||||l|||||||||Ill|||||lllllllllllllll|||||Illll|||l|||||||l|||l||||||||||||
`Us0o5266746A
`
`United States Patent
`
`1191
`
`[11] Patent Number:
`
`5,266,746
`
`Nov. 30, 1993
`Nishihara et al.
`[45] Date of Patent:
`
`[54] FLEXIBLE PRINTED CIRCUIT BOARD
`HAVING A METAL SUBSTRATE
`
`5,130,768 7/1992 Wu et al.
`
`........................ .. 357/75 X
`
`FOREIGN PATENT DOCUMENTS
`
`2724399 11/1978 Fed. Rep. of Germany .
`60-32387 2/1985 Japan .
`61-226995 10/1986 Japan .
`2-24394
`5/1990 Japan .
`2-24395
`5/1990 Japan .
`2-25779
`6/1990 Japan .
`
`Primary Examiner—I_eo P. Picard
`Assistant Examiner-—Cheryl R. Figlin
`Attorney, Agent, or Firm—Burns, Doane, Swecker &
`Mathis
`
`ABSTRACT
`[57]
`A flexible circuit board including a flexible substrate
`having an insulating polyimide sheet and a wiring pat-
`tern portion formed in a mounting portion and a wiring
`pattern portion formed in a connecting portion, and a
`metal substrate on which only the mounting portion of
`the flexible substrate is secured by means of a thermo-
`plastic polyimide film. Electronic devices are mounted
`on the mounting portion of the flexible substrate and
`heat generated by the electric devices can be effectively
`dissipated through the metal substrate. The connecting
`portion of the flexible substrate can be connected to an
`external circuit by means of a connector provided at an
`edge of the connecting portion. Since the connecting
`portion is not secured to the metal substrate, connector
`pins having a large mechanical strength can be used and
`.
`.
`.
`‘he C°““eC‘"‘8 P°m°“ 93“ be hem 3‘ W1“-
`
`13 Claims, 10 Drawing Sheets
`
`[75]
`
`Inventors: Kunio Nishihara, Yokohama; Yoichi
`Hosono, Hiratsuka; Takayuki
`Ishiltawa, Kamakura, all of Japan
`Mitsui Toatsu Chemicals, Inc.,
`Tokyo, Japan
`
`[73] Assignee:
`
`[2]] Appl. No.: 798,958
`
`[22] Filed:
`
`Nov. 27, 1991
`
`Foreign Application Priority Data
`[30]
`Nov. 29, 1990 [JP]
`Japan ................................ .. 2—325753
`
`Int. c1.s ............................................. .. I-105K 1/00
`[51]
`[52] U.S.Cl. .................................. .. 174/254; 174/259;
`361/779; 361/749
`[58] Field ofsearch ............. .. 174/250, 255, 259, 254;
`361/411, 410, 412, 414, 416; 357/70, 71, 74, 75
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`2/1960 Miller ............................ .. 174/262 X
`2,923,860
`7/1966 Osipchak et al,
`.
`3,259,805
`3,409,732 ll/1968 Dahlgren et al.
`.
`4,677,252
`6/1987 Takahashi et a1.
`4,679,122
`7/1987 Belke, Jr. et a1.
`4,839,232
`6/1989 Morita et al.
`.
`5,041,847
`8/1991 Matsumoto et al.
`5,067,004 11/1991 Marshalletal.
`5,068,708 11/1991 Newman .......... ..
`5,077_.595 12/1991 Fukunaga
`5,084,124
`1/1992 Taniguchi
`
`........... .. 361/411 X
`
`.
`
`......... .. 346/76 PH
`..... .. 357/71
`357/71 x
`357/75 X
`174/259 X
`
`
`
`/ la
`
`I /D
`,_..._?Jx:..__.fi
`
`
`
`/30
`
`lab
`'7" ,5, /35 I”
`;5a /30 /7o
`71¢:
`1. .=‘ 1.
`
`
`, , ;,_.=;_‘;;._=_-.;=‘..a‘:;.:.;-,=:.‘:.‘.‘.-_-A-.‘.-::::-.§),:
`
`
`
`
`/2
`
`
`
`
`NU MARK Ex.1028 p.1
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 1 of 10
`
`5,266,746
`
`FIG. I
`PRIOR ART
`
`
`
`FIG. 2
`
`
`
`
`
`NU MARK Ex.1028 p.2
`
`

`
`U.S. Patent
`
`Nov.‘ 30, 1993
`
`Sheet 2 of 10
`
`NU MARK Ex.1028 p.3
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 3 of 10
`
`5,266,746
`
`NU MARK Ex.1028 p.4
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`V Sheet 4 of 10
`
`5,266,746
`
`NU MARK Ex.1028 p.5
`
`

`
`U.S. Patent
`
`Nov. 30, 1993.
`
`Sheet 5 of 10
`
`5,266,746
`
`
`
`
`
`ofifiiftjiiijji
`Q ICICIIICICCJ
`
`
`
`
`O
`
`n\§‘1‘9
`\ I Q\\\‘
`11115 VIIIIIIIIIIIIJI /Q
` ————;——j
`-3-CK
`V §‘V
`5‘ 1 gusts’
`vlapllII’.\"I'Il4 ‘VIII!
`
`TIIII551 VJIIIIJK
`CVICCC C'\‘C§C “““““‘““““‘
`VIIIIAVKXIIIJIIKKIIIJIJJKKIJIJIIZJ
`
`N&\\\\‘
`
`
`
`
`NU MARK Ex.1028 p.6
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 6 of 10
`
`5,266,746
`
`
`
` 6
`
`/////////
`
`
`NU MARK Ex.1028 p.7
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 7 of 10
`
`5,266,746
`
`F/618A
`
`as. as 2
`
`3/0
`
`3/0
`
`FIG. 86‘
`
`
`
`NU MARK Ex.1028 p.8
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 8 of 10
`
`5,266,746
`
`34a
`
`
`34b
`
`-zarmozrtt 32
`
` WIIIIIIIIIIA
` %““38
`
`Fl6‘. 9A
`
`FIG. 98
`
`F/6.96‘
`
`NU MARK Ex.1028 p.9
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 9 of 10
`
`52659746
`
`F/G. /0.4 237
`
`3/
`
`F/62. /03
`
`340
`
`3/0
`
`340
`
`3/0
`
`F/6‘. /06‘
`
`F/6‘. /00 T
`
`NU MARK Ex.1028 p.10
`
`

`
`U.S. Patent
`
`Nov. 30, 1993
`
`‘ Sheet 10 of 10
`
`5,266,746
`
`F/G‘. //,4
`
`3/
`
`
`3/0
`‘L?
`
`3/b
`
`‘
`
`F/G. //[7
`
`3/0
`
`3/0
`
`35
`
`
`
`NU MARK Ex.1028 p.11
`
`

`
`1
`
`5,266,746
`
`FLEXIBLE PRINTED CIRCUIT BOARD HAVING A
`METAL SUBSTRATE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present
`invention relates to a flexible circuit
`board, and more particularly to a flexible circuit board
`comprising a flexible insulating sheet and a metal sub-
`strate on which said insulating sheet is applied by means
`of a thermoplastic polyimide layer.
`2. Description of the Related Art
`Recently, it has been required to dissipate effectively
`heat generated from electronic or electric devices pro-
`vided on a printed circuit board in accordance with the
`increase in the number of electronic devices and the use
`of high power semiconductor devices. Therefore, in the
`case of mounting high power semiconductor devices at
`a high density or in a hybrid IC, it has been proposed to
`use a substrate made of ceramics. In order to connect
`the semiconductor elements and devices mounted on
`the printed circuit board to an external circuit, there
`have been widely used connector pins of such a type
`that the printed circuit board is clamped by the pins at
`its peripheral portion from front and rear surfaces. Such
`connector pins have a large mechanical strength and
`could hardly be damaged even if an external force is
`applied thereto, so that it is possible to attain a very high
`reliability in the electrical connection.
`The packing density of the printed circuit board and
`the power of semiconductor devices have become
`higher and higher, and a substrate made of metal has
`been used in the print circuit board, the heat dissipating
`property of the metal substrate being superior to that of
`the ceramic substrate. In Japanese Patent Application
`Laid-open Publication Kokai Sho 61-226995 and Japa-
`nese Patent Publications Nos. 2-24394, 2-24395 and
`2-25779, there nave been proposed several known flexi-
`ble circuit boards using a metal substrate.
`In the known flexible circuit board including the
`metal substrate, the metal substrate is exposed on the
`rear surface, and thus when the above mentioned con-
`nector pins of such a type that the flexible circuit board
`is clamped by the connector pins from the front and rear
`surfaces are used, all the connector pins would be short-
`circuited by means of the metal substrate, so that such
`connector pins having a large mechanical strength
`could not be utilized.
`In the known flexible circuit
`boards, there have been used surface mounting type
`pins.
`FIG. 1 is a cross sectional view showing an example
`of the known flexible circuit board in which the surface
`mounting type pins are used. On a metal substrate 1 a
`flexible substrate 3 is secured by means of an adhesive
`layer 2. The flexible substrate 3 comprises a flexible
`insulating sheet 4 and a wiring pattern 5 having a prede-
`termined configuration and formed on one surface of
`the metal sheet. An edge of the wiring pattern 5 is re-
`tarded inwardly from an edge of the flexible substrate 3
`so that a distance between the metal pattern 5 and the
`metal substrate 1 measured along the surface of the
`flexible circuit board can be increased. On a surface of
`the wiring pattern 5 near its edge, a tip of a pin 6 is
`secured by soldering.
`In the known flexible circuit
`board including the metal substrate, the metal substrate
`1 and the flexible substrate 3 is secured to each other by
`means of the adhesive agent layer such as an epoxy type
`adhesive agent. Such an epoxy type adhesive agent has
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`a problem of heat resistance and the flexible circuit
`board might be peeled off the metal substrate 1 during
`the wire bonding for connecting bare semiconductor
`chips to the wiring pattern 5 or during the usage of the
`flexible circuit board under a high temperature. In order
`to remove such a drawback, in Japanese Patent Appli-
`cation Laid-open Publication Kokai Sho 61-226995, it
`has been proposed to adhere the flexible circuit sub-
`strate to the metal substrate by means of a thermoplastic
`polyimide film which has a self-adhesive property.
`FIG. 2 is a cross sectional view illustrating another
`example of the known flexible circuit board having a
`metal substrate. In this example, a tip of a pin 6 is ex-
`tended along a surface of a wiring pattern 5 of a flexible
`substrate 3 which is secured to a metal substrate 1 by
`means of an adhesive layer 2, and is connected to the
`wiring pattern 5 by soldering.
`In the known flexible circuit board, the pin of the
`surface mounting type is connected to the surface of the
`wiring pattern by soldering. During the mounting oper-
`ation of the electronic devices such as chip devices and
`semiconductor bear chips onto the flexible circuit board
`or after the mounting operation, when the external
`force is applied to the pin, the flexible substrate might be
`peeled off the metal substrate and the reliability of the
`connection is lost. In the known flexible circuit board
`shown in FIG. 2, since the pin is connected to the wir-
`ing pattern over a relatively long length so that the
`concentration of the external force is mitigated. But in
`this case, the distance between the pin and the metal
`substrate measured along the surface becomes short and
`the pins might be short-circuited via the metal substrate.
`Moreover, in the known flexible circuit board having
`the metal substrate,
`the metal substrate and flexible
`substrate have the same configuration and are adhered
`to each other over their entire surfaces. Therefore, the
`freedom in bending is limited by the metal substrate and
`the application of the flexible circuit board is restricted.
`Further, the known flexible circuit board is connected
`to an external circuit by means of the surface mounting
`type pins, and thus a plurality of flexible circuit boards
`could not be easily stacked one on the other to form a
`laminated unit body.
`SUMMARY OF THE INVENTION
`
`The present invention has for its object to provide a
`novel and useful flexible circuit board in which the
`above mentioned drawbacks of the known flexible cir-
`cuit board can be avoided, highly reliable electrical
`connection can be obtained, freedom in bending can be
`attained lamination is possible even though a metal
`substrate having an excellent heat dissipating property
`is used.
`According to the invention, a flexible circuit board
`comprises:
`a metal substrate having a surface;
`at least one flexible substrate including a flexible insu-
`lating sheet, a first wiring pattern portion formed on a
`part of the insulating sheet which constitutes a mount-
`ing portion for mounting electric devices on the flexible
`circuit board, and a second wiring pattern portion
`formed on the remaining portion of said insulating sheet
`which constitutes at least one connecting portion for
`connecting the flexible circuit board to an external cir-
`cuit; and
`at least one insulating thermoplastic resin film having
`a self-adhesive property and inserted between said
`
`
`NU MARK Ex.1028 p.12
`
`

`
`5,266,746
`
`4
`tutes a mounting portion for mounting at least one elec-
`tric device, and a second wiring pattern portion formed
`on the remaining portion of said insulating sheet which
`constitutes a connecting portion for connecting the
`flexible circuit board to an external circuit, and an insu-
`lating thermoplastic resin film having a self-adhesive
`property arranged between said mounting portion of
`the flexible substrate and said metal substrate, compris-
`ing the steps of:
`providing an insulating sheet on a surface of a metal
`foil;
`stacking said insulating sheet on the metal substrate
`such that only said mounting portion of the flexible
`substrate is placed on the metal substrate by interposing
`the insulating thermoplastic resin film therebetween;
`and
`
`pressing and heating an assembly of said insulating
`sheet with the metal foil and metal substrate to secure
`said mounting portion of the flexible substrate to said
`metal substrate by means of said insulating thermoplas-
`tic resin film; and
`selectively etching the metal foil in accordance with
`a desired pattern to form said first wiring pattern in the
`mounting portion and the second wiring pattern portion
`in the connecting portion.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a cross, sectional view showing a first exam-
`ple of a known flexible circuit board having a metal
`substrate;
`FIG. 2 is cross sectional view illustrating another
`example of a known flexible circuit board;
`FIGS. 3A and 3B are cross sectional and plan views,
`respectively depicting a first embodiment of the flexible
`circuit board according to the invention;
`FIG. 4 is a perspective view showing the condition of
`connecting the flexible circuit board shown in FIGS.
`3A and 3B to an external circuit;
`FIG. 5 is a perspective view illustrating a second
`embodiment of the flexible circuit board according to
`the invention;
`FIG. 6 is a cross sectional view depicting a third
`embodiment of the flexible circuit board according to
`the invention;
`FIG. 7 is a perspective view illustrating a fourth
`embodiment of the flexible circuit board according to
`the invention;
`FIGS. 8A to 8D are cross sectional views represent-
`ing successive steps for manufacturing the flexible cir-
`cuit board according to the invention;
`FIGS. 9A to 9C are cross sectional views illustrating
`successive steps for manufacturing the flexible circuit
`board according to the invention;
`FIGS. 10A to 10D are cross sectional views depicting
`successive steps for manufacturing the flexible circuit
`board according to the invention; and
`FIGS. 11A to 11D are cross sectional views showing
`successive steps of manufacturing the flexible circuit
`board according to the invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`3
`mounting portion of the flexible substrate and said sur-
`face of the metal substrate.
`In the flexible circuit board according to the inven-
`tion,
`the electronic device mounting portion of the
`flexible substrate is secured to the metal substrate hav-
`ing an excellent heat dissipating property, and therefore
`heat generated by electronic devices provide on the
`mounting portion can be effectively dissipated via the
`metal substrate and the electronic devices can be pro-
`tected against the overheating. Thus, a large number of
`electronic devices including high power semiconductor
`elements can be mounted on the flexible circuit board at
`a high packing density.
`Moreover, since the connecting portion of the flexi-
`ble substrate is not connected to the metal substrate, it is
`possible to use connector pins having high mechanical
`strength to increase the reliability in the electrical con-
`nection, and freedom in bending can be attained and
`thus the flexible circuit board according to the inven-
`tion can be used in numerous applications.
`Furthermore, the flexible substrate is divided into the
`mounting portion and the connecting portion and only
`the mounting portion is secured to the metal substrate,
`so that when a plurality of flexible circuit boards are
`laminated to form a_ single body, the connecting por-
`tions can be extended independently from one another
`and the connection to external circuits can be effected
`easily. In this case, a plurality of connecting portions
`may be extended in the same direction or in different
`directions in accordance with particular applications.
`The present invention also relates to a method of
`manufacturing a flexible circuit board having a metal
`substrate and has for its object to provide a novel and
`useful method of manufacturing the above mentioned
`novel flexible circuit board in a simple manner.
`According to the invention, a method of manufactur-
`ing a flexible circuit board having a metal substrate, a
`flexible substrate which includes a flexible insulating
`sheet, a first wiring pattern portion formed on a part of
`the insulating sheet which constitutes a mounting por-
`tion for mounting at least one electric device, and a
`second wiring pattern portion formed on the remaining
`portion of said insulating sheet which constitutes a con-
`necting portion for connecting the flexible circuit board
`to an external circuit, and an insulating thermoplastic
`resin film having a self-adhesive property arranged
`between said mounting portion of the flexible substrate
`and said metal substrate, comprising the steps of:
`providing an insulating sheet on a surface of a metal
`foil;
`selectively etching the metal foil in accordance with
`a desired pattern to form said flexible substrate having
`said first wiring pattern in the mounting portion and the
`second wiring pattern portion in the connecting por-
`tion;
`stacking said flexible substrate on the metal substrate
`such that only said mounting portion is placed on the
`metal substrate by interposing the insulating thermo-
`plastic resin film therebetween; and
`pressing and heating an assembly of said flexible sub-
`strate and metal substrate to secure said mounting por-
`tion of the flexible substrate to said metal substrate by
`means of said insulating thermoplastic resin film.
`According to a further aspect of the invention, a
`method of manufacturing a flexible circuit board having
`a metal substrate, a flexible substrate which includes a
`flexible insulating sheet, a first wiring pattern portion
`formed on a part of the insulating sheet which consti-
`
`65
`
`FIGS. 3A and 3B are cross sectional and plan views,
`respectively showing a first embodiment of the flexible
`circuit board according to the invention. The flexible
`circuit board of the present embodiment comprises a
`flexible substrate 11 and a metal substrate 12. The flexi-
`ble substrate 11 is divided into a mounting portion 11::
`
`
`NU MARK Ex.1028 p.13
`
`

`
`5
`for mounting electric devices such as semiconductor
`chips (bare chips) 13a and chip devices 13b thereon and
`a connecting portion 11b for connecting the flexible
`circuit board to an external circuit. According to the
`invention, only the mounting portion 11a of the flexible
`substrate 11 is secured to the metal substrate 12 by
`means of a thermoplastic polyimide layer 14. The ther-
`moplastic polyimide layer 14 used in this embodiment is
`preferably formed by a resin having an imide construc-
`tion within the molecular skeleton and having a glass
`transition temperature not lower than 160° C. and not
`higher than 300° C. When use is made of a resin having
`a glass transition temperature lower than 160° C., the
`reliability under high temperature becomes low and
`when a resin having a glass transition temperature
`higher than 300° C is used, the adhesive force becomes
`weak. Since the thermoplastic polyimide layer 14 has a
`self-adhesive property, it is possible to adhere the flexi-
`ble substrate 11 to the metal substrate easily by heating
`them at a temperature above the glass transition temper-
`ature with the aid of a heat press or a heat laminate roll
`without using any other adhesive agent. In the present
`embodiment, the metal substrate 12 is made of copper,
`but it may be made of iron, aluminum, alumite, stainless
`steel, silicon steel, molybdenum, 42 alloy and nickel
`silver.
`The flexible substrate 11 comprises an insulating sheet
`15 made of polyimide resin and a wiring pattern having
`a given configuration. The wiring pattern is formed by
`selectively etching a copper foil applied on the insulat-
`ing sheet 15. The wiring pattern is divided into a first
`wiring pattern portion 16a formed in the mounting
`portion 11a and a second wiring pattern portion 16b
`formed in the connecting portion 11b. The first wiring
`pattern portion 160 and the bare semiconductor chips
`13a are connected by means of bonding wires 17a, and
`the second wiring pattern portion 16b and the chip
`devices 13b are connected by solderings 17b. The sec-
`ond wiring pattern portion 16b formed in the connect-
`ing portion llb is extended up to an edge of the con-
`necting portion. A plurality of conductors of the wiring
`pattern are aligned at a given pitch along the edge of the
`connecting portion 11b of the flexible substrate 11.
`In order to connect the portion of wiring pattern 16b
`extending to the edge of the connecting portion 111: of
`the flexible substrate 11 to an external circuit, a connec-
`tor 18 is provided at the edge of the flexible substrate 11
`as shown in FIG. 3A. For the sake of simplicity, the
`connector 18 is not shown in FIG. 3B. The connector
`18 comprises connector pins 18a of such a type that the
`flexible substrate 11 is clamped by the pin from front
`and rear surfaces thereof, and an insulating housing 18b.
`In this manner, according to the present
`invention,
`although the connector pins 180 having a very strong
`mechanical strength are used, they are not short-cir-
`cuited to each other, and further a distance between the
`pins and the metal substrate 12 can be made long, be-
`cause the metal substrate is not existent in the connect-
`ing portion 1117. In this manner, the reliability of the
`electrical connection can be improved. Moreover, the
`connecting portion 11b of the flexible substrate 11 can
`be bent freely and thus the freedom for arranging the
`flexible circuit board relative to the external circuits can
`be increased. It should be noted that according to the
`invention, the second wiring pattern portion 16b may be
`connected to the connector pins 18a by utilizing one of
`known various connecting modes. Further, the conduc-
`tors of the second wiring pattern portion 1617 at the edge
`
`10
`
`I5
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`5,266,746
`
`6
`of the connecting portion 11b may be soldered to con-
`ductors of a wiring pattern of an external flexible circuit
`board without using connector pins.
`In one method of manufacturing the above men-
`tioned flexible circuit board according to the invention,
`after the polyimide sheet 15 has been directly applied on
`a copper foil, the copper foil is selectively etched to
`form the first and second wiring pattern portions 16a
`and 16b having the desired configuration. Then, the
`thermoplastic polyimide film 14 is formed by applying
`polyamic acid to a surface of the metal substrate 12 and
`heating to conduct the imidization. Finally the mount-
`ing portion 11a of the flexible substrate 11 is placed on
`the metal substrate 12 and is heated under pressure. In
`this manner, the mounting portion Ila of the flexible
`substrate 11 can be firmly and easily secured to the
`metal substrate 12.
`In another method of manufacturing the flexible cir-
`cuit board according to the invention, after the poly-
`imide sheet 15 has been directly formed on one surface
`of a copper foil and the copper foil has been selectively
`etched to form the first and second wiring pattern por-
`tions 16a and 16b, the thermoplastic polyimide film 14 is
`applied on a surface of the polyimide sheet 15 which is
`opposite to a surface on which the wiring pattern has
`been formed. Then, only the mounting portion 11a of
`the flexible substrate 11 is brought into contact with the
`metal substrate 12 while the thermoplastic polyimide
`film is interposed between the polyimide sheet 15 and
`the metal substrate 12. Finally, the assembly is heated at
`a temperature higher than the glass transition tempera-
`ture of the thermoplastic polyimide film 14 under pres-
`sure to adhere the flexible substrate 11 to the metal
`substrate 12. In this method, the thermoplastic poly-
`imide film 14 may be applied on a surface of the mount-
`ing portion 11:: of the flexible substrate 11 instead of on
`a whole surface thereof. Further, the first and second
`wiring pattern portions 16a and 16b may be covered
`with an insulating and flexible resist ink or a cover film
`except for portions at which the electronic devices are
`mounted on the flexible substrate 11 and a portion at
`which the connector is provided.
`FIG. 4 is a perspective view showing a condition in
`which the flexible circuit board according to the inven-
`tion is connected to an external circuit board 20 by
`means of the connector 18 which is provided along the
`edge of the connecting portion 11b of the flexible sub-
`strate 11, the connecting portion being extended from
`the mountingportion 11a which is secured to the metal
`substrate 12 by means of the thermoplastic polyimide
`film 14. The connecting portion 11b can be positively
`secured to the external circuit board 20 by inserting the
`connector pins 18a of the connector 18 into through
`holes formed in the external circuit board.
`FIG. 5 is a perspective view illustrating a second
`embodiment of the flexible circuit board according to
`the invention. In the present embodiment, a flexible
`substrate 11 comprises a single mounting portion 11:: for
`mounting electronic devices such as semiconductor
`chips 13a and chip devices 13b, and four connecting
`portions 11c, 11d, He and 11f which extend from the
`rectangular mounting portion 11a in four mutually or-
`thogonal directions. A polyimide sheet 15 of the flexible
`substrate 11 is secured to a metal substrate 12 by means
`of a thermoplastic polyimide film 14. At edges of the
`connecting portions 11c, 11d, He and llf there are
`provided connectors similar to the connector 18 shown
`in FIG. 3A, but in FIG. 5 these connectors are not
`
`
`NU MARK Ex.1028 p.14
`
`

`
`7
`shown for the sake of simplicity. In the present embodi-
`ment, the total number of conductors of the second
`wiring pattern portions 16b can be increased or the
`number of conductors of each of the second wiring
`pattern portions can be reduced.
`FIG. 6 is a cross sectional view showing a third em-
`bodiment of the flexible circuit board according to the
`invention. In the present embodiment,
`three flexible
`substrates 11-1, 11-2 and 11-3 are stacked one on the
`other such that mounting portions of these flexible sub-
`strates are placed on a metal substrate 12. The mounting
`portion of the lowest flexible substrate 11-1 is secured to
`the metal substrate 12 by means of a thermoplastic poly-
`imide film 14. The construction of the flexible substrates
`11-1 to 11-3 is similar to each other and each flexible
`substrate comprises a first polyimide sheet 15 having a
`thermoplastic polyimide film 14 applied on its one sur-
`face, first and second wiring pattern portions 16a and
`16b formed on the other surface of the polyimide sheet
`15, a second thermoplastic polyimide film 21 applied on
`the wiring pattern portions, and a second polyimide
`sheet 22 applied on the second thermoplastic polyimide
`film 21. The configuration of the first and second wiring
`pattern portions 16a and 16b of the flexible substrates
`11-1 to 11-3 differ from each other. In order to connect
`the wiring patterns of different flexible substrates 11-1
`to 11-3 to each other, the wiring patterns are connected
`to each other through via holes. Electronic devices 13a
`and 13b are mounted on the mounting portion of the
`uppermost flexible substrate 11-3.
`Connecting portions 11b-1, llb-2 and 11b-3 of the
`flexible substrates 11-1, 11-2 and 11-3 are not laminated
`each other and are extended independently from each
`other. The construction of these connecting portions
`11b-1 to 11b-3 is similar to each other except for the
`configuration of the second wiring patter portions 161).
`In each of the connecting portions 11b-1 to 11b-3, the
`second thermoplastic polyimide film 21 and second
`polyimide sheet 22 are not extended up to the edge of
`the connecting portion so that conductors of the wiring
`pattern portion 16b are exposed at the edge portion.
`FIG. 7 is a perspective view depicting a fourth em-
`bodiment of the flexible circuit board according to the
`present invention. In the third embodiment shown in
`FIG. 6, the connecting portions llb-1 to llb-3 of the
`flexible substrates 11-1 to 11-3 are extended from the
`mounting portions in the same direction. In the present
`embodiment, each of connecting portions 11b-1 to 11b-3
`is extended from each of three sides of rectangular
`mounting portions. This construction is particularly
`suitable for connecting the flexible circuit board to
`different external circuits. In second thermoplastic film
`21 and second polyimide sheet 22 of the uppermost
`flexible substrate 11-3 there are formed windows for
`mounting electronic devices.
`According to the invention, the electronic devices
`may be mounted not only on the uppermost flexible
`substrate 11-3, but also may be mounted within the
`lower flexible substrates 11-1 and 11-2. In this case, after
`one or more electronic devices have been mounted on
`the first wiring pattern portion 16a of the lower flexible
`substrate, the second polyimide sheet 22 is applied via
`the second thermoplastic polyimide film 21 having a
`large thickness such that
`the electronic devices are
`embedded within the second thermoplastic polyimide
`film. Further the wiring patterns of the flexible sub-
`strates 11-1 to 11-3 are connected to each other by
`means of holes. In this case, the electronic devices em-
`
`l0
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`5,266,746
`
`8
`bedded in the lower flexible substrates 11-1 and 11-2 are
`preferably thin and have good heat resistance.
`Furthermore, according to the invention, wiring pat-
`terns may be provided on opposite surfaces of an insu-
`lating sheet such as the polyimide sheet to form a flexi-
`ble substrate, and then a mounting portion of such a
`flexible substrate may be secured to a metal substrate by
`means of an insulating thermoplastic resin film such as
`the thermoplastic polyimide film.
`Now several methods of manufacturing the flexible
`circuit board shown in FIGS. 3A and 3B will be ex-
`plained.
`As illustrated in FIG. 8.6., on one surface of a copper
`foil 31 having a thickness of 9 to 200 microns is applied
`a polyimide sheet 32 having a thickness of several mi-
`crons to several hundred microns which is formed by a
`casting method. On a surface of this polyimide sheet 32
`is applied a thermoplastic polyimide film 33 having a
`thickness of l to 20 microns.
`
`Then, the copper foil 31 is selectively etched in ac-
`cordance with a desired pattern to form simultaneously
`first and second wiring pattern portions 31a and 31b as
`shown in FIG. 8B. In this case, the first and second
`wiring pattern portions 31:1 and 31b are formed such
`that they are located in mounting and connecting por-
`tions 34a and 34b, respectively.
`Next, as illustrated in FIG. 8C, a flexible resist ink or
`cover sheet 35 having a thickness of 20 to 200 microns
`is formed on the wiring pattern portions 310 and 3112 by
`applying a thermoplastic polyimide layer. In FIGS. 3
`and 4 such a cover sheet 35 is not shown. It should be
`noted that the cover sheet 35 is not formed at portions
`at which the electronic devices are to be mounted and
`
`at a portion at which the connector is to be provided.
`Finally, the mounting portion 34a of the flexible cir-
`cuit board is placed on a metal substrate 36 having a
`thickness of 0.05 to 4 mm as shown in FIG. 8D. Then,
`an assembly of the flexible substrate and the metal sub-
`strate is pressed, while it is heated at a temperature
`which is higher than a glass transition temperature of
`the thermoplastic polyimide film 33 to form the flexible
`circuit board according to the invention. In the present
`embodiment, the polyimide sheet 32 is directly applied
`to the copper foil 31, but a thermoplastic polyimide film
`having a thickness of l to 20 microns may be interposed
`therebetween.
`FIGS. 9A to 9C depict another embodiment of the
`method of manufacturing the flexible circuit board ac-
`cording to the invention. In the present embodiment,
`after the polyimide sheet 32 and thermoplastic poly-
`imide film 33 have been applied on the copper foil 31 as
`shown in FIG. 8A, a mounting portion 34a is secured to
`the metal substrate 36 by a heat press as shown in FIG.
`9A. During this step, a back-up block 38 having the
`same thickness as the metal substrate 36 is placed under-
`neath a connecting portion 34b. However, the copper
`foil 31 and polyimide sheet 32 are not secured to the
`back-up block 38.
`Then, the copper foil 31 is selectively etched to form
`simultaneously a first wiring pattern portion 31a in the
`mounting portion 340 and a second wiring pattern por-
`tion 31b in the connecting portion 3417 as illustrated in
`FIG. 9B.
`Finally, the cover sheet 35 is formed on the flexible
`substrate 34 as shown in FIG. 9C. After all the neces-
`sary steps have been done, the back-up block 38 is re-
`moved.
`
`
`NU MARK Ex.1028 p.15
`
`

`
`5,266,746
`
`9
`FIGS. 10A to 10D represent still another embodi-
`ment of the method of manufacturing the flexible circuit
`board according to the invention. In the present em-
`bodiment. As illustrated in FIG. 10A, on a surface of a
`copper foil 31 is secured a polyimide sheet 32 by means
`of a thermoplastic polyimide film 37 by the heat press or
`heat laminating roller. Then, t