U.S. patent application number 10/802361 was filed with the patent office on 2004-12-30 for adhesiveless transfer lamination method and materials for producing electronic circuits.
Invention is credited to Kydd, Paul H..
Application Number | 20040265549 10/802361 |
Document ID | / |
Family ID | 32329740 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040265549 |
Kind Code |
A1 |
Kydd, Paul H. |
December 30, 2004 |
Adhesiveless transfer lamination method and materials for producing
electronic circuits
Abstract
An electronic circuit is made by printing a Parmod.RTM.
composition on a temporary substrate and curing it to produce a
pattern of metal conductors. The conductors are laminated to a
substrate under heat and pressure to produce a laminate with the
metal prepatterned into the desired circuit configuration. The
conductor can also be coated with a polymer and cured to form a
prepatterned substrate. Single and double-sided circuits or
multilayers can be made this way.
Inventors: |
Kydd, Paul H.;
(Lawrenceville, NJ) |
Correspondence
Address: |
Synnestvedt Lechner & Woodbridge, LLP
P.O. Box 592
Princeton
NJ
08542-0592
US
|
Family ID: |
32329740 |
Appl. No.: |
10/802361 |
Filed: |
March 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10802361 |
Mar 17, 2004 |
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10265513 |
Oct 4, 2002 |
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6743319 |
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10265513 |
Oct 4, 2002 |
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09458929 |
Dec 10, 1999 |
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60111947 |
Dec 11, 1998 |
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60153783 |
Sep 14, 1999 |
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Current U.S.
Class: |
428/201 ;
156/233 |
Current CPC
Class: |
H05K 2203/121 20130101;
H05K 2203/0156 20130101; H05K 2203/016 20130101; Y10T 428/24909
20150115; H05K 3/207 20130101; H05K 3/105 20130101; Y10T 428/24851
20150115; Y10S 428/914 20130101; Y10T 428/2486 20150115 |
Class at
Publication: |
428/201 ;
156/233 |
International
Class: |
H01L 023/48 |
Claims
1-15. (cancelled).
16. A substrate having one or more patterned metal objects on one
or more sides of the substrate made by the method comprising the
steps of: a) applying a metal composition on a thermally resistant
temporary substrate in the patterns of the one or more patterned
metal objects; b) curing said metal composition at a temperature
below about 450.degree. C. for a time less than about 30 minutes to
form the one or more patterned metal objects; c) transferring the
one or more patterned metal objects from said temporary substrate
to one side of the substrate wherein said metal composition is
comprised of one or more metal powders in a reactive organic
medium, said reactive organic medium consisting of one or more
components selected from the group consisting of metallo-organic
compounds which can decompose into elemental metal and volatile
organic fragments, reagents which can react with said one or more
metal powders to form metallo-organic compounds which can decompose
into elemental metal and volatile organic fragments, and mixtures
thereof.
17. A patterned metal object on a substrate, made by the method
comprising the steps of: a) applying a metal composition on a
thermally resistant temporary substrate in the patterns of the one
or more patterned metal objects; b) curing said metal composition
at a temperature below about 450.degree. C. for a time less than
about 30 minutes to form the one or more patterned metal objects;
c) transferring the one or more patterned metal objects from said
temporary substrate to one side of the substrate without the use of
a separately supplied adhesive wherein said metal composition is
comprised of one or more metal powders in a reactive organic
medium, said reactive organic medium consisting of one or more
components selected from the group consisting of metallo-organic
compounds which can decompose into elemental metal and volatile
organic fragments, reagents which can react with said one or more
metal powders to form metallo-organic compounds which can decompose
into elemental metal and volatile organic fragments, and mixtures
thereof.
18. The patterned metal object on a substrate of claim 17 wherein
said patterned metal object is selected from the group consisting
of strain gauges, Tape Automated Bonding Decals, and thermocouples.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
U.S. patent application Ser. No. 09/458,929 filed on Dec. 10, 1999,
the entire contents of which is hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] In the microelectronics industry the basic method for
forming circuit traces on a substrate involves a combination of
photoresist and electroplating steps, which incorporate many
hazardous and expensive compounds and solvents, and involves
extensive processing of the circuit board/substrate. One method
used to avoid the repeated processing of the substrate involves
forming the circuit traces on a metallic board using the
photoresist/electroplating processes or by die cutting the circuit
patterns from a metal foil. An adhesive is then used to transfer
the circuit to the substrate by transfer lamination. Another
alternative is the "lift off" method. In this process an adhesive
image of the circuit traces is formed on the substrate. A metal
foil is then bonded to the adhesive image and the unwanted foil not
bound to the adhesive image is then lifted off by an adhesive film.
It would be desirable to eliminate the use of photoresist and
electroplating steps and the need for an adhesive in a transfer
lamination process.
[0003] There is substantial literature relating to transfer
lamination of patterned circuits from one substrate to another. For
example, Seeger, Jr. (U.S. Pat. No. 4,75,439) discloses "applying a
slurry of a vaporizable solvent, metal particles and a small amount
of binder in the shape of the circuit pattern desired to a
removable layer, vaporizing the solvent, covering the powdered
metal and binder with an adhesive to hold the powdered metal and
carrier in place on the removable layer, laminating the hydrocarbon
containing substrate with pressure and heat to cause compacting of
said powder and bonding of said compacted powder to said substrate
by adhesive layer(sic), said heat being insufficient to destroy
said adhesive, substrate and removable layer, and separation of the
removable layer." Seeger, Jr., discloses that the adhesive is
essential not only to bond the finished circuit to the final
substrate but also to bond the metal particles together.
[0004] Seeger, Jr. discloses, "A metal slurry of metal particles,
e.g. noble metals such as silver, palladium, gold and platinum, is
preferably mixed with the combination of other metal particles such
as nickel or tin. A vaporizable solvent is mixed therewith as well
as a small amount of a curable plastic binder." (Seeger, Jr. column
2, lines 15-20). A particular mixture is given as an example in
Seeger, Jr. column 4, lines 8-18. These mixtures are very similar
to mixtures known as "Ormet" and are described in Capote et al,
U.S. Pat. Nos. 5,538,789 and 5,565,267, among others. The mixtures
are described by Ormet Corp. (formerly Toronaga Technologies) as
"Transient Liquid Phase" materials in that they function by heating
the combination of high melting point and low melting point metal
powders in a fluxing environment to the eutectic temperature at
which the powders alloy and freeze again to form a continuous
conductor. The mixture also includes an epoxy resin, which cures at
the eutectic temperature and acts as a binder to fill the porosity
between the metal particles and to adhere them to the
substrate.
[0005] The mixtures of Seeger, Jr. also resemble conventional
polymer thick film (PTF) mixtures of metal powders and epoxy or
other polymer binders available from Acheson, DuPont and many other
vendors, with the addition of the low melting tin powder. Polymer
thick film materials are cured at approximately 150.degree. C. for
times ranging from a few minutes to an hour. The result is a
conductor in which the epoxy binder is the continuous phase and
electrical conductivity results from adventitious contact between
metal particles. The conductivity typically is less than ten
percent of the conductivity of bulk metal, usually much less, and
is somewhat subject to changes with age and environmental stress.
The traces are not solderable because of the epoxy. However, the
cure conditions are compatible with most polymer substrates, and so
there is no incentive to perform transfer lamination. Virtually all
computer keyboards and other membrane touch switches are now
typically made by printing PTF materials directly on polyester
substrates.
[0006] Ormet type Transient Liquid Phase mixtures cure to metal
traces in which the metal is the continuous phase and the epoxy
binder fills the interstices. Electrical conductivity is
substantially better than most PTF materials, but still only 10% of
bulk metal because the material is an alloy as well as being
porous. Lack of solderability is still a problem because of the
epoxy binder, but the electrical performance is better than PTF,
and there is an incentive to do transfer lamination because the
Ormet cure temperature of 220.degree. C. is well above the
125.degree. C. to which polyester substrates typically are limited.
Seeger, Jr's stated cure temperature of 177.degree. C. (350.degree.
F., column 2, lines 33-43) is lower than quoted by Ormet, and will
result in poorer metallurgical properties, but will still require
transfer lamination.
[0007] A novel family of materials has been developed for printing
on polymer substrates such as those used for printed wiring boards
and flexible circuits. These compositions have been used to produce
metal traces utilized in a transfer laminate process. They offer
the advantage over polymer thick films, Ormet materials, and the
like, by producing electrical conductors consisting of a pure,
single-phase metal can be produced by a simple print-and-heat
process instead of by the usual multi-step photolithographic
etching process. This family of novel compounds is commercially
available as Parmod.RTM. compositions from Parelec, LLC, Rocky
Hill, N.J., USA, and are described in U.S. Pat. Nos. 5,882,722 and
6,036,889 (the total disclosure and contents of each patent being
hereby incorporated by reference); as well as in co-pending PCT
patent application WO98/37133 and in Applicants' co-pending U.S.
application Ser. No. 09/367,783 filed 20 Aug. 1999 (the total
disclosure in its entirety of each application being hereby
incorporated by reference). These compositions can be formulated
for use in a printing process, for example as inks, pastes, toners,
etc. These formulations can be printed on a substrate and cured at
a temperature below 450.degree. C. to well-consolidated films or
traces of pure metal in seconds. The fast curing capability of
these Parmod.RTM. compositions, as well as their easy application,
makes it possible to use them to create complex thin metal objects
by very simple and low-cost processes. An example of such an object
is a pattern of electrical conductors used as an antenna in a radio
frequency identification tag. Another such application is as a TAB
bonding decal for semiconductors. Electronic circuit patterns of
many types can be produced by this process and bonded to various
types of substrates and devices. The method can be used to produce
strain gauges, thermocouples and other types of instrumentation.
Many other such objects and applications will be evident to those
skilled in the art.
[0008] These Parmod.RTM. compositions include printable inks and
pastes, which comprise metal flakes and/or powders mixed with a
Reactive Organic Medium (ROM). The compositions are printed on a
substrate and heated. This decomposes the ROM, which chemically
welds the particulate constituents together, and the residual
organic material leaves as vapor. The result is a pure metallic
deposit which can function as an electrical conductor with low
resistivity and which is solderable. This capability is unique
relative to all other options for printable electronics.
[0009] In contrast to the mixtures described in Seeger, Jr., the
Parmod.RTM. compositions cure to a pure, single-phase metal trace
with no organic content. This is demonstrated in FIG. 4 of U.S.
Pat. No. 5,882,722 for example. The cure temperature of these
mixtures is typically 200-300.degree. C., which definitely requires
transfer lamination to apply them to many polymer substrates,
specifically polyester. The result, however, is a vastly superior
product with electrical conductivity between 25 and 50% of that of
bulk metal. Copper traces prepared from these mixtures are
perfectly solderable because there is no residual organic content.
The organic and metallo-organic constituents of the reactive
organic medium of the present invention are chosen to be thermally
labile and to volatilize completely at the cure temperature.
[0010] Other adhesive-based transfer processes have been described
in the patent literature. Salensky (U.S. Pat. No. 5,045,141)
summarizes the literature on prior transfer processes (column 5,
line 35 to column 6, line 28). His invention is a specific
thixotropic conductive ink, and an adhesive to apply it, used in a
specific inventive transfer process (flow chart shown in column 6,
lines 45-57). Levesoue and Harper (U.S. Pat. No. 3,703,603)
describe conductors similar to those of Seeger Jr. in that the
"metallic particles are intermixed with an appropriate adhesive"
(column 1 lines 43-47). The adhesive is critical to the bonding of
the particles together and to the transfer from the carrier strip
to the final circuit board. The transfer is accomplished by rubbing
or burnishing, which generates sufficient heat and pressure to bond
the circuit element to the board and to release it from the carrier
strip, as described in claims 1 and 2. This is similar to many
transfer materials for applying lettering, for example, which are
consist of a sheet of patterns coated with a pressure sensitive
adhesive which can be transferred to a final substrate by
burnishing.
[0011] The patent of Nakatani (U.S. Pat. No. 5,407,511) discloses
the curing of conventional thick film pastes at required
temperatures in the range 600-1000.degree. C. as recited in his
claims 1, 3 and 6. As noted previously, Parmod.RTM. mixtures can be
cured at temperatures below 450.degree. C. Also, Nakatani
specifically refers to the use of an adhesive layer denoted 103 in
his FIG. 1 and in column 6 line 29 where "a polyimide sheet with an
epoxy adhesive (available from Toray Industries, Inc.)" is cited in
example 1 and by inference in the subsequent examples.
[0012] It has been discovered that Parmod.RTM. compositions can be
transferred and bonded to a substrate without the use of a separate
adhesive coating or layer. Utilizing the unexpected bonding
properties of the Parmod.RTM. compositions, solderable, high
conductivity metal objects, e.g., traces, can be bonded to a broad
range of substrates, saving the costs and extra steps of using an
adhesive.
[0013] Using the Parmod.RTM. compositions result in a pure metallic
deposit which can function as an electrical conductor with low
resistivity and which is solderable. This capability is unique
relative to all other options for printable electronics. In direct
comparisons, Parmod.RTM.-based compositions and the resulting
traces are superior to those used by Seeger, Jr., Salensky,
Nalatani, and Levesoue and Harper. And all of the art currently
require the use of a separate adhesive to bond the circuit trace to
the substrate--where none is needed when using Parmod.RTM.
compositions.
BRIEF SUMMARY OF THE INVENTION
[0014] The present application describes a method of transfer
lamination, which does not use an adhesive. The present invention
decouples the curing and adhesion processes from the substrate by
doing the printing and curing on a temporary substrate and then
transferring the metal object produced to a permanent substrate at
low temperature without the use of an adhesive utilizing metal
objects produced using ROM-metal compositions, such as, Parmod.RTM.
compositions. It is preferred that the temporary substrate: have
approximately the same coefficient of thermal expansion as the
Parmod.RTM.; that it withstand the temperature at which the
Parmod.RTM. cures; that the Parmod.RTM. not permanently bond to it;
and, that it be easily reusable or very inexpensive. The permanent
substrate must have the ability to bond to the printed and cured
metal
[0015] In one embodiment a polymer-based substrate is built up on
the cured circuit traces and cured in place, rather than being
produced as a separate laminate or film. In another embodiment the
cured circuit traces are laminated under heat and pressure to a
premade substrate which can soften and flow into the pores of the
Parmod.RTM. traces, thus eliminating the need for a separate
adhesive layer or coating. These methods simplify the process and
lower costs by using the substrate itself as the adhesive in the
lamination process.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Preferred compositions useful for forming the traces,
components and objects of this invention are ROM-metal mixtures
described in detail below. These compositions can be applied to
thermally stable substrates and cured to well-consolidated circuit
traces and objects by heat treatment. The compositions exhibit a
critical temperature above which they undergo a transformation to
well-consolidated electrical conductors with a resistivity only two
to four times the bulk resistivity of the metal in question. The
electrical conductivity is equal to that obtained by conventional
high temperature metal powder sintering in conventional thick film
compositions on ceramic substrates. Remarkably, this consolidation
process takes place at temperatures 400 to 500 degrees Celsius
lower than with compounds conventionally used in thick film
technology, and in times which are an order of magnitude shorter
than are required for sintering.
[0017] Parmod Compositions and their Characteristics
[0018] Parmod.RTM. compounds contain a Reactive Organic Medium
(ROM) and a source of metal, preferably metal flakes, metal powders
and their mixtures. The ROM consists of either a Metallo-Organic
Decomposition (MOD) compound or an organic reagent, which can form
such a compound upon heating in the presence of the metal
constituents. The ingredients are blended together with organic
vehicles and with rheology modifying organic vehicles if necessary,
to produce printing inks or pasts or toners for electrostatic
printing. These inks and toners can be printed on a
temperature-sensitive substrate and cured to well-consolidated,
well-bonded electrical conductors at a temperature low enough so
that the substrate is not damaged. The curing process occurs in
seconds at temperatures as much as 500.degree. C. below those used
for conventional sintering of thick film inks and pastes. The
process can be performed continuously, for example, using belts and
tapes or webs. Likewise, using a series of belts, tapes and webs,
multilayered units can be produced.
[0019] Suitable metals include copper, silver, gold, zinc, cadmium,
palladium, iridium, ruthenium, osmium, rhodium, platinum, iron,
cobalt, nickel, indium, tin, antimony, lead, bismuth and mixtures
thereof.
[0020] In a preferred embodiment, the metal mixture contains
micron-size metal flake or powder and colloidal or semi-colloidal
metal powder where the total of metal (flake plus powder) is
preferred to be 80-95% of the total mixture, and the colloidal
powder is preferred to be 10-40% of the total metal. Larger amounts
of organic vehicle may be added to reduce viscosity for certain
applications.
[0021] The metal flakes have a major dimension between 2 to 10
micrometers, preferably about 5 micrometers, and a thickness of
less than 1 micrometer. They can be produced by techniques well
known in the art by milling the corresponding metal powder with a
lubricant, which is frequently a fatty acid or fatty acid soap. The
micron-size starting powders are usually produced by chemical
precipitation to obtain the desired particle size and degree of
purity. They can be advantageously used without milling in certain
compositions. The flakes are sold for electronic applications as
constituents of thick film inks and silver-loaded conductive
epoxies.
[0022] The flakes or micron-size powders perform several functions.
They form a skeleton structure in the printed image which holds the
other ingredients together and prevents loss of resolution when the
mixture is heated to cure it. They also provide good electrical
conductivity in the finished trace. The flakes naturally assume a
lamellar structure like a stone wall, which provides electrical
conductivity in the direction parallel to the surface of the
substrate and provides a framework to lessen the amount of metal
transport necessary to achieve well-consolidated pure metal
conductors. They also provide low surface energy, flat surfaces to
which the other constituents of the composition can bond.
[0023] The other metallic powder mixture constituents of the
present invention are preferably colloidal or semi-colloidal
powders with individual particle diameters below about 100
nanometers, preferably less than about 60 nanometers. A primary
function of these powders is to lower the temperature at which the
compositions will consolidate to nearly solid pure metal
conductors. The presence of fine metal powder has been found to be
helpful in advancing this low temperature process with silver and
essential to the consolidation of copper mixtures. It is important
that they be present as individual particles. Metal particles this
small have a strong tendency to agglomerate into aggregates with an
open skeletal structure.
[0024] Colloidal silver particles with a nominal diameter of 20
nanometers have been shown to have an excellent state of dispersion
and have been used in silver compositions and lowered the critical
consolidation temperature from 300 to 260 degrees C.
[0025] To achieve and preserve the desired degree of dispersion of
colloidal metal it is essential to stabilize the particles so that
they cannot aggregate. In the case of the silver particles they
were stabilized by the presence of a surfactant that coated the
surface of the particles and prevented metal-to-metal contact.
Suitable surfactants include carboxylic acids and metal soaps of
carboxylic acids. This favors chemical precipitation as a means of
producing the powders, since they can be exposed to an environment
that promotes stabilization from formation to final
consolidation.
[0026] The Reactive Organic Medium (ROM) provides the environment
in which the metal mixture is bonded together to form
well-consolidated conductors. Many classes of organic compounds can
function as the ROM. The common characteristic, which they share
and which renders them effective, is that they have, or can form, a
bond to the metal via a heteroatom. The heteroatoms can be oxygen,
nitrogen, sulfur, phosphorous, arsenic, selenium and other
nonmetallic elements, preferably oxygen, nitrogen or sulfur. This
bond is weaker than the bonds holding the organic moiety together,
and can be thermally broken to deposit the metal. In most cases the
reaction is reversible, so that the acid or other organic residue
can react with metal to reform the metallo-organic compound, as
shown schematically below:
R-M<=>R+M
[0027] where R is a reactive organic compound and M is the
metal.
[0028] As an illustration of Parmod.RTM. mixtures containing MOD
forming constituents such as organic acids, the reactions that take
place in curing are as follows:
[0029] IIa.) Acid+Metal powder=>MOD+H.sub.2 or
[0030] IIb) Acid+Metal Oxide=>MOD+H.sub.2O and
[0031] III) MOD+heat+H.sub.2O<=>Bulk metal+Acid
[0032] The effect is to consume the small particles and weld
together the big ones to create macroscopic circuit conductors of
pure metal. In place of acid, some other active organic reagent
which will produce an easily decomposed metallo-organic compound
from either the oxide or the metal could be used. An example would
be the use of sulfur compounds to make mercaptides or nitrogen
ligands to produce decomposable complexes.
[0033] Examples of useful compounds are soaps of carboxylic acids,
in which the hetero-atom is oxygen; amino compounds, in which the
hetero-atom is nitrogen; and mercapto compounds, in which the
hetero-atom is sulfur.
[0034] Specific examples of preferred ROM constituents are the
carboxylic acids and the corresponding metallic soaps of
neodecanoic acid and 2-ethyl hexanoic acid with silver and copper,
such as silver neodecanoate:
[0035] Ag C.sub.10H.sub.20O.sub.2
[0036] and silver 2-ethyl hexanoate:
[0037] Ag C.sub.8H.sub.16O.sub.2
[0038] Gold amine 2-ethyl hexanoate is an example of a nitrogen
compound: 1
[0039] Gold amine 2-ethyl hexanoate (gold amine octoate)
[0040] Gold t-dodecyl mercaptide is an example of a sulfur
compound: 2
[0041] where R.sub.1, R.sub.2, and R.sub.3 total
C.sub.11H.sub.23
[0042] These ROM compositions can be made by methods well known in
the art. All of the above compounds are capable of decomposition to
the respective metals at relatively low temperatures. For the
silver neodecanoate and silver 2-ethyl hexanoate (silver octoate),
the decomposition temperature is between 200 and 250.degree. C. For
the corresponding copper compounds, it is between 300 and 315 C.
Gold sulfides decompose at very low temperatures in the
neighborhood of 150.degree. C. Gold amine octoate decomposes
between 300 and 500.degree. C. The copper and silver compounds can
be reformed from the corresponding acids at the same temperature,
so the reaction is reversible, as mentioned above.
[0043] In some cases it is convenient to add rheology-enhancing
compounds well known in the art to improve the printing
characteristics of the compositions of the invention.
Alpha-terpineol has been used to reduce the viscosity of copper and
silver compositions to facilitate screen-printing. Alpha-terpineol
also participates in the consolidation reaction by virtue of the
acid character of the OH group bonded to an unsaturated ring. By
selecting constituents and additives, it has proven possible to
produce a range of printable compositions ranging from fluid inks
with a viscosity of 15 centipoise to solid powders.
[0044] Parmod.RTM. mixtures function by deposition of material from
decomposition of the MOD compound which "chemically welds" the
powder constituents of the mixture together into a monolithic
solid. In the case of metals, this results in a porous but
continuous metal trace which has a density approximately half that
of bulk metal and an electrical conductivity per unit mass which is
also approximately half that of the bulk metal. This demonstrates
that the printed Parmod.RTM. conductors are made up of continuous
well-bonded metal rather than of individual particles that are in
adventitious contact with each other, as in so-called polymer thick
film materials. SEM cross sections of copper, silver and gold
mixtures that have been heated above the critical temperature show
that the metal flake and powder have consolidated into a bonded
network of solid metal.
[0045] The composition is printed on the substrate using any
convenient printing technology. Screen printing and stenciling are
suitable for rigid substrates in relatively small numbers with high
resolution. Gravure printing, impression printing and offset
printing are suitable for high production rates on flexible
substrates. Ink jet printing and electrostatic printing offer the
additional advantage of direct computer control of the printed
image. Examples of Electrostatic Printing are disclosed in
Applicant's co-pending application Ser. No. 09/369,571 filed on
Aug. 6, 1999. This permits circuits to be printed directly from
Computer Aided Design (CAD) files and eliminates the need for
special tooling. Each circuit can be different, if desired, for
coding or prototyping. The same end can be achieved at lower
production rates with computer-controlled dispensing equipment.
This equipment produces dots or lines by moving a needle over the
surface and dispensing printing composition supplied by a pump or
pressurized syringe.
[0046] Compositions of this invention have been applied by screen
printing, stenciling, dispensing, gravure printing, ink jet
printing, impression printing, offset printing and electrostatic
printing. Alternative application methods include coating an
adhesive pattern with a dry powder composition or toner. Screening,
as used in applying conventional thick film pastes has been used
most extensively for preparing samples for evaluation. A
composition with a viscosity of approximately 500 poise is forced
through a fine screen with a photo-defined open image of the
desired conductor pattern in it by a rubber squeegee. The
resolution that has been achieved by this method is approximately
125-micron (5 mil) lines and spaces, although production screen
printers can achieve patterns as fine as 50 microns. Conductive
traces with thicknesses up to 50 microns have been printed, though
most of the test patterns have been in the neighborhood of 12
microns thick, which is equivalent to 0.37 ounces of copper per
square foot.
[0047] Both gold and silver mixtures can be heated in air since the
elemental metals are the stable form at the temperature at which
the metallo-organic constituent decomposes. Copper, however,
requires the use of a protective atmosphere to prevent the
formation of copper oxide that is the stable product of
decomposition in air. A nitrogen atmosphere containing less than
about 20 and most preferably less than 3 ppm by volume of oxygen
has been found to be suitable. Addition of water vapor in the
amount of about 5% has proven to be helpful in improving the
conductivity of the resulting deposits.
[0048] Copper and silver Parmod.RTM. compositions have been
directly printed on polyimide films coated with various adhesive
layers and thermally cured to create well-bonded flexible printed
circuits. Suitable substrates are DuPont Kapton type FN with a FEP
Teflon coating and types KJ and LJ with low melting polyimide
coatings. Copper Parmod.RTM. compositions have been printed on
rigid polyimide-glass laminates coated with a low melting polyimide
adhesive and thermally cured to create rigid printed circuits.
[0049] Transfer Lamination
[0050] To broaden the number of types of substrates which can be
employed, copper and silver Parmod.RTM. compositions have been
printed on temporary substrates, thermally cured, and transfer
laminated to a number of polymers including rigid polyester-glass
and epoxy-glass laminates using an acrylic adhesive film. The
transfer lamination process has the advantage of separating the
thermal cure process for converting the Parmod.RTM. mixture into
well-consolidated pure metallic conductor traces from the adhesion
process for bonding the traces to the polymer-based substrate. This
broadens the choice of substrates and adhesives and allows for more
flexibility in process conditions to optimize the two functions
independently. The use of a separate adhesive coating or film is
undesirable because it increases the cost of the final circuit
substantially and it introduced an additional material that may
degrade the electrical or thermal performance of the finished
circuit. It is highly desirable to use the polymer substrate itself
as the adhesive, and that is the objective of this invention.
[0051] While the Parmod.RTM. should not bond to the thermally
stable substrate, a certain amount of tackiness or adhesion may be
desired when using an automated, continuous process. Substrates
well known in the art will possess the characteristics required for
the temporary substrate. Examples of suitable temporary substrates
include, but are not limited to, polyimide films, polysulfone
films, polyester films, Teflon-coated films, silicone-coated films,
metal foils, glass and ceramic surfaces. Teflon-glass laminate
fulfills all of the requirements very well and will be used in the
examples. This material is available from Taconic Plastics,
Petersburg, N.Y., in both rigid and flexible form, suitable for
both batchwise and continuous belt processing. It has the advantage
that its thermal expansion coefficient matches that of copper, and
the substrates to which it is applied, better than unreinforced
Teflon does.
[0052] The permanent substrate must have the ability to bond
reliably to the transferred metal foil in addition to any other
requirements of the final application such as dielectric strength.
Examples of suitable substrates include, for example, polyethylene,
polypropylene, polystyrene, polyester, polycarbonate, polyurethane,
polyimide, cellulose and paper.
[0053] Conventional Circuit Formation
[0054] Epoxy-glass laminates are the most commonly used substrates
for electronic circuits. Most such circuits are multilayers in
which the circuit traces are etched into copper foil bonded to both
sides of thin epoxy-glass laminate cores. The cores are then
stacked up on lineup pins with layers of B-stage epoxy-glass
prepreg between each layer and bonded under heat and pressure into
a solid multilayer laminate structure. The multilayer is drilled,
and the holes are metallized by electroless plating to make
electrical connections between layers. The outside surfaces are
then circuitized by photolithographic patterning and electroplating
to complete the circuit board.
[0055] The result is a very high quality product but an expensive
one. A major fraction of the expense resides in the purchased
copper-clad laminate itself. This material is made from copper
foil, which is specially treated to have a rough surface that will
bond reliably to the epoxy-glass. The foil is laminated to at least
two layers of epoxy-saturated glass under pressure in a vacuum
autoclave, which is inherently an expensive process. Then, at the
circuit board fabricator, the copper is coated with an expensive
photosensitive resist. The resist is exposed through a phototool
and developed to remove the unexposed material. The copper is then
etched away in the unexposed areas, and the resist is stripped from
the exposed areas. This process produces very fine circuit traces
of well-bonded metal but wastes at least two thirds of the copper
and all of the resist that is applied. In addition to the cost of
the original materials, there are substantial costs associated with
disposing of the hazardous wastes generated as well.
[0056] The Novel Process of this Invention
[0057] A preferred transfer process of this invention involves
printing the circuit traces with Parmod.RTM. on one or two panels
of Teflon-glass or some other temporary substrate. The Parmod.RTM.
is printed only where copper is needed, eliminating waste. The
panels are lined up with one another with a layer of polymer film,
epoxy-glass prepreg or laminate in between and cured in a vacuum
autoclave or press to make a structure similar to inner layer
laminate. However, instead of having a continuous sheet of metal
foil, for example, copper, that must be circuitized, the copper is
prepatterned in the desired configuration, eliminating cost and
waste. The effect of this process is to move the production of
laminate downstream from the material supplier to the circuit board
fabricator, using the vacuum autoclave equipment that the
fabricator has in place to make the individual cores as well as the
finished multilayer.
[0058] Parmod.RTM. circuit traces are well suited to this process
because the metal, while continuous, is porous. As a result of
this, the epoxy will infiltrate the porous metal traces and achieve
a very rugged bond without further pretreatment of the metal. This
infiltration of resin will strengthen the metal traces as well as
bonding them to the surface, and dramatically increase their
mechanical strength and resistance to temperature cycling.
[0059] Yet another advantage of the present process is that the
surface of the finished laminate is flush, with the conductors
inlaid into the laminate. This facilitates making connections to
surface mount components, as well as achieving a uniform lay-up of
a multilayer circuit.
[0060] It has been found, unexpectedly, that Parmod.RTM.
circuitry/traces can be laminated to cured ("C stage") epoxy-glass
laminate under heat and pressure. The lamination process takes
place at temperatures above the glass transition temperature of the
epoxy, typically about 145.degree. C. The laminate is rubbery at
this temperature and under a pressure of about 450 psi (3.0 Mpa) it
flows into the extensive surface porosity of the Parmod.RTM. metal
trace. Using conventional epoxy prepreg materials in contact with
Parmod.RTM. traces suffers from the fact that the very fluid "B
stage" epoxy, which is designed to flow and coat the very fine
microroughness of copper foil, penetrates the entire Parmod.RTM.
trace and renders it difficult to clean up for contact with
subsequent metallization or for soldering. In contrast, the cured
resin of the laminate is cross-linked and cannot flow throughout
the porous metal, but flows into the surface porosity to make an
adequate bond. This bond is enhanced by the fact that the epoxy
continues to crosslink and cure when reheated to do the lamination.
There is a chemical interaction with the surface as well as
physical wetting and interlocking.
[0061] An alternative solution to the problem of polymer
infiltration into the porous Parmod.RTM. metal traces interfering
with solderability is to use a resin that does not penetrate the
porous metal entirely but will bond with it. Polyester and epoxy
resins loaded with particulates to make filling compounds are
effective, nonpenetrating permanent substrate materials.
Thermoplastic polymer films are also suitable by virtue of their
high viscosity.
[0062] Other materials exhibit the same ability to soften and
adhere to Parmod.RTM. circuitry in the lamination process,
including for example, paper and cardboard. It is well known that
ground wood will bond under heat and pressure to form solid
hardboard. Masonite is an example, and there are other such
products produced by similar processes. An Example is given below
showing that Parmod.RTM. silver patterns will adhere to newspaper
and low cost boxboard which contain a high proportion of ground
wood and low proportion of chemical pulp. The advantage of ground
wood in this application is that it still contains the lignin,
which is a natural resin adhesive that binds the cellulose fibers
together in the tree. Under heat and pressure the lignin will flow
and chemically react to bond the fibers together in the new
compressed configuration and to bond them to the circuitry. Kraft
paper, which uses pulp from which the lignin has been removed
chemically, does not bond in this way. Although it is a stronger
product, it does not appear to be suitable for transfer lamination
without added adhesive.
[0063] Because wood is made up of cellulose fibers and crosslinked
lignin it does not melt when heated, although it will char in air.
Circuits laid up on cardboard can be soldered in an inert
atmosphere without significant further damage to the circuit since
the lamination process at 220-250.degree. C. has already taken
place at a higher temperature than the melting point of eutectic
tin lead solder at 183.degree. C.
[0064] An alternative method for achieving the objective of making
the circuit and the substrate in the same process is to lay up the
substrate on a printing plate with the circuit as a raised pattern
of lines similar to a letter press or Flexographic plate. The plate
would be inked by pressing it onto a Parmod.RTM.--coated roller or
plate and heated to cure the Parmod.RTM. to a weakly adhering metal
image. The image could be coated with an epoxy or other resin
composition, reinforced with glass cloth if desired, and cured
thermally or by UV radiation. The result would be a resin-bonded
circuit layer with the circuit lines depressed below the surface.
Alternatively, the image could be pressed into the surface of an
epoxy glass laminate, a sheet of boxboard or the like under heat
and pressure to bond the circuit traces to the substrate. This
configuration is desirable in some instances to improve the
self-aligning of the components to be soldered to the traces and to
improve the strength of the solder joints. The method also provides
more reliable separation between closely spaced traces than a
planar image because the surface relief provides a longer
insulating path for a given line spacing.
[0065] The general method of making circuits by transfer lamination
can be applied to other systems than conventional FR-4 single,
double and multilayers. The characteristic is that the laminate is
manufactured in a patterned state. Clearly, various grades of epoxy
such as BT epoxy with a high glass transition temperature can be
used.
[0066] Polyester resin can be substituted for epoxy to create
polyester-glass circuits. In this case, glass cloth would be wet
down with catalyzed resin as the transfer lay-up is made, rather
than using prepreg. This process lends itself to implementation on
a continuous belt to produce very large volumes of inexpensive
circuitry.
[0067] Polyimide-glass prepreg such as Allied Signal P-25 can be
used in the present process to produce a very high performance
analogue of conventional FR-4 multilayers.
[0068] DuPont Thermount spun bonded aramid fiber prepregged with
polyimide by Arlon can also be used to make a high quality laminate
by the present process.
[0069] Flex Circuits
[0070] The same technology can be adapted to producing flexible
circuitry, which is normally done on a polyimide film substrate.
The polyimide is normally laminated to copper foil using an acrylic
thermosetting adhesive. The circuit is formed as described above
for inner layers. The polyimide laminate is very expensive. While
1-mil polyimide film can be bought for $0.53 per square ft., 1-mil
polyimide coated with 1-mil of acrylic adhesive costs $6.04 and the
acrylic-coated polyimide bonded to 1 ounce per square ft copper
foil costs $10.04. Thinner grades of copper and the so-called
self-bonded grades of flex laminate in which the polyimide is cast
directly on the roughened copper foil are still more expensive.
[0071] The process of this invention can be applied to create
double-sided flex circuitry by again printing and curing the
circuitry for each side on Teflon-glass with Parnod.RTM.. The
panels or belt carrying the circuits would be coated with a
polyimide film to complete the laminate. Other types of film such
as Mylar polyester could also be used with a further reduction in
cost. In making single-sided flex circuits the polyimide, polyester
or other film can be cast onto the cured circuitry on the
Teflon-glass and cured by heat or UV radiation to create the final
circuit. This process lends itself to continuous production on a
belt for high volumes and low cost.
[0072] This process would eliminate the steps of making and
treating specially prepared copper foil and laminating it to
specially coated polyimide film. The coating would be done as the
patterned copper was being laminated, and waste would be
eliminated. No separate adhesive film would be required.
[0073] For still greater economies and higher production rates, the
conductor pattern can be applied to a continuous web of substrate
by a rotary press, much like printing a newspaper but with finer
resolution. Gravure printing can be used in this application.
Offset printing can produce very high resolution also. Ink jet
printing and electrostatic printing at high speeds are candidates.
Following the printing step, the circuits will be cured in an oven,
still as a continuous web. The ability of these mixtures to cure to
solid metal in seconds is critical to realizing this concept.
Longer processing times would make the oven disproportionately
large relative to the press and squander much of the speed
advantage of high speed printing. In a continues process the
Parmod.RTM. compound is printed in the desired patterns onto a belt
of the thermally stable substrate. The belt passes through an oven
in which the Parmod.RTM. is cured and forms solid metal objects. A
continuous "tape" of the permanent substrate is contacted with the
belt and the metal objects are laminated onto the permanent
substrate tape. The tape can then be cut to form individual circuit
boards.
[0074] Multiple layers can readily be produced by this technology
by using, for example, a thermoplastic coverlay that will lift off
another set of images. The process may be continued for as many
layers as desired to make multilayer circuits by a continuous,
low-cost process.
[0075] The lift-on process can also be used to make partially
supported patterned metal foil objects such as Tape Automated
Bonding Decals. The pattern is printed on a nonadhesive material
and lifted onto a die cut substrate tape leaving part of the
pattern exposed. The result is a tape with fine metal fingers which
can be gang bonded to the pads on semiconductor chips. The outer
ends of the fingers, which are supported by the tape, can be
soldered to a semiconductor package or direct to a printed circuit
for chip-on-board mounting.
[0076] Other supported, partially supported and unsupported objects
can be made by the technology of the present invention as can be
appreciated by those skilled in the relevant arts.
[0077] Examples are:
[0078] Instrumentation such as thermocouples and strain gauges
[0079] Resistors, capacitors and inductors printed on polymer
films
[0080] Electric heaters
[0081] Circuitry comprising any or all of the above, such as radio
frequency tags that can be interrogated remotely for identification
of packages and baggage
[0082] Decorative metallic items such as jewelry and Christmas
ornaments
EXAMPLES
[0083] The examples described below indicate how the individual
constituents of the preferred compositions and the conditions for
applying them function to provide the desired result. The examples
will serve to further typify the nature of this invention, but
should not be construed as a limitation in the scope thereof which
scope is defined solely in the appended claims.
Example 1
[0084] A silver Parmod.RTM. screen ink was prepared as follows:
12.0 grams of Degussa silver flake, 3.0 grams of silver
neodecanoate, and 1.35 grams of neodecanoic acid were mixed
together using a spatula. The resulting mixture was then milled on
a roll mill to give a homogeneous paste.
[0085] Images of an eight turn antenna coil and a capacitative
plate were screen printed on separate substrates using silver
Parmod.RTM.. The screen parameters were a 195 mesh screen backed
with a 0.7 mil emulsion. The substrates used were 1-mil thick
sheets of Kaladex.RTM. 2030 polyethylene naphthalate (PEN). The
samples were thermally cured by heating to 210.degree. C. in a box
furnace with an air atmosphere for 2-5 minutes. The resulting
samples were continuous pure silver films with an electrical
resistivity of 3.5 .mu..OMEGA.-cm and poor adhesion to the
substrate.
[0086] The silver films were then transfer laminated to opposite
sides of a 1.3 mil thick polyethylene (PE) substrate. The PE
substrate was placed over the silver eight-turn antenna coil. The
silver capacitative plate was placed face down on the PE and
aligned with the silver image below. The sample was then hot
pressed with a 220.degree. C. iron. The two PEN film substrates
were then carefully peeled away leaving the silver films
transferred intact on either side of the PE substrate. After
transfer, the electrical resistivity properties remained the
same.
Example 2
[0087] The procedure of Example 1 was repeated except that only the
capacitative plate was screen-printed and thermally cured using the
silver screen ink prepared in Example 1. The eight-turn antenna
coil was etched aluminum on a 1-mil thick PE substrate. The silver
capacitative plate was transfer laminated to the aluminum coil as
was done in Example 1 with similar results.
Example 3
[0088] The procedure of Example 2 was repeated except that the
capacitative plate was printed and thermally cured on a DuPont
Kapton.RTM. H polyimide film. Similar results were obtained with
the transfer lamination.
Example 4
[0089] A silver Parmod.RTM. ink similar to the one described in
example 1 was printed as an RFID antenna coil pattern on newspaper,
shirt cardboard and manila envelope stock and cured at a
temperature of 250-260.degree. C. in wet nitrogen to minimize
charring of the paper. The same ink was printed in the same pattern
on Teflon-glass and cured. The coils direct printed on paper had
resistances ranging from 3.5 to 23 ohms and the metal was removed
from the paper in a tape test indicating poor adhesion. The samples
were stacked in such a way that the antenna coils printed on
Teflon-glass were transferred to fresh paper and the direct printed
samples were pressed into the paper by contact with the back side
of the Teflon-glass. The objective was a direct comparison between
direct printed samples that had been post treated by heat and
pressure and transfer laminated samples exposed to the same heat
and pressure. The stack was as follows from the top:
[0090] Blanket
[0091] Separator
[0092] Newspaper
[0093] Teflon-glass
[0094] Printed newspaper
[0095] Separator
[0096] Cardboard
[0097] Teflon-glass
[0098] Printed cardboard
[0099] Separator
[0100] Manila
[0101] Teflon-glass
[0102] Printed manila
[0103] Separator
[0104] The stack was pressed with five tons of force on a 24 square
inch area (416-psi) and heated to 248.degree. C. in approximately
an hour.
[0105] The resistance of the patterns after pressing was as
follows:
1 Ohms Remarks Newspaper transferred 1.9 badly charred and very
fragile Newspaper printed 2.5 ' Cardboard transferred 2.0 Good
appearance good adhesion Cardboard printed 2.5 Good appearance poor
adhesion Manila transferred Broke up No adhesion Manila printed 2.5
Poor adhesion
[0106] It can be concluded from this that heat and pressure improve
the resistivity of all Parmod.RTM. silver patterns. Good resistance
and adhesion can be achieved with newspaper, but the substrate is
too thermally unstable to be useable. Parmod.RTM.) silver patterns
can be transferred to cardboard with good appearance, good adhesion
and good electrical properties. This is important in producing RFID
antennas on packaging without the need for a separate substrate.
Parmod.RTM. silver patterns cannot be transferred to manilla
suggesting that highly refined papers with minimal residual lignin
do not bond well to the printed image.
Example 5
[0107] A copper Parmod.RTM. ink was made by blending the following
constituents:
2 2 micron Cerac copper powder 10 g 9 micron Cerac copper powder 10
g
[0108]
3 Copper nanopowder with a particle size of 11.31 g approximately
60 nanometers made by chemical precipitation and containing 22%
neodecanoic acid Neodecanoic acid, Exxon Chemical Prime 1.18 g
Alpha-terpineol 0.99 g
[0109] The blended material was homogenized by roll milling. The
ink was printed on Teflon-glass in a pattern specified by a
potential user and cured at approximately 300.degree. C. for nine
minutes in a nitrogen atmosphere to create a pure copper image.
[0110] Epoxy prepeg was laminated to the copper and cured under
heat and pressure. The resulting circuit was adhered to the Teflon
glass and could not be removed. Also the porous Parmod.RTM. copper
was impregnated by the fluid B-stage epoxy and was not solderable
after lamination.
[0111] This problem was cured by the use of a filled paste type
epoxy of the type used as a filler and hole plugging marketed as
PC-7 by Protective Coating Co., Allentown, Pa. The filled epoxy
adhered to the copper without infiltrating it and separated from
the Teflon glass. The resulting circuit soldered very well. The
circuit could be reinforced by bonding to a precured epoxy-glass
laminate, by bonding to an epoxy glass prepreg or by laying up
glass cloth and resin on the filled epoxy, either before or after
curing the filled epoxy.
Example 6
[0112] A Parmod.RTM. copper mixture similar to that in Example 5
was printed on Teflon-glass and cured in a nitrogen atmosphere at
temperatures up to 351.degree. C. for 16 minutes. The printed
Parmod.RTM. copper circuit on Teflon-glass was laminated to 0.62
inch thick G-10 epoxy-glass laminate from McMaster Carr Supply Co.
The lamination conditions were temperatures up to 323.degree. C. in
an hour with vacuum to improve contact between the film and the
printed metal. The pressure was approximately 500 psi. While the
conditions were extreme for epoxy and the circuit was extensively
blackened, surprisingly the circuit was solderable and survived a
solder dip test. Subsequent laminations at 230.degree. C. yielded
much better looking circuits on G-10 which could survive a 10
second dip in solder at 245-250.degree. C. The tests were extended
to thinner 0.31 thick G-10 and to 0.004 inch thick inner layer
laminate from Allied Signal. The Parmod.RTM. was printed on
aluminum as well as Teflon-glass.
Example 7
[0113] The same mixture as used in Example 5 was used to print and
cure copper circuit traces on Teflon-glass. The circuits were
transferred to polyester resin coated polyester glass laminates.
Again the resin either infiltrated the copper, or if allowed to
cure, would not bond to it. There was a very narrow range of resin
viscosity that produced acceptable results. A filled polyester,
Formula 27 from Evercoat Division of Illinois Tool works,
Cincinnati, Ohio, was successfully used to coat the circuit traces
without infiltrating them. The coated traces were laminated to
premade polyester glass laminate to make customer samples for
consumer electronics. They were also used as the base on which to
build up a polyester-glass laminate. In both cases the circuits
could be separated from the Teflon-glass and were solderable.
Example 8
[0114] The mixture of Example 5 was used to coat the raised portion
of a nickel printing plate with a pattern of conductors as raised
lines. The raised lines was coated with the copper Parmod.RTM.
mixture by spreading a thin layer of the mixture on a glass plate
and carefully contacting the nickel foil with the coating to
transfer the mixture to the raised portions of the nickel without
coating the nickel between the lines. The sample was cured in a wet
nitrogen atmosphere in the usual way to produce a loosely adhering
patterned copper coating. This coating could then be transferred to
a filled epoxy substrate which is cured in contact with the nickel
and stripped off following cure by the method developed by
Dimensional Circuits of San Diego, Calif.
Example 9
[0115] A Parmod.RTM. copper mixture similar to that in Example 5
was printed on Teflon-glass and cured in a nitrogen atmosphere at
temperatures up to 351.degree. C. for 16 minutes. The sample was
laminated to an 0.8 mil thick film of polyimide resin designated
LARC-SI procured from PAR Technologies, Newport News, Va., in the
same stack as the epoxy-glass laminate in Example 5. The resin is a
soluble polyimide developed by NASA Langley Research Center. The
film was cast from a saturated solution of the resin in
n-methylpyrolidinone. The lamination was performed at temperatures
up to 323.degree. C. in an hour with vacuum to improve contact
between the film and the printed metal. The pressure was
approximately 500 psi.
[0116] The printed circuit transferred to the polyimide film but
did not separate well from the Teflon glass due to its fragility.
The experiment was repeated with other LARC-SI films with better
results. The transferred circuits could be soldered and would
withstand a solder dip test of 20 seconds at 240.degree. C. without
delaminating.
[0117] A similar experiment with General Electric Ultem
polyetherimide film yielded well-bonded circuits of bright
copper.
Example 10
* * * * *