Printed wiring board, and manufacture thereof

A printed wiring board includes an insulating layer, conductive layers respectively formed into predetermined circuit patterns on the upper and lower surfaces of at least the insulating layer, and a conducting section formed in a portion of the insulating layer so as to enable electrical connection between the upper and lower conductive layers. A thickness of the insulating layer is varied to change the electric characteristics of the printed wiring board according to a circuit configuration of the conductive layers.

BACKGROUND OF THE INVENTION
 The present invention relates to a printed wiring board and manufacture
 thereof and, more particularly, to an improved printed wiring board and an
 improved manufacturing method thereof in which conductive layers are
 formed into predetermined circuit patterns on both the upper and lower
 sides of at least the insulating layer, and a conducting section is formed
 in a portion of the insulating layer so as to enable electrical connection
 between the upper and lower conductive layers.
 An a result of the debut of digital copiers and color copiers,
 high-frequency signals are now used. With this situation, countermeasures
 against EMI (Electromagnetic Interference) according to the VCCI
 regulations (Voluntary Control Council Interference) pose technically
 important problem to the business machine industry.
 For example, a wiring component used for the transmission of communication
 and image signals within a digital copier in required to simultaneously
 satisfy demands for an electromagnetic shielding capability and for the
 capability of stably transmitting high-frequency signals. In many cases, a
 shielded electric wire is usually employed an the wiring component.
 However, the electric wire itself in expensive, and it is difficult to
 assemble an electric wire for a power source and electric wires for a
 high-frequency signal into a single wiring component, which in turn
 prevents reduction in the size of the product.
 Printed wiring boards provided with the same noise leak prevention function
 as that of the shielded electric wire are proposed as means for solving
 the previously-described problem. For example, a double-sided flexible
 printed wiring board (hereinafter referred to as a double-sided FPC as
 required) is formed into a microstrip line structure. One of two sides of
 the double-sided FPC in used as a signal layer, and the other side is used
 an a shielding layer (a ground layer). Further, there in proposed a
 two-layer single-sided flexible printed wiring board (hereinafter referred
 to an a two-layer single-sided PPC) [the Examined Japanese Patent
 Application Publication No. Hei 6-48753]. This FPC is produced by forming
 a signal layer and a shielding layer on respective insulating bases, and
 by stacking the thus-formed signal layer and the shielding layer into a
 two-layer structure. The two-layer single-sided flexible printed wiring
 board is provided with a strip line structure.
 The shielding layers of the microstrip line structure and the strip line
 structure shield the signal layer and suppress the leakage of noise by
 distorting high-frequency signals to such an extent as to provide no
 practical problems.
 According to a known method of manufacturing the double-sided FPC, an
 insulating resin base (insulating base), such as polyimide, having both
 sides coated with metal foil, such as iron, aluminum, or copper, is used.
 Holes layer are formed in the insulating resin base in order to enable
 electrical connection between conductor circuits provided on both sides of
 the insulating resin. The insulating resin base is entirely plated, and
 conductive circuits (such as a signal layer and a shielding layer) are
 formed by a known method such as etching.
 In contrast, according to another known method of manufacturing the
 two-layer single-sided FPC, two single-sided metal-foiled plates are
 formed by coating with iron, aluminum, or copper one side of each of two
 insulating resin bases (insulating bases) made of polyimide or polyester.
 Conductor circuits (such an a signal layer and a shielding layer) are
 formed on the respective FPCs by a known method such as etching. Holes are
 formed in the desired locations of an insulating resin layer of the
 respective single-sided metal-foiled plates. Subsequently, the
 single-sided metal-foiled plates are laminated, and electrical connection
 between the signal layer and the shielding layer is established by
 applying conductive paste to the holes.
 There often arises the case of handling both a digital signal and an analog
 signal in one printed wiring board.
 However, the required electric characteristics of the printed wiring board
 change according to whether the printed wiring board handles a digital
 signal or an analog signal. It is very difficult to handle the signals
 mixedly.
 More specifically, it is necessary to round the waveform of a digital
 signal to a desired extent while suppressing ringing or reflection. In
 contrast, it is necessary to faithfully transmit an analog signal without
 rounding or distorting its waveform and to protect the analog signal from
 external noise.
 At this time, the electric characteristics of the printed wiring board are
 determined by the thickness and dielectric constant of an insulating layer
 and by the width and geometry of a wiring pattern. If the electric
 characteristics are determined according to either the analog or digital
 signal, the other signal, for example, the analog signal, may be rounded
 or distorted too much to be practically used. Conversely, in the case of
 the digital signal, the leakage of noise such an electromagnetic waves
 increases because of ringing or reflection.
 As described above, it in difficult to control the electric characteristics
 of the printed wiring board so as to satisfy both signals. Particularly,
 in a case where the frequency of the signal is speeded up, this phenomenon
 becomes noticeable.
 To solve the previously described technical problems, it in conceivable
 that a printed wiring board for handling a digital signal in independent
 of a printed wiring board for handling an analog signal, and the electric
 characteristics of each of the printed wiring boards are controlled so an
 to match with the corresponding signal. This method adds to the cost of
 components and therefore is impractical.
 Another means for solving the problems in a conductor circuit (a conductor
 circuit which require small electric capacitance) for handling, e.g., an
 analog signal, in which only an conductive layer of a shielding layer
 provided on a circuit pattern possessing small electric capacitance is
 formed into a mesh or a lattice. As a result, the analog signal becomes
 more susceptible to external electromagnetic-wave problems, and there
 arisen another technical problem such as unstable characteristic impedance
 of the conductor circuit.
 SUMMARY OF THE INVENTION
 The present invention has boon conceived to solve the previously described
 technical problems, and the object of the prevent invention is to provide
 a printed wiring board and a manufacturing method thereof capable of
 easily controlling the electric characteristics of the wiring board itself
 so as to match with the characteristics of conductor circuits of a
 plurality of system while reducing the cost of the printed wiring board.
 As illustrated in FIG. 1A, the present invention provides a printed wiring
 board including an insulating layer 1, conductive layers 2 and 3 (one of
 them is a signal layer, and the other in a shielding layer) respectively
 formed into predetermined circuit patterns on the upper and lower surfaces
 of at least the insulating layer 1, and a conducting section 4 formed in a
 portion of the insulating layer 1 so as to enable electrical connection
 between the upper and lower conductive layers 2 and 3, the improvement
 being characterized by the fact that the thickness of the insulating layer
 1 is varied to change the electric characteristics of the printed wiring
 board according to the circuit configuration of the conductive layers 2
 and 3.
 In such a technological means, the printed wiring board may be formed into
 any configuration so long as the conductive layers 2 and 3 are formed on
 the upper and lower surfaces of the insulating layer 1. If consideration
 in given to the ease of manufacture of the printed wiring board, as
 illustrated in FIGS. 1B and 1C, it is desirable for the printed wiring
 board to comprise a two-layer single-sided printed board 7 [more
 specifically, 7(1) and 7(2)] in which the conductive layers 2 and 3 are
 each formed on one side of the respective insulating bases 5 and 6 in
 predetermined circuit patterns.
 So long as electrical conduction between the conductive layers 2 and 3 is
 established, any type of through-hole such as a via-hole comprising a
 plated through-hole may be selected as the conducting section 4 of the
 present invention as required. In a mode in which patterned printed board
 materials are stacked, it in desirable to fill the via-hole with
 conductive paste in order to ensure conductivity.
 A mode of the printed circuit board, in which its electric characteristics
 are changed according to the circuit configuration of the conductive
 layers 2 and 3, is represented by a printed circuit board an illustrated
 in; e.g., FIG. 1A. In this printed circuit board, a thick portion 1a of
 the insulating layer 1 correspond to an analog circuit configuration, and
 a thin portion 1b corresponds to a digital circuit configuration.
 Further, although the availability of flexibility (the flexible
 characteristics) is not required by the printed wiring board of the
 present invention, it in desirable to provide; e.g., the two-layer
 single-sided printed wiring board 7 [(more specifically 7(1) and 7(2)],
 with flexibility if consideration is given to compactness, low profile,
 and ease of mounting.
 One mode of the previously described technical means, in which the
 thickness of the insulating layer 1 is changed, is represented by a
 printed circuit board as illustrated in, e.g., FIG. 1B. In this printed
 circuit board, an auxiliary insulating sheet 8 is interposed in the
 two-layer single-sided printed board 7. An insulating base 6 (or 5) and
 the auxiliary insulating sheet 8 interposed between the conductive layers
 2 and 3 form the insulating layer 1 having uneven thickness.
 The auxiliary insulating shoot 8 may be provided in the area where the
 thickness of the two-layer single-sided printed board is increased.
 Alternatively, the auxiliary insulating sheet 8 may be a sheet which has
 substantially the same size an that of the single-sided printed board 7
 and has undesired portions thereof punched.
 The dielectric constant of the auxiliary insulating sheet 8 may be selected
 as required. However, in light of the fine control of the electric
 characteristics of the printed wiring board, the dielectric constant of
 the auxiliary insulating sheet 8 should preferably be set to be smaller
 than that of the insulating base 6 (or 5) interposed between the
 conductive layers 2 and 3.
 Further, although the material of the auxiliary insulating sheet 8 may be
 selected as require, in light of effective absorption of a stop between a
 nonlaminated area and a laminated area of the printed wiring board, it in
 desirable to form at least either the insulating base 6 (or 5) or the
 auxiliary insulating sheet 8 which forms the insulating layer 1, from a
 material which in capable of resiliently deforming in a thicknesswise
 direction; for example, an aromatic-polyamide-based nonwoven fabric
 impregnated with thermosetting resin.
 There are various types of methods of manufacturing a printed wiring board
 so as to for the insulating layer 1 having an uneven thickness by use of
 the previously described auxiliary insulating sheet 8. In view of simple
 manufacture of the printed wiring board having an uneven thickness by
 utilization of a single-sided printed wiring board, first and second
 single-sided printed boards 7 [more specifically, 7(1) and 7(2)] are
 formed in such a way that predetermined circuit patterns 2 and 3 are
 formed on one side of the respective insulating bases 5 and 6 in the way
 as illustrated in FIG. 1B. A via-hole 4a for use as the conducting section
 4 in formed beforehand in either the first single-sided printed wiring
 board 7(1) or the second single-sided printed wiring board 7(2). The
 auxiliary insulating sheet 8 in which the via-hole 4a for use as the
 conducting section 4 is formed as required, is fixed to either the upper
 or lower surface of the first single-sided printed wiring board 7(1).
 Subsequently, the second single-sided printed wiring board 7(2) is fixed
 to either the upper or lower surface of the first single-sided printed
 board 7(1) and of the auxiliary insulating sheet 8.
 According to the previously described manufacturing method, it in possible
 to select any process as a process of bonding the single-sided printed
 board 7(2) to the auxiliary insulating sheet 8 with an inclusion such as
 an adhesive sandwiched between them, as required. However, in view of the
 simplification of the bonding step, it is desirable to impregnate at least
 either the single-sided printed board 7(2) or the auxiliary insulating
 sheet 8 with thermosetting resin and to utilize the bonding action of the
 thermosetting resin in a state in which the impregnated resin is in a
 semi-cured state (or in stage B).
 In another made of forming the insulating layer 1 so as to have an uneven
 thickness, an auxiliary insulating layer 9 such as (thermosetting melamine
 resin-based, epoxy resin-based, radical polymer-based, or cationic
 polymer-based) solder resist is printed on a portion between the two-layer
 single-sided printed board 7. The insulating base 6 (or 5) and the
 auxiliary insulating layer 9 sandwiched be the conductive layers 2 and 3
 form the insulating layer 1 having an uneven thickness.
 With regard to this mode, there are mentioned various types of
 manufacturing methods. In light of simple manufacture of the printed
 wiring board by utilization of a single-sided printed board, as
 illustrated in FIG. 1C, first and second single-sided printed boards 7
 [more specifically, 7(1) and 7(2)] are formed such that predetermined
 circuit pattern 2 and 3 are formed on one side of the respective
 insulating bases 5 and 6. A via-hole 4a for use as the conducting section
 4 is formed beforehand in either the first single-sided printed wiring
 board 7(1) or the second single-sided printed wiring board 7(2). The
 auxiliary insulating layer 9 in which the via-hole 4a for use as the
 conducting section 4 in formed as required, is fixed to either the upper
 or lower surface of the first single-sided printed wiring board 7(1).
 Subsequently, the second single-sided printed wiring 7(2) is fixed to
 either the upper or lower surface of the first single-sided printed board
 7(1) and the auxiliary insulating layer 9.
 According to the previously described manufacturing method, in view of
 simplification of the step for bonding the single-sided printed board 7(2)
 to the auxiliary insulating layer 9, it in desirable to impregnate at
 least either the single-sided printed board 7(2) or the auxiliary
 insulating layer 9 with thermosetting resin and to utilize the bonding
 action of the thermosetting resin in a state in which the impregnated
 resin is in a seat-cured state (or in stage B).
 Next, the operation of these technical means will be described.
 As illustrated in FIG. 1A, the printed wiring board of the present
 invention in comprised of the insulating layer 1 and the conductive layers
 2 and 3 (one of them in a signal layer, and the other is a shielding
 layer) respectively formed into predetermined circuit patterns on the
 upper and lower surfaces of the insulating layer 1. The conductive layers
 2 and 3 are electrically connected together through the conducting section
 4. According to the circuit configuration of the conductor layers 2 and 3,
 the thickness of the insulating layer 1 is changed by use of; e.g., the
 auxiliary insulating sheet 8 or the printed auxiliary insulating layer 9
 an illustrated in FIGS. 1B or 1C.
 For example, even if the single-sided printed board 7(1), including the
 insulating base 5 of a given thickness and the conductive layer 2 formed
 into a predetermined circuit pattern on one side of the insulating bass 5,
 and the single-sided printed board 7(2), including the insulating base 6
 of a given thickness and the conductive layer 3 formed into a
 predetermined circuit pattern on one side of the insulating base 6, are
 stacked, the thickness of the insulating layer 1 is set to a desired
 thickness according to the circuit configuration of the conductive layers
 2 and 3, for example, according to whether the conductive layers 2 and 3
 form a digital or analog circuit, so long as the thickness of the
 auxiliary insulating sheet 8 or the auxiliary insulating layer 9 in
 selected an required. As a result, there in formed a printed wiring board
 having the electric characteristics corresponding to the circuit
 configuration of a plurality of systems such as an analog circuit or a
 digital circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 With reference to the accompanying drawings, the present invention will be
 described in detail hereinbelow in view of its several aspects.
 FIRST ASPECT
 FIG. 2 is an explanatory plan view of a printed wiring board according to a
 first aspect of the present invention. FIG. 3A is a cross-sectional view
 of the printed wiring board taken across line A--A in FIG. 2, and FIG. 3B
 is a cross-sectional view of the printed wiring board taken across line
 B--B in FIG. 2. FIG. 4 is a schematic representation of the printed wiring
 board as view in the direction designated IV in FIG. 3B.
 In the drawings, a printed wiring board 20 has a two-layer structure
 (including two single-sided FPCs) comprising two flexible single-sided
 printed boards 21 and 22 stacked on each other. A plurality of circuit
 systems; e.g., a digital circuit auction 23 and an analog circuit section
 24, are mounted on the printed wiring board 20. A surface protecting layer
 29 (see FIGS. 3A and 3B) is omitted from FIG. 2.
 In the first aspect, as illustrated in FIGS. 3A and 3B, the lower
 single-sided printed board 21 is comprised of a insulating base 211 having
 a given thickness, and a conductive layer (corresponding to a signal
 layer) 212 formed in a predetermined circuit pattern (the digital circuit
 section 23 and the analog circuit section 24) on one side (the upper
 surface) of the insulating base 211. The upper single-sided printed board
 22 in comprised of an insulating bass 221 having a given thickness, and an
 conductive layer 222 (corresponding to a shielding layer) provided on one
 side (the upper surface) of the insulating bass 221 so an to correspond to
 the overall signal layer 212.
 In the first aspect of the present invention, as illustrated in FIG. 3A,
 the digital circuit auction 23 is comprised of the single-sided printed
 boards 21 and 22 that are directly stacked on each other, and the signal
 layer 212 electrically connected to the shielding layer 222 through
 conducting sections 26.
 Reference numeral 29 designates a surface protecting layer which covers the
 surface of the printed wiring board.
 In the first aspect of the present invention, a nonwoven fabric which
 includes; e.g., aromatic polyamide, as a core material and in impregnated
 with epoxy and a rubber-based resin is used as the insulating bases 211
 and 221 (see the Unexamined Japanese Patent Application Publication No.
 Hei 6-334339). However, the insulating bases 211 and 221 are not limited
 to such a material but may be formed from various types of material which
 start with unsaturated polyester impregnated with a thermosetting resin.
 Materials differing from the insulating bases 211 and 221 may be used as an
 auxiliary shieiding sheet 27. As in the case of the insulating bases 211
 and 221, a nonwoven fabric which includes; e.g., aromatic polyamide, as a
 core material and is impregnated with epoxy and a rubber-based resin, is
 used as the auxiliary insulating sheet 27 in the first aspect of the
 invention.
 In order to reduce the dielectric constant of the auxiliary insulating
 sheet 27, a non oven fabric which includes a fluorine-based or
 polyphenylene-ether-based resin as a core material, may be impregnated
 with epoxy an a rubber-based resin.
 Further, the conducting sections 26 and 28 are formed by forming via-hole
 261 and 281 in the insulating bass 221 so as to connect the signal layer
 212 with the shielding layer 222, and filling the thus-formed via-holes
 261 and 281 with conductive pastes 262 and 282.
 Next, a method of manufacturing the printed wiring board in the first
 aspect of the invention will be described with reference to FIG. 5.
 Wiring patterns corresponding to the signal layers 212 and 222 are formed
 on single-sided metal-foiled platen 41 and 42, each of which is comprised
 of metal foil (copper foil in the first aspect) laminated on one side of
 an insulating base, by a known method such as etching, thereby forming the
 single-sided printed wiring boards 21 and 22. The signal layer 212 (the
 digital circuit section 23 and the analog circuit section 24) of the first
 layer single-sided printed board 21 is roughened by blackening (reference
 numeral 43 designates a blackened film). On the other hand, the via-holes
 261 and 281 are formed in the second layer single-sided printed board 22
 by pressing or drilling.
 Similarly, the via-holes 281 are also formed in the auxiliary insulating
 sheet 27.
 Subsequently, the auxiliary insulating sheet 27 is placed on the area of
 the first layer single-sided printed board 21 corresponding to the analog
 circuit 24. The second layer single-sided printed board 22 is stacked on
 the first layer single-sided printed board 21 by heat pressing. At the
 time of lamination, the single-sided printed boards 21 and 22 and the
 auxiliary insulating sheet 27 are relatively positioned by a known method.
 For example, positioning holes are formed in suitable places of the
 single-sided printed boards 21 and 22 and the auxiliary insulating sheet
 27 which are to be stacked up. The single-sided printed boards 21 and 22
 and the auxiliary insulating sheet 27 are stacked on a job having
 positioning pins provided so as to correspond to the positioning holes.
 The thus-stacked boards and sheet are temporarily hold and integrally
 stacked up.
 At this time, if the single-sided printed board 22 and the auxiliary
 insulating sheet 27 are stacked while they are in a semi-cured state (or
 in stage B), the single-sided printed boards 21 and 22 and the auxiliary
 insulating sheet 27 are integrally stacked without use of an adhesive.
 Further, in the first aspect of the invention, the surface of the signal
 layer 212; i.e., copper foil, is roughened by blackening. As a result,
 stable bond strength is obtained in stacking up the single-sided printed
 boards 21 and 22 and in stacking up the auxiliary insulating sheet 27 and
 the single sided-printed board 21.
 Moreover, in the first aspect of the present invention, a nonwoven fabric
 is used for the insulating bases 211 and 221 of the single-sided printed
 boards 21 and 22 (see FIGS. 3A and 3B) and the auxiliary insulating sheet
 27. Therefore, it is possible to absorb a step of about 0.1 mm. An a
 result, the single-sided printed boards 21 and 22 and the auxiliary
 insulating sheet 27 are stably stacked without causing a problem in the
 wiring patterns.
 The blackened film 43 is formed on the copper foil below the bottoms of the
 via-holes 261 and 281 of the single-sided printed board 22. Since the
 blackened film 43 is non-conductive, it does not permit electrical
 conduction as it is.
 For this reason, the blackened film 43 exposed through the surface is
 removed by acid cleaning using; e.g., hydrochloric acid, in the first
 aspect of the present invention.
 Subsequently, the via-holes 261 and 281 are filled with the conductive
 pastes 262 and 282 such an a copper or silver paste by use of; e.g., a
 screen printing machine. Then, the conductive pastes are cured.
 Finally, a polyamide film or a solder resist cover (corresponding to the
 surface protecting layer 29 in FIG. 3) is placed to protect the
 copper-toiled surface and the pasted surface of the second layer
 single-sided printed board 22.
 According to the printed wiring board in the first aspect of the present
 invention, the electric characteristics of the printed wiring board
 respectively matching up with the digital circuit section 23 and the
 analog circuit section 24 are obtained as is evident from the descriptions
 of embodiment which will be described later.
 SECOND ASPECT
 FIG. 6 illustrates a printed wiring board according to a second aspect of
 the present invention.
 Although the printed wiring board has been manufactured by use of the two
 single-sided printed boards 21 and 22 in the first aspect of the present
 invention, a double-sided printed board 44 is used in lieu of the
 single-sided printed boards as illustrated in FIG. 6 if the wiring pattern
 of the signal layer 212 exceeds the single-sided printed board 21.
 In this case, an insulating base 441 having both its surfaces covered with
 a metal foil by a known method in used as the double-sided printed board
 44. Through-holes 443 are formed in tho areas of the double-sided printed
 board 44 which are to be opened into via-hole. The overall double-sided
 printed board 44 is then plated, thereby forming the via-holes from the
 plated through-holes. In addition, for example, the pattern of a signal
 layer 442 is formed on the metal foil on both sides of the printed board
 44. The blackened film 43 is formed on the surface of the signal layer
 442. Protecting covers provided on the upper and lower surfaces of the
 printed wiring board are omitted from FIG. 6.
 THIRD ASPECT
 FIG. 7 illustrates a printed wiring board according to a third aspect of
 the present invention.
 In the drawing, the basic configuration of the printed wiring board in the
 third aspect is substantially the same as that of the printed wiring board
 in the first aspect of the invention. As opposed to the auxiliary
 insulating sheet in the first aspect of the invention, the auxiliary
 insulating sheet 27 is not a sheet material having a size corresponding to
 the required area but an auxiliary insulating sheet 51 which has a size
 substantially corresponding to the size of the single-sided printed boards
 21 and 22 and has unnecessary portions thereof punched into openings 52.
 The same elements as those of the printed wiring board in the first aspect
 are assigned the same reference numerals, and their detailed explanations
 will be omitted here. The same applies to the other aspects of the present
 invention which will be provided below.
 Therefore, according to the third aspect of the invention, it becomes easy
 to position the auxiliary insulating sheet 27 interposed between the
 single-sided printed boards 21 and 22 when compared to the first aspect of
 the invention.
 FOURTH ASPECT
 The basic configuration of the printed wiring board in a fourth aspect of
 the invention is substantially the same as that of the printed wiring
 board in the first aspect of the invention. Instead of the auxiliary
 insulating sheet 27 an in the first aspect of the invention, the printed
 wiring board in the fourth aspect employs an auxiliary insulating layer 60
 (see FIG. 8) which is formed by printing.
 FIG. 8 illustrates a method of manufacturing the printed wiring board in
 the fourth aspect of the present invention.
 As in the case of the manufacturing method in the first aspect of the
 present invention, the single-sided printed boards 21 and 22 are formed
 such that the wiring patterns of the signal layer 212 and the shielding
 layer 222 are formed on one surface of each of the printed boards. The
 single-sided printed boards 21 and 22 are blackened, and holes are formed
 in the single-sided printed boards 21 and 22.
 An insulating resin layer 60 composed of; e.g., thermosetting solder resist
 (thermosetting melamine resin-based, epoxy resin-based, radical
 polymer-based, or cationic polymer-based solder resist) is printed on the
 area of the first layer single-sided printed board 21 corresponding to the
 analog circuit section 24. At this time, the insulating resin layer 60 in
 formed into a printed pattern from which the via-holes 281 have been
 removed.
 Subsequently, the second layer single-sided printed board 22 is stacked on
 the first layer single-sided printed board 21 having the insulating resin
 layer 60 printed thereon.
 If the single-sided printed board 22 in stacked on the single-sided printed
 board 21 while it is in a semi-cured state (or in stage B) at this time,
 the single-sided printed boards 21 and 22 are integrally stacked up
 without use of an adhesive as they are in the first aspect of the
 invention.
 As in the case of the first aspect of the invention, the blackened film 43
 formed on the copper foil below the bottom of the via-holes formed in the
 single-sided printed board 22 is removed by acid. Subsequently, the
 via-holes 261 and 281 are filled with the conductive pastes 262 and 282
 such as a copper or silver paste by use of; e.g., a screen printing
 machine. Then, the conductive pastes are cured.
 Finally, a polyamide film or a solder resist cover (not shown) is placed to
 protect the copper-foiled surface and the pasted surface of the second
 layer single-sided printed board 22.
 Even in the case of the printed wiring board in the fourth aspect of the
 invention, the electric characteristics of the printed wiring board
 respectively matching up with the digital circuit section 23 and the
 analog circuit section 24 are obtained as they are in the first aspect of
 the invention.
 FIFTH ASPECT
 As opposed to the printed wiring boards in the first through fourth aspects
 of the present invention, a printed wiring board in a fifth aspect of the
 invention is formed by sequentially stacking required layers on one
 single-sided metal-foiled plate, and by finally forming wiring patterns in
 the metal foil (e.g., copper foil) formed on the upper and lower surfaces
 of the insulating layer having different thicknesses by use of a known
 method such an etching.
 FIG. 9 shows a method of manufacturing a printed wiring board according to
 the fifth aspect of the present invention.
 First, an auxiliary insulating sheet 73 is stacked on a non-metal-foiled
 side (or the upper surface) of a single-sided metal-foiled plate 71 which
 is comprised of an insulating base having a given thickness and metal foil
 72 stacked on one side (or the lower surface) of the insulating base.
 Then, metal foil (e.g., copper foil) 74 in additionally stacked on the
 upper surfaces of the single-sided metal-foiled plate 71 and the auxiliary
 insulating sheet 73.
 After via-holes 75 and 76 have been formed in the single-sided metal-foiled
 plate 71, the surface of the metal foil 72 and 74 and the via-holes 75 and
 78 is entirely coated with plating 77 by means of electroless or
 electrolytic plating. Wiring patterns corresponding to a signal layer 78
 and a shielding layer 79 are formed in the metal foil 72 and 74 by a known
 method such as etching.
 Even in the thus-manufactured printed wiring board, the same electric
 characteristics as those of the printed wiring board in the first aspect
 are obtained.
 In the fifth aspect of the present invention, an auxiliary insulating layer
 similar to that in the fourth aspect may be used in lieu of tho auxiliary
 insulating sheet 73.
 SIXTH ASPECT
 In contrast to the printed wiring boards according to the first through
 fourth aspects of the invention, a printed wiring board in a sixth aspect
 of the invention is formed by forming wiring patterns on one single-sided
 metal-foiled plate by a known method such as etching, and by sequentially
 stacking the required layers on the single-sided metal-foiled plate.
 FIG. 10 illustrates a method of manufacturing the printed wiring board in
 the sixth aspect of the present invention.
 First, a wiring pattern corresponding to a signal layer 82 (e.g., a digital
 circuit section and an analog circuit section) is formed on a single-sided
 metal-foiled plate 81, which is comprised of an insulating bass having a
 predetermined thickness and metal foil (copper foil in the sixth aspect of
 the invention) stacked on one surface of the insulating base, by means of
 a known method such as etching. Then, an auxiliary insulating sheet 83 is
 additionally stacked on the area of the wiring pattern corresponding to
 the analog circuit section. Via-holes 84 have previously been formed in
 the auxiliary insulating sheet 83.
 Next, after via-holes 86 and 87 have been formed in an insulating base 85
 plate having a given thickness, the insulating base plate 85 is stacked so
 as to cover the wiring pattern of the single-sided metal-foiled plate 81
 and the auxiliary insulating sheet 83. The upper surface of the insulating
 base plate 85 and the via-holes 84, 86, and 87 are coated with conductive
 paste by screen printing, thereby forming a shielding layer 88.
 Finally, for example, a polyimide film or a solder resist cover (not shown)
 is provided to cover the surface of the conductive paste.
 Even in the case of the printed wiring board in the sixth aspect of the
 invention, the same electric characteristics as those of the printed
 wiring board in the first aspect are obtained.
 SEVENTH ASPECT
 As in the case of the printed wiring boards in the first through fourth
 aspects of the invention, a printed wiring board in a seventh aspect of
 the invention is made up of two single-sided metal-foiled plates. In
 contrast to the printed wiring boards in the first through fourth aspects
 of the invention, the printed wiring board in the seventh aspect is formed
 by finally forming a wiring pattern on metal foil (e.g., copper foil)
 formed on the upper and lower surfaces of the insulating layer having
 different thicknesses by means of a known method such as etching.
 FIG. 11 illustrates a method of manufacturing the printed wiring board in
 the seventh aspect of the invention.
 First, two single-sided metal-foiled plates 91 and 92, each of which is
 comprised of an insulating bass having a given thickness and metal foil
 stacked on one side of the insulating base, are positioned so as to be
 opposite to each other with their metal-foiled sides outside. An auxiliary
 insulating sheet 95 is interposed between the unfoiled sides of the two
 single-sided metal-foiled plates 91 and 92 in the area corresponding to;
 e.g., an analog circuit. Then, the two single-sided metal-failed plates 91
 and 92 are stacked up.
 After via-holes 96 and 97 have been formed in the single-sided metal-foiled
 plates 91 and 92, the surface of the metal foil 93 and 94 and the
 via-holes 96 and 97 are entirely coated with plating 98 by means of
 electroless or electrolytic plating. Wiring patterns corresponding to a
 signal layer 99 and a shielding layer 100 are formed in the metal foil 93
 and 94 by a known method such as etching.
 Even in the case of the printed wiring board in the seventh aspect of the
 invention, the same electric characteristics as those of the printed
 wiring board as in the first aspect are obtained.
 In the seventh aspect of the present invention, an auxiliary insulating
 layer similar to that in the fourth aspect may be used in lieu of the
 auxiliary insulating sheet 95.
 FIRST EMBODIMENT
 In a first embodiment, a printed wiring board as in the first aspect of the
 present invention.
 Copper foil is laminated on an insulating sheet (corresponding to the
 insulating base of the lower single-sided printed board) which has a
 thickness of about 100 .mu.m and is comprised of a nonwoven aromatic
 polyamide fabric impregnated with epoxy and a rubber-based resin. For
 example, conductor circuit patterns for a signal layer are formed in the
 insulating sheet by a subtractive process.
 Subsequently, a blackened film is formed on the surface of the conductor
 circuit patterns.
 An insulating sheet (corresponding to the auxiliary insulating sheet) which
 has a thickness of about 100 .mu.m and is comprised of a nonwoven aromatic
 polyamide fabric impregnated with epoxy and a rubber-based resin, is
 selectively thermocompression-bonded particularly to a circuit pattern
 (i.e., an analog circuit pattern) which requires small electric
 capacitance of the thus-formed circuit patterns, while holes for
 interlayer connection purposes are formed in the insulating sheet as
 required.
 An insulating sheet having the following structure is further laid on the
 previously-described insulating sheet by thermocompression bonding.
 Specifically, the insulating sheet (corresponding to the insulating base
 of the upper single-sided printed board) is formed into a thickness of
 about 100 .mu.m from a nonwoven aromatic polyamide fabric impregnated with
 epoxy and a rubber-based resin and is in a semi-cure state. Copper foil is
 laminated on this insulating sheet, and conductor circuit patterns for a
 shielding layer are formed on the insulating sheet by a subtractive
 process. Holes for interlayer connection purposes are further formed in
 the insulating sheet.
 Thermocompression bonding of the insulating sheets is carried out under
 conditions; namely, a pressure of 30 to 50 Kg/cm.sup.2 and a temperature
 of 140 to 160.degree. C.
 The blackened film formed below the bottom of the via-holes formed for
 interlayer connection purposes is removed by acid. The via-holes are then
 filled with a polymer-type copper paste E-1000 (trade name) manufactured
 by Mitsui Metal Coating Chemicals Co., Ltd. After the polymer type copper
 paste has been thermally set, a polyimide cover is provided to protect the
 copper foil wiring pattern on the topmost layer of the multilayered
 printed wiring board by thermocompression bonding.
 SECOND EMBODIMENT
 A second embodiment shows an illustrative example of the printed wiring
 board in the second aspect of the present invention.
 Holes are formed in a glass epoxy plate having both surfaces thereof foiled
 with copper in order to ensure electrical connection between the copper
 layers. The overall glass epoxy plate is plated.
 For example, conductor circuit patterns for a signal layer are formed on
 both surfaces of the thus-plated glass epoxy plate by a subtractive
 process, and a blackened film is formed on the surface of the conductor
 circuit patterns.
 Further, in a desired location of the conductor circuit pattern at least
 one of the surfaces of the conductor is placed an insulating sheet
 (Corresponding to an auxiliary insulating sheet) which has a thickness of
 about 100 .mu.m, is comprised of a nonwoven aromatic polyamide fabric
 impregnated with epoxy and a rubber-based resin, and is in a semi-cured
 state.
 An insulating sheet having the following structure is further laid on the
 previously-described insulating sheet by thermocompression bonding.
 Specifically, the insulating sheet (corresponding to an auxiliary
 insulating sheet) is formed into a thickness of about 100 .mu.m from a
 nonwoven aromatic polyamide fabric impregnated with epoxy and a
 rubber-based resin and is in a semi-cured state. Copper foil is laminated
 on this insulating sheet, and conductor circuit patterns for a shielding
 layer are formed on the insulating sheet by a subtractive process. Holes
 for interlayer connection purposes are further formed in the insulating
 sheet.
 Thermocompression bonding of the insulating sheets is carried out under
 conditions; namely, a pressure of 30 to 50 Kg/cm.sup.2 and a temperature
 of 140 to 160.degree. C.
 The blackened film formed below the bottom of the via-holes formed for
 interlayer connection purposes is removed by acid. The via-holes are then
 filled with a polymer-type copper paste E-1000 (trade name) manufactured
 by Mitsui Metal Coating Chemicals Co., Ltd. As a result, the interlayer
 connection is accomplished by combination of the plated through-holes and
 the copper paste.
 Finally, a polyimide cover and resist ink are laid on and applied to the
 upper surface of the laminated substrate in order to protect the exposed
 copper foil surface.
 Measurement of Electric Characteristics of the First (or Second) Embodiment
 Measurement Method
 With regard to the first embodiment (the insulating layer of the digital
 circuit section has a thickness of 100 .mu.m, and the insulating layer of
 the analog circuit section has a thickness of 200 .mu.m) and a comparative
 example (the insulating layers of the digital and analog circuit sections
 have a thickness of 100 .mu.m), the electric characteristics of the analog
 circuit section is measured by use of an HP4194A impedance analyzer, and
 the frequency characteristics of a transmission circuit is measured by use
 of an EP4185A network analyzer). Further, the waveform of a transmitted
 signal is measured by use of an oscilloscope.
 Measurement Result
 Electrical Characteristics
 Table 1 provides the results of the measurements of the electrical
 characteristics of the printed wiring boards.
 TABLE 1
 COMA-
 EMBODI- TIVE RATE OF
 MENT EXAMPLE INCREASE OR
 ITEM t = 200 .mu.m t = 100 .mu.m DECREASE
 ELECTRIC 66 100 -34%
 CAITANCE
 (pF)
 INDUCTANCE 179 139 +29%
 (nH)
 CHARACTERISTIC 51 37 +38%
 IMPEDANCE
 (.OMEGA.)
 RESONANCE 46 42 +9%
 FREQUENCY
 (MHz)
 As illustrated in FIGS. 14A and 14B, it is understood that the electric
 capacitance is reduced by about 35% as a result of a change in the
 thickness of the insulating layer from 100 micrometers to 200 micrometers.
 Since the electric capacitance is expressed by Equation (1) given below,
 the electric capacitance should be ideally reduced to half its original
 capacitance if the thickness of the insulating layer is doubled. However,
 a measurement value is slightly greater than the expected value.
EQU C=.epsilon.S/d (1)
 (where .epsilon. is a dielectric constant, S is an area, and "d" is a
 thickness)
 In contrast, as illustrated in FIG. 15, the inductance of the printed
 wiring board is increased by about 30% as a result of doubling of the
 thickness of the insulating layer.
 An has been described above, since the rate of reduction of the electric
 capacitance in greater than that of the inductance, both the
 characteristic impedance (Z0=L/C) and a resonance frequency (fR=1/2.pi.LC)
 of the printed wiring board itself increase. Therefore, the
 characteristics of the printed wiring board become optimum for
 transmission of a high-frequency signal. It could have confirmed that the
 electric characteristics of the printed wiring board were sufficiently
 controlled by controlling the thickness of the printed wiring board.
 Frequency Characteristics of a Transmission Circuit
 The frequency characteristics of transmission circuits of the embodiment
 and comparative example are measured by use of attenuation coefficients of
 the transmission circuits as parameters. The attenuation coefficients
 .zeta. are set to the range of 0.5 to 1.0, and the damping resistance of
 an element to be inserted into the circuit is set to become as close to
 the resistance calculated from the characteristic impedance and preset
 attenuation constants .zeta. of the comparative example as possible. An
 illustrated in FIG. 16, the frequency characteristics used herein
 designate a frequency attenuated from a gain peak value by -3 dB.
 The results of the measurements are illustrated in FIGS. 12 and 13.
 FIG. 12 is a plot illustrating the attenuation coefficients .zeta. vs. the
 frequency characteristics, and FIG. 13 is a plot illustrating the
 attenuation coefficients .zeta. vs. the gain peak values.
 It is necessary for an analog signal transmission circuit to have
 sufficiently flat frequency characteristics with respect to the frequency
 components of a transmission signal. It is seen that, when compared with
 the comparative example, the band frequency of the transmission circuit of
 the embodiment in which the capacity components of the circuit are
 reduced, is extended with respect to all the attenuation coefficients by
 about 10 MHz.
 To make the frequency characteristics of the transmission path flat, it is
 necessary to set the attenuation coefficient to a value more than 0.707
 (=1/2). If the attenuation coefficient has decreased to become smaller
 than 0.7 as illustrated in FIG. 13, a peak arises in the frequency
 characteristics, thereby resulting in possibility of the development of
 ringing. In FIG. 13, the slight increase in the gain peak value of the
 embodiment compared to that of the comparative example is considered to be
 attributable to the difference between the resistance of the printed
 wiring board itself and the damping resistance set without consideration
 of the resistance of the printed wiring board.
 Ideally, the gain peak value Gp can be calculated from the attenuation
 coefficients.
EQU Gp=1/{2.zeta..times.(1-.zeta..sup.2)}
 where .zeta..ltoreq.1/2.
 The frequency band in the vicinity of the attenuation coefficient
 .zeta.=0.7 is about 60 MHz in the embodiment and about 46 MHz in the
 comparative example.
 If an analog signal such as an output of a charge-coupled device (CCD) is
 transmitted, a band which is three times as wide as a drive frequency of
 the CCD becomes necessary. In contrast to transmission of a signal of 15
 MHz in the comparative example, a signal of about 20 MHz can be
 transmitted in the embodiment.
 As describes above, it becomes possible to transmit a signal at a high
 speed by use of a printed wiring board having a partial increase in its
 thickness.
 Waveform of Transmitted Signal
 Next, the waveform of the analog signal (having a drive frequency of
 f.phi.=12.5 MHz) is measured while it is transmitted from the CCD through
 the previously-described transmission circuit. The attenuation coefficient
 of the transmission circuit is set to one. The frequency band of the
 transmission circuit when the attenuation coefficient is set to one, is
 about 40 MHz in the embodiment and is about 30 MHz in the comparative
 example.
 Although the CCD output signal has an analog waveform analogous to a
 rectangular waveform, transient response becomes important as the
 characteristics of the transmission circuit. There is a relationship
 between a response time T of the waveform and the frequency band fT of the
 transmission circuit; namely, T=0.35/ft. Theoretically, it is possible to
 expect an improvement of about 3 ns in the response time in the embodiment
 and the comparative example.
 As a result of comparison between the waveforms of the transmitted CCD
 output signals of the embodiment and the comparative example according to
 the result of the measurement, it could have confirmed an increase in
 response to trailing of the waveform of the embodiment; namely, an
 improvement of the response time about 4 ns.
 In view of the previous descriptions, it is understood that the reduction
 in the electric capacity of the printed wiring board due to an increase in
 the thickness of the printed wiring board has sufficiently contributed to
 an improvement in the transient response.
 The similar test was performed with regard to the second embodiment, and
 substantially the same result as that of the first embodiment was
 obtained.
 An has been described above, even in a case where circuit sections which
 require different electric characteristics with respect to the printed
 wiring board mixedly exist; for example, in a case where an analog circuit
 section and a digital circuit section are mounted one printed wiring
 board, the present invention allows matching of the electric
 characteristics of one printed wiring board to the respective requirements
 by controlling the thickness of the insulating layer interposed between
 two conductive layers for each circuit.
 As a result, it becomes unnecessary to provide a printed wiring board for
 each circuit configuration, which in turn enables cost reduction and easy
 control of the electric characteristics of the printed wiring board itself
 according to the characteristics of each of a plurality of systems of
 conductor circuits.
 Further, if the printed wiring board is formed by stacking two single-sided
 printed wiring boards, each of which is comprised of an insulating base
 and a conductive layer formed on one side of the insulating base in a
 given circuit pattern, the present invention allows easy manufacture of a
 printed wiring board having an uneven thickness by interposing an
 insulating layer formation member (e.g., an auxiliary insulating sheet or
 an auxiliary insulating layer) for increasing the thickness of the layer
 between the single-sided printed boards.
 Further, if the single-sided printed board is provided with flexibility,
 the present invention makes it possible to readily provide a compact and
 thin printed wiring board which is easy to mount.
 In the aspect of the present invention in which an auxiliary insulating
 sheet is sandwiched between the single-sided printed boards, if a sheet
 having unnecessary portions thereof punched is used as the auxiliary
 insulating sheet, it is possible to very easily position the auxiliary
 insulating sheet with respect to the single-sided printed board.
 If an auxiliary insulating sheet whose dielectric constant in smaller than
 that of the insulating base of the single-sided printed board is used, it
 is possible to finely control the electric characteristics of the printed
 wiring board by adjusting the thickness of the auxiliary insulating sheet.
 If at least either the auxiliary insulating sheet or the single-sided
 printed board is formed from material elastically deformable in a
 thicknesswise direction, the single-sided printed board absorbs a stop
 resulting from lamination of the auxiliary insulating sheet in order to
 control the thickness of the printed wiring board. Consequently, the
 laminated state of the single-sided printed board or the auxiliary
 insulating sheet can become more stable.
 If either the auxiliary insulating sheet or the auxiliary insulating layer
 is impregnated with thermosetting resin when either of them in fixed to
 the single-sided printed board, and if the auxiliary insulating layer or
 sheet is stacked on the single-sided printed board while it is in a
 semi-cured state (in stage B), the impregnated resin performs the function
 of an adhesive. The member to be stacked burrows into the surface on which
 the member will be stacked, as a result of which high bond strength is
 obtained. This makes it possible to reliably stack the members without use
 of an adhesive by simple thermocompression bonding.