provide structural support to the circuit assembly, typically have thermal
coefficients of expansion much different than that of the microelectronic
chips
being attached thereto. As a result, failure of the circuit assembly after
repeated use is a risk due to failure of adhesive joints between the layers of
the assembly.
[ooo5] Likewise, dielectric materials used on the substrates must meet several
requirements, including conformality, flame resistance, and compatible
thermal expansion properties. Conventional dielectric materials include, for
example, polyimides, polyepoxides, phenolics, and fluorocarbons. These
polymeric dielectrics typically have thermal coefficients of expansion much
higher than that of the adjacent layers.
[0006] There has been an increasing need for circuit panel structures, which
provide high density, complex interconnections. In applications wherein
circuit layers are built one on top of another, a dielectric material
typically
separates the circuitized layers. Polymeric dielectric materials that
typically
are used in circuit assembly manufacture are thermoplastic or thermoset
polymers. Thermoset materials are typically cured first to form a conformal
coating. As density and complexity of interconnected circuitry increases,
there
is an increasing need for dielectric materials with increasingly lower
dielectric
constants and dielectric loss factors.
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SUMMARY OF THE INVENTION
(0007] The present invention is directed toward a method for preparing a
circuit assembly. The method comprises: (a) applying a curable coating
composition to a substrate, (b) curing the coating composition to form a
coating on the substrate, and (c) applying a conductive layer to all surfaces.
The curable coating composition is comprised of (i) one or more ungelled
active hydrogen-containing resins, (ii) one or more polyester curing agents,
and (iii) optionally one or more transesterification catalysts. In a further
embodiment, the method also comprises: (d) applying a resist to the
conductive layer applied in step (c), (e) processing said resist to form a
predetermined pattern of exposed underlying metal, (f) etching said exposed
metal, and (g) stripping the remaining second resist to form an electrical
circuit pattern.
DETAILED DESCRIPTION OF THE INVENTION
[0008] Other than in the operating examples, or where otherwise indicated, all
numbers expressing quantities of ingredients, reaction conditions and so forth
used in the specification and claims are to be understood as being modified in
all instances by the term "about." Accordingly, unless indicated to the
contrary, the numerical parameters set forth in the following specification
and
attached claims are approximations that may vary depending upon the
desired properties sought to be obtained by the present invention. At the very
least, and not as an attempt to limit the application of the doctrine of
equivalents to the scope of the claims, each numerical parameter should at
least be construed in light of the number of reported significant digits and
by
applying ordinary rounding techniques.
[ooos] Notwithstanding that the numerical ranges and parameters setting forth
the broad scope of the invention are approximations, the numerical values set
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forth in the specific examples are reported as precisely as possible. Any
numerical values, however, inherently contain certain errors necessarily
resulting from the standard deviation found in their respective testing
measurements.
[ooolo]Also, it should be understood that any numerical range recited herein
is
intended to include all sub-ranges subsumed therein. For example, a range
of "1 to 10" is intended to include all sub-ranges between and including the
recited minimum value of 1 and the recited maximum value of 10, that is,
having a minimum value equal to or greater than I and a maximum value of
equal to or less than 10.
[oooll]As previously mentioned, in one embodiment, the present invention is
directed to a method for preparing a circuit assembly. The method comprises:
(a) applying a curable coating composition to a substrate, (b) curing the
curable coating composition to form a coating on the substrate, and (c)
applying a conductive layer to all surfaces. The curable coating composition
is comprised of (i) one or more ungelled active hydrogen-containing resins,
(ii)
one or more polyester curing agents, and (iii) optionally one or more
transesterification catalysts.
[00012] The curable coating compositions useful in the methods of the present
invention comprise, as a main film-former, an ungelled, active hydrogen-
containing resin (i). A wide variety of film-forming polymers are known and
can be used in the curable coating compositions of the present invention
provided they comprise active hydrogen groups, as determined by the
Zerewitinoff test, described in the JOURNAL OF THE AMERICAN CHEMICAL
SOCIETY, Vol. 49, page 3181 (1927). In one embodiment, the active
hydrogens are derived from hydroxyl groups, thiol groups, primary amine
groups and/or secondary amine groups.
[000131 By "ungeNed" is meant the resins are substantiaily free of
crosslinking
and have an intrinsic viscosity when dissolved in a suitable solvent, as
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determined, for example, in accordance with ASTM-D1795 or ASTM-D4243.
The intrinsic viscosity of the reaction product is an indication of its
molecular
weight. A gelled reaction product, on the other hand, since it is of
essentiaily
infinitely high molecular weight, will have an intrinsic viscosity too high to
measure. As used herein, a reaction product that is''substantially free of
crosslinking" refers to a reaction product that has a weight average molecular
weight (Mw), as determined by gel permeation chromatography, of less than
1,000,000.
[oooi4]A variety of active hydrogen-containing resin materials are suitable
for
use in the present invention. Non-limiting examples of suitable resins
include:
polyepoxide polymers, acrylic polymers, polyester polymers, urethane
polymers, silicon-based polymers, polyether polymers, polyurea polymers,
vinyl polymers, polyamide'polymers, polyimide polymers, mixtures thereof and
copolymers thereof. As used herein, by "silicon-based polymers" is meant a
polymer comprising one or more -SiO- units in the backbone. Such silicon-
based polymers can include hybrid polymers, such as those comprising
organic polymeric blocks with one or more -SiO- units in the backbone.
[ooois] The polymer is typically a water-dispersible, electrode posita ble
film-
forming polymer. The water-dispersible polymer may be ionic in nature; that
is, the polymer can contain anionic functional groups to impart a negative
charge or cationic functional groups to impart a positive charge. Most often,
the polymer contains cationic salt groups, usually cationic amine salt groups.
[00016] Non-limiting examples of film-forming resins suitable for use as the
polymer in the composition of the present invention, in particular in anionic
electrodepositable coating compositions, include base-solubilized, carboxylic
acid group-containing polymers such as the reaction product or adduct of a
drying oil or semi-drying fatty acid ester with a dicarboxylic acid or
anhydride;
and the reaction product of a fatty acid ester, unsaturated acid or anhydride
and any additional unsaturated modifying materials which are further reacted
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with polyol. Also suitable are the at least partially neutralized
interpolymers of
hydroxy-alkyl esters of unsaturated carboxylic acids, unsaturated carboxylic
acid and at least one other ethylenically unsaturated monomer. Still another
suitable electrodepositable resin comprises an alkyd-aminoplast vehicle, i.e.,
a vehicle containing an alkyd resin and an amine-aidehyde resin. Another
suitable anionic electrodepositable resin composition comprises mixed esters
of a resinous polyol. These compositions are described in detail in U.S. Pat.
No. 3,749,657 at col. 9, lines 1 to 75 and col. 10, lines 1 to 13. Other acid
functional polymers also can be used such as phosphatized polyepoxide or
phosphatized acrylic polymers as are well known to those skilled in the art.
Additionally, suitable for use as the polymer are those resins comprising one
or more pendent carbamate functional groups, for example, those described
in U.S. Patent No. 6,165,338.
[00017] In one particular embodiment of the present invention, the polymer is
a
cationic, active hydrogen-containing ionic eiectrodepositable resin capable of
deposition on a cathode. Non-limiting examples of such cationic film-forming
resins include amine salt group-containing resins such as the acid-solubilized
reaction products of polyepoxides and primary or secondary amines such as
those described in U.S. Pat. Nos. 3,663,389; 3,984,299; 3,947,338; and
3,947,339. Besides the epoxy-amine reaction products discussed
immediately above, the polymer can also be selected from cationic acrylic
resins such as those described in U.S. Pat. Nos. 3,455,806 and 3,928,157.
[oool 8) Besides amine salt group-containing resins, quaternary ammonium salt
group-containing resins can also be empioyed. Examples of these resins
include those which are formed from reacting an organic polyepoxide with a
tertiary amine salt. Such resins are described in U.S. Pat. Nos. 3,962,165;
3,975,346; and 4,001,101. Examples of other cationic resins are ternary
sulfonium salt group-containing resins and quaternary phosphonium sait-
group containing resins such as those described in U.S. Pat. Nos. 3,793,278
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and 3,984,922, respectively. Also, film-forming resins such as described in
European Application No. 12463 can be used. Further, cationic compositions
prepared from Mannich bases such as described in U.S. Pat. No. 4,134,932
can be used.
[oooisj In one embodiment of the present invention, the polymer can comprise
one or more positively charged resins which contain primary and/or secondary
amine groups. Such resins are described in U.S. Pat. Nos. 3,663,389;
3,947,339; and 4,116,900. In U.S. Pat. No. 3,947,339, a polyketimine
derivative of a polyamine such as diethylenetriamine or triethylenetetraamine
is reacted with a polyepoxide. When the reaction product is neutralized with
acid and dispersed in water, free primary amine groups are generated. Also,
equivalent products are formed when a polyepoxide is reacted with excess
polyamines such as diethylenetriamine and triethyienetetraamine and the
excess polyamine vacuum stripped from the reaction mixture. Such products
are described in U.S. Pat. Nos. 3,663,389 and 4,116,900.
[000201 Mixtures of the above-described ionic resins also can be used
advantageously. In one embodiment of the present invention, the polymer
has cationic salt groups and is selected from a polyepoxide-based polymer
having primary, secondary and/or tertiary amine groups (such as those
described above) and an acrylic polymer having hydroxyl and/or amine
functional groups.
[00021]As previously discussed, in one particular embodiment of the present
invention, the polymer has cationic salt groups. In this instance, such
cationic
salt groups typically are formed by solubilizing the resin with an inorganic
or
organic acid such as those conventionally used in electrodepositable
compositions. Suitable examples of solubilizing acids include, but are not
limited to, sulfamic, acetic, lactic, and formic acids. In an embodiment of
the
invention the solubilizing acid comprises sulfamic acid and/or lactic acid.
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[00022] In a particular embodiment, the coating compositions useful in the
methods of the present invention comprise one or more components
comprising covalently bonded halogen atoms. It should be understood that
for purposes of the present invention, by "covalently bonded halogen atom" is
meant a halogen atom that is covalently bonded as opposed to a halogen ion,
for example, a chloride ion in aqueous solution.
[00023] The coating composition used in the methods of the present invention
can have a covalently bonded halogen content of at least I weight percent, or
at least 2 weight percent, or at least 5 weight percent, or at least 10 weight
percent, based on total weight of resin solids. Also, the coating composition
used in the methods of the present invention can have a covalently bonded
halogen content of less than or equal to 50 weight percent, or less than or
equal to 30 weight percent, or less or equal to 25 weight percent, or less
than
or equal to 20 weight percent. The coating composition can have a covalently
bonded halogen content which can.range between any combination of these
values, inclusive of the recited values.
[00024] In an embodiment of the present invention, the coating composition is
an e{ectrodepositable coating composition comprising,.a resinous phase
dispersed in an aqueous medium. The covalently bonded halogen content of
the resinous phase of the electrodepositable coating composition can be
derived from haiogen atoms covalently bonded to the resin (i). In such
instances, the covalently bonded halogen content can be attributed to a
reactant used to form any of the film-forming resins described above. For
example, the resin may be the reaction product of a halogenated phenol, for
example a halogenated polyhydric phenol such as chlorinated or brominated
bisphenol A with an epoxy group-containing material such as those described
above with reference to the resin (i). In the case of an anionic group-
containing polymer, solubilization with phosphoric acid may follow.
Alternatively, an epoxy containing compound reacted with a halogenated
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carboxylic acid followed by reaction of any residual epoxy groups with
phosphoric acid would yield a suitable polymer. The acid groups can then be
solubilized with amine. Likewise, in the case of a cationic salt group-
containing polymer, the resin may be the reaction product of an epoxy
functional material such as those described above with a halogenated phenol
followed by reaction of any residual epoxy groups with an amine. The
reaction product can then be solubilized with an acid.
[00025] In one embodiment of the present invention, the covalently bonded
halogen content of the resin (i) can be derived from a halogenated compound
selected from at least one of a halogenated phenol, halogenated polyepoxide,
halogenated acrylic polymer, halogenated polyolefin, halogenated phosphate
ester, and mixtures thereof. In another embodiment of the present invention,
the covalently bonded halogen content of the resin (i) is derived from a
halogenated polyhydric phenol, for example, a chlorinated bisphenol A such
as tetrachlorobisphenol A, or a brominated: bisphenol A such as
tetrabromobisphenol A. Additionally, the covalently bonded halogen content
may be derived from a halogenated epoxy compound, for example, the
diglycidyl ether of a halogenated bisphenol A.
[00026] The active hydrogen-containing resin (i) described above can be
present in the curable coating composition of the present invention in amounts
ranging from 10 to 90 percent by weight, or 30 to 45 percent by weight based
on total weight of the curable coating composition.
[000271As previously discussed, the composition used in the methods of the
present invention further comprises one or more polyester curing agents (ii).
The polyester curing agent (ii) is a material having greater than one ester
group per molecule. The ester groups are present in an amount sufficient to
effect cross-linking at acceptable cure temperatures and cure times, for
example at temperatures up to 250 C, and curing times of up to 90 minutes.
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It should be understood that acceptable cure temperatures and cure times will
be dependent upon the substrates to be coated and their end uses.
(000281 Compounds generally suitable as the polyester curing agent (ii) are
polyesters of polycarboxylic acids. Non-limiting examples include bis(2-
hydroxyalkyl)esters of dicarboxylic acids, such as bis(2-hydroxybutyl) azelate
and bis(2-hydroxyethyl)terephthalate; tri(2-ethylhexanoyl)trimellitate; and
poly(2-hydroxyalkyl)esters of acidic half-esters prepared from a dicarboxylic
acid anhydride and an alcohoi, including polyhydric alcohols. The latter type
is
particularly suitable to provide a polyester with a final functionality of
more
than 2. One suitable example includes a polyester prepared by first reacting
equivalent amounts of the dicarboxylic acid anhydride ( for example, succinic
anhydride or phthalic anhydride) with a trihydric or tetrahydric alcohol, such
as
glycerol, trimethylolpropane or pentaerythritol, at temperatures below 150 C,
and then reacting the acidic polyester with at least an equivalent amount of
an
epoxy alkane, such as 1,2-epoxy butane, ethylene oxide, or propylene oxide.
The polyester curing agent (ii) can comprise an anhydride. Another suitable
polyester comprises a lower 2-hydroxy-akylterminated poly-alkyleneglycol
terephthalate.
[000291 In a particular embodiment, the polyester comprises at least one ester
group per molecule in which the carbon atom adjacent to the esterified
hydroxyl has a free hydroxyl group.
[0003o]Also suitable is the tetrafunctional polyester prepared from the half-
ester intermediate prepared by reacting trimellitic anhydride and propylene
glycol (molar ratio 2:1), then reacting the intermediate with 1,2-epoxy butane
and the glycidyl ester of branched monocarboxylic acids.
[00031] In one particular embodiment, where the active hydrogen-containing
resin (i) comprises cationic salt groups, the polyester curing agent (ii) is
substantially free of acid. For purposes of the present invention, by
"substantially free of acid" is meant having less than 0.2 meq/g acid. For
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aqueous systems, for example for cathodic electrodepositable, coating
compositions, suitable polyester curing agents can include non-acidic
polyesters prepared from a polycarboxylic acid anhydride, one or more
glycols, alcohols, glycol mono-ethers, polyols, and/or monoepoxides.
[00032] Suitable polycarboxylic anhydrides can include dicarboxylic acid
anhydrides, such as succinic anhydride, phthalic anhydride, tetrahydrophthalic
anhydride, trimellitic anhydride, hexahydrophthalic anhydride,
methylhexahydrophthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic
dianhydride, and pyromellitic dianhydride. Mixtures of anhydrides can be
used.
[00033] Suitable alcohols can include linear, cyclic or branched alcohols. The
alcohols may be aliphatic, aromatic or araliphatic in nature. As used herein,
the terms glycols and mono-epoxides are intended to include compounds
containing not more than two alcohol groups per molecule which can be
reacted with carboxylic acid or anhydride functions below the temperature of
150 C.'
[00034] Suitable mono-epoxides can include glycidyl esters of branched
monocarboxylic acids. Further, alkylene oxides, such as ethylene oxide or
propylene oxide may be used. Suitable glycols can include, for example
ethylene glycol and polyethylene glycols, propylene glycol and polypropylene
glycols, and 1,6-hexanediol. Mixtures of glycols may be used.
[00035] Non-acidic polyesters can be prepared, for example, by reacting, in
one or more steps, trimellitic anhydride (TMA) with glycidyl esters of
branched
monocarboxylic acids in a molar ratio of 1:1.5 to 1:3, if desired with the aid
of
an esterification catalyst such as stannous octoate or benzyl dimethyl amine,
at temperatures of 50-150 C. Additionally, trimellitic anhydride can be
reacted
with 3 molar equivalents of a monoalcohol such as 2-ethylhexanol.
[00036]Alternatively, trimellitic anhydride (1 mol.) can be reacted first with
a
glycol or a glycol monoalkyl ether, such as ethylene glycol monobutyl ether in
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a molar ratio of 1:0.5 to 1:1, after which the product is allowed to react
with 2
moles of glycidyl esters of branched monocarboxy(ic acids. Furthermore, the
polycarboxylic acid anhydride i.e., those containing two or three carboxyl
functions per molecule or a mixture of polycarboxylic acid anhydrides can be
reacted simultaneously with a glycol, such as 1,6-hexane diol andlor glycol
mono-ether and monoepoxide, after which the product can be reacted with
mono-epoxides, if desired. For aqueous compositions these non-acid
polyesters can also be modified with polyamines such as diethylene triamine
to form amide polyesters. Such "amine-modified" polyesters may be
incorporated in the linear or branched amine adducts described above to form
self-curing amine adduct esters.
[00037] The non-acidic poiyesters of the types described above typically are
soluble in organic solvents, and typically can be mixed readily with the
active
hydrogen-containing resin (i) previously described.
[000381 Polyesters suitable for use,in an aqueous systerra or mixtures of such
materials disperse in water typically in the presence of resins comprising
cationic or anionic salt groups such as any of those described previously.
[00039] A transesterification catalyst (iii) may optionally be present in the
compositions used in the methods of the present invention. The catalyst (iii)
can be any suitable catalyst known for catalysis of the transesterification
reaction. In an embodiment of the present invention the catalyst (iii)
comprises a metal oxide, metal complex or metal salt.
100040) Suitable metal oxides include, for example, oxides of lead, bismuth,
and tin, including dialkyltin oxides such as dioctyltin oxide or dibutyltin
oxide.
Alternatively, lead oxide and bismuth oxide can also be used when dissolved
in an aqueous acid solution for example, an aqueous solution of a sulfonic
acid.
[00041] Suitable salts may inciude carboxylate salts ( for example, octoates
or
naphthenates) of lead, zinc, calcium, barium, iron, bismuth and tin, including
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dialkyltin dicarboxylates. Non-limiting examples of salts include lead
octoate,
zinc octoate, and dioctyltin formate. A suitable example of a metal complex is
titanium acetyl acetonate.
[00042]Also suitable are salts e.g., octoates, and naphthenates, of the alkali
and earth alkali metals, of the lanthanides, and of zirconium, cadmium,
chromium; acetyl acetonate complexes of lead, zinc, cadmium, cerium,
thorium, copper; alkali alurninium alcoholates and titanium tetraisopropoxide.
[000431 Mixtures of any of the salts, oxides and/or complexes described above
can also be used.
[00044] In view of the varying metal content of available metal oxides, salts
or
complexes, or solutions thereof, the amount of catalyst may be indicated by
the metal content contained in the compositions. Metal contents of 0.1 to 3.0
percent by weight are suitable, or metal contents of 0.3 to 1.6 percent by
weight may be used, based on the total weight of the curable coating
composition.
[00045]
[00046]As mentioned above, the method of the present invention comprises:
(a) applying any of the curable coating compositions described above to a
substrate, (b) curing the coating composition to form a coating on the
substrate, and (c) applying a conductive layer to all surfaces.
[00047] The substrate (or "core") can comprise any of a variety of substrates.
In one embodiment, the substrate is electrically conductive. Suitable
electroconductive substrates can comprise metal substrates, for example,
iron, aluminum, gold, nickel, copper, magnesium or alloys of any of the
foregoing metals, as well as substrates coated with a conductive material,
e.g., conductive carbon-coated materials. An example of a suitable iron-
nickel alloy is INVAR, (trademark owned by lmphy S. A., 168 Rue de Rivoli,
Paris, France) comprising approximately 64 weight percent iron and 36 weight
percent nickel. This alloy has a low coefficient of thermal expansion,
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comparable to that of silicon materials used to prepare chips. This property
is
desirable for example to prevent failure of adhesive joints between
successively larger or smaller scale layers of a chip scale package, due to
thermal cycling during normal use. When a nickel-iron alloy is used as the
electrically conductive core, a layer of copper metal can be applied to all
surfaces of the electrically conductive core to ensure optimum conductivity.
The layer of copper metal may be applied by conventional means, such as
electroplating or metal vapor deposition. The layer of copper typically has a
thickness of from 1 to 8 microns. In another embodiment, circuitized materials
such as printed circuit boards are suitable as substrates.
[00048] The aforementioned coating compositions can be applied by a variety
of application techniques well known in the art, for example, by roll-coating
or
'spray application techniques. In such instances, the resinous binder may or
may not include solubilizing or neutralizing acids and amines to form cationic
and anionic salt groups, respectively.
[00049]Any of the previously described ionic group-containing compositions
can be electrophoretically applied to an electroconductive substrate. The
applied voltage for electrodeposition may be varied and can be, for example,
as low as 1 volt to as high as several thousand volts, but typically between
50
and 500 volts. The current density is usually between 0.5 ampere and 5
amperes per square foot (0.5 to 5 milliamperes per square centimeter) and
tends to decrease during electrodeposition indicating the formation of an
insulating conformal film on all exposed surfaces of the core. As used herein
and in the specification and in the claims, by "conformal" film or coating is
meant a film or coating having a substantially uniform thickness which
conforms to the substrate topography, including the surfaces within (but not
occluding) any holes that may be present.
[0005o] After the coating has been applied by an appropriate method, such as
those mentioned above, it is cured. The coating can be cured at ambient
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temperatures or thermally cured. at elevated temperatures ranging from 90 to
300 C for a period of 5 to 90 minutes to form a dielectric coating over the
substrate.
[ooosl]The dielectric coating thickness can be no more than 50 microns, or
no more than 25 microns, or no more than 20 microns.
[00052] Those skilled in the art would recognize that prior to the application
of
the dielectric coating, the core surface may be pretreated or otherwise
prepared for the application of the dielectric coating. For example, cleaning,
rinsing, and/or treatment with an adhesion promoter prior to application of
the
dielectric may be appropriate.
[00053]After application of the dielectric coating, the surface of the
dielectric
coating is optionally ablated in a predetermined pattern to expose sections of
the substrate. Such ablation can be performed using a laser or by other
conventional techniques, for example, mechanical drilling and chemical or
plasma etching techniques.
[00054]A conductive layer can be applied to all surfacesafter the optional
ablation step. The conductive layer may comprise a conductive paste or ink
or metaE.,
[00055] Suitable conductive pastes and inks can include, for example,
conductive silver coating copper pastes such as DD PASTE SAP510 and
Conductor Ink P2000, respectPully, both of which are available from TATSUTA
SYSTEM ELECTRONICS CO., LTD. Such materials can be applied by
screen-printing techniques.
[00056] Suitable metals include copper or any metal or alloy with sufficient
conductive properties. The conductive material can be applied by
electroplating or any other suitable method known in the art to provide a
uniform conductive layer. Alternatively, the conductive layer can be applied
in
a predetermined pattern, such as a circuit pattern. The thickness of this
conductive layer can range from 1 to 50 microns, or from 5 to 25 microns. In
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the case where the dielectric coating is ablated prior to the application of
the
conductive layer, conductive or metallized vias are formed.
[00057] To enhance the adhesion of the conductive layer to the dielectric
polymer, prior to the application of the conductive layer, all surfaces can be
treated with ion beam, electron beam, corona discharge or plasma
bombardment followed by application of an adhesion promoter layer to all
surfaces. The adhesion promoter layer can range from 50 to 5000 angstroms
thick and can be a metal or metal oxide selected from chromium, titanium,
nickel, cobalt, cesium, iron, aluminum, copper, gold, tungsten, and zinc, and
alloys and oxides thereof.
[00058) In a further embodiment, the method of the present invention also
comprises: (d) applying a resist to the conductive layer applied in step (c),
(e)
processing said resist to form a predetermined pattern of exposed underlying
conductive layer, (f) etching said exposed conductive layer, and (g) stripping
the remaining second resist to form an electrical circuit pattern.
[ooo5s]After application of the conductive layer, a resinous photosensitive
layer (i.e. "photoresist" or'"resist ) can be applied to the conductive layer.
Optionally, prior to application of the photoresist, the coated substrate can
be
cleaned and/or pretreated; e.g., treated with an acid etchant to remove
oxidized metal. The resinous photosensitive layer can be a positive or
negative photoresist. The photoresist layer can have a thickness ranging from
I to 50 microns, or 5 to 25 microns, and can be applied by any method known
to those skilled in the photolithographic processing art. Additive or
subtractive
processing methods may be used to create the desired circuit patterns.
[000601 Suitable positive-acting photosensitive resins include any of those
known to practitioners skilled in the art. Examples include dinitrobenzyl
functional polymers such as those disclosed in U.S. Pat. No. 5,600,035,
columns 3-15. Such resins have a high degree of photosensitivity. In one
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embodiment, the resinous photosensitive layer is a composition comprising a
dinitrobenzyl functional polymer, typically applied by spraying.
foom] In a separate embodiment, the resinous photosensitive layer comprises
an electrodepositable composition comprising a dinitrobenzyl functional
polyurethane and an epoxy-amine polymer such as that described in
Examples 3-6 of U.S. Pat. No. 5,600,035.
[00062] Negative-acting photoresists include liquid or dry-film type
compositions. Any of the previously described liquid compositions may be
applied by spray, roll-coating, spin coating, curtain coating, screen coating,
immersion coating, or electrodeposition application techniques. Preferably,
liquid photoresists are applied by electrodeposition, more preferably cationic
electrodeposition. Electrodepositable photoresist compositions comprise an
ionic, polymeric material which may be cationic or anionic, and may be
selected from polyesters, polyurethanes, acrylics, and polyepoxides.
Examples of photoresists applied by anionic electrodeposition are shown in
U.S. Pat. No. 3,738,835. Photoresists applied by cationic electrodeposition
are described in U.S. Pat. No. 4, 592,816. Examples of dry-film photoresists
include those disclosed in U.S. Pat. Nos. 3,469,982, 4,378,264, and
4,343,885. Dry-film photoresists are typically laminated onto the surface such
as by application of hot rollers.
[00063] Note that after application of the photosensitive layer, the multi-
layer
substrate may be packaged at this point allowing for transport and processing
of any subsequent steps at a remote location.
[00064] In a separate embodiment of the invention, after the photosensitive
layer is applied, a photo-mask having a desired pattern may be placed over
the photosensitive layer and the layered substrate exposed to a sufficient
level of a suitable radiation source, typically an actinic radiation source.
As
used herein, the term "sufficient level of radiation" refers to that level of
radiation which polymerizes the monomers in the radiation-exposed areas in
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the case of negative acting resists, or which depolymerizes the polymer or
renders the polymer more soluble in the case of positive acting resists. This
results in a solubility differential between the radiation-exposed and
radiation-
shielded areas.
[00065]The photo-mask may be removed after exposure to the radiation
source and the layered substrate developed using conventional developing
solutions to remove more soluble portions of the photosensitive layer, and
uncover selected areas of the underiying conductive iayer. Theconductive
layer thus uncovered may then be etched using metal etchants which convert
metal to water soluble metal complexes. The soluble complexes then may be
removed by water spraying.
[00066]The photosensitive layer protects the underlying substrate during the
etching step. The remaining photosensitive layer, which is impervious to the
etchants, may then be removed by a chemical stripping process to provide a
circuit pattern connected by the conductive vias.
[000671After preparation of the circuit pattern on the multi-layered
substrate,
other circuit components may be attached to form a circuit assembly.
Additional components include, for example, one or more smaller scale
components such as semiconductor chips, interposer layers, larger scale
circuit cards or mother boards and active or passive components. Note that
interposers used in the preparation of the circuit assembly may be prepared
using appropriate steps of the process of the present invention. Components
may be attached using conventional adhesives, surface mount techniques,
wire bonding or flip chip techniques.
[00068] Illustrating the invention are the following examples which are not to
be
considered as limiting the invention to their details. Unless otherwise
indicated, all parts and percentages in the following examples, as well as
throughout the specification, are by weight.
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EXAMPLES
[00069] It will be appreciated by those skilled in the art that changes could
be
made to the embodiments described above without departing from the broad
inventive concept thereof. It is understood, therefore, that this invention is
not
limited to the particular embodiments disclosed, but it is intended to cover
modifications which are within the spirit and scope of the invention, as
defined
by the appended claims.
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