Flexible printed circuits

The present invention provides flexible, tear-resistant printed circuits comprising first and second electrical conductors and a flexible, insulating core separating and supporting said first and second conductors. The core preferably comprises aromatic polyamide fiber in a resin binder matrix. Rigid/flex circuits including the flexible, tear-resistant printed circuits are disclosed.

The present invention relates to flexible printed circuits and, more 
particularly, to printed circuit boards having a flexible printed circuit 
connected to a rigid printed circuit. 
Printed circuit boards are now commonly used in numerous and varied 
applications, ranging from telecommunications equipment to toys. Printed 
circuit boards typically comprise an insulating substrate and a conductive 
circuit printed on the substrate. Because printed circuit boards 
frequently support electronic components, the substrates of such boards 
are very often configured to possess mechanical strength and rigidity. 
Thus, most printed circuit boards are rigid, that is, they are strongly 
resistant to bending and deformation. 
Relatively recently, however, the demands of certain circuit board 
applications have resulted in the development of boards containing both 
flexible and rigid sections, frequently referred to as rigid/flex circuit 
boards. In complex electronic equipment, for example, rigid printed 
circuit boards are very often mounted in a rack or sub-rack which includes 
grooves for receiving the boards. In the past, such rigid circuit boards 
were frequently electrically interconnected to each other and to the rest 
of the electronic equipment by means of surface, or inter-rack, wiring. 
This configuration was generally acceptable for equipment in which size 
and cost was not a dominant factor. However, there has been an increasing 
demand for electronic equipment that is both compact and low cost. Since 
inter-rack wiring is frequently a painstakingly slow and arduous task, 
especially as the space between boards is reduced, such demands are very 
difficult to satisfy with standard rigid printed circuit board 
configurations. 
The relatively recent development of printed circuit boards containing both 
rigid and flexible sections has alleviated many of the problems associated 
with inter-rack wiring of rigid printed circuit boards. Rigid/flex circuit 
boards are disclosed, for example, in U.S. Pat. No. 4,931,134. According 
to the design disclosed in the patent, the printed circuit board comprises 
a series of flexible cables or circuit sections connected selectively 
between a plurality of rigid circuit sections. One method utilized to 
produce such rigid/flex circuit boards includes the steps of laminating a 
superstructure comprising rigid, insulative boards to a substrate 
comprising individual, flexible layers such that the superstructure forms 
the outermost layers of a rigid laminate composite. Subsequent to the 
formation of this rigid board, selected portions of the rigid 
superstructure are severed and removed from the rigid laminate to expose 
pre-selected areas of the flexible circuit layers. These layers are 
relatively readily deformable and therefore comprise flexible sections of 
the circuit board. 
Because they are formed during the overall manufacturing process of the 
printed circuit board, the flexible sections of such rigid/flex circuit 
boards provide the potential for creating compact, cost-effective 
electronic equipment. In order for these potential advantages to be 
achieved, however, such circuit boards must possess certain 
characteristics. For example, the flexible portions of these printed 
circuit boards must be capable of withstanding the frequent bending and 
deformation associated with the processes used to 1) manufacture the board 
and 2) incorporate it into the electronic equipment in which it will be 
used. If the circuit board does not possess these characteristics, the 
overall cost of producing such circuit boards rises dramatically due to 
the high incidence of waste associated with the manufacture and 
installation of such units. Furthermore, in order to readily use the 
rigid/flex circuit board in compact configurations, the flexible portions 
of the printed circuit should have the ability to bend around very small 
radii without failure. If the minimum bend radius of the flexible section 
is too large, the ability to use the boards in compact locations will be 
undesirably diminished. Accordingly, the flexible circuit section should 
have the smallest possible minimum bend radius. 
While printed circuit boards containing both rigid and flexible circuit 
sections have heretofore possessed desirable features and properties, 
prior rigid/flex circuit board configurations have nevertheless been found 
to exhibit disadvantages. For example, applicant has discovered that the 
flexible portions of the heretofore used circuit boards are not 
sufficiently resistant to tear, especially in the region where the 
flexible section enters the rigid section of the board. Certain of the 
heretofore used designs have been so susceptible to tear, in fact, that 
almost half of the boards manufactured according to such designs have been 
found to be incapable of surviving the manufacturing process. This, in 
turn, results in low manufacturing yield and/or high incidence of circuit 
failure during use. 
Previous attempts to overcome the lack of tear resistance associated with 
rigid/flex circuit boards have been only partially successful. For 
example, attempts have been made to improve the tear resistance of 
flexible circuits by covering the flexible sections thereof with one or 
more layers of relatively flexible material, such as 
polytetrafluoroethylene film, aromatic polyamide fabric, fiberglass 
fabric, polyimide film, and certain combinations of these. Such attempts 
have been reported in the paper entitled "The Effect of Reinforcement on 
the Tear Properties of Flexible Circuits", Andra E. Acton, First 
International SAMPE Electronics Conference, Jun. 23-25, 1987. This paper 
reports that while an improvement in tear-resistant properties was 
observed With the use of certain of the above-noted materials as cover 
layers, a substantial and unacceptable decrease in the flexibility and 
processibility of such flexible circuits was also observed. For example, 
this paper concludes that while Kevlar covering layers provided the 
greatest benefit in tear resistance, such covering layers also presented 
the greatest processing difficulties and highest loss of flexibility. 
Accordingly, such arrangements are generally not acceptable for 
wide-spread commercial applications. 
Accordingly, it is object of the present invention to provide 
tear-resistant flexible printed circuits. 
It is a further object of the present invention to provide tear-resistant 
flexible printed circuits which have low values of minimum bend radius. 
It is another object of the present invention to provide rigid/flex printed 
circuit boards that have highly tear-resistant flexible sections. 
It is yet another object of the present invention to provide improved 
rigid/flex printed circuit boards that are adaptable for production by 
standard manufacturing techniques. 
It is a still further object of the present invention to provide computing 
means, such as compact personal computers, containing rigid/flex printed 
circuit boards that have flexible sections which are highly tear-resistant 
and have low values of minimum bend radius. 
SUMMARY OF THE INVENTION 
Applicant has found that these and other objects are achieved and the 
disadvantages of the prior art are overcome by flexible circuit boards 
comprising at least one flexible, highly tear-resistant core and first and 
second electrical conductors separated by said core. According to 
preferred embodiments, such highly tear-resistant cores comprise woven 
aromatic polyamide fiber in a matrix of resin binder, preferably 
thermosetting plastic resin binder. 
According to especially desirable embodiments of the present invention, the 
circuit board comprises a rigid printed circuit section and a flexible 
printed circuit configured as described above and connected to the rigid 
circuit section. In such embodiments, applicant has found that substantial 
improvement in performance is realized by attaching the highly 
tear-resistant core of the present flexible circuit to the rigid circuit 
section. In particular, it is preferred in such embodiments that at least 
a first portion of the core is bonded to the rigid circuit section. 
In an even further preferred embodiment, the circuit board comprises at 
least one flexible circuit section and at least first and second rigid 
circuit sections. In such embodiments, the first and second rigid sections 
are mechanically and electronically connected to one another by the 
flexible circuit section of the present invention. Connection of the two 
rigid circuit sections is preferably achieved by having at least a first 
portion of said flexible circuit core connected, by lamination for 
example, to said first rigid section and a second portion of the flexible 
section core connected to the second rigid circuit section.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention provides flexible circuits having improved tear 
resistance and flexibility. As those skilled in the art are aware, 
tear-resistance has heretofore been achieved by adding a tear resistant 
covering layer to the flexible circuit. Applicant has found that the tear 
resistance provided by such coverings is less than fully satisfactory, as 
explained more fully hereinafter. Furthermore, the use of tear resistant 
covering layers according to the prior art also produces a highly 
undesirable decrease in printed circuit flexibility. In particular, the 
additional layers of covering material generally result in significant 
increase in the overall thickness of the final printed circuit and hence a 
decrease in flexibility. During the bending process, this additional 
material tends to compress when on the inside of the bend and to stretch 
when on the outside of the bend. Applicant believes that this bending and 
stretching of the covering layers not only makes it more lo difficult to 
deform the printed circuit, but that it also has a tendency to crush the 
printed circuit on the inside of the bend and to tear the printed circuit 
on the outside of the bend. Thus, the prior art has provided flexible 
printed circuits with (1) less than satisfactory tear resistance and (2) 
diminished flexibility. 
According to the present invention, flexible printed circuits are provided 
which overcome the shortcomings of the prior art. The present flexible 
circuits generally comprise first and second electrical conductors and a 
flexible insulating core separating and preferably supporting said 
conductors. The core is preferably a generally planar layer or sheet of 
electrically insulating material having first and second generally planar 
surfaces, said planar surfaces typically being substantially parallel. 
Conductors are formed on and preferably bound to the first and second 
surfaces of the core by techniques well known in the art. It is preferred, 
however, that the conductors be formed by adhering a continuous sheet of 
conductive material, such as copper, to each side of the core. The layers 
of conductive material are then imaged and etched in known fashion to 
provide conductive pads and/or connectors. The number and type of 
conductors will vary widely and is a matter of design choice which depends 
upon the particular application involved. 
One important aspect of the present invention resides in the properties and 
characteristics of the insulating core of the flexible printed circuit. In 
particular, it is preferred that the insulating core comprise a highly 
tear-resistant core. As used herein, the term highly tear-resistant 
generally includes materials having a load-at-tear (LAT) of at least about 
25 lb./mil as measured by the following test procedure. A sample of the 
material to be tested is placed in an Instron Model 4204 Mechanical Tester 
with Series IX automated material testing software. This equipment is used 
to produce a rotational tear in the materials. The sample is generally 
rectangular in shape and has a width of about one inch and a length of 
about 12 inch. For the testing of laminate materials, the one-inch wide 
strips should be cut from larger sheets at an angle of about 45.degree. to 
the ply of the laminate. Samples are provided in this manner so as to 
simulate a rigid/flex circuit configuration in which the core material of 
the flexible circuit is laid up at a 45.degree. angle to the rigid printed 
circuit section. The LAT value is then determined by dividing the 
load-at-tear by the thickness of the sample. It is contemplated that the 
samples tested according to this method will typically have a thickness of 
from about 2 to about 12 mil. 
Core materials of the present invention preferably have an LAT of at least 
about 28 lb./mil, with LATs of at least about 30 lb./mil being even more 
preferred. The flexible circuits of the present invention are themselves 
preferably highly tear-resistant. That is, the flexible circuits have an 
LAT of at least about 25 lb./mil, with at least about 28 and 30 lb./mil 
being even more preferred. 
The core of the flexible circuits of the present invention, as well as the 
flexible circuits themselves, also preferably exhibit improved or at least 
substantially undiminished flexibility. As described above, prior attempts 
to improve tear-resistance have resulted in thicker and less flexible 
printed circuits. Since the present invention does not require the 
previously used cover layers to impart tear resistance, the present 
printed circuits are generally thinner and hence more flexible than prior 
art configurations. Thus, the herein described flexible printed circuits 
are at once more tear resistant and more flexible than previous flexible 
circuits. In particular, the tear resistant flexible printed circuit 
boards of the present invention are preferably less than about 12 mil in 
overall thickness, and even more preferably less than about 10 mil. It is 
contemplated that the overall thickness of the flexible circuit may even 
be less than about 8 mil according to certain embodiments. Furthermore, 
the core of the flexible circuits of the present invention, as well as the 
flexible circuits themselves, possess low values for minimum bend radius. 
It is preferred that the flexible circuits of the present invention have a 
minimum bend radius no greater than about 10 times, and even more 
preferably no greater than about 8 times, the overall thickness of the 
flexible circuit. As the term is used herein, bend radius refers to the 
inside radius of the arcuate portion of the bend in a sample sheet of 
flexible material. The bend in the sample sheet is produced by bending the 
sheet around a smooth cylinder of constant and known radius. The term 
minimum bend radius refers to the smallest bend radius such a sample sheet 
is able to withstand without failure. As the term is used herein, failure 
refers to the formation of cracks, tears or other strength-weakening 
deformations in the sample being tested after at least 25 cycles of 
bending. The preferred cores of the present invention have a minimum bend 
radius of no greater than about 50 mil, more preferably no greater than 
about 30 mil, and even more preferably no greater than about 25 mil. 
Furthermore, the preferred cores of the present invention are capable of 
withstanding at least about 50 cycles, and even more preferably 100 
cycles, of bending to the minimum radius without failure. 
Similarly, the flexible circuits of the present invention preferably have a 
minimum bend radius of no greater than about 120 mil, more preferably no 
greater than about 70 mil, and even more preferably no greater than about 
60 mil. Furthermore, the preferred flexible circuits are capable of 
withstanding at least about 50 cycles, and even more preferably 100 
cycles, of bending to the minimum radius without failure. 
The flexible circuits of the present invention and the cores thereof are 
preferably both highly tear-resistant and flexible. Thus, it is preferred, 
for example, that the cores of the present invention have an LAT of at 
least about 25 lb./mil and a minimum bend radius of no greater than about 
50 mil. It is even more preferred that the core material of the present 
invention have an LAT of at least about 30 lb./mil and a minimum bend 
radius of no greater than about 25 mil. Furthermore, it is preferred, for 
example, that the flexible circuits of the present invention have an LAT 
of at least about 30 lb./mil and a minimum bend radius of no greater than 
about 120 mil. 
In view of the disclosure contained herein, it is contemplated that those 
skilled in the art will be capable of readily selecting, without undue 
experimentation, materials which provide the beneficial features and 
characteristics of the present invention. While all such materials are 
within the scope of the present invention, applicant has found that 
especially beneficial results are obtained with core materials comprising 
polyamide fibers, and preferably aromatic polyamide fibers. Especially 
preferred among the aromatic polyamide fibers are those long chain 
synthetic polyamide fibers in which at least 85% of the amide linkages 
have an aromatic group attached to the carbon atom and an aromatic group 
attached to the nitrogen atom. Such preferred fibers are sold under the 
trademark KEVLAR by E.I. Du Pont Nemours and Company of Wilmington, Del. 
The particular configuration and orientation of the aramid fibers may be 
varied within the scope of the present invention. It is preferred, 
however, that the insulating core of the present invention comprise aramid 
fibers formed into a regular interlocking pattern, such as by weaving, 
knitting, or braiding, with woven fibers being preferred. 
The polyamide fibers are preferably bound in a matrix of resin binder. 
Materials which are acceptable as the resin binder of the present 
invention will vary depending upon the particulars of each individual 
application. It is preferred, however, that the resin binder comprise 
epoxy resin, and even more preferably tetrafunctional epoxy resin. 
Accordingly, the cores of the present invention preferably comprises woven 
aramid fiber contained in a binder matrix of tetrafunctional epoxy resin. 
Such a material has been made available by Arlon-ESD under the designation 
"4093". 
The advantages of the present invention are especially evident according to 
embodiments in which the present flexible circuits are used in combination 
with rigid circuit sections. Accordingly, a preferred aspect of the 
present invention provides printed circuit boards having at least one 
rigid section and at least one flexible circuit connected thereto. As 
described hereinbefore, previous rigid/flex circuit boards were extremely 
susceptible to failure, especially as a result of flexible circuit tear. 
Applicant has found that tearing of the flexible circuit sections of 
heretofore used rigid/flex circuit boards was initiated and/or 
concentrated around the interface or area of connection between the rigid 
and flexible printed circuits. While applicant does not intend to be bound 
by or limited to any particular theory, it is believed that the previous 
attempts to address the tearing problem have been unsuccessful, at least 
in part, because the source of the tearing problem went unrecognized. In 
particular, prior attempts to improve tear resistance typically involved 
the application of a covering layer only to that portion of the flexible 
circuit not connected to and/or not in contact with the rigid portion of 
the rigid/flex circuit board. For example, U.S. Pat. No. 4,800,461 
describes the covering the flexible printed circuit with Kapton 
(polyamide) sheets in order to provide tear resistance characteristics to 
the flex sections. These Kapton sheets are disclosed as covering 
substantially only the flexible portion of the flexible printed circuit 
section, that is, substantially only the portion not in contact with the 
rigid circuit sections. Applicant believes that such an approach has been 
less than successful, at least in part, because it does not provide tear 
resistance to the flexible circuit in the region where most tearing has 
been found to originate--at the boundary or interface between the flexible 
and rigid sections. 
Applicant has found that the failures of the prior art are overcome and 
tear resistance substantially improved by providing rigid/flex circuit 
board configurations in which the flexible circuit section comprises a 
highly tear-resistant core wherein at least a portion of this highly 
tear-resistant core is connected to the rigid circuit section. According 
to preferred embodiments, a portion of at least one of the planar surfaces 
of the core of the present invention is connected to at least a portion, 
and preferably a substantial portion, of at least one of the planar 
surfaces of the rigid circuit of the present invention. Connection of the 
planar surfaces can be achieved directly and/or indirectly. It is 
contemplated, for example, that connection of the core surface and the 
rigid circuit surface will include direct core/rigid circuit connection 
and indirect connection resulting from the core conductors being bound or 
otherwise connected to the surface of the rigid circuit section. 
Furthermore, it is preferred that at least about 5 percent of the area of 
the planar surface of the rigid circuit section be covered by, connected 
to or otherwise in contact with the core section of the flexible circuit, 
with at least 10 percent being more preferred and at least 20 percent 
being even more preferred. In this way, the rigid/flex circuit boards of 
the present invention possess high tear resistance in the area most 
susceptible to the initiation of tearing, that is, in the area of 
intersection between the rigid section and the flexible printed circuit. 
In especially preferred arrangements, a portion of the flexible section is 
substantially coextensive with the rigid circuit section, that is, about 
100% of at least one planar surface area of the rigid circuit section is 
covered by, connected to or other wise in contact with a portion of the 
core of the flexible circuit section. 
With particular reference now to FIGS. 1 and 2, a rigid/flex printed 
circuit board according to the present invention, generally designated 10, 
has a plurality of rigid printed circuit sections 11 connected by a 
plurality of flexible printed circuit sections 12. The rigid printed 
circuits 11 are generally planar in construction and have relatively 
closely spaced upper and lower surfaces 11A and 11B, respectively, as 
shown in FIG. 2. Likewise, flexible printed circuits 12 are also generally 
planar in construction and have relatively closely spaced upper and lower 
surfaces. Thus, both the rigid and flexible printed circuits comprise 
relatively thin sheets in which the dimensions of the planar surfaces are 
many times larger than the edge dimensions. The rigid portions 11 carry 
various electronic components and hardware on one or more of the planar 
surfaces 11A and 11B. For the purposes of illustration, some of the 
electronic components shown in FIG. 1 have been designated with the 
reference numeral 13. As shown in cross-section in FIG. 2, the rigid 
circuit sections preferably comprise multiple layers of rigid circuits 
laminated together to form a unitary rigid circuit section. 
As best shown in FIG. 2, the rigid circuit sections 11 generally comprise a 
series of conductors, such as printed circuits, separated by relatively 
rigid insulating sheets or layers. Thus, the rigid circuit sections 11 are 
multi-layer rigid circuit sections having conductors 40-45 separated from 
one another and/or other conductors by insulating layers 30-35. In a 
typical and well-known configuration, the insulating layers 30-35 comprise 
layers of fiberglass impregnated with epoxy adhesive. Such glass/epoxy 
layers result in a relatively rigid multi-layer circuit board when 
laminated together by heat and pressure according to well-known laminating 
techniques. 
The flexible circuit sections 12 comprise conductors 23 and 22 laminated 
according to conventional methods to the upper and lower planar surfaces 
21A and 21B of core layer 21, respectively. Core 21 extends into and is 
part of the rigid circuit sections 11. Conductors 22 and 23 also extend 
into rigid sections 11 and are electrically connected to rigid circuit 
conductors 40-45 by, for example, copper barrel 51. Pad 50 in turn 
provides means for obtaining electrical connection to electronic 
components 13. Conductors 22 and 23 may comprise conductive copper 
circuits printed according to techniques well known in the art on each 
side of core 21. Core 21 is preferably one or more layers of woven aramid 
fiber contained in a binder of tetrafunctional epoxy resin. According to 
preferred embodiments, core 21 has a thickness of from about 3 to about 5 
mil. 
According to an important aspect of the present rigid/flex circuit boards, 
core layer 21 comprises highly tear-resistant insulating material 
connected to the rigid circuit sections 11. More particularly, at least 
one of the upper and lower planar surfaces 21A and 21B of core 21 are 
bound, preferably adhesively bound, to at least one corresponding planar 
surface of the rigid circuit board. It is preferred that planar surfaces 
21A and 21B are each bound to a mating planar surface of the rigid printed 
circuit board. Thus, at least a portion of upper surface 21A of core 21 is 
adhesively bound to at least a portion, and preferably a substantial 
portion, of the lower planar surface of prepreg layer 33, and at least a 
portion, and preferably a substantial portion, of lower surface 21B of 
core 21 is adhesively bound to at least a portion, and preferably a 
substantial portion, of the upper planar surface of prepreg layer 32. It 
is seen, therefore, that connection of core 21 to the rigid sections 11 is 
achieved by having substantial bonding between the planar surfaces of the 
core and the planar surfaces of the rigid printed circuit section. As 
mentioned hereinbefore, such bonding includes indirect bonding which 
results from the bonding of conductors 22 and 23 with the corresponding 
surfaces of prepreg layers 32 and 33, respectively. In multilayer 
embodiments of the type shown in the figures, this planar bonding is 
achieved by having the core 21 and the rigid circuit section laminated 
together. That is, both planar surfaces of the highly tear-resistant 
material of core 21 are bound during the lamination process to a planar 
surface of the rigid circuit board. 
Copper conductors 22 and 23 are optionally but preferably covered by 
flexible insulator layers 24 and 25. These layers are covered with or have 
impregnated therein suitable adhesive for bonding to core 21 and 
conductors 22 and 23. The insulating layers 24 and 25 are thereafter 
laminated to the flexible portion of the sheet in known fashion. The 
present flexible printed circuits have an advantage over previously known 
circuits in that insulating layers 24 and 25 are optional, thereby 
allowing flexible printed circuits of reduced thickness and enhanced 
flexibility. In particular, since the tear resistance of the present 
flexible printed circuits is provided by the core layer and the manner in 
which the core layer is attached to the rigid circuit sections, the need 
for tear-resistant covering layers is eliminated. Elimination of the 
tear-resistant covering layers, in turn, provides circuit sections 12 with 
enhanced flexibility due to the reduction in the overall thickness of the 
flexible printed circuit layer. Furthermore, the use of highly tear 
resistant core 21 allows the thickness of any covering layer that is used 
to be substantially reduced relative to previous configurations. As a 
result, overall printed circuit thickness may be substantially reduced and 
flexibility increased even when insulating layers 24 and 25 are used. 
Another advantage of the rigid/flex circuit boards of the present invention 
is that standard manufacturing techniques may be utilized, with little or 
no modification, to produce the printed circuit board. For example, the 
circuit boards may be produced using an initial processing step which 
includes providing the highly tear-resistant core 21. Copper sheets are 
then laminated to the core 21 in known fashion. Following lamination, the 
copper sheets are imaged and etched to provide copper conductors 22 and 23 
on the core. Insulated prepreg sheets 30-32 and their associated 
conductors 40-42 are positioned below core 21, and insulated prepreg 
sheets 33-35 and their associated conductors 43-45 are positioned above 
core 21. The sandwich formed by the foregoing sheets is then laminated 
together and cut as required to provide, in this example, a multi-layer 
rigid/flex circuit. Holes are drilled at appropriate locations to 
interconnect desired conductors and then typically de-smeared by a 
suitable process to expose fully the copper conductors. The holes are then 
plated through to interconnect desired conductors via plated through 
barrel 51. 
Many methods are known and available for manufacturing the rigid/flex 
printed circuit boards of the present invention. See, for example, the 
following patents, each of which is incorporated herein by reference: U.S. 
Pat. No. 4,931,134--Hatkevitz et al; 4,800,461--Dixon et al; 
4,338,149--Quaschner; 3,409,732--Dahlgren et al. 
The flexible printed circuits of the present inventions are especially well 
suited for use in complex electronic equipment. Accordingly, this 
invention provides computing equipment, preferably portable computing 
equipment such as lap-top and notebook computers, comprising the flexible 
circuits of the present invention. 
It will be understood that the above-described embodiments of the present 
invention are illustrative only of the present invention and not limiting 
thereof. Accordingly, it will be understood that various changes and 
alterations may be made in the above-described embodiments without 
departing from the essential features of the invention, which are defined 
only by the claims which follow.