A multilayer laminate having at least one layer of a no longer formable fully aromatic polyimide and at least one layer of substrate material, the layer of no longer formable polyimide adhering directly on one side to the layer of substrate material with a peel strength of at least 4.0 N/cm. The layer of no longer also formable polyimide is also insoluble in phenolic solvents, has a tensile strength of from 100 to 150 N/mm.sup.2, a breaking elongation of from 15 to 100%, a dielectric dissipation factor of from 1.5.times.10.sup.-3 to 5.times.10.sup.-3 at 1 kHz. Additionally, a layer of heat-sealable high-temperature adhesive selected from the class of polyacrylates, polysulfone resins, epoxy resins, fluoropolymer resins, silicone resins or butyl rubbers is joined to that side of the polyimide layer which is remote from the substrate material.

BACKGROUND OF THE INVENTION 
The invention relates to flexible multilayer laminates comprising at least 
one layer of a no longer formable, fully aromatic polyimide and at least 
one layer of a substrate material. The invention also relates to a process 
for producing these laminates. 
Laminates comprising one or more layers of polyimide and one or more layers 
of substrate material may be used for a variety of applications, such as, 
for example, reinforcing materials. In addition, laminates of the type in 
question, in the form of polyimide-coated metal foils, are used for 
printed electrical circuits. In that case, use is made of the flexibility 
and outstanding mechanical, thermal and electrical properties of the 
polyimides. This is because the laminates are frequently exposed to high 
temperatures during further processing, for example during soldering or 
drilling. The laminates also have to satisfy stringent requirements 
regarding their electrical and mechanical properties. 
Laminates comprising only one substrate layer of metal or a metal alloy and 
a layer of polyimide, so-called single clads, may be used for printed 
electrical circuits. The same applies to multilayer laminates, so-called 
double clads or multiclads, which comprise several metal layers and/or 
several polyimide layers. In certain cases, however, multilayer laminates 
are superior to single clads. In the case of printed circuits for example, 
it is often necessary to make printed conductor lines which intersect one 
another. The high packing densities often required, i.e. overall layer 
thicknesses, cannot be obtained where single clads are used, but only 
where double clads or multiclads are used. The present invention is 
concerned with multilayer laminates which are eminently suitable for the 
production of double clads and multiclads. In the context of the 
invention, double clads are understood to be laminates comprising two 
(metallic) substrate layers, while multiclads are understood to be 
laminates comprising more than two (metallic) substrate layers. 
Laminates containing polyimides and substrate materials are known. In this 
case, the polyimide layers are often bonded to the substrate materials by 
a conventional adhesive. For example, U.S. Pat. No. 3,900,662 describes 
the bonding of polyimide to metal by an acrylate-based adhesive. Use is 
also made of this possibility in the laminates described in U.S. Pat. No. 
3,822,175. 
If double clads or multiclads are produced in accordance with the 
above-mentioned patent specifications in which a layer of an acrylate 
adhesive is situated between each metal layer and each polyimide layer, 
the products obtained have a number of disadvantages, namely: 
(a) The overall layer thickness of the clads is considerable on account of 
the necessary adhesive layers, whereas low overall layer thicknesses are 
required for multiclads. 
(b) The metal (substrate material) layer is directly joined to acrylate 
which is inferior to the polyimide in its dimensional stability under 
heat. Thus, undesirable decomposition of the acrylate often occurs during 
preparation of the clads for printed circuits. This decomposition occurs 
with the acrylate layer at the high temperatures which the metal layer 
encounters, for example during soldering and drilling. Since the acrylate 
is directly joined to the metal layer, it is not adequately protected 
against those temperatures. 
(c) Since the acrylate has poorer electrical insulating properties than the 
polyimide, the adhesive layer(s) between the polyimide and substrate 
material (metal) adversely affect(s) the dielectric properties. 
It has been found that, where conventional adhesives, such as those based 
on acrylate, epoxide, polyamide, phenolic resin, etc. are used, the 
laminates in which the polyimide is bonded to the metal by an intermediate 
layer of one of these adhesives do not show entirely satisfactory 
properties which meet the stringent demands often imposed. 
On account of the disadvantages of laminates comprising layers of 
conventional adhesives between polyimide and metal, multilayer laminates 
have been proposed wherein the polyimide is bonded directly to metal, i.e. 
without a layer of adhesive. For example, DE-OS No. 32 15 944 describes 
laminates in which two metal layers are bonded by an intermediate layer of 
polyimide. The polyimide used in this case predominantly consists of 
diphenyl tetracarboxylic acid and may be bonded to a metal foil by 
applying high temperature and pressure. In other words, the polyimide is 
formable. It has now been discovered that formable polyimides or 
polyimides which are soluble in phenolic solvents are inferior in their 
thermal stability to fully aromatic, no longer formable polyimides which 
are insoluble in phenolic solvents. In double clads which only contain 
these formable polyimides as an insulating layer(s) the polyimide may flow 
away in the process of laminating, resulting in an undesirable direct 
contact between the metal layers. Accordingly, clads containing only 
formable polyimides are inferior to products containing no longer formable 
polyimides as an insulating layer(s). 
Because of the disadvantages associated with clads containing a layer of 
adhesive between metal and polyimide, single clads of a substrate material 
to which a no longer formable, fully aromatic polyimide which is insoluble 
in phenolic solvents is directly bonded have already been proposed. These 
single clads show excellent mechanical, thermal and electrical properties. 
Starting out from these single clads, it would be desirable to produce 
multilayer laminates which like their single clad counterparts consist 
only of substrate materials and these no longer formable, fully aromatic 
polyimides and which would thus show the same mechanical, thermal and 
electrical properties. However, it has been found that two or more single 
clads of this type cannot be directly bonded to one another or one single 
clad directly bonded to a metallic substrate material. i.e. without an 
intermediate layer of adhesive, because it is not possible to apply 
another layer of substrate material or another single clad to the fully 
hardened polyimide layer without a coupling layer sufficient to impart a 
high peel strength, i.e. high adhesion between the polyimide and the 
additional layer. Although application of the other layer of substrate 
material before the polyimide has completely hardened is possible in 
principle and leads to an increase in peel strength, bubbles can be formed 
in the polyimide layer because volatile constituents such as, for example, 
water have to escape during its hardening and the release of these 
volatile constituents can be impeded by the additional layer of substrate 
material. 
SUMMARY OF THE INVENTION 
It has now surprisingly been found that the disadvantages attending known 
multilayer laminates can be overcome by laminates comprising at least one 
layer of a no longer formable fully aromatic polyimide and at least one 
layer of substrate material, wherein the layer of no longer formable 
polyimide adheres directly on one side to the layer of substrate material 
with a peel strength of at least 4.0 N/cm, is insoluble in phenolic 
solvents, has a tensile strength of from 100 to 150 N/mm.sup.2, a breaking 
elongation of from 15 to 100%, a dielectric dissipation factor of from 
1.5.times.10.sup.-3 to 5.times..sup.10-3 at 1 kHz. Furthermore, a layer of 
heat-sealable high-temperature adhesive selected from the class of 
polyacrylates, polysulfone resins, epoxy resins, fluoropolymer resins, 
silicone resins or butyl rubbers is joined to that side of the polyimide 
layer which is remote from the substrate material. 
In these laminates, the high-temperature adhesive, which is inferior in its 
thermal and electrical (insulating) properties to the no longer formable 
polyimide, is not joined to the substrate material (metal). On the one 
hand, it is protected by the polyimide layer against the high temperatures 
which can arise during further processing of the metal surface. On the 
other hand, it was surprisingly found that the dielectric properties of 
the laminates can also be improved if there is a layer of polyimide 
between the metal layer and the adhesive layer. For example, when 
polyacrylates for example are used for the adhesive layer, the dielectric 
(insulating) properties of the laminates according to the invention are 
distinctly better than might be estimated from the sum of the dielectric 
properties of the individual products. This is shown very clearly, for 
example, in a preferred embodiment of the invention in which two layers of 
polyimide (each directly joined on one side to substrate material) are 
bonded to one another by a layer of adhesive. These double clads show 
dielectric properties which come very close to those of laminates (for 
example single clads) containing only substrate material and polyimide. 
Surprisingly, the poorer dielectric properties of polyacrylates have very 
little effect on the product of the invention, in complete contrast to 
products in which polyacrylate is directly joined to substrate material 
(metal). It is assumed that the "embedding" of the acrylate layer between 
two polyimide layers is responsible for this. 
In addition, the number of necessary adhesive layers in the laminates 
according to the invention and in the double clads or multiclads 
containing the basic element of the laminates according to the invention 
is reduced by comparison with products containing a layer of adhesive 
between each polyimide layer and each layer of substrate material. This 
increases the relative amount of polyimide present in the insulating 
layers and hence the thermal stability, improves the dielectric properties 
and provides for lower overall layer thickness of the double clads and 
multiclads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Accordingly, the laminates according to the invention comprise at least one 
layer of no longer formable polyimide which, on one of its sides, adheres 
directly, i.e. without an intermediate layer, to a layer of substrate 
material. The basic element of no longer formable polyimide and substrate 
material has a peel strength of at least 4.0 N/cm, as measured by the 
method described in IPC TM 650, 2.4.9. The other side or surface of the no 
longer formable polyimide is covered by a layer of a heat-sealable 
high-temperature adhesive. Accordingly, the laminates according to the 
invention contain at least one element which forms the basis of the 
laminates and which has the following construction: substrate material/no 
longer formable polyimide/heat-sealable high-temperature adhesive. 
The layer of no longer formable polyimide has a tensile strength of from 
100 to 150 N/mm.sup.2, as measured in accordance with ASTM D 882, a 
breaking elongation of from 15 to 100%, as measured in accordance with 
ASTM D 882 and a dielectric dissipation factor of from 1.5.times.10.sup.-3 
to 5.times.10.sup.-3 at 1 kHz, as measured in accordance with ASTM D 150. 
In the context of the invention, "fully aromatic, no longer formable 
polyimides and insoluble in phenolic solvents" are understood to be 
polyimides which are obtained from aromatic tetracarboxylic acids or their 
dianhydrides and primary aromatic diamines, each of the carboxyl groups 
and the primary amino groups being directly attached to an aromatic ring. 
In addition, the polyimides cannot be melted without decomposition and are 
insoluble in conventional solvents, including phenolic solvents, such as 
phenol, cresols and halogenated phenols. Accordingly, these polyimides 
cannot be formed again by melting or by dissolving. 
Double clads and multiclads may be produced advantageously from the 
laminates according to the invention, comprising the basic element of a 
substrate material/no longer formable polyimide/heat-sealable 
high-temperature adhesive. Thus, the following products inter alia may be 
obtained with excellent electrical, mechanical and thermal properties from 
these laminates all of which contain the above-described basic element: 
(a) double clads comprising substrate material/no longer formable 
polyimide/heat-sealable high-temperature adhesive/no longer formable 
polyimide/substrate material. 
(b) double clads comprising substrate material/no longer formable 
polyimide/heat-sealable high-temperature adhesive/heat-sealable 
high-temperature adhesive/no longer formable polyimide/substrate material. 
(c) multiclads in which the outer surface of one or both layer(s) of 
substrate material of the laminates (a) or (b) is directly joined to no 
longer formable polyimide. The outer surface of the substrate layer of the 
basic element (consisting of substrate material/no longer formable 
polyimide/heat-sealable high-temperature adhesive) may also be directly 
joined to a layer of no longer formable polyimide. Accordingly, the 
products contain at least one layer of substrate material which is joined 
on either side to a layer of no longer formable polyimide. 
The laminates (a) to (c) are preferred embodiments of the laminates 
according to the invention. Their production is described hereinafter. 
Products (a) and (b) differ from one another in the fact that, in one case, 
there is only one layer of heat-sealable high-temperature adhesive and, in 
the other case, two layers. These two layers of heat-sealable high 
temperature adhesive may merge with one another to a greater or lesser 
extent. In products (a) and (b), both layers of no longer formable 
polyimide and both layers of substrate material may each have the same or 
different chemical structure and/or layer thickness. In the case of 
product (b), this also applies to the two layers of heat-sealable 
high-temperature adhesive. 
Accordingly, in products (a) and (b), two layers of no longer formable 
polyimides (both directly joined on one side to substrate material) are 
joined to one another by heat-sealable high-temperature adhesive on that 
side remote from the substrate material. They all comprise the basic 
element of the laminates according to the invention. The assembly of two 
such (identical or different elements at the layer of heat-sealable 
high-temperature adhesive gives the products mentioned in (b). The 
products mentioned in (a) are formed for example when two of the basic 
elements are joined together in such a way that the two originally 
separated layers of heat-sealable high-temperature adhesive merge into one 
another, forming a single, defined layer in the end product. 
Since, the laminates according to the present invention may be used for 
printed circuits, metals or alloys are used as the substrate materials and 
since high temperatures are applied during further processing of the 
laminates, it is thus advantageous that the basic element of the laminates 
according to the invention and of the double clads and multiclads 
containing same, have the heat-sealable high-temperature adhesive joined 
to the no longer formable polyimide and not to the metal. Accordingly, the 
heat-sealable high-temperature adhesive which is less stable under heat is 
protected by the more stable polyimide because the high temperatures are 
generated at the metal layer, such as, for example, during soldering. By 
virtue of the fact that layers of substrate material, such as, for example 
metal, directly joined on one or both sides to no longer formable 
polyimides are present in the basic element of the laminates according to 
the invention, the number of adhesive layers required is reduced to a 
minimum. This is of considerable significance because the thermal 
stability of the products can be increased and their overall layer 
thickness reduced in this way. 
As mentioned above it is advantageous for one or both layer(s) of substrate 
material to be joined on either side to a layer of fully aromatic, no 
longer formable polyimide. In this way, it is possible to obtain 
multiclads which provide for a high packing density, even in complex 
printed circuit boards. In this case, other layers, including layers of 
materials other than polyimides, may be present on one or both outer 
surface(s), which now consist(s) of no longer formable polyimides, 
providing this is compatible with the application envisaged. 
Preferable no longer formable fully aromatic polyimides have the following 
recurring structure: 
##STR1## 
wherein R is a tetrafunctional aromatic group and R' is a difunctional 
aromatic group. More specifically, R represents: 
##STR2## 
and R' represents 
##STR3## 
These polyimides may be obtained by reaction of tetracarboxylic acids or 
their mono- or di- anhydrides with diamines. Examples of suitable 
dianhydrides are pyromellitic acid dianhydride, 2,3,6,7-naphthalene 
tetra-carboxylic acid dianhydride, 3,4,3',4'-diphenyl sulfone 
tetracarboxylic acid dianhydride, perylene-3,4,9,10-tetra-carboxylic acid 
dianhydride, 3,4,3',4'-diphenyl ether tetracarboxylic acid dianhydride. 
Examples of diamines which may be reacted with the tetracarboxylic acids or 
their derivatives to give suitable, no longer formable, fully aromatic 
polyimides are 4,4'-diamino-diphenyl ether, 
5-amino-2-(p-aminophenyl)-benzothiazole, 
4-amino-2-(p-aminophenyl)-benzothiazole, 
5-amino-2-(m-amino-phenyl)-benzothiazole, 
5-amino-2-(p-aminophenyl)-benzoxazole, 
4-amino-2-(m-aminophenyl)-benzothiazole, p- and m-phenylene diamine, 
4,4'-diaminodiphenyl, bis-(4-aminophenyl)-methane, 
4-amino-2-(p-aminophenyl)-benzoxazole, 
4-amino-2-(m-aminophenyl)-benzoxazole, 
5-amino-2-(m-aminophenyl)-benzoxazole, 2,5-diaminobenzoxazole, 
2,5-diaminobenzothiazole. 
The polyimide obtainable by reaction of pyromellitic acid dianhydride 
(PMDA) with 4,4'-diaminodiphenyl ether (DADE) has proven to be 
particularly suitable. 
The laminates according to the invention contain layer(s) of heat-sealable 
high-temperature adhesive. This adhesive is selected from the class of 
polyacrylates, polysulfone resins, epoxy resins, fluoropolymer resins, 
silicone resins or butyl rubbers. 
According to the invention, heat-sealable high-temperature adhesives are 
understood to be products of the above-mentioned types which are formable 
at a temperature in the range from 140.degree. to 500.degree. C., 
optionally under pressure, and at the same time have a bonding effect. In 
addition, they should not melt at temperatures below 200.degree. C. 
However, the products used as high-temperature adhesives do not 
necessarily have to show a defined melting point or melting range. It is 
sufficient if they can be formed without melting at a temperature in the 
above-mentioned range. As already mentioned, the high-temperature 
adhesives must have a bonding effect. This means that a laminate of 
polyimide and adhesive produced as described below must show a peel 
strength, as measured by the method described in IPC TM 650, 2.4.9, of at 
least 2.0 N/cm. The laminate used for this test is produced as follows: 
A single clad of metal and polyimide is produced by one of the methods 
described in Examples 1 to 3. The adhesive to be tested is applied to the 
polyimide layer of this single clad in the form of a solution or film; if 
the adhesive is applied as a solution, the solvent is removed by heating. 
The adhesive is then heat-sealed at a temperature of from 140.degree. to 
500.degree. C., optionally under pressure. The suitable temperature and 
pressure conditions depend upon the nature of the adhesive and may be 
determined by simple tests. After removal of the metal layer, for example 
by etching, the peel strength may be determined. Products which do not 
have a peel strength of at least 2.0 N/cm over the above-mentioned 
temperature range, even where pressure is applied, are unsuitable as 
adhesives for the laminates according to the invention. 
The requirement that the adhesives should be heat-sealable, i.e. formable, 
at a temperature of from 140.degree. to 500.degree. C. does not mean that 
all adhesives which satisfy this requirement are suitable for every 
application of the laminates according to the invention. On the contrary, 
adhesives which can only be formed at 250.degree. C. or higher may have to 
be used for a specific application. 
The basic element of polyimide and adhesive in the laminates according to 
the invention advantageously has a peel strength of more than 4.0 N/cm. 
The thickness of the layer(s) of no longer formable polyimide, which 
perform(s) an insulating function, for example where the laminates are 
used for printed circuits, may be varied within wide limits. This is 
because the preferred processes for producing the laminates according to 
the invention, which are described hereinafter also make it possible to 
produce laminates comprising relatively thick layers of these polyimides 
which satisfy the stringent demands imposed on these laminates. The 
thickness of each layer of no longer formable polyimide is preferably 
between 1 .mu.m and 1 mm. Where the laminates according to the invention 
are used for standard printed circuits in the electronics field, layer 
thicknesses for the no longer formable polyimides of from 10 .mu.m to 1 mm 
and preferably from 50 .mu.m to 250 .mu.m have proved to be particularly 
suitable. 
In another preferred embodiment, all the layers of no longer formable 
polyimide (providing there is more than one layer) have the same 
thickness. This is the case inter alia when the laminates in question are 
multilayer laminates produced from identical single clads of the same 
quality. 
In one preferred embodiment of the laminates, a foil of a metal or a metal 
alloy and/or a polymer film and/or a sheet-form fibrous material is/are 
used as the substrate material. 
Suitable polymer films are, for example, films of aromatic polyamides or 
polyimides. Suitable fibers for the sheetform material are metal fibers, 
synthetic fibers, for example of aromatic polyamides, and mineral fibers, 
such as glass fibers quartz fibers or asbestos fibers or carbon fibers. 
Particularly preferred substrate materials, especially where the laminates 
are used for printed circuit boards are foils of copper, nickel, 
aluminium, or foils of an alloy containing one or more of these metals as 
an essential constituent, for example a chrome/nickel alloy. Foils of 
steel have also proven to be very suitable. In one special embodiment, the 
substrate material is a foil of rolled, tempered copper or a rolled, 
tempered copper alloy. In another preferred embodiment of the process 
according to the invention, a foil of amorphous metal is used as the 
substrate material. Special properties of the laminates may be obtained in 
this way, being produced by the amorphous metals. These amorphous metals 
do not have the crystal structures typical of metals. Because of this, 
they are also known as "metallic glasses". They may be produced by 
quenching metal melts or melts of alloys. Amorphous metals suitable as 
substrate material for the laminates according to the invention are, for 
example, amorphous alloys containing iron. Other suitable amorphous metals 
are described in the Article in "Spektrum der Wissenschaft", June 1980, 
page 46-61. 
The layer thickness of the foil(s) used as substrate material is preferably 
between 5 and 110 .mu.m in the case of metal or alloy foils. Layer 
thicknesses of between 10 to 50 .mu.m have been found to be still more 
advantageous. 
In one advantageous embodiment of th elaminates according to the invention, 
the layer(s) of heat-sealable high-temperature adhesive contain(s) a 
fibrous material. This material performs a reinforcing function. Suitable 
fibrous materials are, in particular, temperature-stable glass fibers 
(sodium-aluminium silicate fibers), aramide fibers (fibers of aromatic 
polyamides), carbon fibers and/or silica (SiO.sub.2.nH.sub.2 O) fibers. 
The fibers are preferably present as fabrics woven from end-less 
filaments. However, the fibrs may also be used in the form of nonwoven 
structures or in the form of loose staple fibers. 
It is of course only possible or sensible to use reinforcing fibers above a 
minimum ratio of polyimide layer thickness to fiber or fabric diameter. 
Furthermore, it has been found that laminates the polyimide and/or 
adhesive layer of which contain(s) particles of polytetrafluorethylene 
(PTFE) are still more suitable for some uses, the PTFE particles acting as 
a reinforcing medium and/or improving the electrical, i.e., insulating 
properties. 
The laminates according to the invention may be produced by the process 
comprising: 
(a) applying a coating of polyamide acid solution to a substrate layer 
without a coupling layer, said polyamide acid solution being formed by 
reacting an aromatic tetracarboxylic acid or its dianhydride and a primary 
aromatic diamine, in a molar ratio of from 0.95:1 to 1.05:1 in a polar 
solvent, to form a solution of a polyamide acid corresponding to the 
following formula: 
##STR4## 
wherein R is an aromatic tetrafunctional group, R' is a difunctional 
aromatic group and the value of n is sufficient to obtain a polyamide acid 
having an .eta..sub.red -value of at least 0.5; 
(b) heating the coated substrate layer; 
(c) removing said solvent in situ from the polyamide acid solution coated 
on said substrate layer in a first stage to form a film, said first stage 
being at a temperature of from 100.RTM. to 200.degree. C., wherein 
virtually the entire quantity of solvent is removed; 
(d) hardening the film is situ in a second stage, said second stage being 
at a temperature above 200.degree. C. to give a no longer formable 
polyimide, said polyimide layer being insoluble in phenolic solvents, and 
wherein at least 95% of said polyamide acid is reacted to polyimide; and 
(e) applying a heat-sealable high-temperature adhesive selected from the 
group consisting of polyacrylates, polysulfone resins, epoxy resins, 
fluoropolymer resins, silicone resins and butyl rubbers to said polyimide 
layer to produce a basic element, said heat-sealable adhesive applied to 
that side of said polyimide layer which is remote from the substrate 
material. 
Accordingly, the first step of the process comprises producing single clads 
from a substrate material and a no longer formable, fully aromatic 
polyimide directly joined to the substrate material. 
This first step of the process will now be described. 
The polyamide acid is produced by reaction of an aromatic tetracarboxylic 
acid, preferably pyromellitic acid, or preferably its dianhydride, 
pyromellitic acid dianhydride (PMDA), with a primary aromatic diamine, 
preferably 4,4'-diaminodiphenyl ether (DADE), in a solvent, for example 
dimethyl acetamide (DMAc). The single clad is obtained by applying a film 
of the polyamide acid solution to a substrate material, such as a metal 
foil or a polymer material or a sheet-form fibrous material, and hardening 
the film in situ by heat treatment in at least two stages, so that a 
single clad is obtained of which the polyimide layer adheres firmly to the 
above-mentioned substrate material, without requiring an intermediate 
layer of adhesive to join the polyimide film to the substrate. 
The single clad may be a sheet-form structure i.e. a flexible polyimide 
layer which adheres to a foil of copper or other metal, for example 
aluminium, nickel, or steel, or an alloy containing one or more of these 
metals as an essential constituent or to a foil of amorphous metal. In all 
cases, the polyimide layer adheres firmly to the substrate and has peel 
strength of 4.0 N/cm and higher. 
Materials of metals or synthetic polymers for example may be used as the 
substrate. The metals do not have to be used as elements in pure form, 
i.e. it is also possible in particular to use substrates of metal alloys, 
such as alloys containing nickel, chromium or iron or nickel and copper or 
of amorphous alloys containing iron. Particularly suitable substrate 
materials are foils of rolled, tempered copper or of a rolled, tempered 
copper alloy. In many cases, it has proven to be advantageous to pretreat 
the substrate material before coating. This pretreatment may consist of a 
chemical treatment, for example with an acidic salt solution, or of a 
mechanical roughening treatment. It has been found that this pretreatment 
enables the adhesion of the polyimide layer and, hence, the peel strength 
to be further increased. Apart form roughening the surface, the chemical 
pretreatment may lead to the formation of metal oxide groups on the 
surface of the substrate material to be coated, thus enabling the 
increased adhesion of a metallic substrate material to the polyimide 
layer. It has proven to be favorable to carry out the pretreatment in such 
a way that a center-line average height (R.sub.a) of at least 0.2 .mu.m is 
obtained. 
In one embodiment of the invention, the single clads are obtained by 
reacting a primary aromatic diamine with an aromatic tetracarboxylic acid 
or its dianhydride in an extruder under conditions which lead to the 
formation of a solution of polyamide acid in a solvent. A layer of 
polyamide acid solution may then be extruded directly onto the substrate, 
after which time most of the solvent may be removed in situ from the 
polyamide acid layer in a first heating zone and the polyamide acid layer 
subsequently hardened in situ by another heat treatment in at least one 
second heating zone at a higher temperature leading to almost complete 
imidization. Instead of applying the polyamide acid solution to the 
substrate material by extrusion, it may also be applied by doctoring. The 
subsequent heat treatment, which results in removal of the solvent and in 
formation of the polyimide, is the same as described above. A polyimide 
layer more than 10 .mu.m thick which does not have any interruptions or 
defects due to bubbles produced by the combination of a skin effect and 
overrapid evaporation of the solvent or of the steam formed during 
imidization or hardening and which adheres firmly to the substrate may be 
obtained by a particular sequence of heat treatments. 
The polyamide acid precursors used in accordance with the invention and 
obtained by reacting an aromatic tetracarboxylic acid or its dianhydride 
with a primary aromatic diamine in a polar organic solvent have the 
following structural formula: 
##STR5## 
wherein R is an aromatic tetrafunctional group, R' is a difunctional 
aromatic group and n has a value sufficient for the formation of a 
polyamide acid having a reduced viscosity of 0.5 or higher, as measured on 
a 0.5% solution in dimethyl acetamide containing 0.1 mole/liter of lithium 
bromide. After application to the substrate, the polyamide acid is 
hardened by the described heating process, resulting in the formation of a 
no longer formable polyimide insoluble in phenol or phenolic solvents and 
having the following recurring structure 
##STR6## 
wherein R and R' represent the same groups as previously described. 
Pyromellitic acid dianhydride and 4,4'-diaminodiphenyl ether are preferably 
used as starting materials and dimethyl acetamide as solvent in the 
production of the polyamide acid. 
Other reactants which produce no longer formable polyimides insoluble in 
conventional phenolic solvents, for example phenol or substituted phenols 
(halogenated phenols) may also be extruded by the process according to the 
invention for producing the single clads. 
Although dimethyl acetamide (DMAc) is preferably used as the solvent, it is 
also possible to use other polar organic solvents, for example 
N,N-dimethyl methoxy acetamide, dimethyl formamide (DMF), diethyl 
formamide, N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). 
Other suitable solvents are, for example, N-methyl caprolactam, dimethyl 
sulfone, pyridine, hexamethyl phosphoramide, N-acetyl-2-pyrrolidone, 
tetramethyl urea and tetramethylene sulfone. 
The polyamide acid may be produced by known methods, for example by the 
methods described in U.S. Pat. Nos. 3,179,614 and 3,179,634. 
In the apparatus illustrated in FIG. 1, a dry mixture is prepared for 
example from the dianhydride (pyromellitic acid dianhydride or PMDA) and 
the diamine (4,4'-diaminodiphenyl ether or DADE) in a molar ratio of from 
0.95:1 to 1.05:1. This mixture is delivered to a gravimetric metering unit 
3. The mixture is then introduced at an accurately controlled rate into a 
reaction vessel 4 in the form of an extruder. A polar solvent is added by 
means of a metering pump 5 to the dry mixture accommodated in the extruder 
4. The molecular weight of the polyamide acid is determined by the molar 
ratio of dianhydride to diamine. The optimal molecular weight range of the 
polyamide acid is reached at a molar ratio of from 0.98 to 1.02 and is 
measured as the reduced viscosity (.eta..sub.red) of a 0.5% solution in 
dimethyl acetamide containing 0.1 mole/liter of lithium bromide. The 
reduced viscosity of the polyamide acid is of the order of 0.5 for molar 
ratios of PMDA to DADE of from 0.95 to 1.05 and is in the range from about 
1.0 to 4.0 at the optimal ratio (PMDA:DADE 0.98:1 to 1.02:1). The average 
molecular weight of the polyamide acid formed was 32,000 for a molar ratio 
of 0.95, approximately 200,000 for a molar ratio of 1.0 and approximately 
35,000 for a molar ratio of 1.03 (as determined with a FIKA light 
scattering photometer, model PGD 42,000, at .lambda.=436 nm). 
The temperature in the extruder 4 should be kept at a level below about 
80.degree. C. In practice, however, it may be gradually increased, 
starting from about 20.degree. C., or raised to at most 80.degree. C. in 
zones of increasing temperature. The solvent is added in the first zone of 
the extruder 4. The residence time in the extruder 4 is from 1 to 5 
minutes. At the end of this residence time, the reaction by which the 
polyamide acid is formed is over. The polyamide acid with a reduced 
viscosity of from 0.5 to 4.0 and preferably of more than 1.0 may be 
extruded through a flat die 6 onto a substrate material 7 in the form of a 
foil of copper or another metal or an alloy run off from a roll 8 or in 
the form of a synthetic film or in form of a sheet-form fibrous material. 
The substrate coated with the polyamide acid solution then passes through a 
furnace 10, to which nitrogen is fed by means of a supply pipe 11, for 5 
to 20 minutes or longer for the purpose of condensation to the polyimide. 
The residence time in the furnace depends on the thickness of the film 
because longer times are required for relatively thick films. 
It has proven to be essential to control the temperatures in successive 
zones in the furnace. However, if the temperature is controlled within the 
above-mentioned range, a no longer formable, bubble-free polyimide layer 
showing excellent electrical and mechanical properties and adhering to the 
substrate with a peel strength of more than 4.0 N/cm is formed on the 
substrate 7 in a very short time. Beyond a purely theoretical explanation 
of this result, it may be assumed that it is necessary for the solvent to 
diffuse through the polyamide acid layer and to be released from the 
exposed layer surface so slowly that it does not form any bubbles which 
increase in size and remain trapped in the matrix of the polymer layer. 
Also, a large part of the solvent must be released from the exposed side 
of the polyamide acid layer before imidization is complete. In addition, 
from 80 to 90% of the imidization reaction must be completed at 
temperatures below about 180.degree. C. so that the majority of the water 
formed during the cyclization reaction also diffuses to and is released 
from the surface of the layer. 
To achieve the objective stated above, the following heating zones are 
established in the condensation furnace by means of resistance elements 
12, 13, 14 and 15: 
In the first zone, the temperature is kept at 100.degree. to 150.degree. C. 
by an electrical resistance element 12; in the second zone, the 
temperature is increased to between about 130.degree. C. and about 
200.degree. C., but preferably below 180.degree. C.; in the third zone the 
temperature is increased to between about 200.degree. and 400.degree. C. 
after virtually all the solvent and the majority of the water formed 
during the cyclization reaction have diffused to the surface and have been 
removed. In the fourth zone, the temperature is again increased, 
preferably to between about 300.degree. and 600.degree. C. These heating 
zones are approximately equal in length, so that the residence times in 
the individual zones is approximately the same. However the progress rate 
and hence the throughput may be increased by extending the first and/or 
second zone(s) or by preceding the first zone with an additional heating 
zone kept at a temperature above 50.degree. C., but below the temperature 
of the first zone. In the apparatus shown in FIG. 2, the furnace 10 may be 
provided with a removable cover 16 to provide easy access to the laminated 
element in the furnance. 
In a second process step, a heat-sealable high-temperature adhesive 
selected from the class of acrylates, polysulfone resins, epoxy resins, 
fluoropolymer resins, silicone resins or butyl rubbers is applied to the 
layer of no longer formable polyimide, on the side remote from the 
substrate material. The adhesive may be applied from a solution, in which 
case the solvent is subsequently removed by heating. In a preferred 
embodiment, however, the adhesive is applied in the form of a film. After 
application, the film is either heat-sealed with the single clad or, 
alternatively, two single clads are sealed by the film to form a double 
clad with the following layer sequence: substrate 
material/polyimide/adhesive/polyimide/substrate material. 
The basic elements of the flexible multilayer laminates according to the 
invention may be further processed in various ways. For example: 
(a) Two of these basic elements, which may be the same or different, are 
joined at their exposed surfaces of heat-sealable high-temperature 
adhesive to form a double clad. This operation takes place at a 
temperature of from 140.degree. C. to 500.degree. C. and optionally under 
pressure. A preferred temperature range is from 180.degree. to 450.degree. 
C. The two basic elements used for this purpose may differ in the nature 
of the substrate material used and/or the no longer formable polyimide 
and/or the heat-sealable adhesive and/or in the thickness of the 
individual layers. Depending on the nature of the two heat-sealable 
adhesives and/or the process conditions (temperature, pressure), the end 
products obtained are double clads, in which two defined layers of 
heat-sealable adhesives can still be detected, or double clads in which 
the originally separate layers of heat-sealable adhesives have merged to 
form a single, defined layer. In this variant of the process, therefore, 
the heat-sealable high-temperature adhesive is applied to both the layers 
to be joined. 
(b) One of the basic elements is joined to a single clad obtained by the 
first process step, i.e. consisting solely of substrate material and no 
longer formable polyimide. In this case, the layer of heat-sealable 
adhesive of the basic element of the laminates according to the invention 
is joined to the layer of no longer formable polyimide of the single clad, 
again at the temperatures mentioned in (a) and optionally under pressure. 
The products formed correspond to those mentioned as the second 
alternative in (a) (single-defined layer of heat-sealable adhesive). In 
this variant of the process, therefore, the layer of heat-sealable 
adhesive is applied to only one of the layers to be joined. 
(c) Starting out from the basic element of the laminates or from products 
obtained by the process variants described above, other layers may 
optionally be applied to the exposed outer surfaces to obtain multiclads. 
The basic element is joined to other layers at a temperature at which the 
heat-sealable high-temperature adhesive is formable. Depending on the 
nature of the layers to be joined, the nature of the heat-sealable 
adhesive and the desired properties of the laminate, joining may be 
carried out by applying a light or relatively heavy pressure. The adhesive 
may optionally be applied before the polyimide has fully hardened. In some 
cases, the adhesion of the polyimide to the adhesive can be improved in 
this way. The polyimide may then be hardened to its no longer formable 
state. 
After the above-described process steps leading to the basic element of the 
laminates according to the invention, further layers may be applied if 
desired. In another embodiment of the process, it is possible to produce 
laminates of the type described above in which both sides of one or both 
layers of substrate material are directly joined to no longer formable 
polyimide. To this end, a single clad of substrate material and polyimide 
is produced and the polyimide completely cured in the first step of the 
process as described above. Thereafter, the second side of the substrate 
material is coated with a polyimide and the solvent evaporated. Complete 
curing may then be carried out directly or, alternatively, the 
heat-sealable adhesive may be applied before complete curing. 
The further procedure may then be, as described above, to obtain further 
embodiments of the laminates. The laminates obtained in this embodiment 
thus have the following layer sequence: no longer formable 
polyimide/substrate material/no longer formable polyimide/heat-sealable 
high-temperature adhesive, optionally followed by further layers. 
The invention is illustrated by the following Examples. 
EXAMPLES 1 TO 3 
These Examples illustrate the first step of the process leading to single 
clads which may be further processed to the laminates according to the 
invention by the process variants described in the following Examples. 
EXAMPLE 1 
A dry mixture of pyromellitic acid dianhydride (PMDA) and 
4,4'-diaminodiphenyl ether (DADE) was prepared in a standard commerical 
powder mixer. In all, 5.0 kg of PMDA and 4.54 kg of DADE (molar ratio of 
PMDA to DADE 1.01) were weighed into the mixer and then mixed for 48 hours 
at the highest speed setting. Approximately 1.6 kg of the mixture were 
then introduced into a gravimetric metering unit which delivered the 
mixture to a negative-feed twin-screw extruder at a rate of approximately 
200 g/h. DMAc was introduced into the first extruder zone kept at 
20.degree. C. at a rate of approximately 430 g/h, so that a solids 
concentration of 31.7% by weight was obtained. During the remaining 
residence time in the extruder, the temperature was increased in 
successive zones to 50.degree. C. A polyamide acid having a reduced 
viscosity of 1.67 was obtained, being extruded from the extruder barrel 
through a die for thin films. The die orifice had a rectangular 
cross-section measuring 200.times.0.35 mm. The pressure at the die head 
was 85 bar. The polyamide acid solution was extruded onto a 35 .mu.m thick 
sheet of rolled, tempered copper foil (Oak F-111), after which the 
laminate was introduced under nitrogen into a furnace having four equally 
long temperature zones of 140.degree. C., 180.degree. C., 350.degree. C. 
and 400.degree. C. respectively. The total residence time of the laminate 
was 10 minutes. During this time, the polyamide acid was reacted almost 
completely into the polyimide. The polyimide film adhered firmly to the 
copper substrate and was free from bubbles and interruptions. 
The above-mentioned Oak F-111 copper foil is a product of Oak Materials 
Group Inc., USA, which meets the requirements of IPC-CF 150 E. 
EXAMPLE 2 
A second 1.6-kg sample of the mixture was subjected to the same treatment 
as in Example 1, except that on this occasion a 70 .mu.m thick copper foil 
(Oak F-111) was used as the substrate. The polyimide film adhered firmly 
to the copper foil and was free from bubbles and interruptions. The 
properties of the laminates of Examples 1 and 2 are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Example 1 
Example 2 
Text method 
__________________________________________________________________________ 
Property (polyimide layer) 
Electric strength, KV/10.sup.-3 inch at 60 Hz 
4.4 4.35 ASTM D-149 
Dielectric constant at 1 kHz/25.degree. C. 
4.0 3.9 ASTM D-150 
Dielectric loss factor at 1 kHz/25.degree. C. 
0.0047 0.0039 ASTM D-150 
Tensile strength, N/mm.sup.2 
105 110 ASTM D882 
Breaking elongation, % 45 31 ASTM D882 
Density, g/cm.sup.3 1.42 1.42 ASTM D1505 
Thickness, .mu.m 66 61 ASTM D374 
LOI 40 40 ASTM D2863 
Property (laminate) 
Peel strength, N/cm 8.2 4.8 IPC TM 650 
2.4.9 
Behavior on soldering without further pretreatment 
no bubble 
no bubble 
IPC TM 650 
formation, 
formation, 
2.4.13 (slightly 
no separation 
no separation 
modified) 
__________________________________________________________________________ 
EXAMPLE 3 
A three-necked flask was charged with 8.17 g of PMDA to which 7.58 g of 
DADE dissolved in 60 g of DMAc was added. The DADE had been dissolved 
beforehand in DMAc with continuous stirring at full speed. The molar ratio 
of PMDA to DADE was 0.99:1.00. Another 29.25 g of the DMAc which had been 
previously used for flushing out the flask and in which DADE was dissolved 
were then introduced into the reaction vessel. The reaction was continued 
with stirring for 80 minutes under nitrogen at a temperature of 22.degree. 
C. Part of the polyamide acid solution formed was cast onto a 23 .mu.m 
thick nickel-chrome foil (Inconel, a product of the Somers 
Thin-Strip/Brass Group, Olin Corp. Waterbury, Conn.) which had been 
previously etched with an iron (III) chloride solution of 30 g of 
FeCl.sub.3, 60 cc of 12 N HCl and 180 cc of water. The Inconel foil 
consisted of an alloy containing nickel as its principal constituent, 
chromium and iron. The polyamide acid solution thus applied was drawn out 
to a thickness of 356 .mu.m by means of a glass rod onto which copper wire 
356 .mu.m in diameter had been wound. The alloy foil was applied to a 
glass plate and attached by adhesive tape. The film was dried for 20 
minutes at 70.degree. C. and then treated under a reduced pressure of 
approximately 2 mm Hg at 160.degree. C. This treatment was carried out 
under nitrogen in a vacuum dryer. The temperature of the dryer was then 
increased to 310.degree. C. over a period of 4.5 hours. By the time the 
film had reached a temperature of 160.degree. C., which took about 1-2 
minutes, most of the solvent had already been driven out, as could be seen 
from the color of the film, a clear light yellow. The hardened, dry film 
was 25 .mu.m thick. 
In addition, a polyamide acid sample obtained in accordance with Example 1 
was diluted with DMAc to 22% by weight of polyamide acid and a reduced 
viscosity (.eta..sub.red) of 1.22, cast onto a 58 .mu.m thick 
machine-scrubbed, i.e. roughened, alloy foil of a copper-nickel alloy 
containing approximately 70% Cu and approximately 30% Ni (Cupro-Nickel 30 
#715, a product of Somers Thin-Strip/Brass Group, Olin Corp., Waterbury, 
Conn.) and spread by doctoring to a wet film thickness of 356 .mu.m. The 
film thus applied was also dried and hardened by the method described in 
this Example. Both films had an extremely high peel strength, whereas a 
similar film sample on a bright untreated alloy foil was easy to peel off 
(peel strength 0.7 N/cm). Neither the polyimide layer on the etched foil 
nor the polyimide layer on the machine-scrubbed foil could be separated 
without damage to the polyimide film for the purpose of measuring peel 
strength. After treatment for 7 days at 260.degree. C., the polyimide film 
on the machine-scrubbed foil showed excellent adhesion and flexibility. 
EXAMPLE 4 
Two 10.times.20 cm large single clads of 35 .mu.m thick brass-clad copper 
foil (Gould) and polyimide of PMDA and DADE were laminated by means of a 
commercial 50 .mu.m thick polyacrylate adhesive film, the layer sequence 
being as follows: copper foil, polyimide, adhesive film, polyimide, copper 
foil. For lamination, the layers were initially cold-pressed in a platen 
press under a pressure of 50 kp/cm.sup.2, after which the foil stack was 
heated for 1 hour to 200.degree. C. under that pressure, kept under these 
conditions for 1 hour and then cooled under pressure. 
The two single clads were satisfactorily bonded into a double clad. 
The properties of the double clad are shown in Table 2 which, for each 
property, shows three values obtained from three tests: 
TABLE 2 
______________________________________ 
Total layer 
thickness Dielectric loss 
(.mu.m) Dielectric constant 
factor (.times. 10.sup.3) 
______________________________________ 
98/97/98 3.7/3.8/3.9 37.2/41.6/45.5 
______________________________________ 
The tests were repeated using a polyacrylate film in which a fabric had 
been embedded. The values obtained are shown in Table 3: 
TABLE 3 
______________________________________ 
Total layer Dielectric 
thickness Dielectric loss factor 
SAMPLE (.mu.m) constant (.times. 10.sup.3) 
______________________________________ 
(a) Double clad 
160/163 3.8/3.9 29/30 
(b) Adhesive 115/115 5.1/5.6 95/117 
film alone 
(comparison) 
(c) Comparison* 
120/112/112 
5.9/5.4/5.0 
67.5/72.7/63.3 
______________________________________ 
*Adhesive film laminated directly (without polyimide) between two copper 
foils. 
The film used in this example was a polyacrylate film in which a glass 
fiber web was embedded, the individual fibers of which were approximately 
5 .mu.m thick. The film was obtained from Brand Rex Company USA. 
The results show that clads in which a layer of acrylate is directly joined 
to copper (case c) are clearly inferior in their electrical properties to 
the laminates according to the invention (case a) in which a polyimide 
layer is present between the acrylate and the copper. 
EXAMPLE 5 
Two 10.times.20 cm large single clads having the same specification as in 
Example 4 were dried for 2 hours at 200.degree. C. in a nitrogen 
atmosphere and then laminated in a platen press using a 100 .mu.m thick 
intermediate adhesive film (Hostaflon.RTM.-TFA, a fluoropolymer 
manufactured by Hoechst, Frankfurt). The polyimide layers of the single 
clads were in contact with the adhesive film. The laminating conditions 
are shown in Table 4. 
TABLE 4 
______________________________________ 
Laminating time 
Temperature 
Pressure 
______________________________________ 
1h 340.degree. C. 
20-30 kp/cm.sup.2 
1h 370.degree. C. 
30-60 kp/cm.sup.2 
______________________________________ 
In the sample laminated at 340.degree. C., the average peel strength of the 
double clads was 7.0 N/cm between polyimide and Hostaflon.RTM. and 6.0 
N/cm between polyimide and copper. The dielectric loss factor of clads 
approximately 75 .mu.m thick (three samples produced by the above method) 
was 14.0.times.10.sup."3, 16.2.times.10.sup.-3 and 16.6.times.10.sup.-3 
and the corresponding dielectric constants of 2.8, 2.9 and 2.8 
respectively. 
The abbreviations used in the preceding Examples have the following 
meanings: 
PMDA=pyromellitic acid dianhydride 
DADE=4,4'-diaminodiphenyl ether 
DMAc=N,N-dimethyl acetamide.