Note: Descriptions are shown in the official language in which they were submitted.
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w o 96/09158 P~ ss~36s2
FOILED UD-PREPREG AND PWB LAMINATE PREPARED THEREFROM
The present invention pertains to a basic material for making a PWB
laminate comprising at least one layer of parallel, unidirectionally
oriented (UD) reinforcing fibres impregnated with not yet fully
consolidated matrix resin, i.e., a UD prepreg layer. The invention
also pertains to laminates for use in printed wire boards (PWBs)
prepared from such a UD prepreg layer.
A UD prepreg material for making PWBs is known from US 4,814,945. This
disclosure relates to a PWB laminate comprising a matrix resin
reinforced with parallel aramid fibres. The laminate is built up from
layers of unidirectional aramid tape stacked one on top of the other
in crosswise fashion. The aramid tape is formed by arranging a single
layer of parallel aramid fibres to form fibre strips, coating the
fibre strips with resin, and heating them to a semi-cured or
"B"-stage.
A problem easily incurred when making UD crossply laminates is
disorientation of the UD layers. Retaining proper orientation is
necessary for obtaining a laminate having sufficient flatness, which
is a property of particular importance to a PWB laminate. Particularly
if a still flowable matrix resin is used, e.g., B-stage material,
there is a substantial risk of disorientation occurring since, on
account of the flow which occurs during lamination, the tension, and
hence the orientation of the UD layers, cannot be adequately
controlled.
Still, it is desired to make UD-reinforced crossply composite
laminates on the basis of prepreg. For, making laminates on the basis
of prepreg (usually woven glass-fabric prepreg) is customary in the
field of PWB laminates, and it may be advantageous to be able to
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WO 96/09158 PCT/EP95/03652
employ resin systems and lamination technology of proven viability in
this field. Of course, by virtue of the woven fabric structure the
customary prepregs are less prone to disorientation problems. However,
the replacement of the woven fabric structure by a UD-crossply
structure leads to considerable advantages such as an improved surface
area quality, a comparatively low linear thermal coefficient of
expansion (TCE) in the x and y directions, the option of incorporating
a high content of fibres, and a favourable dimensional stability. In
these respects UD-crossply laminates are pre-eminently suitable as PW8
substrate. Of course, this holds only if these laminates can be
manufactured by means of a method that enables retaining proper
orientation.
Several such methods are known in the art, e.g., from EP 478 051,
W0 92/22191, and US 4,943,334. None of these is based on prepreg
lamination, whereas the present invention is directed to providing UD
crossply laminates on the basis of UD prepreg.
A problem associated with the orientation of UD fibres is addressed in
DE 3542295. It pertains to photographic shutter materials on the basis
of a substrate layer of oriented fibres contained in a matrix resin.
It is disclosed that by applying a layer of a heat-shrinkable
synthetic foil onto the substrate layer, positional deviations of the
parallel fibres during shaping under pressure may be suppressed. The
use of a heat-shrinkable foil will cause the UD prepreg to slightly
bend in the fibre direction. While this may be desirable in the case
of a shutter material, in a PWB laminate it should be avoided
entirely.
Yet another problem that may be incurred when utilizing UD prepreg,
notably if thin laminates are to be made, is that a single layer of UD
fibres impregnated with still flowable resin is hard to handle (even
if the resin is solid at handling temperature). This problem is
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WO 96/09158 PCT/EP95/03652
enhanced by the fact that UD-layers have a tendency to tear in the
longitudinal direction. This is due to the UD layer's lack of strength
in the direction perpendicular to that of the fibres.
An additional problem is encountered when the not yet fully
consolidated matrix resin is a thermally curable resin. In
contradistinction to a thermoplastic resin, such a not yet fully
consolidated thermoset resin does not yet have its final properties,
i.e. all mechnical properties are as yet inferior. This leads to an
additional problem in handling the prepreg, since it may easily be
damaged.
It should be noted that other background art in which UD prepregs are
employed pertains to making shaped, round articles such as golf club
shafts. Thus JP-Hei-4-329,132 teaches a hybrid prepreg article for use
therein. The hybrid prepreg comprises two different kinds of parallel
fibres, essentially thick ones having a diameter of from 30 to 500 ~m
and thin ones of 5 to 30 ym, and a metal layer of 5-100 ym thickness.
For PWBs that should meet modern requirements, it is important that
any reinforcing fibres be filaments having a diameter below 30 ym, and
preferably of from 3 to 15 ym, since thicker fibres are prohibitive
for suitable drilling of holes and for obtaining a desirable surface
flatness. It is particularly with thin prepregs, having thin
reinforcing fibers, that handling problems occur.
Other such background art is JP-Hei-6-008,240. It teaches structural
composites, the outermost layer of which has been covered with a metal
or metal-compound film. While the core of the composite can be
reinforced with parallel, unidirectional yarns, the outer layers are
reinforced with glass cloth. The disclosure is directed to shaped,
round articles such as golf club shafts or antennas.
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WO 96/09158 PCT/EP95/03652
Japanese Patent Application Laid-Open No. 201699/1985 discloses a
heat-bondable electric shielding material which comprises a metal
foil, a heat-bondable resin layer formed thereon, and a multiplicity
of parallel reinforcing wires fixed to the resin layer. The conductive
wires, which have a diameter of from 0.03 to 0.5 mm, are spaced apart
10 to 15 cm. It is disclosed that if the metal foil is to be prevented
from wrinkling, the shielding material should be wound up together
with a cushioning material, e.g. a polyurethane foam sheet.
Further background art on PWB laminates includes EP O 372 505. It
essentially discloses a fibre-reinforced thermoplastic laminate. The
fibre reinforcement can be in any form. The thermoplastic laminate is
provided with a metal foil when in the molten state. It generally is a
high temperature thermoplastic, which is solid at room temperature.
The disclosed laminate is not the type of basic material for making
PWBs that the invention is aiming at, as it serves as a PWB laminate
itself. Hence, the laminate manufactured according to EP 372 505 is a
laminate having the final properties of a PWB substrate. The invention
essentially aims at UD prepregs which can be used to make PWBs, but
are not suitable as PWB laminates in themselves. Being thin, and
having fibre-reinforcement in just a single direction, it will have
suitable properties in that direction only. Hence, it is hard to
handle, and will easily tear. As indicated above, this problem is even
more manifest when a not yet fully consolidated thermally curable
resin is used.
The invention now seeks to provide a UD prepreg layer that allows
further handling and processing without incurring problems such as
indicated above, and is of a type essentially suitable for making a
PWB laminate. Furthermore, the invention seeks to provide a UD prepreglayer in which it is possible to employ a thermally curable resin as
the not yet fully consolidated resin without suffering from the
additional problems associated therewith.
WO 96/09158 2~ 2 ~ ~ 3 ~1 4
PCT/EP95/03652
To this end the invention provides a basic material for making a PWB
laminate comprising a UD prepreg layer of the type indicated above,
wherein the reinforcing fibres have a diameter of below 30 ~m, and a
layer of a conductive metal foil, such as copper foil, the layer of
conductive metal foil being bonded to the UD prepreg layer.
The layer of conductive metal foil makes for a UD prepreg material
having sufficient strength perpendicular to the fibres direction to
prevent tearing during handling. If the foil is laminated onto the UD
prepreg prior to its being cut to size, the problem of handling a thin
copper foil is solved too.
As indicated above, the invention pertains to a basic material for
making a PWB laminate. This basic material comprises a layered
structure, the two consecutive layers bonded to each other being a
layer of a metal foil, such as copper foil, and a layer of parallel,
unidirectionally oriented fibres impregnated with not yet fully
consolidated matrix resin.
The term "prepreg" is well-known in the art and generally indicates a
reinforcing material impregnated with resin and (semi)cured. It
usually is still in a tacky stage. The term "not yet fully
consolidated matrix material" indicates that the resin can be further
cured still. In the case of a thermoset resin, it generally refers to
the matrix resin being in the B stage. The several matrix material
(matrix resin) stages are customarily identified in the art as the
"A", "B", and "C" stages, the A stage indicating unsolidified resin
(i.e., in the case of a thermoset resin: the uncured stage), the B
stage generally indicating partial solidification (in the case of a
thermoset resin: the reaction has proceeded through the formation of
longer chains, but not to full network formation), and the C stage
indicating a solidified (cured) stage. The terms A stage, B stage and
C stage are known to the person of ordinary skill in the art and
require no further elucidation here.
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WO 96/09158 PCT/I~P95/03652
The foiled prepreg of the invention can be laminated with other
prepreg layers or with layers of a consolidated material. Of course,
it is quite possible that such other layers comprise a woven fabric
reinforcement, but if the advantages of UD reinforcement are to be
enjoyed in full, the other layers should have UD parallel fibres as
well, i.e., should be UD prepreg layers or consolidated (non-flowing)
UD composite layers such as disclosed in W0 92/22191.
The current standard basic materials for printed wire boards are
generally manufactured according to the process described in, e.g.,
C.F. Coombs, Jr.'s Printed Circuits Handbook (McGraw-Hill), which
includes the following steps:
- Woven glass fibres are impregnated with a solution of epoxy resin
in MEK.
5 - Next, the solvent is evaporated and the resin partially cured up
to a so-called B-stage.
- The resulting prepreg is cut to length and stacked between two
copper foils.
- This package is cured under pressure at elevated temperature in a
multidaylight press.
- The laminate coated with copper on both sides manufactured in this
manner is then formed into a printed wire board by etching.
PWB laminates on the basis of the UD prepreg according to the
invention can be manufactured in essentially analogous manner. Of
course, the preparation of the UD prepreg basic material deviates from
the process of impregnating and curing a woven fabric. The UD prepreg
can be conveniently prepared by coating a copper foil with matrix
resin to form a foiled resin layer, heating the foiled resin layer so
as to ensure that the resin is sufficiently flowable for impregnation
of filaments to occur, and applying parallel filaments onto the resin
to form a foiled UD-reinforced resin layer. The impregnation can also
be carried through inversely, viz. applying the parallel filaments
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WO 96/09lS8
PCT/EP9S/03652
onto a not necessarily flowable resin layer, and then heating the
resin so as to render it sufficiently flowable for impregnation to
occur. Depending on the type of resin used, the foiled UD-reinforced
resin layer is either further heated or subjected to actinic radiation
to effect partial curing of the resin (e.g., to the B-stage) or cooled
down in order for the resin to solidify (e.g., with a thermoplastic
resin that is solid at room temperature). Surprisingly, the resulting
foiled UD prepreg is easier to handle than both the bare copper foil
and the corresponding non-foiled UD prepreg. The foiled prepreg is cut
to length and ready to be stacked and laminated with other, non-
foiled, UD layers. The non-foiled UD layers will form the inner
laminae and be sandwiched between two foiled UD prepregs (with the
Cu-foil layers on the outer surfaces).
In this respect the invention also pertains to a method of making a
PWB laminate wherein several layers comprising parallel,
unidirectionally oriented reinforcing fibres contained in a resin
matrix are stacked and pressed. In this method, the layers forming the
outer surfaces of the laminate are formed of a foiled UD-prepreg
comprising a layer of a conductive metal foil bonded to a UD-prepreg
layer, the conductive metal foil being on the outside of the laminate.
In one embodiment, the layers forming the inner laminae of the PWB
laminate are prepreg layers comprising parallel, unidirectionally
oriented reinforcing fibres impregnated with not yet fully
consolidated matrix resin, i.e., non-foiled UD prepreg. In another
embodiment, the layers forming the inner laminae of the laminate are
formed of non-flowing UD-composite layers or non-flowing UD crossply
laminates.
The term "non-flowing UD composite" is used to indicate a composite
material comprising unidirectionally oriented reinforcing fibres
enclosed in a matrix material which has been solidified (consolidated)
to the extent that it is not brought to flow again during the
W096/09158 ~2 n o 3 t 4
PCTIEP95/03652
remainder of the manufacturing process. In general, this means that
during storage and processing the non-flowing UD composite is under
such conditions of pressure and temperature as to be in a state below
its softening point (i.e., below Tg or apparent Tg), or solidified to
a stage in which flow no longer can occur. For convenience of storage
and processing, it is preferred for the solidification of the non-
flowing UD composite to have reached the C stage, or for such resins
to be used as those comprising rigid molecular chains in which, under
regular storage and processing conditions, a non-flowing state may
already be attained at a stage still called the B stage. However,
notably when pressing in the laminating zone is conducted under
isobaric conditions, also A stage material can be employed.
In the embodiments where the inner laminae are formed of non-flowing
UD composites, these laminae can be prepared in accordance with
W0 92/22191. It is also possible to stack and laminate foiled UD
prepreg in accordance with the invention using intermediate substrates
such as disclosed in W0 92/22192, which may be coated with adhesive or
not.
As has long been known, UD crossply laminates preferably are balanced
and symmetric. The term "balanced" indicates equal properties in
perpendicular directions (e.g., an equal number of filaments in the x
and y directions), the term "symmetric" indicates mirror image
symmetry over the thickness of the laminate, i.e., the laminate is
mid-plane symmetric. The plane of symmetry, which runs through the
centre of the laminate and is parallel to the laminate's outer
surfaces, is either the boundary between two UD layers or an imaginary
plane running through one UD layer, depending on the number and order
of UD layers over the thickness of the laminate. A major advantage of
such a balanced and mid-plane symmetric laminate provided with
crosswise applied UD-reinforced layers consists in the isomorphism of
its properties in the x and y directions (i.e., the two fibre
directions perpendicular to each other).
WO961091S8 ~ 3 ~ ~ PCT/EP95/03652
More particular preference is given to the laminate being so composed
that the UD-reinforced layers are oriented as specified in one of the
following models, with 0 and 90 standing for orthogonal
orientational directions and the relative thickness of the layers
being indicated by repeating the given orientation where necessary:
00/9oo9ooloo
oo/soosoo/oooolsoosooloo
In general, for utilisation in PWBs the UD-reinforced layers in the
laminate according to the invention will each have a thickness in the
range of 6 to 800 ~m, preferably of about 12.5 to 400 ym.
The outer layers of the crossply laminate will be formed by a foiled
UD prepreg in accordance with the present invention, i.e., a layered
structure having a layer of metal foil (say; Cu) and a UD layer (say;
0). In the above example, in which the inner UD layers have a double
thickness as compared with the outer UD layers, the inner layers can
be built up of a UD prepreg. Of course, in that case the
aforementioned danger of disorientation applies, but the UD layers of
double thickness do not display the same handling problems as a UD
layer of single thickness (which problem is solved in accordance with
the invention by applying metal foil). In this embodiment it is
preferred that lamination be conducted in an isobaric press, so as to
avoid a driving force for flow. Preferably, though, the inner layers
are the above-identified non-flowing UD composite layers in accordance
with W0 92/22191.
As is clear from the above, it is preferred that the stack of non-
foiled UD layers sandwiched between two foiled UD prepregs (with the
Cu-foil layers on the outer surfaces) is such that the UD-reinforced
layers are oriented as specified in one of the above models, i.e.,
CuO/9090/0Cu, or
Cu0/90goo/oooo/9oo9ooloocu
WO 96/09158 2~ 2 ~ ~ 3 1 4 p~/Ep95103652
The lamination may be conducted in a multidaylight press, an
autoclave, a vacuum press, a double belt press, or in any other
suitable apparatus.
The PWB laminates made on the basis of the foiled UD-prepreg inacccordance with the present invention are suitable to be used in
multilayer PWBs (MLBs), e.g., as disclosed in W0 92/22192.
The materials employed in carrying through the present invention are
not especially critical. Preferably, use is made of the materials
discussed hereinafter.
The matrix material is a thermoplastic or a thermosetting polymer,
preference being given to thermosetting resins. More preferred is the
use of an epoxy resin based matrix material, but other resins are also
useful in principle. Examples include cyanate esters, unsaturated
polyester (UP) resins, vinyl ester resins, acrylate resins, BT epoxy
resin, bismaleimide resin (BMI), polyimide (PI), phenol resins,
triazines, polyurethanes, silicone resin, biscitraconic resin (BCI).
Alternatively, combinations of said resins may be employed, and it is
also possible to mix the aforementioned resins with certain
appropriate thermoplasts, such as PP0, PES, PSU, and PEI among others.
Also interpenetrating polymer networks (IPNs) may be suitable. It is
of advantage to incorporate compounds into the matrix material to
render it flame resistant, such as phosphorus or halogen-(particularly
bromine-) containing compounds. A particular matrix material which is
preferred for its favourable flow and curing properties comprises
about 100 parts by weight of Epikote~828 EL, about 73 parts by weight
of Epikote~5050, and about 4 parts by weight of a complex of boron
trifluoride and monoethyl amine.
While the preferred reinforcing material consists of filament yarns,
non-continuous fibres may also be employed. According to the
WO 96/09158 ~ 4 PCT/EP95/03652
invention, the reinforcing yarns are preferably selected from the
following group of materials: glass, e.g., E-glass, A-glass, C-glass,
D-glass, AR-glass, R-glass, S1-glass, and S2-glass, and various
ceramic materials, such as alumina and silicon carbide. Also suited to
be used are polymer based fibres, more particularly so-called liquid-
crystalline polymers, such as paraphenylene terephthalamide (PPDT),
polybenzobisoxazole (PB0), polybenzobisthiazole (PBT), and
polybenzoimidazole (PBI), as are fibres based on polyethylene
naphthalate (PEN), polyethylene terephthalate (PET), and polyphenylene
sulphide (PPS). The fibres (filaments) should have a diameter of below
30 ~m, e.g. 20 ~m. Typical diameters more particularly range from 3 to
15 ~m, and preferably are of from 5 to 13 ~m.
In general, the fibre content in the matrix is about 10-90 vol.%,
preferably in the range of about 40 to about 70 vol.%. A fibre volume
fraction of about 50 vol.% is highly satisfactory.
In contradistinction to woven fabric-reinforced laminates, the
composite laminates manufactured using the process according to the
invention are also suited to be used in a flexible panel or laminate
and in rigid-flex laminates. When used in a flexible panel, woven
fabrics undergo cracking at the iunctions of warp and weft fibres, due
to the fact that fibres oriented in the bending direction are
interwoven with fibres perpendicular to the bending direction, this
adverse effect being enhanced by the high fibre concentration at these
junctions, which leads to cracking at a relatively low degree of
bending. Such cracks cause a high concentration of stress in the
conductive traces present on the flexible laminate, and consequently a
high risk of cracking, which leads to circuit breakage. In a flexible
laminate (or in the flexible portion of a rigid-flex laminate) the
orientation of the outer UD layers preferably parallels the desired
bending direction.
WO96/09158 ~2 ~ ~ 3 ~ ~
PCI/EP95/03652
In addition, the present UD crossply laminates are pre-eminently
suited to be used as supporting material in devices with various
integrated circuits provided thereon (multichip modules). This is
notably due to the favourable TCEs, which mostly are the result of the
high fibre volume fraction that can be obtained when crossply
laminates are used and may be close to the TCEs of electronic
components (chips) used in conjunction with PWBs, more particularly
MLBs. Such components may be provided on top of an MLB (chip-on-board)
or else be embedded in a substrate such as an intermediate substrate
according to W0 92/22192 (chip-in-board).
The build-up of laminates made on the basis of the foiled UD-prepreg
of the present invention is further illustrated in the schematic
drawings.
Fig. 1 shows a foiled UD prepreg (1) in accordance with the invention.
Indicated in the Figure, which shows cross-sections in x and y
direction. are copper foil (2), which is applied onto a UD prepreg
layer (3) made up of parallel, unidirectionally oriented reinforcing
fibres (4) impregnated with not yet fully consolidated matrix resin
(5).
Fig. 2 shows a non-flowing UD composite (6) in accordance with
W0 92/22191. Indicated in the Figure (x and y cross-sections) are two
layers made up of UD fibres (7) impregnated with non-flowing matrix
resin (8).
Fig. 3 shows a CuO/9090/0Cu PWB laminate made by stacking and
laminating the non-flowing UD composite (6), with two foiled UD
prepregs (1).
