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`Patent and Trademark Office, US DEPARTMENT OF COMMERCE
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`
`Patm.34
`
`Hirokazu Yoshioka
` TRANSMITTAL
`
`PATENT APPLICATION
`
`—_
`>
`
`Title
`
`“Multilayer Printed Wiring Board and Method|for sms
`4
`”
`e =
`Manufacturing Same
`% —
`.
`> =e
`Express Mail Label No.
`EL250848316US
`aA SSS
`Assistant Commissionerfor Paférttsas~..
`Box Patent Application
`oon ee
`Washington, D.C. 20231 wo B=—
`
`ADDRESSTO:
`
`C] Microfiche Computer Program (Appendix)
`Nucleotide and/or Amino Acid Sequence Submission
`(If applicable,alt necessary)
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`b. U Paper Copy (identical to computercopy)
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`
`ACCOMPANYING APPLICATION PARTS
`
`
`& Assignment Papers (cover sheet & document(s))
`PowerofAttomey
`s
`[1 37C.F.R§3.73() Statement
`[1]
`(when there is an assignee)
`9. O English Translation Document(if applicable)
`10.
`CO Information Disclosure
`C] Copies of IDS
`Statement
`Citations
`[1 Preliminary Amendment
`BX Return Receipt Postcard (MPEP 503)
`(Should be specifically itemized)
`[1 <smattentity
`(71 Statementfiled in prior application,
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`Statusstill proper and desired
`CO Certified Copy of Priority Document(s)
`(if foreign priority is claimed)
`15. Cc Other:
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`43.
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`14.
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`5.
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`11.
`12,
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`2
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`35
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`1 & *Fee transmittal Form (e.g., PTO/SB/17)
`(Submut an orginal and a duplicate for fee processing)
`DB Specification
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`(preferred arrangementset forth below)
`—Descriptivetitle of invention
`—Cross References to Related Applications
`-Statement regarding Federally sponsored R & D
`—Reference to Microfiche Appendix
`—Backgroundof the Invention
`—Brief Description of the Drawings(iffiled)
`—Brief Summary of the Invention
`—Detailed Description
`—Claim(s)
`—Abstract of the Disclosure
`Total Sheets
`&X Drawing(s) (35 U.S.C. 113)
`3.
`Total Pages
`“4. Oath or Declaration
`a.
`EX] Newly executed (original or copy)
`b,
`CL Copy from a prior application (37 C.F. R. §1.63(d))
`(for continuation/divisional with Box 16 completed)
`I. | DELETION OF INVENTOR(S)
`Signed statementattached deleting
`inventor(s) named in the prior application,
`see 37 C.F.R. §§ 1.63(d)(2) and 1.33(b).
`
`12
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`4
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`HuOH
`
`
`IN ORDER TO BE ENTITLED TO PAY SMALL
`‘NOTE FOR ITEMS 1 &13__
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`ENTITY FEES, A SMALL ENTITY STATEMENTIS REQUIRED (37 CFR
`§ 1.27), EXCEPT IF ONE FILED IN A PRIOR APPLICATION IS RELIED
`
`UPON (37 C.F. R_§1.28)
`
`f 46.-
`
`IF A CONTINUING APPLICATION,check appropriate box, and supply the requisite information below and in an preliminary amendment:
`
`O Continuation—in—-part (CIP) of prior application No.:
`O Divisional
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`Examiner:
`FOR CONTINUATION ORDIVISIONAL APPS ONLY: Theentire disclosure of the prior application, from which an oath or declaration is supplied
`under Box 4b,is considered a part of the disclosure ofthe accompanying continuation ordivisional application and is hereby incorporated by
`reference. Theincorporation can only berelied upon when a portion has been inadvertently omitted from the submitted application parts.
`17. CORRESPONDENCE ADDRESS
`
`pom|2915RedhillAvenue
`
`John H Lynn
`Suite F200
`
`Name
`Address
`
`Teas FICO
`
`FET
`
`Name(Print/Type)
`
`
`
`
`
`
`
`
`
`John H. Lynn
`
`Registration No.:(Attorney/Agent)
`
`
`
`
` 05/28/1999
`
`
`
`
`

`

`
`
`
`"Multilayer Printed Wiring Board and
`
`Method For Manufacturing Same"
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to a multilayer printed
`
`wiring board constituted for electric connection
`
`between mutiple conductor layers by means of a so-called
`
`via hole of non-penetrating type, and to a method for
`
`manufacutring same.
`
`DESCRIPTION OF RELATED ART
`
`10
`
`In the multilayer printed wiring board comprising
`
`a lamination of a plurality of conductor layers forming
`
`a circuit with insulating layers
`
`interposed,
`
`the
`
`electric connection between the conductor
`
`layers is
`
`generally attained by forming through holes with a
`
`15
`
`drilling work, and providing a plated metal
`
`layer on
`
`inner peripheries of the through holes.
`
`In order
`
`to establish the wiring at
`
`a higher
`
`density in recent years, however, there has been started
`
`to be widely employed a measure in which the non-
`
`20
`
`penetrating via hole instead of
`
`the through hole is
`
`formed in the insulating layer,
`
`and an electrically
`
`conducting substance is provided to the inner periphery
`
`of the via hole.
`
`For
`
`
`a process of
`
`forming this non-
`
`penetrating via hole,
`
`it has been general
`
`to remove
`
`29
`
`through an etching a portion of a metal layer formed on
`
`a surface of the insulating layer, at which portion the
`
`via hole is to be formed, and then to irradiate a carbon
`
`dioxide gas laser onto the thus exposed insulating layer
`
`-2-
`
`

`

`
`
`
`formed by a thermosetting resin or the like so as to
`
`provide the via hole in the insulating layer at
`
`the
`
`portion where the metal
`
`layer has been removed.
`
`The method
`
`for manufacturing the multilayer
`
`printed wiring board with the via hole formed by means
`
`of such laser irradiation has been disclosed in,
`
`for
`
`example, Japanese Patent Publication No. 4-3676 by Y.
`
`Ohnsachi et al., according to which an aperture of the
`
`same diameter as that of the via hole is formed first
`
`10
`
`through the top metal layer by means of the etching or
`
`the like at the position where the via hole is to be made,
`
`then the non-penetrating via hole bottomed by
`
`an
`
`internal conductor layer is formed in the insulating
`
`layer with the laser irradiation of the carbon dioxide
`
`gas laser or the like at the position where the aperture
`
`has been formed,
`
`thereafter a metal foil is formed on
`
`inner periphery of the via hole by means of a plating,
`
`the metal foil is processed through the etching to form
`
`a circuit which connects the metal layer andthe internal
`
`20
`
`conductor
`
`layer
`
`through the metal
`
`foil,
`
`and
`
`the
`
`multilayer printed wiring board in which a plurality of
`
`the metal
`
`and
`
`conductor
`
`layers
`
`are electrically
`
`connected through the plated metal foils inthe via holes
`
`thus formed is obtained.
`
`25
`
`In providing the via hole in the insulating layer
`
`with the laser irradiation as has been described,
`
`a
`
`continuing zone of the inner periphery of the via hole
`
`to the bottoming internal conductor layer becomes an
`
`

`

`
`
`
`angular shape close to be 90° in an axially sectioned
`
`view of the via hole, so long as the inner periphery of
`
`the via hole is of a vetical wall.
`
`In providing to such
`
`periphery of the via hole the plated metal foil, a thin,
`
`electrolessly plated film is first formed through an
`
`electroless copper plating or the like plating on the
`
`inner periphery of the via hole, andthereafter the metal
`
`foil is formed by means of an electro-copper-plating or
`
`the like electroplating with a current passed through
`
`10
`
`the plate film.
`
`
`Equipotential
`
`surfaces occurring
`
`inside the via hole during the electroplating wili be
`
`parallel
`
`to the inner periphery and to the bottom
`
`conductor layer of the via hole but will deflect at the
`
`continuing zone of the inner periphery of the via hole
`
`15
`
`to the bottom conductor
`
`layer, whereby the electric
`
`lines
`
`of
`
`force
`
`intersecting at
`
`right
`
`angles’
`
`the
`
`eguipotential surfaces are caused to be coarse at the
`
`deflected part of the equipotential surfaces so as to
`
`render the current density to be smaller at this part.
`
`20
`
`Also, plating solution is deteriorated in the fluidity
`
`at
`
`the continuing zone.
`
`Consequently,
`
`
`the plated metal foil formed in the
`
`via hole becomes extremely thin at the continuing zone
`
`of
`
`the inner periphery of the via hole to the bottom
`
`25
`
`conductor layer so as to lower the reliability of the
`
`electric connection between the respective conductor
`
`layers through the plated metal foil, so that a problem
`
`rises in that the connection is broken frequently at
`
`

`

`such extremely thin part of
`
`the metal foil due to a
`
`heating upon mounting cnstituent parts or a generated
`
`heat at
`
`the mounted parts being used.
`
`In U.S. Patent No. 4,070,501 to V. Carbine et al.,
`
`on the other hand, there has been disclosed a provision
`
`of the via hole, which is an element in a semiconductor
`
`device and is not suggestive to any technical matter
`
`characteristic to the multilayer printed wiring board.
`
`Japanese Patent Laid-Open Publication No. 8-279679 of
`
`H. Takagi et al. and assigned to the same assignee as
`
`the present invention also disclose a formation of the
`
`via hole but
`
`involves
`
`the same problem as
`
`in the
`
`foregoings.
`
`
`
`
`SUMMARY OF THE INVENTION
`
`The present
`
`invention has been suggested to
`
`overcome the foregoing problem, and an object of
`
`the
`
`invention is to provide a multilayer printed wiring
`
`board high in the reliability of the electric connection
`
`between the conductor layers through the plated metal
`
`10
`
`15
`
`
`
`
`20
`
`foil inthe via hole as well as amethod for manufacturing
`
`such multilayer printed wiring board.
`
`According to the present
`
`invention,
`
`the above
`
`object can be realized by means of a multilayer printed
`
`wiring board in which a plurality of conductor layers
`
`25
`
`are laminated as a whole with insulating resin layers
`
`respectively interposed, a non-penetrating via hole is
`
`provided in the respective insulating layers as bottomed
`
`by each conductor layer exposed, and a plated layer is
`
`

`

`
`
`
`ai
`
`provided in the via hole for electric connection between
`
`the respective conductor layers, wherein the via hole
`
`is
`
`formed at
`
`least at
`
`a continuing zone of
`
`inner
`
`periphery to bottom surface to be of a concave curved
`
`surface of a radius of 20 to 100 pmin axially sectioned
`
`view of the via hole.
`
`Other objects
`
`and advantages of
`
`the present
`
`invention shall become clear as the description advances
`
`as detailed with reference to embodiments
`
`shown in
`
`10
`
`accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGURES l1(a)
`
`to l(c) are fragmentary sectioned
`
`views showing the board in an embodiment according to
`
`the present invention in a sequence of respective steps
`
`15
`
`of manufacturing method for the board;
`
`FIGS. 2(a) to 2(f) are fragmentary sectioned views
`
`showing more concretely the repective steps in the
`
`embodiment of FIG.
`
`1 of the present
`
`invention;
`
`FIG. 2A is an explanatory view for that a plated
`
`20
`
`layer becomes larger in the thickness than in the past
`
`at concave curved surface in continuing zone of inner
`
`periphery to bottom surface of via hole;
`
`FIG. 2Ba is an explanatory view for a plated layer
`
`formed according to the present
`
`invention;
`
`25
`
`FIG. 2Bb is an explanatory view for a plated layer
`
`formed according to a conventional manner;
`
`FIGS. 3(a) to 3(f) are fragmentary sectioned views
`
`showing the board in the respective steps in another
`
`-6-
`
`

`

`
`
`
`embodiment according to the present
`
`invention;
`
`FIGS. 4(a) to 4({f) are fragmentary sectioned views
`
`showing the board in the respective steps in another
`
`embodiment according to the present
`
`invention;
`
`FIGS. 5(a) to 5(d) are fragmentary sectioned views
`
`showing the board in the respective steps in another
`
`embodiment according to the present
`
`invention;
`
`FIGS. 6(a) to 6(qg) are fragmentary sectioned views
`
`showing the board in the respective steps in another
`
`10
`
`embodiment according to the present
`
`invention;
`
`FIGS. 7(a)
`
`to
`
`T(g) are fragmentary sectioned
`
`views
`
`showing the board in the respective steps in
`
`another embodiment according to the present invention;
`
`FIGS. 8(a) to 8(g) are fragmentary sectioned views
`
`15
`
`showing the board in the respective steps in another
`
`embodiment according to the present
`
`invention;
`
`
`FIGS. 9(a) to $(g) are fragmentary sectioned views
`
`showing the board in the respective steps in another
`
`embodiment according to the present
`
`invention;
`
`20
`
`FIGS. 10(a) and 10(b) are fragmentary sectioned
`
`views
`
`showing the board in the respective steps in
`
`another embodiment according to the present invention;
`
`and
`
`FIGS. 11 to 13 are diagrams showing respectively
`
`25
`
`each of examples of
`
`irradiation aspects of
`
`carbon
`
`dioxide gas laser in further embodiments according to
`
`the present
`
`invention.
`
`While the present invention shall now be described
`
`

`

`with reference to the respective embodiments shown in
`
`the accompanying drawings, it should be appreciated that
`
`the intention is not to limit the invention only to these
`
`embodiments shown but rather to include all alterrations,
`
`modifications
`
`and equivalent arrangements possible
`
`within the scope of appended claims.
`
`DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
`
`FIGS. lta) to l(c) show an embodiment of the method
`
`for manufacturing the multilayer printed wiring board
`
`10
`
`according to the present invention, and FIGS. 2(a)
`
`to
`
`2(f) show more concrete aspects of the steps in FIG. 1.
`
`Referring in detail
`
`to the present
`
`embodiment
`
`by
`
`references to FIGS.
`
`1 and 2, a multilayer substrate 11
`
`shown in FIG. 2(a) is formed by laminating a metal layer
`
`LS
`
`16 through an insulating layer 2 on an inner layering
`
`substrate 15 provided on a surface with a conductor layer
`
`la by a wiring pattern formation.
`
`The inner layering
`
`substrate 15 employable here is one prepared by forming
`
`
`
`
`
`
`gu
`
`the conductor layer la in a circuit through a wiring
`
`for example,
`
`pattern processing made with respect
`
`to,
`
`20
`
`a copper foil on a copper clad laminate of glass-cloth
`
`base epoxy resin,
`
`and the conductor
`
`layer
`
`la should
`
`preferably be subjected to such surface treatment as a
`
`surface
`
`roughening
`
`or
`
`the
`
`like.
`
`Further,
`
`the
`
`25
`
`insulating layer 2 can be formed with such thermosetting
`
`resinor the like as epoxy resin, polyimide and the like,
`and the metal layer 16 laminated on the surface of the
`
`insulating layer 2 may be formed by means of a plating,
`
`-8-
`
`
`
`

`

`
`
`thermocompression bonding of a metal foil and so on.
`
`Further,
`
`the multilayer
`
`substrate 11
`
`can be also
`
`manufactured by providing preliminarily athermosetting
`
`resin layer
`
`in a half-set
`
`state for
`
`forming the
`
`insulating layer 2 on one surface of a metal foil for
`
`forming the metal
`
`layer 16,
`
`and bonding under
`
`the
`
`thermocompression the metal foil on its side of
`
`the
`
`thermosetting resin layer onto the surface of the inner
`
`layering substrate 15.
`
`In this manner,
`
`the insulating
`
`10
`
`layer 2 may only be formed on either side or on both sides
`
`of the inner layering substrate 15, and either one of
`
`the steps can be properly selected.
`
`The insulating layer 2 may contain a filler of an
`
`inorganic powder, if required. While not specifically
`
`i5
`
`limited, such inorganic fibers as glass fibers whichare
`
`apt to be hindering to the formation of a via hole 3 by
`
`means of a carbon dioxide gas laser should preferably
`
`be not used.
`
`
`In an event when the insulating layer 2
`
`is formed by means of a coating or the like of a resin
`
`20
`
`composition containing a solvent, it is preferable that
`
`the solvent is removed with a heating therefor, prior
`
`to a setting with a heat.
`
`For the metal foil to be used
`
`as the metal layeri16, copper foil, aluminum foil, nickel
`
`foil and the like may be enumerated but, fromaview point
`
`25
`
`of availability or
`
`function,
`
`copper
`
`foil
`
`should
`
`desirably be used.
`
`With
`
`the
`
`foregoing multilayer
`
`substrate
`
`il
`
`employed,
`
`the metal layer 16 at a position where the via
`
`-9-
`
`

`

`
`
`hole 3 is to be formed is removed as shown in FIG. 2(a)
`
`to form an opening 13.
`
`The position where the via hole
`
`3 is formed is to be set at part corresponding to the
`
`conductor layer la on the inner layering substrate 15,
`
`and, while not specifically limited, an etching or the
`
`like process may be employed as a process for removing
`
`the metal
`
`layer 16.
`
`A concrete example of the steps for manufacturing
`
`the multilayer substrate 11 and for forming the opening
`
`10
`
`13 inthe metal layer 16 is shown. First, aboth-surface
`
`copper clad laminate of FR-4 type (the laminate being
`
`1.0mm thick, and the copper foil being 18 pm thick) was
`
`used,
`
`the conductor
`
`layer
`
`la to be
`
`the inner
`
`layer
`
`circuit was
`
`formed by means
`
`of
`
`a wiring pattern
`
`15
`
`processing with respect to the copper foil on one surface,
`
`the copper foil on the other surface was etched off
`
`overall,
`
`and the
`
`inner
`
`layering substrate 15 was
`
`prepared.
`
`This substrate was dipped in an aqueous
`
`solution of 2% copper chloride and 7% hydrochloric acid
`
`20
`
`at 30C,
`
`to have a surface of the conductor
`
`iayer la
`
`subjected to an etching surface-roughening process.
`
`On
`
`the other
`
`hand,
`
`an
`
`epoxy
`
`resin varnish
`
`containing as main components bisphenol A epoxy resin
`
`ana dicyandiamide and generally used for manufacturing
`
`25
`
`the FR-4 type laminate was applied onto the roughened
`
`bonding surface (mat finished) of the 18 um thick copper
`
`foil with a comma coater employed, soas tobe 70 pmthick
`
`after being dried,
`
`the varnish was heated to be dried
`
`-10-
`
`

`

`
`
`
`
`
`under conditions of 1507 and 20 minutes,
`
`so as to be
`
`half-set (to be in B stage), anda copper foil lined with
`
`resin was prepared.
`
`The resin-lined copper foils were placed over both
`
`surfaces of the inner layering substrate 15 with the
`
`resin side of the foils faced to the substrate, and the
`
`multilayer substrate 11
`
`in which the inner
`
`layering
`
`substrate 15 carries the metal
`
`layer
`
`16
`
`laminated
`
`through the insulating layer 2, as shown in FIG. 2(a),
`
`10
`
`through a heat/pressure molding under conditions of 30
`
`kg/om?, at 170° and for 120 minutes.
`
`Next, anetching resist film (dry film) was applied
`
`to top surface of
`
`the metal
`
`layer 16,
`
`the film was
`
`subjected to an exposure to light and an development,
`
`15
`
`the metal layer 16 at the portion where the via hole 3
`
`was to be formed was removed through an etching, and the
`
`opening 13 of
`
`@
`
`i100 pm was
`
`formed therein as in FIG.
`
`2(b).
`
`
`After the formatin of the opening 13 in the metal
`
`20
`
`layer 16 of the multilayer substrate 1i as in the above,
`
`a carbon dioxide gas
`
`laser L was
`
`irradiated to the
`
`insulating layer 2 exposed in the opening 13 as in FIG.
`
`2(c), and the non-penetrating via hole 3 bottomed by the
`
`conductor layer la was formed.
`
`The 1rradiation of the
`
`25
`
`carbon dioxide gas laser was executed under conditions
`
` of more thanl ps in pulse width so that the overall energy
`
`would be 8 mJ at working surface at an output of 150W.
`
`While 1 shot irradiation should cause no trouble so long
`
`-1l-
`
`

`

`
`
`
`as the overall energy of
`
`8 mJ could be attained,
`
`the
`
`irradiation may be performed sequentially as divided
`
`into a few shots as shown in FIG. 11 so that the overall
`energy would be 8 mJ, at a frequency of more than 100
`
`Hz.
`
`In FIG. 11, the abscissa denotes time, the ordinate
`
`
`denotes peak power of the carbon dioxide gas laser L,
`
`P denotes one-shot pulse width,
`
`L denotes time (sec.)
`
`between respective shots,
`
`and the frequency is L/L.
`
`Further, E is an energy per one shot and is proportional
`
`10
`
`to an area of the pulse.
`
`In the working aspect of FIG. 2,
`
`the opening 13
`
`made in the metal layer 16 is formed to be of the same
`
`diameter as that of the via hole 3, and a beam diameter
`
`of the carbon dioxide gas laser L is set to be larger
`
`i5
`
`than the diameter of
`
`the opening 13,
`
`so that
`
`the
`
`insulating layer 2 will be worked with the carbon dioxide
`
`gas laser L which passes through the opening 13 to form
`
`the via hole 3.
`
`In the present instance, therefore, the
`
`via hole 3 can be formed in accurate dimension and at
`
`20
`
`an accurate position without requiring the beam diameter
`
`of the carbon dioxide gas laser L tobe set dimensionally
`
`accurately nor requiring the positionof rrradiation of
`
`the laser L to be set accurately.
`
`In an event where the
`
`irradiation is executed with the beam diameter set to
`
`be larger than that of
`
`the opening 13,
`
`it should be
`
`preferable that the carbon dioxide gas laser Lof a beam
`
`uniform in the energy distribution on working surface
`
`is employed. With the use of such carbon dioxide gas
`
`-12-
`
`

`

`
`
`
`laser Luniforminthe energy distribution, itis enabled
`
`to eliminate any variation in the configuration of the
`
`via hole 3 being formed,
`even when the irradiating
`position of the laser L is deviated.
`
`While a plated layer 4 is to be formed next inside
`
`the via hole 3,
`
`there is adhered on exposed surface of
`
`the bottoming conductor layer la in the via hole 3 an
`
`insulating substance 5 of
`
`the insulating layer 2 as
`
`remained without being completely removed during the
`
`10
`
`formation of the via hole 3
`
`in the insulating layer 2
`
`by means of the foregoing carbon dioxide gas laser L,
`
`asin FIG. 2(c), andthis remaining insulating substance
`
`5
`
`
`is to be removed in a manner as will be referred to
`
`later,
`
`so as to be as shown in FIG. 2(d).
`
`15
`
`According to the present
`
`invention, here,
`
`the
`
`inner periphery of the via hole 3 is so formed as to be
`
`a concave curved surface in the inner surface shape of
`
`the via hole 3 as axially sectioned, and this concave
`
`curved surface is set to be of a radius R preferably less
`
`20
`
`than 100 pm,
`
`in particular, about 20 um. While the
`
`radius R of the concave curved surface may be in this
`
`range so long as the via hole 3 is of an aspect ratio
`
`(depth of the hole/diameter of the hole) less than 0.4,
`
`the radius R of
`
`the concave curved surface should
`
`preferably be set to be ina range of 30 to 100 pm when
`
`the aspect ratio is more than 0.4.
`
`That is, it should
`
`be appreciated that, as shown in FIG. 2A,
`
`the thickness
`
`of the plated layer in the via hole with respect to the
`
`-13-
`
`

`

`
`
`
`plated thickness on the top conductor
`
`layer remains
`
`smaller in such circular curved surface of a radius close
`
`to and less than 10 pm as shown by a circle O', whereas
`the thickness of the plated layer in continuing zone of
`
`the inner periphery to the bottom surface with respect
`
`to the thickness of top conductor layer is remarkably
`
`increased in such circular curved surface of a radius
`
`close toandmore than 30 ymas shownbyacircle oO. While
`
`the upper limit of the aspect ratio is not specifically
`
`10
`
`limited, an aspect ratio 1 will be the substantial upper
`
`limit, froma view point of the possibility of formation
`
`of the hole by means of the carbon dioxide gas laser L
`
`and of provision of uniform plating inside the via hole.
`
`In executing the plating with respect to the via
`
`15
`
`hole 3,
`
`such electroless plating as
`
`an electroless
`
`copper plating is first carried out
`
`to form a
`
`thin
`
`electrolessly plated film on the inner periphery of the
`
`via hole 3.
`
`Thereafter,
`
`such electroplating as
`
`an
`
`electro-copper-plating is performed with an electric
`
`20
`
`current passed, and the plated layer 4 is formed as in
`
`FIG. 2(e).
`
`The plated layer 4 is formed not only on the
`
`inner wall and on the conductor layer la exposed at the
`
`bottom of the via hole 3, but also on the top surface
`
`of the metai foil 16. While the egquipotential surfaces
`
`25
`
`occurring
`
`inside
`
`the
`
`via
`
`hole
`
`3
`
`during
`
`this
`
`electroplating will be parallel to the inner periphery
`
`
`and bottoming surface of the conductor layer la of the
`
`via hole 3, the zone in which the inner periphery of the
`
`-14-
`
`

`

`
`
`
`via hole 3 continues to the bottoming conductor layer
`
`la
`
`consists
`
`of
`
`a
`
`curved
`
`surface,
`
`so
`
`that
`
`the
`
`equipotential surfaces at this curved surface zone will
`be also curved and not angularly bent, as seen in FIG.
`
`1(b).
`
`As the equipotential surfaces at such continuing
`
`zone of the inner periphery of the via hole 3
`
`to the
`
`bottoming conductor
`
`layer
`
`la are thus curved,
`
`the
`
`electric lines of force intersecting at right angles the
`
`equipotential surfaces will not become coarse even at
`
`10
`
`the continuing zone of the inner periphery of the via
`
`hole 3
`
`to the bottoming conductor
`
`layer la, and the
`
`current density at the continuing zone is not reduced
`
`but can be rather unified.
`
`Further, the fluidity of the
`
`plating solution is improved at
`
`the continuing zone.
`
`15
`
`Consequently,
`
`the plated layer 4 formed inside the via
`
`hole 3 through the electroplating is never thinned at
`
`the continuing zone of the inner periphery of the via
`
`hole 3
`
`to the bottoming conductor layer la, and is to
`
`be formed with a uniform thickness.
`
`20
`
`In forming the plated layer
`
`4
`
`in the uniform
`
`thickness in this manner, it is necessary that the via
`
`hole 3 has an inner peripheral shape formed,
`
`in the
`
`axially section at
`
`the continuing zone of
`
`the inner
`
`
`periphery of the via hole 3 to the bottoming conductor
`
`25
`
`layer la,
`
`to be the concave curved surface of a radius
`
`R more than 20 um. When the via hole 3 is of an aspect
`
`ratio more
`
`than 0.4,
`
`then the via hole
`
`3
`
`should
`
`preferably be formed to be more than 30 pmin the radius
`
`-15-
`
`

`

`
`
`
`R. Further,
`
`the via hole 3 should be of a diameter at
`
`the most of ¢ 200 pm and, with the radius R made larger
`
`the bottoming conductor layer la is not to
`than 100 pm,
`be exposed in the via hole 3, the upper limit of the radius
`
`R should preferably be 100 pm.
`
`Then, as has been referred to,
`
`the via hole 3 of
`
`the concave curved surface at the continuing zone of the
`
`inner periphery to the bottoming conductor layer la is
`
`formed through the processing of the via hole 3 with the
`
`10
`
`irradiation of the carbon dioxide gas laser of the pulse
`
`width more thanl us. While the upper limit of the pulse
`
`width of
`
`the carbon dioxide gas is not specifically
`
`limited,
`
`it should be preferably less than 100 us.
`
`Further, even when the insulating layer 2 is of varying
`
`15
`
`thickness,
`
`the formation of
`
`the via hole 3 with the
`
`sequential irradiation of the carbon dioxide gas laser
`
`at a frequency of more than 100 Hz allows the via nole
`
`3
`
`to be formed in the concave curved surface at
`
`the
`
`intersecting, continuing zone of the inner periphery to
`
`20
`
`the bottoming conductor layer la.
`
`The upper limit of
`
`the radiation frequency should preferably be less than
`
`10 kHz, while not specifically limited.
`
`After forming the plated layer 4
`
`inside the via
`
`hole 3 as has been described,
`
`the etching resist film
`
`25
`
`(dry film)
`
`is applied to the surface of the multilayer
`
`substrate ll, the filmis exposedto light and developed,
`
`and the metal
`
`layer 16 and plated layer 4
`
`thereon are
`
`etched, so that amultilayer printed wiring board formed
`
`-16-
`
`

`

`
`
`
`as in FIG. 2(f) can be obtained with a conductor layer
`
`1b comprising the metal layer 16 and plated layer 4 made
`
`as an outer layer circuit.
`In this multilayer printed
`wiring board, the conductor layers 1a and 1b are mutually
`
`electrically connected through the plated layer
`
`4
`
`provided in the via hole 3,
`
`the plated layer 4 can be
`
`formed in the uniform thickness as shown in FIG. 2Ba in
`
`contrast to a finish of conventional plated layer as in
`
`FIG.
`
`2Bb, because the continuing zone of
`
`the inner
`
`10
`
`periphery to the bottom surface of the conductor layer
`
`la is concave-curved inthe via hole 3 to prevent plating
`
`liguid from staying in such zone, sothat the reliability
`
`of the electric connection between the conductor layers
`
`la and 1b can be elevated, and there arises no problem
`
`15
`
`that any disconnection is caused by the heating upon
`
`mounting or by generated heat during use of mounted
`
`parts.
`
`In another embodiment shown in FIGS. 3(a) to 3(f),
`
`the working with respect to the insulating layer 2 for
`
`20
`
`forming the via hole 3 with the irradiation of the carbon
`
`dioxide gas laser Lis carried out, as shown in FIG. 3(c),
`
`by setting the diameter of beam of the laser L
`
`to be
`
`smaller than the diameter of the opening 13 provided in
`
`the metal layer 16, and irradiating the laser L to the
`
`insulating layer 2 within the opening 13, to thereby form
`
`25
`
`the via hole 3
`
`in the layer 2. All other respects are
`
`the same as those in FIG. 2. While there has been a risk
`
`in the case of FIG. 2 that, as the via hole 3 and opening
`
`-17-
`
`

`

`
`
`13 are substantially equal to each other inthe diameter,
`
`the metal layer 16 around the opening 13 may happen to
`
`protrude into the via hole 3 so as to render the plating
`performance deteriorated, the embodiment of FIG. 3 forms
`
`the via hole 3 in the smaller diameter than the opening
`
`13 so that the metal layer 16 around the opening 13 does
`
`not protrude
`
`to the via hole
`
`3
`
`and
`
`the plating
`
`performance inside the via hole 3 can be made excellent.
`
`In the event where the beam diameter of the carbon
`
`10
`
`dioxide gas laser L is made smaller than that of the
`
`opening 13 so as not to reach the metal layer 16, it is
`
`desirable
`
`to employ
`
`a
`
`beam which
`
`has
`
`an
`
`energy
`
`distribution on the working surface to be higher at
`
`positions closer to the center of the beam. When the
`
`15
`
`working is performed to form the via hole 3 with the
`
`carbon dioxide gas laser L of the beam of
`
`the higher
`
`energy distribution towards the center of the beam, it
`
`is possible to form the concave curved surface of
`
`a
`
`larger radius R in the continuing zone of
`
`the inner
`
`20
`
`periphery of the via hole 3 to the bottoming conductor
`
`layer la.
`
`In an event where the via hole 3
`
`
`is formed
`
`by irradiating the carbon dioxide gas laser L so as not
`
`to reach the metal
`
`layer 16 as seen in FIGS. 4(c) and
`
`5(b)
`
`later described,
`
`it is similarly preferable to
`
`25
`
`
`employ the laser L of
`
`the beam of
`
`the higher energy
`
`distribution towards the center of the beam.
`
`FIGS. 4(a) to 4(£) show another embodiment of the
`
`present
`
`invention,
`
`in which,
`
`as
`
`in FIG. 4(a),
`
`the
`
`-18-
`
`

`

`
`
`multilayer substrate li is first prepared by laminating
`
`the metal layer 16 on the inner layering substrate 15
`
`with the insulating layer 2 interposed similarly to FIG.
`
`2{(a).
`
`That is,
`
`an epoxy resin varnish comprising as
`
`main
`
`components
`
`bisphenol
`
`A
`
`epoxy
`
`resin
`
`and
`
`dicyandiamide and generally employed for manufacturing
`
`the FR-4 type laminate is applied to the bonding surface
`
`(mat finished) of a copper foil which is 18 pm thick by
`
`means of the comma coater, so as to be 70 pm thick after
`
`10
`
`being dried,
`
`the applied varnish was heated to be dried
`
`under conditions of 1507 and 20 minutes,
`
`so as to be
`
`half-set (B stage), and a copper foil lined with resin
`
`is prepared.
`
`These resin-lLined copper foils are placed
`
`over both surfaces of the inner layering substrate 15
`
`15
`
`with the resin side of the foils faced to the substrate,
`
`and the multilayer substrate 11
`
`in which the inner
`
`layering substrate 15 carries
`
`the metal
`
`layer
`
`16
`
`Laminated through the insulating layer 2, as shown in
`
`FIG. 4(a),
`
`is obtained.
`
`Since in the present instance
`
`20
`
`the insulating layer 2
`
`is laminated on the surface of
`
`the inner layering substrate 15 through the metal layer
`
`16 as heated and compressed,
`
`a high bond strength can
`
`be attained between the inner layering substrate 15 and
`
`the insulating layer 2.
`
`25
`
`Next, as in FIG. 4(b),
`
`the metal
`
`layer 16 on the
`
`surface is removed overall by means of the etching.
`
`In
`
`this state where the whole surface of the insulating
`
`layer 2 is exposed with the metal layer 16 removed,
`
`the
`
`-~19-
`
`

`

`
`
`carbon dioxide gas laser L is irradiated in the same
`
`manner as in FIG. 3(c), and the via hole 3 is formed in
`
`the insulating layer 2.
`As
`in the case of FIG. 2(d),
`further,
`the insulating substance 5 remaining on the
`
`surface of the bottoming conductor layer la in the via
`
`hole 3
`
`is removed, as in FIG. 4(d).
`
`Thereafter,
`
`
`in the same manner as in FIG. 2(e),
`
`the electroless plating and electro-plating are carried
`
`out
`
`to form the plated layer 4 within the via hole 3,
`
`10
`
`upon which the plated layer 4 is formed not only inside
`
`the via hole 3 but also ailover the surface of
`
`the
`
`insulating layer 2, as in FIG. 4(e).
`
`Then,
`
`the etching
`
`resist film (dry film) is applied to the surface of the
`
`plated layer 4
`
`formed allover the insulating layer 2,
`
`
`
`15
`
`such film is subjected to the exposure to light and to
`
`the development, and a multilayer printed wiring board
`
`in which a conductor layer lb comprising tne plated layer
`
`4
`
`is formed as the outer layered circuit through the
`
`etching as in FIG. 4(f)
`
`is obtained.
`
`20
`
`Since in this multilayer printed wiring board the
`
`conductor layers la and lb are mutually electrically
`
`connected through the plated layer 4 provided inside the
`
`viahole 3andthis plated layer 4is formed inthe uniform
`
`thickness as has been disclosed, itis possible to render
`
`25
`
`the reliability of the electric connection between the
`
`conductor layers la and 1b to be high, and to eliminate
`
`the risk of causing disconnection due to the heating upon
`
`mounting or generated heat of the mounted parts being
`
`- 20 -
`
`

`

`
`
`used.
`
`In the foregoing embodiment of FIG. 2 or 3 (as well
`
`as conventional ones),
`
`the plated layer 4 is formed not
`
`only inside the via hole 3 but also allover the metal
`
`layer 16 as in FIG. 2(e) or 3(e)
`
`so that the conductor
`
`layer 1b forming the outer layered circuit will comprise
`
`two layers of the metal layer 16