Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K7/00—Constructional details common to different types of electric apparatus
  • H05K7/20—Modifications to facilitate cooling, ventilating, or heating
  • H05K7/2039—Modifications to facilitate cooling, ventilating, or heating characterised by the heat transfer by conduction from the heat generating element to a dissipating body
  • H05K7/20509—Multiple-component heat spreaders; Multi-component heat-conducting support plates; Multi-component non-closed heat-conducting structures
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K1/00—Printed circuits
  • H05K1/02—Details
  • H05K1/03—Use of materials for the substrate
  • H05K1/05—Insulated conductive substrates, e.g. insulated metal substrate
  • H05K1/056—Insulated conductive substrates, e.g. insulated metal substrate the metal substrate being covered by an organic insulating layer
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/0058—Laminating printed circuit boards onto other substrates, e.g. metallic substrates
  • H05K3/0061—Laminating printed circuit boards onto other substrates, e.g. metallic substrates onto a metallic substrate, e.g. a heat sink
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
  • H05K2201/02—Fillers; Particles; Fibers; Reinforcement materials
  • H05K2201/0203—Fillers and particles
  • H05K2201/0206—Materials
  • H05K2201/0209—Inorganic, non-metallic particles
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/10—Apparatus or processes for manufacturing printed circuits in which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern
  • H05K3/18—Apparatus or processes for manufacturing printed circuits in which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern using precipitation techniques to apply the conductive material
  • H05K3/181—Apparatus or processes for manufacturing printed circuits in which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern using precipitation techniques to apply the conductive material by electroless plating
  • H05K3/182—Apparatus or processes for manufacturing printed circuits in which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern using precipitation techniques to apply the conductive material by electroless plating characterised by the patterning method
  • H05K3/184—Apparatus or processes for manufacturing printed circuits in which conductive material is applied to the insulating support in such a manner as to form the desired conductive pattern using precipitation techniques to apply the conductive material by electroless plating characterised by the patterning method using masks

Definitions

• This invention relates to improved means for thermal dissipation for printed circuit boards and electrical devices mounted thereon.
• This invention relates to an improved means for dissipating heat from electrical devices mounted on printed circuit boards.
• electrical devices When using electrical devices operating at elevated temperatures, it may be necessary to use a heat sink to avoid thermal destruction of the devices. Industry requirements for heat dissipation are increasing rapidly, and it is not uncommon to be required to dissipate as much as 50 watts per square inch.
• a typical example is an aluminum heat sink which is affixed to a sheet of copper foil by means of a thermally conducting epoxy resin.
• the desired circuit is then formed by etching the copper foil to form a conductive pattern. While providing good heat transfer, a serious disadvantage to this construction is that it is useful for printed circuit boards having only a single conductive surface.
• the majority of printed circuit boards are required to have electrical conductors on more than one surface.
• the conductors are interconnected by plated through holes through which much of the heat is transferred, and such holes may even be filled with conductive material to further facilitate heat transfer.
• Prior art methods have employed an adhesive layer of material, in the form of a partially cured epoxy or polyester resin to bond heat sinks to printed circuit boards. While these materials have excellent electrical insulating properties, they lack the thermal transfer properties required to efficiently transfer heat to the heat sink.
• thermally conducting adhesives made by incorporation of a thermally conductive, yet electrically insulating material into the adhesives.
• U. S. Patent No. 5,288,769 to Davis et al. discloses an epoxy resin comprising metal-coated aluminum nitride particles, the adhesive employed to bond an electrical component to a circuit carrying substrate to dissipate heat.
• the adhesive layer acts as a barrier for heat flow, it is important to use as thin a layer as possible for efficient heat transfer, while at the same time employing enough adhesive to ensure good bonding and electrical insulation. It is difficult to coat thermally conductive adhesives on a printed circuit board having a conductive pattern raised above the plane of the dielectric base material on which the circuit board is fabricated. The flow characteristics of the adhesives must be rigidly controlled. If the adhesive does not flow enough to conform to the surface topography of such a printed circuit board, voids will occur at the intersection of the base material and the conductor, and will become filled with air. This increases the thermal resistance of the bonding layer since air is a poorer thermal conductor than the adhesive.
• the present invention comprising a thermally conductive fiber reinforced resin sheet, an additive printed circuit board and a planar heat sink solves the above mentioned problems. Because the surfaces of the dielectric base material and the conductive pattern of an additive printed circuit board are coplanar, intimate contact between the resin sheet and the printed circuit board, without intermediate voids, is maintained and heat transfer is significantly enhanced.
• An objective of this invention is to provide a highly efficient heat-dissipating means for interconnecting electrical components.
• a further objective of this invention is to provide an additive printed circuit board integrally mounted to a heat sink.
• a further objective of this invention is to provide an additive printed circuit board integrally mounted to a heat sink, said resulting article being highly resistant to thermal stress and delamination.
• a further objective of this invention to provide an additive printed circuit board integrally mounted to a heat sink by means of a fiber-reinforced, thermally conductive organic dielectric adhesive.
• Heat transfer efficiency will depend on the thermal characteristics of both the printed circuit board and the means used to bond the heat sink to the board. Since heat transfer through a printed wiring board having a plated-through hole occurs, for the most part, by the thermally conducting path created by the plated- through hole, the overall efficiency of heat transfer from the heat-generating device to the heat sink depends mainly on the thermal resistance of the means used to bond the heat sink to the printed circuit board. The lower the thermal resistance, the more efficient is the heat transfer.
• the bonding means may con-prise an adhesive in the form of a non-reinforced paste
• the preferable form of bonding means is a fiber-reinforced, thermally conductive organic dielectric adhesive sheet, commonly known as a "prepreg", containing a thermally conductive filler.
• prepreg thermally conductive organic dielectric adhesive sheet
• the thermal conductivity of a prepreg comprising a thermally conductive filler may be calculated, based on simple heat transfer considerations, as follows:
• the rate of heat flow (Q) in watts is proportional to the thermal conductivity of the prepreg sheet (K) , the area of such a sheet through which heat is transferred (A) , the thickness of the sheet (L) and the temperature difference between the printed circuit board interface and the heat sink ( ⁇ T expressed in °C) . This can be expressed as:
• the thermal conductivity is then equal to
• the thermal conductivity of a composite material comprising a matrix material and a thermally conductive filler, will depend on the thermal conductivity' s and volume fractions of the individual components of the composite, namely the matrix material and the thermally conductive filler.
• the composite thermal conductivity (Kc) can be approximated by the following equation derived by the inventors from a more complex expression taken from D. M. Bigg, Polymer Composites, 7, 125 (1986) :
• Km is the thermal conductivity of the matrix material and F is the volume fraction of the filler.
• F is the volume fraction of the filler.
• the thermal conductivity of a fiber-reinforced FR-4 epoxy resin prepreg sheet absent a thermally conductive filler is about 0.25 (W/M °K) and the thermal conductivity of aluminum nitride is about 150 (W/M °K) .
• a metal plate serves as the heat sink.
• the metal plate may have openings fabricated by punching or drilling if needed for mounting or electrical feedthrough.
• a minimum metal thickness of about 0.025 inches is desirable to maintain rigidity and ease of handling.
• the inventors have found that aluminum of Type 5052-H32, having a thickness of about 0.1 inches is a preferred metal, although other metals, such as copper, may be employed.
• the aluminum may be anodized.
• the aluminum plate is degreased with solvent. Following degreasing, the surface of the aluminum plate to be bonded to the printed circuit board is mechanically roughened by scrubbing with pumice.
• the aluminum plate is bonded to a surface of an additive printed circuit board opposite the surface upon which heat generating components are mounted by using, as a bonding means, a thermally conductive prepreg.
• the prepreg is used in the form of a partially cured sheet, which upon the application of heat and pressure becomes fully cured. During the cure, an excellent adhesive bond is formed between the additive printed circuit boaxd and the heat sink .
• Methods for the production of partially cured prepreg sheets are well known in the art and an elucidation of the prepreg production process to which the present invention is applicable, to the extent that the subsequent description does not specifically describe such a process, is to be found in Printed Circuits Handbook, 3rd Edition, Chapter 13, Clyde F. Coombs, ed. , McGraw-Hill, New York (1988), which is incorporated herein by reference.
• Suitable resins are of the thermosetting type, typical examples of which are epoxy resins, phenolic resins and silicone resins, although acrylic resins may also be used
• the preferred resin is the epoxy resin comprising the reaction product of the diglycidyl ether of bisphenol A and dicyandiamide, commonly called "FR-4". A more complete description of this material is found in Printed Circuits Handbook, 3rd Edition, Chapter 6, Clyde F. Coombs, ed. , McGraw-Hill, New York (1988).
• the thermally conductive prepreg sheet is prepared by incorporation of a thermally conductive filler into the organic resin.
• Suitable fillers are the group of inorganic ceramics having thermal conductivity of at least 20 (W/M °K) examples of which are aluminum oxide and aluminum nitride.
• the most preferable embodiment employs aluminum nitride in particulate form as the thermally conductive filler.
• Aluminum nitride, Grade A-100, (Reade Advanced Material Company, Riverside, CA) was obtained as spheroidal particles of average diameter of about 2 micrometers to about 4 micrometers. Optionally, average diameters as small as 0.1 micrometer may be used.
• the optimal amount of aluminum nitride to be incorporated into the prepreg sheet was calculated as described above.
• a coupling agent selected from among the group of organic zirconates, titanates and silicates can be used.
• the most preferable agent is a complex ammonium pyrophosphato titanate, commercially available as
• Lica 38TM Coupling Agent from the Kenrich Corporation.
• the opti al concentration of Lica 38TM Coupling Agent was found to be 0.3 parts per hundred parts of resin.
• the optimum thickness of the partially cured prepreg has been found to be in the range of 0.001 inches to 0.004 inches, and most optimally 0.003".
• the thermal conductivity of the prepreg was measured by the Metal-Surfaced Hot Plate method of ASTM Test No. D- 177.
• EXAMPLE 3 Preparation of Prepreg
• a 0.003 inch thick prepreg sheet was prepared by the method of Example 2 with the exception that aluminum oxide was used in place of the aluminum nitride filler.
• Table 1 shows the results obtained for each of the aluminum nitride, aluminum oxide and unfilled prepreg sheets.
• the aluminum heat sink is bonded to the additive printed circuit board.
• Methods for the production of additive printed circuits are well known in the art and an elucidation of the additive process to which the present invention is applicable, to the extent that the subsequent description does not specifically describe such a process, is to be found in Printed Circuits Handbook, 3rd Edition, Chapter 13, Clyde F. Coombs, ed. , McGraw-Hill, New York (1988).
• Lamination was effected in a Wabash laminating press under the following preferred conditions. Two FR-4 epoxy resin prepreg sheets, prepared as above with a total thickness of 0.003 inches were employed. The press platen is preheated to a temperature of about 300 F. A lamination temperature of 300 F and lam nation pressure of 200 PSI were employed. The lamination cycle time is preferably 80 minutes for a 0.003 inch thick prepreg sheet. After the end of the 80 minute cycle, a further cooldown cycle of 60 minutes is performed at atmospheric pressure with no additional heating. Following the preparation described above, the thermal resistance of the completed assembly was measured by the Thermal Resistivity Test Method of MIL-STD 750C.
• This test measures the thermal resistance between a semiconductor die junction of a transistor mounted on an upper surface of the additive printed circuit board and the bottom surface of the bonded aluminum heat sink.
• the transistor was soldered to a 0.35 inch by 0.40 inch rectangular copper mounting pad on the upper surface of the additive printed circuit board.
• the printed circuit board contains a rectangular array of 63 plated-through holes connecting an identical copper pad on the bottom surface of the printed circuit board. The holes are used to transfer heat through the printed circuit board interface.
• a thermocouple was affixed to the bottom surface of the aluminum heat sink to measure the temperature at the bottom surface.
• FR-4 epoxy prepreg was employed in each example with the only difference being in the filler material and content. Table 2 shows the measured results.

Landscapes

• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Manufacturing & Machinery (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Laminated Bodies (AREA)
• Cooling Or The Like Of Semiconductors Or Solid State Devices (AREA)
• Structure Of Printed Boards (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (3)

Cited By (6)

Families Citing this family (2)

Citations (3)

Family Cites Families (5)

• 1995
  • 1995-10-03 JP JP8512684A patent/JPH10509277A/ja active Pending
  • 1995-10-03 WO PCT/US1995/012990 patent/WO1996011105A1/en not_active Application Discontinuation
  • 1995-10-03 EP EP95937408A patent/EP0784539A4/en not_active Withdrawn

Patent Citations (3)

Non-Patent Citations (1)

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