Note: Descriptions are shown in the official language in which they were submitted.
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FLEXIBLE CABLE ASSEMBLY
Background of the Invention
This invention relates generally to printed circuits and
in particular to a flexible ribbon cable for interconnecting
portions of a display system such as between a matrix display
element and its drive electronics~
Prior packaging techniques for electronic equipment have
typically involved mounting acti~e and passive electronic
components on a single layer or multilayer printed wiring
board (PWB). A plurality of such printed wiring boards are
interconnected by plugging the plurality of boards into a
"mother" interconnect panel. In some applications where
space is at a premium the interconnect panel is replaced by
straight runs of flexible ribbon cable such as in a flat
panel matrix display system for interconnecting the contacts
on the edge of a display element to the contacts on the edge
of a display drive electronics printed wiring board. Typi-
cally there may be between 500 to 6000 electrical connections
between the display element and the drive electronics resulting
in considerable space required for the drive electronics,
significant assembly costs and limits on the degree of
maintainability in the field. If connections between the
drive electronics and t~ display ele~ent need to be undone
in the field for testing or replacement of only a single
~5 inex2ensive faulty device, the display asse~bly must be
æ~
returned to a service center for reassembly. As a result,
the display element and drive electronics are spared in the
field as a single line replaceable unit (LRU), which is
expensive. If the display system parts are designed to be
readily separable, then repair of the drive electronics is
feasible in the field requiring only the sparing of relatively
inexpensive parts or devices.
The high voltage driver circuits have accounted for most
of the drive electronics volume requirements in a matrix
display system. The packaging of high voltage driver circuits
in a leadless chip carrier (LCC) configuration can achieve
significant volume reductions in such a display system;
however, an assembly problem arises if a surface mounted LCC
and conventional thru-board integrated circuit ~IC) devices
such as dual in-line packages (DIPS) are mounted on the same
printed wiring board because two incompatible assembly tech-
niques are required. This approach by itself does not reduce
the large number of interconnections required between the
display drive electronics and the display element.
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62901-675D
In accordance with the present invention there is
provided a flexible cable assembly comprising: a flexible
insulating substrate having formed thereon a plurality of
conductors; a prearranged group of solder pads disposed on ~aid
subs~rate, a plwrality of said pads beiny electrically connec~ed
to said conductors; circuit means having solder pads for attaching
to said prearranged group of solder pads disposed on said
subætrate; and means at~ached under said circuit means and said
substrate means for providing ~upport to ~aid flexible cable
assembly in the area o~ said circuit means and ~or removing heat
from ~aid circuit means.
Pins inserted into one end of the flexible cable
assembly provide the means for connectlng to a printed wiring
board and exposed conductors with conductive plating on one slde
of a second end of the flexible cable assembly provide the means
for clamping this second end to matcbing conductors. The means
for providing support and for removing heat preferabl~ comprises
heat sink/backer laminated to the area under the circuit package
and the substrate. A plurality of flexible cable assemblies
interconnect a drive electronics module to a matrix display
element in the display assambly. The circuit package comprises
high vol~age driver circuits mounted in a leadless chip carrier
(LCC) resulting in a reduction in the volume required for the
drive electronics. In additlon, ~he circuit package at~ached to
the ~lexible cable provides the mean~ for reducing the number o~
connections required between the driva electronics and each
flexible cable assembly. The flexlble
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62901-675D
cable assembly provides the means for a more readily repair-
able display assembly in a field applieation environment.
The invention also provides the method of making a
flexible cable assembly comprising the steps of:
providing a flexible eable having formed therein a
plurality of strip conductors arranged in one or more layers
with flexible insulating means attached to each side of said
layers of eonduetors and having arranged thereon a group of
solder pads on a portion of a first side in a region of a
first end of said flexible eable, a portion of said solder pads
being eoupled to a first portion of said strip eonductors and
a noninsulated second portion of said strip conductors on a
second side of a second end of said assembly being gold
conductive-plated;
soldering a circuit means to said arranged solder pads
on said first side of said flexible cable;
laminating a heat sink/backer means on a portion of
said second side of said flexible cable under said cable and
said cireuit means; and
soldering term.inal pin means Oll said first end of
said flexible eable immediately adjaeent to said eireuit means,
said pin means being eoupled to a portion of said strip
eonduetors and a portion of said solder pads.
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Brief Description of the Drawings
The above-mentioned aspects and other features of the
invention are explained more fully in the ~ollowing descrip-
tion taken in connection with the accompanying drawings, in
which:
FIG. 1 is a top view of a flexible ca~le assembly
according to the invention showing two layers of conductors.
FIG. 2 is a side view of the flexible cable assembly
showing a leadless chip carrier mounted on a first end of
said cable assembly next to insertion pins and a heat sink/
backer laminated immediately under both the flexible cahle
and the leadless chip carrier.
FIG. 3 is an exploded perspective view of the flexible
cable assembly according to the invention.
FIG. 4 identifies the material layers of the flexibl~
~; cable shown in FIG. 3.
FIG. S is a layout of a top layer of copper strip
; conductors according to the invention~
~IG. 6 is a layout of a bottom layer of copper ~trip
conductors according to the invention.
FIG. 7 is an exploded perspective of a matrix display
assembly showing the flexible cable assembly interconnecting
between a drive electro~ics board and a matrix display element.
.
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_ the Preferred Embodiment
Referring to FIGS. 1, 2 and 7 there i~ shown a flexible
cable assembly 10 comprising a ~lexible cable 11 having formed
therein layers of insulated strip conductors 24 and 32, a
leadless chip carrier 12 attached to one end of the flexible
cable 11, a heat sink/backer 14 attached to the flexible cable
11 under the area covered by the leadless chip carrier 12, and
connection means including pins 15 and strip conductor end
connector 16 on the ends of the flexible cable 11 for connect-
ing between portions of a display assembly 50. The leadless
chip carrier 12 provides the means for interconnecting one
or more circuit chips or other devices within said carrier.
Pins 15 provide the means for electrically connecting one end
of the flexible cable assembly 10 to a printed wiring board.
The other end connector 16 of the flexible cable assembly 10
is embodied by a one-quarter inch end portion of the strip
conductor layer 32. End connector 16 does not have a coating
of insulating material but instead has a conductive plating
which enables this end of the assembly 10 to be clamped against
mating conductors on a display element 58 of the display assembly
50 as shown in FIG. 7. The flexible cable assembly 10 is used
in the display assembly 50 to interconnect a portion of a
drive electronics module 54 to a portion of the display element
58. Pins 15 plug into the drive electronics module 54 and end
connector 16 is wrapped around the edge of the display element
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58 while its exposed strip conductor contacts are aligned
with mating contacts on the bottom side tnot shown) of display
element 58. ~ clamp 62 secures end connector 16 to the mating
contacts on the display element 58. Referring now to FIG. 2
a side view of the flexible cable assembly 10 shows the heat
sink/backer 14 and the pins 15 for connecting to a prlnted
wiring board.
Referring no~ to FIG. 3 and FIG. 4, an exploded perspec-
tive view of the flexible cable assembly 10 is shown in FIG. 3
and in particular the flexible cab?e 11 comprising a plura-
lity of layers o materials. It is noted that the vertical
scale in FIG. 3 is distorted and the thicknesses of the
material layers is greatly exaggerated. FIG. 4 conveniently
identifies the layers of, materials used to form the two
conductive layer flexible cable 11. The base material in
the flexible cable 11 forming insulating layers 20, 28, 36
is a condensate polyimide which may be embodied by a Kapton~
polyimide manufactured by E. I. duPont de Nemours & Co. Inc.
of Fairfield, CT 06433. A Kapton~ polyimide is selected
because of its flexibility, tensile strength and excellent
insulating prope~ties in thin sheets o 1 to 2 mils. The
conductors 24 and 32 are formed from soft copper foil gen-
erally 1 or 2 ounce ~ or 2.8 mils), and it is rolled
; ~ annealed thereby providing.a relatively soft copper. Layer,s
22, 26, 30, 34 o an acrylic adhesive such as ParaluxTM
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also manufactured by E.I. du Pont Nemours Co. Inc. are
used to bond together the Kapton0 insulating layers 20, 28,
36 and the copper conductor layers 24, 32. Such an acrylic
adhesive is used in thicknesses of 1 and 2 mils. Leadless
chip carrier 12 which i5 a familiar device to those skilled
in the art o~ microelectronics has leadless terminals 23
disposed around its perimeter, and it is soldered to spe-
cifically arranged corresponding pads 25 on the flexible
cable 11. Apertures 27 are cut in the Kapton~ layer 20 to
avoid insulating the tops of solder pads 25 which protrude
through the mating apertures 27 and to enable pins 15 to be
inserted into the flexible cable 11 and soldered. The heat
sink/backer 14 may be embodied with an epoxy glass such as
G10 material known to one skilled in the art or other alter-
nate heat conductive material such as alumina or coe~ficient
of expansion matched materiai such as a copper-invar-copper
laminate depending on the cooling needs of a particular LCC.
The heat sink/backer 14 may be of a size as shown in ~IG. 1
and 3 covering the area under the LCC 12 or it may be larger
covering the area around the connection pins 15. It is
attached to the flexible cahle 11 by means of an adhesive.
The heat sink/backer 14 provides a means to restrain the
flexible cable assembly 10 to assure that the input connector
pins 15 stay mated, that the LCC 12 is in proper contact
with the heat sink/backer 14 and that the ~lexible cable 11
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is restrained in the area of the LCC 12 in order to prevent
fatiguing of the LCC 12 solder connections. Pins 15 are
inserted, swagged and soldered into each one of the nine
plated-thru holes 29 of a first end of flexible cable assembly
lO and these pins 15 form the connecting means for attaching
the first end of the cable assembly lO to a printed circuit
board. A second end of flexible cable assembly 10 comprising
connector 16 is clearly shown in the cut-away section of
FIG. l. Approximately one-quarter inch of the bottom ends
of the plurality of copper strip conductors of layer 32 are
noninsulated as noted hereinbefore and gold-plated, thereby
permitting these conductors to be used as an end connector
16 for clamping this end of cable assembly lO to mating
conductors on the display element 58 by a V-shaped clamp 62
; 15 as shown in FIG. 7.
The method for making a flexible cable 11 is similar to
the method for making single, double sided and multilayer
printed wiring boards and known to one skilled in the art
because both methods include etched copper on a dielectric
material. Printed wiring boards are generally used to inter-
connect components whereas flexible cables are generally used
to interconnect printed wiring boards to other printed wiring
boards or other active ~evices. Because they are so similar,
most of ~he method steps are the same even though some of the
material may be diferent. The method of fabricating a
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flexible cable 11 comprises the following steps:
1. Select material, thickness and assemble: insu-
lator (Kapton~), adhesive ~ParaluxTM), conductor
tsoft copper).
2. Punch or drill tooling noles: Holes shall
match the artworX and drill tape for back-to-
front registration and cover sheet lamination.
3. Drill thru-holes: Special attention mus~ be
given to drill feeds and speeds because of the
burring of the soft copper.
4. Desmear excess adhesive: Smear is caused by the
drill heating up going through the copper,
melting the acrylic, and smearing acrylic on
the copper during the exit stroke of the drill.
5. Electroless and electroplate copper for plated
thru-holes: Only used when plated thru-holes
are present on the circuit and is the same
process used for printed wiring boardsO
6. Image Conductor Pattern: The flexible cable
now has copper on two sides with plated thru-
holes It is then coated with dry film photo
resist, the artwork aligned, exposed and
developed leaving a positive image of the
desired circuit with photo resist tenting the
thru-holes for protection during subsequent
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processing.
70 Etch Conductor Pattern: Standard as for printed
wiring board fabrication; etching chemically
removes the excess copper leaving the desired
circuit.
8. Clean: Cleanliness is the single most important
process in successfully producing printed wiring
boards, multilayer boards (MLB), or flexible cables.
A cleaning step or several cleaning steps take place
beore each operation to insure that subsequent
process steps can be performed without interfer-
ence from undesired dirt, oxide, or residuesO
9. Cover Coat: To protect the copper conductors
from handling damage and electrical shorting, a
lS coating over the conductors is necessary. To main-
tain the flexibility, the same base materials are
used for the cover coating. A thin KaptonTM layer
is laminated over the conductors using the proper
amount oE acrylic adhesive. In this case, the
adhesive is predrilled or punched to leave pads
exposed for soldering.
10. Laminate heat sink/backer: Acrylic adhesive
is used to adh~esively attach the heat sinkJbacker
to the flexible cable.
11. Solder Coat: Since there is no need for solder
anywhere but on the pads, flexible cables are
solder coated after the cover coat is laminated.
This is done by dipping the part in molten solder
and removing the excess solder by blowing with hot
air or hot oil or wiping with a roller to remove
the excess solder.
12. Profile: Routing with templates and carbide
tools is qenerally used for printed wiring boards
and multilayer boards but tends to be expensive.
However, flexible cables can be cut very effectively
using inexpensive steel rule dies, or in small
quantities can be knifie cut.
Referring now to FIGS. 3, 5 and 6, typical layouts for
the top and bottom layers of copper conductors 24 and 32 of
flexible cable 11 on the Kapton~ dielectric material layer 28.
are shown after the etching step of the fabrication method. Also
shown in FIG. 5 are the circular dots which will become plated
thru~holes 29, 31 and the copper pad arrangemen~ 25 for attach-
ina the leadless chip carrier 12 to the flexible cable ll.
Referrlng again to FIGS. 1, 2 and 3, when the flexible
cable 11 is made the steps for completing the assembly of
the flexible cable assembly 10 are as follows:
1. First Inspection: The flexible cable 11 is
inspected for workmanship and confoxmance
to specifications.
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2. Solder Paste Application: Solder paste is
screen printed onto the solder pads 25 utilizing
common screening equipment.
3. Placement of LCC: A leadless chip carrier 12
having pre-tinned pads 23 is placed with proper
orientation on the arranged solder pads 25 on
the flexible cable 11.
4. Second inspection: The placement of the LCC 12
onto the flexible cable ll is inspected to
assure that the pre-tinned pads 23 on the
LCC 12 are aligned properly to the solder
pads 25 on the flexible cable 11.
5. Vapor phase soldering: The flexible cable 11
with the LCC 12 attached is vapor phase reflow
soldered.
6. Third inspection: The soldered assembly 10 is
inspected for proper solder joints (fillets).
7. Cleaning: A cleaning is performed to remove
solder flux and other contaminates.
8. Connector pin attachment: The terminal pins
15 are swagged into each of the 9 plated-thru
holes 29 and soldered into position.
9. Fourth Cleanin~: A cleaning is performed to
remove flux resid,ues resulting from the
previous soldering.
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10. Final In.spection: A final inspection of the
flexible cable assembly is performed to verify
proper workmanship and compliance with assembly
procedures.
Referring now to FIG. 7 the flexible cable assembly 10
is shown interconnecting two portions of a flat panel matrix
display assembly 50 which is shown in an exploded perspective
view. The first end of the flexible cable assembly 10 having
the pins 15 adjacent to the leadless chip carrier 12, is in-
serted into a driver electronics module 54, and the secon2
end connector 16 of the flexible cable assembly 10 having
exposed strip conductors on one side of the flexible ~able
11 is wrapped around the edge of a flat glass panel matrix
display element 58 while simultaneously aligning the exposed
conductors of end connector 16 to the mating conductors on
the edge of the bottom side (not shown) of the display element
58. End connector 16 is secured to the display element 58
by a U-shaped clamp 62. A plurality of flexible cable assem-
blies 10 are used in such a flat panel matrix display assembly
50 around the perimeter of the driver module 54 and display
element 58 to make all the necessary interconnections. The
- matrix display assembly 50 also comprises a controller module
52 for controlling signals to the driver module 54 and hence
the display element 58, a stifener panel 56 and a frame/bezel
2; 60 for protecting and supporting the integrity of the display
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element 58. The display element 58 may be embodied by a
flat panel plasma display element, part No. PDM256512-062,
manufactured by Electro-Plasma Inc. (EPI) of Milbury, Ohio.
The input signals to the LCC 12 on the flexible cable
assembly 10 from the driver module 54 are all via the con-
nector means formed by pins 15. The output signals from the
LCC 12 to the display element 5R are via the flexible cable
end connector 16 having the exposed or noninsulated strip
conductors which are aligned with the mating conductors on
the peripheral ed~e of the display element 58 and clamped in
place. The number of connector pins 15 needed for connection
between flexible cable assembly 10 and the driver module 54
is significantly less than what would be reyuired without
the placement of the active LCC 12 device on the cable assem-
bly 10. Only ~ small number of pins are required to handle
control drive signal inputs to the flexible cable assembly
10 and the larger group of decoded output signals from the
LCC 12 are handled by the layers of strip conductors 24, 32
and the noninsulated end connector 16 formed from layer
; 32.' The use of a flexible cable assembly 10 in the matrix
display assembly 50 not only reduces the number of dlrect
electrical connections required between the driver module 54
and the display element~58, but also significantly decreases
the overall volume of the matrix display assembly 50. In
addition, maintainability of the matrix display assembly 50
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is improved by allowing the driver module 54, the display
element 5~ and the flexible cable assembly 10 to be dis-
assembled and reassembled in the field. Repairs can now be
readily performed in the field on ~he driver module 58 and
the flexible cable assembly lO by the replacement of failed
circuit parts or devices with relatively inexpensive spare
circuit parts or devices.
This concludes the description of the preferred embodi-
ment. However, many modifications and alterations would be
obvious to one of ordinary skiil in the art without departing
from the spirit and the scope of the inventive concept. For
example the flexible cable may have one conductor layer or
multiple conductor layers, the material in the conductive
layers as well as the plating on the connecting end to the
matrix display may be copper! gold, silver, aluminum or other
similar electrically conductive material or material compound.
The surface area of the heat sink may be smaller or larger
or the type of connecting means on each end of the flexible
cable assembly may vary depending on the particular applica-
tions. Therefore, it is intended that the scope of ~his
invention be limited only by the appended claims.
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