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Description  |
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BACKGROUND OF THE INVENTION
Metal-clad laminates which are used in various electrical apparatus have
been formed by various means in the prior art. For example, one method of
preparing a copper-clad laminate is to press a resin bonded to a filler
with a metal component, a thin film of thermoplastic polymeric material
being deposited between the resin and the metal component. Another prior
art method is to utilize various laminates such as glass, fiber, paper,
phenolic resins, etc. to which a metal layer may be deposited thereon, the
surface of the laminate being treated in various ways before bonding the
metal thereto. Yet another method of preparing metal-clad laminates is to
apply a plurality of resins to a sheet of fibrous material to produce a
smooth surfaced sheet member and thereafter applying a resinous adhesive
coating to the smooth surface followed by heat treating and application of
a metal foil followed by pressing to produce the desired metal-clad
laminate. However, in these prior art metal-clad laminates, the metal foil
which is utilized as the coating or conductive layer is relatively thick
due to the inability of the present methods of operation to handle a
relatively thin foil. Currently, the limiting factor in utilizing thinner
foils is the inability to handle any metal foil such as copper which is
thinner than one-half ounce per square foot by manual or mechanical means.
However, as will be hereinafter set forth in greater detail, it has now
been discovered that relatively thin metal coatings having a thickness of
from about 1 micron to about 20 microns may be utilized as a conductor for
a laminate.
In addition, it is also known that a thin film of conductive metal may be
applied to a carrier metal which is temporary in nature, the temporary
carrier being of such material so that it can be thrown away after one
use. The thickness of this temporary carrier will depend upon the
stiffness of the material which is used. After preparing the conductive
metal film on the temporary carrier, it is then bonded to a laminate
following which the clad laminate will then undergo a number of
manufacturing operations which are involved in printed circuitry, said
operations including drilling, punching, etching, etc. However, before the
printed circuit board can be processed further the temporary carrier must
be removed by chemical or mechanical means and, due to the fact that it is
still present when the various operational steps are performed and is not
reusable when the temporary carrier is removed by physical means, it is
necessary that the adhesion between the temporary carrier and the
conductive metal be kept within certain limits which will insure the
possibility of an easy removal of the carrier after the conductive metal
has been bonded to the laminate. However, certain factors are present in
the lamination process which can give rise to a metal-clad laminate in
which it is difficult to remove the temporary carrier from the face of the
conductive metal. One such factor which is involved in the removal could
be when the resin is removed from beneath the conductive metal coating and
the glass fiber weave of the laminate forces it into the temporary
carrier. This can happen when the high spots in the glass fiber mat cause
relatively large localized pressures which tend, at the elevated
temperatures necessary for lamination, to weld the conductive metal such
as copper to the temporary metallic carrier such as aluminum. This welding
will then cause some difficulty in stripping the temporary carrier from
the finished laminate. It then becomes necessary that a minimum dwell time
is required in order to obtain laminates which will permit the ready
stripping of the temporary carrier from the bonded laminate. The dwell
time may be defined as the time during which the resin is in a liquid or
semiliquid phase and is free to move against the mat. The longer the time
period exists that the resin is in a fluid state, the more the resin will
flow to the edge of the pack in order to remove or relieve the pressure
and therefore the more resin that is removed from between the mat and the
conductive metal, the greater will be the difficulty in removing the
temporary carrier. In commercial operations, it is extremely difficult to
achieve a relatively short dwell time in a laminating operation. In
addition, another difficulty is present when a double-clad laminate is to
be prepared inasmuch as in most pre-preg materials containing a low
percentage of resins there is a disparity between the amount of the resins
between the two surfaces of the material. In many instances there is a
rough side and a smooth side of the pre-preg, the rough side tending to be
deficient in resin and the smooth side having a surplus thereof. Therefore
for the reasons hereinbefore set forth a situation may exist in the case
of doublesided laminates where one side will be easily stripped from the
supporting carrier while the other side will be difficult to strip of the
supporting material. These difficulties can be overcome be preparing a
metal-clad laminate using the process of the present invention which will
be hereinafter described in greater detail.
This invention relates to metal coated laminates and to a method for the
preparation thereof. More specifically, the present invention is concerned
with metal coated laminates in which the metal coating has a thickness of
from about 1 to about 20 microns, and to a novel method for the
preparation of the same.
Metal-clad laminates, and particularly copper-clad laminates, are widely
used in the electrical industry in the preparation of printed electric
circuit boards. In these electric circuit boards the copper or other metal
which is deposited on the surface of the laminate must be bonded to the
laminate so that subsequent handling and usage thereof will not affect the
electrical properties of the board. In addition to the necessary bonding
or adhesion of the metal to the laminate, it is also preferred that the
printed circuit boards be prepared comprising metal-clad laminates in
which the metal is as thin as possible. By utilizing a reltively thin
layer of metal such as copper, i.e., a layer which is from about 1 to
about 20 microns in thickness, it will be possible to greatly reduce the
etching time with a concomitant advantage of using less of the etching
solutions as well as smaller amount of spent etchant which need to be
disposed as well as a reduced rejection rate due to copper surface
defects. In addition, less undercutting will occur along with a finer line
definition with finer or narrower circuit lines and closer spacing
allowing greater circuit density. An added advantage to the use of a thin
layer will be that there is a lesser problem with the availability of
hard-to-obtain thick copper foil.
A current problem in the use of thinner foils is the inability to handle
anything which is thinner than one-half ounce per square foot by manual or
mechanical means. However, by utilizing the process of the present
invention, it will be possible to prepare a metal-clad laminate in which
the metal is bonded to the laminate wherein the metal layer will have a
thickness ranging from about 1 to about 20 microns. The circuit boards
which are prepared according to the process of this invention will, as
hereinbefore set forth, be used in the electrical and electronics
industries in radios, televisions, computers, etc.
It it therefore an object of this invention to provide a metal-clad
laminate in which the metal is in the form of a relatively thin coating on
the laminate.
A further object of this invention is found in a process for producing a
metal-clad laminate in which the metal on the laminate is in the form of a
relatively thin coating, the thickness of said coating being from about 1
to about 20 microns.
In one aspect an embodiment of this invention resides in a metal-clad
laminate in which the metal has a thickness of from about 1 to about 20
microns prepared by depositing a layer of conductive metal on the surface
of a substrate which has been treated with a release agent, treating the
upper side of said conductive metal to improve the adhesive properties
thereof, bonding said conductive metal to a laminate and removing said
substrate.
Another embodiment of this invention is found in a method for preparing a
metal-clad laminate which comprises treating a substrate with a release
agent, depositing a layer of conductive metal on the surface of said
substrate, treating the upper side of said conductive metal to improve the
adhesive properties thereof, bonding said conductive metal to a laminate,
removing said substrate, and recovering the resultant metal-clad laminate.
A specific embodiment of this invention resides in a metal-clad laminate in
which the metal such as copper possesses a thickness of from 1 to about 20
microns, said laminate having been prepared by depositing a layer of
copper on the surface of a substrate such a stainless steel which has been
treated with a silane release agent, thereafter treating the upper side of
said copper to improve the adhesive properties thereof, bonding said
copper to a laminate comprising a glass epoxy resin, and removing said
stainless steel.
Another specific embodiment of this invention is found in a method for
preparing a metal-clad laminate which comprises treating a stainless steel
plate with a silane release agent, depositing a layer of copper on the
surface of stainless steel, treating the upper side of said copper by
subjecting said copper to a high current density, oxidizing the surface by
the addition of heat, thereafter treating the oxidized surface with a
silane bonding agent, bonding said copper to a glass epoxy resin laminate,
removing the stainless steel and recovering the resultant copper-clad
glass epoxy resin laminate.
Other objects and embodiments will be found in the following further
detailed description of the present invention.
The metal coated laminate of the present invention is prepared by forming a
thin coat of copper or other desired conductive metals such as nickel,
tin, gold, etc. on the treated surface of a predetermined substrate. This
substrate may be metallic or non-metallic in nature, said substrate
comprising steel, aluminum, or other metals or, if so desired, it may be
non-metallic in composition such as plastics, including polyethylene,
polypropylene, epoxy resin, etc. The substrate is treated with a material
which will insure the formation of a relatively poor bond between the
substrate and the electro- or electrolessly deposited metal coating. This
treatment will consist in applying a release agent to the surface of the
substrate. The release agent which may also be characterized as a parting
agent, slip agent, etc. will comprise a film which will prevent or reduce
the adhesion between the surface of the metal coating and the substrate.
Suitable release agents which may be utilized will include calcium
fluoride, alkoxy silanes, polysiloxanes, silica colloids, etc. One method
of applying the release agent to the surface of the substrate is from a
water solution which is followed by other steps to insure the removal of
excess release agent from the surface of the substrate, the removal of the
excess being accomplished by washing or wiping the surface of the
substrate. Following this, the coating of the conductive metal such as
copper or the like is then accomplished by electroplating at appropriate
conditions whereby the full and relatively uniform coverage of the
substrate with the desired thickness of metal is insured. For example, one
method of obtaining a uniform relatively thin copper coating on the
substrate is to utilize a copper pyrophosphate plating bath which contains
copper pyrophosphate as well as nitrates, ammonia and orthophosphates. The
weight ratio of pyrophosphate ion to copper ion should be maintained in a
range of from about 7.0:1 to about 8.0:1. The plating is effected at a pH
usually in the range of from about 8.1 to about 8.8 at an elevated
temperature of from about 120.degree. to about 140.degree. F. The electric
current necessary to effect the electrolytic deposition of copper on the
substrate will include a voltage of from 1.4 to about 4.0 volts, a cathode
current density of from about 10 to about 80 amps per square foot and an
anode current density of from about 20 to about 40 amps per square foot.
Likewise when utilizing a copper sulfate plating bath in the presence of
sulfuric acid and any addition agents which may be required at operating
conditions of the bath will include a temperature in the range of from
about 65.degree. to 125.degree. F., a cathode current density of from
about 20 to about 50 amps per square foot and an anode current density of
from about 15 to about 45 amps per square foot. The plating of the copper
from either the copper pyrophosphate or copper sulfate bath is
accomplished during an agitation of the bath by means of an air or, in the
preferred embodiment, a nitrogen current flowing into said bath. In
addition, it is also contemplated within the scope of this invention that
various material or additives may be present in the copper sulfate bath to
improve the deposit of copper on the substrate. Other baths which may be
utilized to effect an electrolytic plating of the conductive metal on the
substrate will include acid copper fluoroborate, alkaline gold cyanine,
acid gold, tin-nickel, nickel sulfamate, rhodium sulfate, silver cyanine,
etc. As in the case of the various copper plating baths hereinbefore set
forth, it is to be understood that the typical composition and operating
conditions of the various conductive metal baths will vary as to pH,
temperature, voltage, as well as cathode and anode current densities,
these variables being well known in the plating art. It is to be
understood that the aforementioned metals are only representative of the
class of metals which may be utilized for the conductive metal portion of
the metal-clad laminate and that the present invention is not ncessarily
limited thereto. While, as hereinbefore set forth, a specific example of a
copper electroplating system has been set forth, it is to be understood
that each plating operation must be operated and maintained in order to
obtain the desired relatively thin metal coating which constitutes the
metal portion of the metal-clad laminate, the specific characteristics of
each metal coating being independent of a different metal coating.
It is also contemplated within the scope of this invention that the metal
of the type hereinbefore set forth may also be deposited on the substrate
in an electroless manner. This type of plating may be accomplished by
immersing the substrate which has been pretreated and which may, in this
instance, comprise a non-metallic substrate in a bath comprising stannous
chloride. The substrate is then removed, washed with water and dipped in a
solution of slightly acidic palladium chloride. Following this, the
substrate is then dipped in an electroless bath such as, for example,
copper sulfate-formaldehyde which may contain other additives. If so
desired, the substrate may be first dipped in a bath comprising a mixture
of stannous chloride and palladium chloride, there being a colloidal
suspension of the palladium metal in the bath followed by washing and
immersion in a bath of the metal which is to be plated on the substrate.
When the metal coating on the treated substrate has reached the desired
thickness, that is, being from about 1 to about 20 microns in thickness,
the plating step is terminated and the upper surface of the metal coating
is then treated in order to provide a surface which will possess increased
adhesive properties thereof. The first step of treating the upper surface
of the conductive metal foil comprises subjecting said metal to a high
current density with no agitation or stirring of the plating solution This
high current density comprises increasing the amperage of the bath from
about 18 amps per square foot up to about 105 or more amps per square
foot. The result of this increased or high current density in the absence
of agitation or stirring will result in a normally undesirable condition
in which the upper surface is slightly roughened, said roughening
resulting in a greater surface area for bonding the exposed surface of the
metal to the laminate in a subsequent step. Upon completion of the step of
subjecting the metal coating to a high current density treatment, the
exposed roughened surface of the metal is subjected to the action of an
oxidizing agent such as heat, oxygen, hydrogen peroxide, etc., this step
resulting in the preparation of surface which is capable of interacting
with a bonding agent such as those containing an aliphatic amine group of
a type hereinafter set forth in greater detail which is capable of being
incorporated into the curing laminate.
Following the oxidation of the upper surface of the metal coating, the
surface is then treated with a bonding agent. Although it is contemplated
within the scope of this invention that any type of bonding agent known in
the art may be used which acts as an interface between the surface of the
metal and the laminate by forming a resin bond, the preferred bonding
agent comprises a silane, a particularly applicable silane being a
gamma-aminopropyltriethoxysilane. The thus treated metal coating
containing the bonding agent thereon in a relatively thin film is then
bonded to a laminate of a type hereinafter set forth in greater detail by
conventional bonding methods such as heat and pressure, the bonding step
being performed at an elevated temperature in the range of from about
250.degree. to about 500.degree. F. or more and at a pressure ranging from
about 200 to about 1000 pounds per square inch or more. The time during
which the metal coating is being bonded to the laminate may vary over a
relatively wide range of from about 0.5 up to about 2 hours or more, the
bonding time being dependent upon the other parameters of the step such as
temperature, pressure and resin composition. The treatment of the upper
surface of the metal utilizing a high current density, an oxidation,
followed by a thin film of bonding agent, the bonding agent being
preferably present in a thickness of 1 molecule, differs from the prior
art which teaches that relatively thick coats of bonding agents are
utilized to improve the adhesion. Furthermore, the oxidation of the
surface also differs from the prior art which teaches deoxygenation of the
surface followed by polymerization of the bonding agent such as the silane
on the surface of the metal in a thick film.
Examples of laminates which may be used as the base for the conductive
metal on one or both sides thereof preferably consist of thermosetting
resins. The thermosetting resins are preferably impregnated on a base
material which may, in the preferred embodiment of the present invention
include paper which imparts a good mechanical strength combined with low
cost, glass fiber which is used in either low pressure or high pressure
laminates, especially where electrical properties are important, as well
as low moisture absorption, high tensile, flexural and comprehensive
strengths are required, fabrics, lignin, asbestos, or synthetic fibers
such as Rayon, Nylon, etc. The aforementioned base material is impregnated
with the thermosetting resins such as phenolic resins, melamine resins,
epoxy resins, silicones, polyimides, acrylic resins, polyesters, etc. It
is to be understood that the aforementioned base materials and
thermosetting resins are only representative of the class of laminates
which may be used, and are given only for purposes of illustration rather
than for restricting the present invention.
The preparation of the metal-clad laminate in the final step is
accomplished by heat pressing the metal coated substrate, the upper
surface of which having been treated in a manner hereinbefore set forth,
with the laminate at a temperature and pressure within the range
hereinbefore set forth. Upon completion of the desired residence time at
the temperature and pressure previously selected, the metal-clad laminate
in which the metal is on one or both sides thereof is removed from the
press and the substrate is thereafter removed from the metal, the removal
of the substrate being facilitated by the presence of the release agent on
the surface of the substrate. The substrate which still contains the
release agent in a thin film thereon is then ready for repeated plating
and pressing cycles, it being not necessary to refinish the substrate
after every cycle.
As will hereinafter be shown in the following examples by utilizing the
process of the present invention a metal-clad laminate will be formed
which will possess many desirable physical characteristics such as a
relatively thin metal coating of from about 1 to about 20 microns in
thickness, said coating possessing peel strengths sufficient to meet the
requirements placed upon the completed printed circuits, a feat not easily
achieved without the use of other processes currently utilized on
commercial foils used in laminate cladding. These examples will show the
effect which is obtained by utilizing the steps of preparing the
conductive metal subsequent to plating on a substrate and prior to being
bonded to the desired laminate, namely, subjecting the conductive metal
coating to a high current density without agitation followed by oxidation
and treatment with a bonding agent.
The following examples are given to illustrate the metal-clad laminates
which may be prepared according to the invention and the process for
preparing the same. However, these examples are given merely for purposes
of illustration and are not intended to unduly limit the same.
EXAMPLE I
A stainless steel caul plate which was normally used in the forming of high
pressure laminates was treated with a silane known in the trade as A-151
sold by Union Carbide Company. The treatment was effected by applying the
silane as a water solution which had been buffered to a pH of about 2
followed by washing the surface to effect the removal of excess silane
therefrom. Following this, the steel plate was copper plated by immersion
in a copper sulfate-sulfuric acid bath, the bath containing 1980 grams of
copper sulfate, 900 grams of sulfuric acid and 12 liters of water. The
plating was effected at room temperature and a current of 18 amps per
square foot, the bath being stirred by nitrogen bubbling while the anodes
were separated by filter paper supported by Plexiglass plates which were
drilled so as to be provided with 1 inch diameter holes. When the desired
thickness of copper had been plated on the stainless steel plate, stirring
was discontinued by halting the nitrogen input and increasing the current
density by raising the charge to 5 volts for a period of 15 seconds.
Further treatment of the copper plate comprised placing the copper plated
steel caul plate in an oven at a temperature of from 200.degree. to
300.degree. F. for a period ranging from about several minutes to several
hours, ideally until a noticeable, but firmly adherent, oxide coating was
formed on the exposed surface of the metal film. An alternative method of
oxidizing the exposed surface of the copper was to subject the aforesaid
copper plated steel plate to the action of hydrogen peroxide, said
hydrogen peroxide being added in a strength of 1% to 30% concentration of
hydrogen peroxide for a period of time such that a noticeable oxide film
was observed on the metal surface, the duration of the hydrogen peroxide
treatment being dependent upon the strength of the solution.
The final step in treating the exposed surface of the copper was to add a
bonding agent, said bonding agent comprising a silane known as A-1100 and
sold by Union Carbide Company. This silane comprised, as hereinbefore set
forth, a gamma-aminopropyltriethoxysilane. The laminate was then prepared
for pressing by laying up suitable prepregs with the metal-clad caul plate
and pressing the composite for a period of 1 hour at a pressure of 1000
pounds per square inch while maintaining the temperature of the pressure
apparatus in a range of from about 340.degree. to 350.degree. F.
The metal-clad laminate was then separated from the caul plate by removal
of the flow (the excess resin which was squeezed from the prepregs by the
pressure applied during curing) and the copper which coated the edges of
the caul plate was removed by grinding or cutting the material and
removing the caul plate from the laminate. The thickness of the copper
coating on the laminate was measured with a micrometer and was found to be
in a range of from about 5 to about 12 microns. Other samples were then
prepared in a similar manner with the exception that the thickness of the
copper coating was allowed to reach a range of from about 15 to about 45
microns for use in a peel value test.
In addition other metal-clad laminates were also prepared for use in a peel
value test, however, certain of the steps of the present process were
omitted during the preparation thereof. For example, the high current
density step was omitted with some laminates, in other laminates the
oxidation step was omitted while in a third series of laminates the
treatment with the bonding agent was omitted. The results of these tests
are set forth in Table I below.
TABLE I
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Lami- High Current Oxidation Bonding
Peel Value
nate Density Treatment
Treatment Agent lbs/inch
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A 5 volts for 15 sec.
-- -- 4.82
B -- Yes -- 5.35
C -- -- Silane 1.50
D -- Yes Silane 3.69
E -- -- Silane 1.90
F -- Yes Silane 2.85
G -- -- Silane 4.49
H 5 volts for 15 sec.
Yes Silane 8.62
I 5 volts for 15 sec.
Yes Silane 8.00
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From the above table which discloses the peel value of a copper strip which
had an average width normallized to 1 inch and an average thickness of
about 1.5 mls, it is evident that the copper which was subjected to the
three individual steps of the present process, namely, high current
density, oxidation and the addition of a bonding agent possessed peel
values which were greatly in excess over the peel value of those strips
which were not subjected to all of the various steps of the process. When
more severe treatments such as subjecting the coated substrate to a high
current density of 105 amps per square foot for a longer period of time,
it is possible to attain a peel value of 14 pounds per inch.
EXAMPLE II
In like manner a thin coating of gold is plated on a steel caul plate by
treating said caul plate with a release agent comprising a silane
comprising a vinyl silane. The plating is accomplished by placing said
caul plate in an acid gold bath at a pH in the range of from about 3.5 to
4.5 while maintaining the temperature of the bath in a range of from about
80.degree. to 120.degree. F. utilizing a voltage of about 2 volts and a
cathode current density of about 10 amps per square foot. Upon reaching
the desired thickness of gold on the steel caul plate, that is, a
thickness of from 1 to about 20 microns, agitation is discontinued and the
voltage is raised to provide a high current density treatment. Following
this the exposed surface of the gold coating is subjected to an oxidation
treatment by heating in an oven or under a flame at a temperature of about
900.degree. F. Alternatively the oxidation may be accomplished by an
anodic oxidation in a suitable electrolyte. Following this a bonding agent
comprising a silane similar in nature to that set forth in Example I above
is placed on the surface of the gold and the thus treated gold coating is
pressed to a laminate comprising a glass epoxy resin which is in a B stage
prepreg condition. The composite is subjected to a pressure of 1000 pounds
per square inch at a temperature of about 350.degree. F. until the desired
stage of curing is realized. Following this the laminate is then separated
from the caul plate, etched and recovered. Other gold coated laminates are
prepared according to the above process with the exception of a thicker
coat of gold and subjected to a peel value test. It will be determined
that the peel value of the gold coating which has been prepared according
to the above process will be greater than that of gold coating which has
been pressed on a glass epoxy resin in a process whereby one or more of
the above steps in the preparation have been omitted.
EXAMPLE III
In like manner a nickel coating having a thickness of from 1 to 20 microns
is plated on a steel caul plate which has been treated with a release
agent comprising a vinyl silane. Upon reaching the desired thickness
plating is discontinued, the agitation of the plating bath is also
discontinued and the voltage is increased so that the exposed surface of
the nickel is treated with a high current density. Following this, the
nickel plated caul plate is removed from the bath, washed and treated with
hydrogen peroxide in the oxidation or second step of the process.
Thereafter a bonding agent is coated on the exposed surface of the nickel
and the nickel coating is laminated to a glass polyimide resin, the
pressing of the nickel coating to the laminated being accomplished in a
manner similar to that set forth in Example I above. When the pressing is
completed, the caul plate is removed from the metal-clad laminate. Other
laminates are prepared according to the above process possessing a
slightly thicker nickel coating, these laminates then being etched and the
nickel coated on the laminate is subjected to a peel value test. It will
be determined that the peel value of the nickel coating on the laminate
which was prepared according to the above process is greater than the peel
value which is possessed by a nickel coating pressed on a laminate which
has not undergone the treatment comprising the steps of high current
density, oxidation, and the placing of a bonding agent thereon.
EXAMPLE IV
In this example a tin coating is plated on a steel caul plate in a manner
similar to that set forth in Example I above. The caul plate comprises a
stainless steel plate which has been pretreated with a release agent such
as a vinyl silane. The release agent will act as a point of fracture. By
utilizing the vinyl silane, the formation of the release agent on both
surfaces is prevented and the fracture which will occur later in the
process between the conductive metal and | | |
