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`4,591,659
`[11] Patent Number:
`11)
`United States Patent
`Leibowitz
`[45] Date of Patent: May 27, 1986
`
`
`4,464,704
`4,472,762
`4,526,835
`
`8 Claims, 3 Drawing Figures
`
`APPLE 1006
`
` 1
`
`8/1984 Huie et AL, veeesessesesereteres 174/68.5 X
`
`9/1984 Spinelli et al. screen 174/68.5 X
`............. 428/901 X
`7/1985 Takahashi et al.
`.
`.
`.
`Joseph D. Leibowitz, Culver City,
`Primary Examiner—R.R. Kucia
`Calif.
`Attorney, Agent, or Firm—James W. Paul; Donald R.
`Nyhagen; Noel F. Heal
`[73] Assignee: TRW Inc., Redondo Beach,Calif.
`[57]
`ABSTRACT
`(21] Appl. No.: 564,952
`A composite printed circuit board structure including
`[22] Filed:
`Dec. 22, 1983
`‘multiple layers of graphite interleaved with layers of a
`[SU] Lit, C14 ccccccesssscsscscsessscsseseecsssesenssses HOSK 1/03
`dielectric material, such as a polytetrafluoroethylene
`(52) U.S. Ch saccccsccssccccccseeseeterereeeees 174/68.5; 361/387,
`(PTFE) and woven glass laminate. Someof the dielec-
`361/414; 427/96; 428/408; 4287901
`‘tic layers are copper clad, and at least some ofthe
`[58] Field of Search ............. 174/68.5; 361/414, 387,
`graphite layers are positioned in close proximity to the
`361/388; 427/96, 97; 428/209, 408, 901
`coppercladding layers, to provide good heatdissipation
`,
`>
`properties. The PTFE provides a desirably low dielec-
`References Cited
`tric constant and the graphite also provides good me-
`U.S. PATENT DOCUMENTS
`chanical strength and a low or negative coefficient of
`.
`thermal expansion, to permit matchingofthe coefficient
`4/1976 Davidson et al. occ. 361/386
`3,952,231
`:
`.
`:
`4,054,939 10/1977 Ammon..........
`w» 361/414
`
`with that of chip carriers used to mount components on
`4.294.877 10/1981 Graham ......................... 428/209
`
`4,318,954 3/1952 Jensen ssercssscesssesssesnsee 174/68.5 X_the circuit board.
`
`4,327,126 4/1982 Ogden o...cecccscceccseseseeeeee 427/97
`4,456,712 6/1984 Christie et al... 428/901 X
`
`[54] MULTILAYER PRINTED CIRCUIT BOARD
`STRUCTURE
`
`[75]
`
`Inventor:
`
`[56]
`
`APPLE 1006
`
`1
`
`

`

`U.S. Patent
`
`May27, 1986
`
`4,591,659
`
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`1
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`4,591,659
`
`MULTILAYER PRINTED CIRCUIT BOARD
`STRUCTURE
`
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to printed circuit
`boards, and moreparticularly, to printed circuit boards
`on which microelectronic components or chips are
`mounted. One desirable characteristic of printed circuit
`boards used for this purposeis that they should be me-
`chanically stable under conditions of varying tempera-
`ture. This is particularly important in circuit boards
`used to support microelectronic components housed in
`chip carriers.
`,
`Chip carriers are usually fabricated from a ceramic
`material, such as aluminum oxide, and are produced in
`the form of a hermetically sealed package for each chip.
`Bondedleads are broughtout from the chip to the edges
`of the chip carrier, and the carrier is then usually
`soldered, by its leads, directly to a circuit board. The
`principal advantage of this structure is a significantly
`highercircuit density. Also, the use of shorter and more
`uniform lead lengths results in improved speed and
`impedance characterisitics. Another consideration is
`that the use of chip carriers substantially reduces the
`overall cost of a circuit package. Package size reduc-
`tions as high as a five-to-one ratio can be obtained,
`compared with an equivalent dual in-line package con-
`struction.
`A major drawbackto the use of chip carriers is that
`the coefficient of thermal expansion of aluminum oxide,
`the most commonly used chip carrier material,
`is ap-
`proximately one-half the coefficient of thermal expan-
`sion for glass/epoxy laminates typically used in the
`manufactureof circuit boards. Whentheresulting struc-
`ture is exposed to anysignificant range of temperatures,
`the thermal cycling of the structure can crack soldered
`joints and renderthecircuit inoperative. One solutionto
`this problem is to use an intermediate member between
`the chip carrier and the circuit board. The circuit board
`is sometimes referred to as a mother board, and the
`intermediate memberas a baby board. The intermediate
`membermayalso take the form of a hybrid package on
`whichthe chip carrier is mounted. Another techniqueis
`to use a compliant lead structure between the chip car-
`rier and the circuit board, although this clearly in-
`creases the cost of the package andresults in inherently
`long lead lengths.
`Accordingly, an ideal circuit board should have a
`coefficient of thermal expansion that closely matches
`that of chip carriers mounted on the board.If there is a
`substantial mismatch in coefficients of thermal expan-
`sion, the chip carrier may break loose from the board,
`or the electrical connections may be damaged.
`Anotherdifficulty that has arisen as larger numbers
`of components are mounted oncircuit boards,is that the
`heat produced by the components mustbedissipated in
`some manner, whether by conduction throughthecir-
`cuit board or by radiative, convective, or forced-air
`cooling. Since the principal materials used in circuit
`boardsare insulators, the boards themselves havetradi-
`tionally played no significant role in dissipating heat
`from the components that they support.
`A third factor in the design of circuit boards is that
`they should ideally be of a material with a relatively low
`dielectric constant, to enhance the board’s ability to
`propagate signals overrelatively long distances.
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`such as polytetrafluoroethylene
`Some materials,
`(PTFE)have good dielectric properties but an undesir-
`ably high coefficent of thermal expansion. Kevlar
`(trademark of E.I. du Pont de Nemours & Co., Inc.) has
`a negative coefficient of thermal expansion, and may be
`used to reduce the average coefficient of thermal expan-
`sion in a composite circuit board structure. However,
`Kevlar is a poor thermal conductor, and therefore does
`nothing to enhance the thermal conduction properties,
`of the board.
`U.S. Pat. No. 4,318,954 issued to Jensen, proposes the
`use ofa single thick layer of graphite reinforced with a
`resin, to adjust the coefficient of thermal expansion of a
`circuit board. The technique disclosed in the patentis to
`use a large bulk of graphite, such that the composite
`expansion coefficient approaches that of the graphite
`alone. However, the Jensen patent does not provide any
`solution to the increasing problem of heat dissipation.
`It will be appreciated from the foregoing that thereis
`an ever increasing need for a multilayer printed circuit
`board structure that addresses these problems. Specifi-
`cally, the ideal circuit board structure should have low
`dielectric properties, a low or negative thermal coeffici-
`ent of thermal expansion, and good thermal conduction
`properties to enhance heat conduction from devices
`mounted on the board. The present invention satisfies
`all of these needs.
`
`SUMMARYOF THE INVENTION
`
`The present invention resides in a multilayer printed
`circuit board structure in which multiple layers of
`graphite are employed both to reduce the coefficient of
`thermal expansion and to provide enhanced thermal
`conductivity, and multiple layers of a PTFE material
`are used to provide the necessary dielectric properties.
`More specifically,
`the layers of graphite are spaced
`symmetrically across the thickness of the circuit board,
`to minimize the possibility of bowing of the board dur-
`ing temperature changes, and at least some ofthe layers
`of graphite are positioned in close proximity to copper
`layers in the board, to provide enhanced thermal con-
`duction from the mounted components.
`The graphite layers in the structure of the invention
`take the form of woven sheets of fabric that have been
`impregnated with a bonding material, such as an epoxy
`resin, An alternative preferred construction, in which
`high modulus carbon fiber material is used, takes the
`form of alternative layers of unidirectional fibers, the
`fibers in each layer being more orless parallel to one
`another and the alternate layer fiber directions being
`arranged to provide stiffness and strength along the
`major axes of the structure of the invention. Each
`graphite sheet is separated from adjacent copper or
`dielectric layers by a thin layer of insulating adhesive
`material. The adhesive serves both to bond the two
`layers together and to electrically insulate the copper
`layers from the graphite. Holes may be formed through
`the graphite, either for plated-through connectors be-
`tween copper layers, on to accommodate mounting
`screws used to secure the board structure to a housing.
`At each of the predetermined hole positions, the graph-
`ite sheets are pre-drilled and the holesare filled with a
`bonding adhesive material, such as the epoxy resin used
`to form the sheets. When via holes or mounting screw
`holes are subsequently drilled through the composite
`board, a drill of smaller diameter is used. In this way,
`each hole through a graphite layer has an annular
`sheath of insulating material around it, and no inadver-
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`4,591,659
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`avoid having the graphite act as an unwanted electrical
`tent connection is made between copperlayers. How-
`connection between portions of the copper layers. The
`ever, the insulation around the mounting screwsis thin
`adhesive layers 22 are, however, so thin that they offer
`enoughto provide only a small resistance to the flow of
`little resistance to the flow of heat between the copper
`heat.
`and the graphite.
`It will be appreciated from the foregoing that the
`The graphite layers 16 are formed from fibrous car-
`present invention represents a significant advancein the
`bon material, such as “Thorne!” P100 or P75S, made by
`field of multiple-layer printed circuit boards. In particu-
`Union Carbide Corporation, Carbon Products Division,
`lar, the invention provides a circuit board in which the
`Chicago, Ill. 60606. The graphite or carbon. yarn is
`coefficient of thermal expansion may be substantially
`woveninto a cloth, which is then impregnated with a
`reduced, and matched to that of an adjacent chip car-
`bonding material, such as an epoxyresin. In an alterna-
`rier. Moreover, the board of the invention has good
`tive preferred embodiment, which incorporates high
`thermal conductivity properties, excellent mechanical
`moduluscarbonfibers, the carbon fibers are arranged in
`strength, and doesnotsacrifice dielectric performance.
`each layer so that they are moreorless parallel to one
`Other aspects and advantages ofthe invention will be-
`another and are not woveninto cloth, since such weav-
`come apparent from the following more detailed de-
`ing may result in damage to the brittle high modulus
`scription, taken in conjunction with the accompanying
`fibers. Alternate layers are then arranged so that the
`drawings.
`combination of fiber directions in the layers provides
`BRIEF DESCRIPTION OF THE DRAWINGS
`the required regidity and strength along the major axes
`20
`of the circuit board. The resin-impregnated graphite
`FIG.1 is a simplified perspective view showing four
`cloth cures to a hard, regid sheet, with practically pla-
`chip carriers mounted onacircuit board;
`nar surfaces. Hole locations, such as indicated at 30 in
`FIG. 2 is a fragmentary cross-sectional view of a
`FIG.3, are predetermined, and the graphite sheets are
`multilayer circuit board made in accordance with the
`pre-drilled with oversized holes 32, which are immedi-
`invention; and
`ately filled with moreresin material. Then the graphite
`FIG.3 is a fragmentary view showing how via holes
`boards 16 are ready to be laminated with the PTFE
`and mounting screw holes are formed in the board of
`layers 18, using the adhesive 22.
`the invention.
`Prior to lamination, the copper layers 20 are appro-
`priately patterned as dictated by the design ofcircuits
`mounted on the board. After lamination, holes are made
`through the entire board 10, but using a smaller diame-
`ter drill than the one used to pre-drill the graphite layers
`16. Thus, each newly formed hole 34 is surrounded by
`an annular sheath ofinsulating material, which serves to
`electrically separate the graphite layers 16 from the
`copperlayers20, butis thin enough to permit transmis-
`sion of heat from the copper.
`.
`The number and thickness of the graphite layers 16
`are selected to provide a desired effect on the composite
`coefficient of thermal expansion of the board 10. In
`addition, the graphite layers 16 are disposed in a sym-
`metrical fashion across the thickness of the board 10, to
`minimizethe possibility of bending of the board during
`temperature changes. The coefficient of thermal expan-
`sion of the graphite layers 16 is close to zero after im-
`pregnation with epoxyresin. The coefficient of thermal
`expansion for copper is 9.4 10-6 in/in/°F., and the
`coefficient
`for copper-clad PTFE sheets is much
`higher. The goalin selecting the number and thickness
`ofthe graphite layers 16 is to match the coefficient of
`thermal expansion to that of the chip carrier material.
`The most common material, aluminum oxide, has a
`coefficient of thermal expansion of 3.33 10-6 in-
`“/in/°F.
`The PTFE layers 18 may be part of a suitable mate-
`rial, such as CU-CLAD 233, manufactured by the Elec-
`tronics Products Division of the 3M Company,St. Paul,
`Minn. 55144. This material is a laminate of PTFE and
`wovenglass. It has a low dielectric constant of 2.33, but
`a relatively high coefficient of thermal expansion. Ap-
`propriate selection and positioning of the graphite lay-
`ers 16 results in a composite coefficient of thermal ex-
`pansion that closely matches that of the chip carrier
`material.
`Oneofthe principal advantagesof the use of graphite
`in the circuit board structure is that it serves as a good
`conductor of heat, which normally can flow from the
`mounted components through the copperlayers of the
`
`As shownin the drawingsfor purposesofillustration,
`the present
`invention is principally concerned with
`multilayer printed circuit boards. There are three desir-
`able properties of circuit boards, especially those used
`to support electronic components on ceramic chip car-
`riers. First, the board should havea relatively low di-
`electric constant,
`to facilitate transmission of signals
`over relatively long distances. Second, the coefficent of
`thermal expansion of the board should be controllable
`to some degree, to match the coefficient of adjacent
`materials, such as chip carriers. Finally,
`the board
`should ideally be a sufficiently good thermal conductor
`to assist in the dissipation of heat generated in mounted
`components. Unfortunately, these three desired charac-
`teristics cannot be found in a single material.
`FIG. 1 showsa multilayer circuit board, indicated by
`reference numeral 10, used to support four chip carriers
`12. The chip carriers 12 have leads broughtoutto their
`edges and extending around beneath the carriers. The
`leads are then attached to: the circuit board 10 by a
`reflow soldering process in which the leads are secured
`to respective pads 14 on the top ofthe circuit board 10.
`Any mismatch betweenthe coefficients of thermal ex-
`pansion of the chip carriers 12 and the board 10 can
`result in damageto the electrical chip connections.
`In accordance with the invention, and as showndia-
`grammatically in FIG.2, the circuit board 10 includes a
`plurality of layers of graphite 16 interleaved between
`layers 18 ofa dielectric material that includes polytetra-
`fluoroethylene (PTFE). Someof the layers 18 are cop-
`per coated, as indicated at 20. The PTFE layers 18
`provide the basic dielectric material of the board 10,
`and the graphite layers 16 provide both thermal con-
`ductivity and control of thermal coefficient of expan-
`sion. The graphite layers 16 are bonded to adjacent
`copper layers 20 or PTFE layers 18 with a suitable
`adhesive layer 22. For the copper-to-graphite bond, the
`adhesive 22 also functions as an electrical insulator, to
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
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`4,591,659
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`board, through mounting bolts, and thence to a housing
`or otherheatsink. In the structure of the invention, the
`graphite layers 16 provide a parallel path for the flow of
`heat, and thereby improve the heat dissipation charac-
`teristics of the circuit package. Another advantage of
`the use of graphite is that it is mechanically extremely
`strong. The tensile modulusofelasticity for single yarns
`of
`the graphite specified above is
`in the range
`75-100 x 106 p.s.i., but this is naturally reduced when
`the graphite is impregnated with resin. Nevertheless,
`the use of graphite greatly stengthens the circuit board
`structure.
`
`6
`a plurality of layers of conductive metal used to es-
`tablish connections between components to be
`mounted on the board;
`a plurality of layers of graphite, at least some of
`which are positioned in close proximity to some of
`said layers of conductive metal, to provide a rela-
`tively low resistance path for the flow of heat from
`said layers of conductive metal; and
`a plurality of layers of a dielectric material bonded
`together with said layers of conductive metal and
`graphite, to yield a composite multilayer printed
`circuit board;
`wherein said layers of graphite are positioned in a
`symmetrical manner with respect to the thickness
`of the composite board, and selected in numberto
`provide a desired composite coefficient of thermal
`expansion;
`and wherein said graphite layers are formed from
`unidirectional fibers of graphite impregnated with
`a bonding material; and
`said graphite sheets are bonded to adjacent sheets of
`conductive metal bya thin layer of insulating adhe-
`sive, wherebythe layer of adhesive serves the addi-
`tional purpose of electrically separating the con-
`ductive metal from the conductive graphite layer.
`3. A multilayer printed circuit board as set forth in
`claim 2, wherein:
`said sheets of conductive metal are of copper; and
`said sheets of dielectric material include a polytetra-
`fluoroethylene (PTFE) material.
`4. A multilayer printed circuit board having a desired
`coefficient of thermal expansion, good thermal conduc-
`tivity and low dielectric constant, said circuit board
`comprising:
`a plurality of layers of a dielectric material that in-
`cludes polytetrafluoroethylene, someof said layers
`being coated with patterned copper, as dictated by
`the nature of the components supported on the
`circuit board; and
`_
`a plurality of layers of graphite, at least some of
`whichare positioned in close proximity to some of
`said copper layers, to provide good thermal con-
`duction for the dissipation of heat generated in the
`components mounted on the board;
`wherein said graphite layers are selected to provide a
`desired composite coefficient of thermal conduc-
`tivity and are positioned across the thickness of the
`board in a symmetrical manner, to minimize bow-
`ing of the board in conditions of varying tempera-
`ture.
`5. A printed circuit board as set forth in claim 4,
`wherein:
`said said layers of graphite are formed from unidirec-
`tional graphite fibers impregnated with a plastic
`bonding material.
`6. A printed circuit board as set forth in claim 5,
`wherein:
`said plastic bonding material in an epoxyresin.
`7. A printed circuit board as set forth in claim 4,
`wherein:
`said layers of graphite are formed from woven sheets
`of graphite impregnated with a plastic bonding
`material.
`8. A printed circuit board as set forth in claim 7,
`wherein:
`said plastic bonding material is an epoxy resin.
`*
`*
`&
`*
`
`It will be appreciated from the foregoing that the
`present invention represents a significant advance in the
`field of multilayer printed circuit boards. In particular,
`the use of multiple layers of graphite in a circuit board
`is used to control the coefficient of thermal expansion
`and to provide a supplementary thermal path for the
`dissipation of heat from components mounted on the
`board. The base material used in the board is PTFE
`with a desirably low dielectric constant.
`It will also be appreciated that, although a specific
`embodiment of the invention has been described in
`detail for purposes ofilustration, various modifications
`may be made without departing from the spirit and
`scope of the invention. Accordingly, the invention is
`not to be limited except as by the appended claims.
`I claim:
`1. A multilayer printed circuit board having a desired
`coefficient of thermal expansion, good thermal conduc-
`tivity and low dielectric constant, said circuit board
`comprising:
`a plurality of layers of conductive metal used to es-
`tablish connections between components to be
`mounted on the board;
`a plurality of layers of graphite, at least some of
`which are positioned in close proximity to some of
`said layers of conductive metal, to provide a rela-
`tively low resistance path for the flow of heat from
`said layers of conductive metal; and
`a plurality of layers of a dielectric material bonded
`together with said layers of conductive metal and
`graphite, to yield a composite multilayer printed
`circuit board;
`wherein said layers of graphite are positioned in a
`symmetrical manner with respect to the thickness
`of the composite board, and selected in numberto
`provide a desired composite coefficient of thermal
`expansion, and formed from woven sheets of
`graphite impregnated with a bonding material;
`and wherein said graphite sheets are bonded to adja-
`cent sheets of conductive metal by a thin layer of
`insulating adhesive, whereby the layer of adhesive
`serves the additional purpose of electrically sepa-
`rating the conductive metal from the conductive
`graphite layer;
`and wherein said sheets of conductive metal are of
`copper; and
`said sheets of dielectric material include a polytetra-
`fluoroethylene (PTFE) material.
`2. A multilayer printed circuit board having a desired
`coefficient of thermal expansion, good thermal conduc-
`tivity and low dielectric constant, said circuit board
`comprising:
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