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WO2006032836A1 - Thick-film hybrid production process - Google Patents

Thick-film hybrid production process Download PDF

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Publication number
WO2006032836A1
WO2006032836A1 PCT/GB2005/003290 GB2005003290W WO2006032836A1 WO 2006032836 A1 WO2006032836 A1 WO 2006032836A1 GB 2005003290 W GB2005003290 W GB 2005003290W WO 2006032836 A1 WO2006032836 A1 WO 2006032836A1
Authority
WO
WIPO (PCT)
Prior art keywords
thick
process according
film
pads
overglaze
Prior art date
Application number
PCT/GB2005/003290
Other languages
French (fr)
Inventor
Jens Helfrich
Rolf DISSELNKÖTTER
Hans-Joachim Krokoszinski
Dieter Gilbers
Original Assignee
Vetco Gray Controls Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Vetco Gray Controls Limited filed Critical Vetco Gray Controls Limited
Priority to GB0706875A priority Critical patent/GB2432978A/en
Publication of WO2006032836A1 publication Critical patent/WO2006032836A1/en
Priority to NO20071954A priority patent/NO20071954L/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
    • C04B41/81—Coating or impregnation
    • C04B41/89—Coating or impregnation for obtaining at least two superposed coatings having different compositions
    • C04B41/90—Coating or impregnation for obtaining at least two superposed coatings having different compositions at least one coating being a metal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/48—Manufacture or treatment of parts, e.g. containers, prior to assembly of the devices, using processes not provided for in a single one of the subgroups H01L21/06 - H01L21/326
    • H01L21/4814—Conductive parts
    • H01L21/4846—Leads on or in insulating or insulated substrates, e.g. metallisation
    • H01L21/4867—Applying pastes or inks, e.g. screen printing
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
    • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
    • C04B41/52—Multiple coating or impregnating multiple coating or impregnating with the same composition or with compositions only differing in the concentration of the constituents, is classified as single coating or impregnation
    • 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
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/24—Reinforcing the conductive pattern
    • H05K3/245—Reinforcing conductive patterns made by printing techniques or by other techniques for applying conductive pastes, inks or powders; Reinforcing other conductive patterns by such techniques
    • 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
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/24—Reinforcing the conductive pattern
    • H05K3/245—Reinforcing conductive patterns made by printing techniques or by other techniques for applying conductive pastes, inks or powders; Reinforcing other conductive patterns by such techniques
    • H05K3/246—Reinforcing conductive paste, ink or powder patterns by other methods, e.g. by plating
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
    • C04B2111/00844—Uses not provided for elsewhere in C04B2111/00 for electronic applications
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
    • H01L2224/44—Structure, shape, material or disposition of the wire connectors prior to the connecting process
    • H01L2224/45—Structure, shape, material or disposition of the wire connectors prior to the connecting process of an individual wire connector
    • H01L2224/45001—Core members of the connector
    • H01L2224/45099—Material
    • H01L2224/451—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof
    • H01L2224/45117—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof the principal constituent melting at a temperature of greater than or equal to 400°C and less than 950°C
    • H01L2224/45124—Aluminium (Al) as principal constituent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
    • H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
    • H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
    • H01L2224/4805—Shape
    • H01L2224/4809—Loop shape
    • H01L2224/48091—Arched
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/73—Means for bonding being of different types provided for in two or more of groups H01L2224/10, H01L2224/18, H01L2224/26, H01L2224/34, H01L2224/42, H01L2224/50, H01L2224/63, H01L2224/71
    • H01L2224/732—Location after the connecting process
    • H01L2224/73251—Location after the connecting process on different surfaces
    • H01L2224/73265—Layer and wire connectors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/19—Details of hybrid assemblies other than the semiconductor or other solid state devices to be connected
    • H01L2924/191—Disposition
    • H01L2924/19101—Disposition of discrete passive components
    • H01L2924/19107—Disposition of discrete passive components off-chip wires
    • 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
    • H05K1/09—Use of materials for the conductive, e.g. metallic pattern
    • H05K1/092—Dispersed materials, e.g. conductive pastes or inks
    • 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
    • H05K1/16—Printed circuits incorporating printed electric components, e.g. printed resistor, capacitor, inductor
    • H05K1/167—Printed circuits incorporating printed electric components, e.g. printed resistor, capacitor, inductor incorporating printed resistors
    • 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
    • H05K2201/01—Dielectrics
    • H05K2201/0137—Materials
    • H05K2201/017—Glass ceramic coating, e.g. formed on inorganic substrate
    • 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
    • H05K2201/03—Conductive materials
    • H05K2201/0332—Structure of the conductor
    • H05K2201/0335—Layered conductors or foils
    • H05K2201/0347—Overplating, e.g. for reinforcing conductors or bumps; Plating over filled vias
    • 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
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/24—Reinforcing the conductive pattern
    • H05K3/243—Reinforcing the conductive pattern characterised by selective plating, e.g. for finish plating of pads
    • 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
    • H05K3/22—Secondary treatment of printed circuits
    • H05K3/28—Applying non-metallic protective coatings

Definitions

  • the present invention relates to a process for producing a thick-film hybrid.
  • the inventive process enables thick-film hybrids which operate satisfactorily at high temperatures, for example ambient temperatures above 150 0 C.
  • the state of the art integration technology proposed for temperatures above 150 0 C is Thick-film Hybrid Technology on ceramic substrates, for example Al 2 O 3 , using silver- based ink systems and insulating overglazes for covering the silver and enabling pad layer reinforcement.
  • the most common technology to produce metal films on a ceramic substrate is screen printing.
  • the pattern to be printed is produced by photolithographic means in the film layer on top of a steel wire mesh with appropriate mesh size.
  • Typical layer thicknesses are in the order of 8 to 18 ⁇ m. If larger thicknesses are required for lower conductor resistance, multiple printing and burning of layers on top of each other is possible.
  • the most important ceramic substrates used in thick film technology are alumina (Al 2 O 3 ) and aluminium nitride (AlN).
  • Standard thick-film materials are the silver palladium (AgPd) air fireable inks.
  • the palladium content is needed to reduce the tendency of migration and leaching of pure silver, but it is costly and increases the resistivity of the films.
  • copper based systems have been introduced which have to be burned in nitrogen to avoid oxidation. This is no problem for gold-based inks which are, of course, a high-end system both with respect to cost and performance (low ohmic, non-oxidizing, no corrosion). Due to cost, only small gold film thicknesses are produced resulting in a moderate sheet resistance of the track.
  • a good (low-priced) alternative is the family of pure silver and silver platinum inks which combine the advantages of low ohmic tracks with firing in air and low material cost.
  • a further problem is to find a suited process that allows the simultaneous use of the different technologies mentioned above at high operation temperatures.
  • the inventive process and material selection for applying thick-film technology on ceramic substrates produces reliable electrical conductor tracks and thick-film resistors for high temperature applications with ambient temperatures above 15O 0 C. Based on these substrates, electrical hybrids can be built with surface mountable components and long lasting reliable interconnections to the substrate.
  • a process for producing a thick-film hybrid comprising the steps of: providing a ceramic substrate; printing at least one thick-film conducting layer onto the substrate to form a conductive pattern on the substrate, the pattern comprising at least one conductive pad located for subsequent connection to an electrical component; covering at least a portion of the thick- film conductor with an insulating overglaze, leaving the or each pad uncovered by the overglaze; metallising the pad(s); and electrically connecting an electrical component to the or each metallised pad.
  • the thick-film conductor comprises silver.
  • a resistor film is printed onto the substrate, to form at least one resistor.
  • the step of covering the thick-film conductor with an overglaze preferably includes covering the resistor film with the overglaze.
  • the at least one overglazed resistor may then be trimmed to adjust its electrical characteristics. This trimming may be carried out using a laser. The trimmed resistor may then be re-covered by overglaze.
  • the step of metallising the pads comprises applying a layer of nickel to the pads.
  • a layer of gold may be applied to the layer of nickel.
  • the step of electrically connecting electrical components to the metallised pads may comprise affixing components to the pads using solder and / or electrically conductive glue.
  • the step of electrically connecting electrical components to the metallised pads may comprise affixing wire bonds to the pads, for example aluminium wire bonds. These may be affixed ultrasonically.
  • the step of electrically connecting electrical components to the metallised pads may comprise affixing metallic strips to the pads by welding.
  • the metallic strips may comprise nickel.
  • a thick-film hybrid produced by the inventive process.
  • a thick-film hybrid comprising a ceramic substrate, at least one thick-film conducting layer forming a conductive pattern arranged on the substrate, the pattern comprising at least one conductive pad, wherein at least a portion of the thick-film conductor is covered with an insulating overglaze leaving the or each pad uncovered by said overglaze, said at least one pad is metallised, and an electrical component is electrically connected to the or each metallised pad.
  • Figure 1 shows a ceramic substrate with overglazed conductive tracks
  • Figure 2 shows a section of a hybrid with a surface mounted component
  • Figure 3 shows a section of a hybrid with various connections.
  • a thick-film hybrid is produced using the following steps, illustrated in Figs 1 to 3:
  • a ceramic substrate for example Al 2 O 3
  • the conductor pattern to be printed is produced with a standard process as described above, using silver-based thick-film inks.
  • the substrates are passed through a belt dryer at around 15O 0 C for about lOmin and subsequently through a multizone belt furnace with maximum temperature of about 850 0 C and with air or N 2 -atmosphere.
  • the substrates should be present for about lOmin in order to completely burn out all organic constituents and to melt the glass frit of the ink, which then tends to flow into the interface between the substrate and the thick film layer thus acting as glue.
  • Typical layer thicknesses are in the order of about 8 to 18 ⁇ m. If larger thicknesses are required, multiple printing and burning of layers on t ⁇ p of each other is possible. Fine line patterns down to track widths of about 75 ⁇ m are feasible. Complex wiring can therefore be accomplished. To maintain low levels of silver migration the distance d, see Fig.l, between two tracks 2 should be about 1 mm / 100V or a minimum of about 0.5 mm.
  • the resistor ink family used has to be compatible with the AgPd or AgPt termination metallurgies as specified by the suppliers.
  • the production of resistors with different resistivity ranges is accomplished by printing and drying for about 10-15 min at around 150 0 C the different pastes successively, which are then simultaneously co-fired at about 850 0 C in air for about 10 min at peak temperature.
  • the overglaze 3 is a multifunctional layer. Apart from protecting most of the silver layer printed on the alumina substrate (from migration) it simultaneously serves as a mask for the subsequent metallisation of the I/O-pads and as a solderstop during the reflow soldering process of the final assembly. In both processes it prevents the additional material, for example nickel/gold plated onto the silver and, respectively, solder wetting the nickel/gold surface, from contacting the regions of the hybrid where it is not supposed to be deposited.
  • the printed resistors may be laser-trimmed to adjust the electrical characteristics with an accuracy of ⁇ 1%.
  • the resistor material and the overglaze are cut (e.g. L-cut) with a resulting open cut that is very susceptible to any contamination by the ambient or subsequent processes.
  • This step is necessary to close the open cuts to protect the resistors against an intrusion of gas or bath solutions during the following steps or long-term operation. Only the resistor pattern is screen printed again and the same overglaze may be used as before.
  • a nickel-gold interface 4 is introduced between the contact areas and the silver-based conductor tracks 2 below.
  • a suitable electroless metallization process is described for example in UK Patent Application No. 0417917.2.
  • a layer of nickel is applied to the pads, followed by a thin layer of gold.
  • solder 6 such as semiconductor 7 may be affixed by applying solder 6 to the substrate contacts, by using screen printing as mentioned above.
  • the solder is ideally a high temperature solder with a high lead content (Pb95Sn5) and a solidus temperature above 30O 0 C. In this case, only bare chips and components with non-organic housings can be used.
  • the components are placed into the wet solder and then soldered in a batch oven with an oxygen-reducing atmosphere.
  • the peak temperature is about 350 0 C for around 2 minutes.
  • soft solders like lead are used which can act as an elastic mechanical connection, to minimize the mechanical stress on the components during the cool-down process, due to thermal mismatch between the substrate and the components and the shrinkage of the solid solder itself.
  • Gluing with an electrical conductive material is an alternative or additional process, which does not need the high process temperature compared to soldering. Therefore additional devices can be used which would not withstand the solder process.
  • the composition may be for example a metal filled epoxy that concurrently provides the mechanical fixation and electrical connection of the component. The epoxy is applied to the corresponding contacts after any soldering has been done. The component is then placed on the substrate and then cured at elevated temperatures (below 200 0 C).
  • An ultrasonic bonding process with aluminium wire 8 is used to make electrical contacts between bare semiconductor chips and the nickel gold pads.
  • An advantage of the aluminium wire is the monometallic connection to the chip contacts and the proven and known long-time reliability with the nickel connection on top of the silver tracks.
  • the interconnections between different substrates as shown in Fig. 3, to connectors or to flying leads are made by thin (low current) or thick (high current) nickel strips 9, which are micro-welded on the nickel-Au pads.
  • the welding process may be achieved by ohmic heating on the tip of the weld tool, with the welding voltage and time selected as necessary.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Ceramic Engineering (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Structural Engineering (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • Power Engineering (AREA)
  • Manufacturing Of Printed Wiring (AREA)
  • Parts Printed On Printed Circuit Boards (AREA)

Abstract

A process for producing a thick-film hybrid, comprises the steps of: providing a ceramic substrate (1); printing at least one thick-film conducting layer (2) onto the substrate to form a conductive pattern on the substrate, the pattern comprising at least one conductive pad located for subsequent connection to an electrical component; covering at least a portion of the thick-film conductor with an insulating overglaze (3), leaving each pad uncovered by the overglaze; metallising the pads (4); and electrically connecting electrical components (7) to the metallised pads. The hybrid produced is suitable for use at ambient temperatures above 150°C.

Description

Thick-Film Hybrid Production Process
The present invention relates to a process for producing a thick-film hybrid. The inventive process enables thick-film hybrids which operate satisfactorily at high temperatures, for example ambient temperatures above 1500C.
The state of the art integration technology proposed for temperatures above 1500C is Thick-film Hybrid Technology on ceramic substrates, for example Al2O3, using silver- based ink systems and insulating overglazes for covering the silver and enabling pad layer reinforcement.
The most common technology to produce metal films on a ceramic substrate is screen printing. The pattern to be printed is produced by photolithographic means in the film layer on top of a steel wire mesh with appropriate mesh size. Typical layer thicknesses are in the order of 8 to 18 μm. If larger thicknesses are required for lower conductor resistance, multiple printing and burning of layers on top of each other is possible.
The most important ceramic substrates used in thick film technology are alumina (Al2O3) and aluminium nitride (AlN).
Standard thick-film materials are the silver palladium (AgPd) air fireable inks. The palladium content is needed to reduce the tendency of migration and leaching of pure silver, but it is costly and increases the resistivity of the films. Also, copper based systems have been introduced which have to be burned in nitrogen to avoid oxidation. This is no problem for gold-based inks which are, of course, a high-end system both with respect to cost and performance (low ohmic, non-oxidizing, no corrosion). Due to cost, only small gold film thicknesses are produced resulting in a moderate sheet resistance of the track. A good (low-priced) alternative is the family of pure silver and silver platinum inks which combine the advantages of low ohmic tracks with firing in air and low material cost. Since they are fired at T=850°C both materials can be used up to high ambient temperatures. Beside the low cost, another advantage of silver-based inks is the chemical and physical compatibility with a broad spectrum of available thick film resistor compositions and the very good layer tightness and adhesion to the ceramic substrates, compared to e.g. gold systems.
However, well-known problems are known to exist. For example, especially at high temperature, silver is known to migrate across surfaces when humidity and / or an electric field is present. To avoid migration of silver due to surface moisture and resulting short- circuiting by dendritic growth, passivation by glass layers is mandatory to inhibit the contact of humidity to the silver-based tracks and to prevent migration of the silver in an electric field. However, the available and suited passivation glasses generally show a leakage current flow if an electric field is applied at high temperatures, i.e. there is decreasing electrical resistance behaviour with increasing temperature. The current is mainly generated by the migration of ions (e.g. lead) in the glass, which leads to an accumulation at the negative electrode and a growth of highly conductive dendrites and therefore the risk of a short circuit arising after some time between neighbouring conductor tracks.
A further problem is to find a suited process that allows the simultaneous use of the different technologies mentioned above at high operation temperatures.
It is an aim of the present invention to provide a new process which allows the production of reliable thick-film hybrids suitable for high temperature applications with high voltages and current densities. The inventive process and material selection for applying thick-film technology on ceramic substrates produces reliable electrical conductor tracks and thick-film resistors for high temperature applications with ambient temperatures above 15O0C. Based on these substrates, electrical hybrids can be built with surface mountable components and long lasting reliable interconnections to the substrate. According to a first aspect of the present invention, there is provided a process for producing a thick-film hybrid, comprising the steps of: providing a ceramic substrate; printing at least one thick-film conducting layer onto the substrate to form a conductive pattern on the substrate, the pattern comprising at least one conductive pad located for subsequent connection to an electrical component; covering at least a portion of the thick- film conductor with an insulating overglaze, leaving the or each pad uncovered by the overglaze; metallising the pad(s); and electrically connecting an electrical component to the or each metallised pad.
Preferably, the thick-film conductor comprises silver.
Advantageously, after the at least one thick-film conductor layer is printed onto the substrate, a resistor film is printed onto the substrate, to form at least one resistor. The step of covering the thick-film conductor with an overglaze preferably includes covering the resistor film with the overglaze. The at least one overglazed resistor may then be trimmed to adjust its electrical characteristics. This trimming may be carried out using a laser. The trimmed resistor may then be re-covered by overglaze.
Preferably, the step of metallising the pads comprises applying a layer of nickel to the pads. A layer of gold may be applied to the layer of nickel.
The step of electrically connecting electrical components to the metallised pads may comprise affixing components to the pads using solder and / or electrically conductive glue.
The step of electrically connecting electrical components to the metallised pads may comprise affixing wire bonds to the pads, for example aluminium wire bonds. These may be affixed ultrasonically. The step of electrically connecting electrical components to the metallised pads may comprise affixing metallic strips to the pads by welding. The metallic strips may comprise nickel.
According to a second aspect of the present invention, there is provided a thick-film hybrid produced by the inventive process.
According to a third aspect of the present invention, there is provided a thick-film hybrid comprising a ceramic substrate, at least one thick-film conducting layer forming a conductive pattern arranged on the substrate, the pattern comprising at least one conductive pad, wherein at least a portion of the thick-film conductor is covered with an insulating overglaze leaving the or each pad uncovered by said overglaze, said at least one pad is metallised, and an electrical component is electrically connected to the or each metallised pad.
The invention will now be described with reference to the accompanying drawings, in which:
Figure 1 shows a ceramic substrate with overglazed conductive tracks;
Figure 2 shows a section of a hybrid with a surface mounted component; and Figure 3 shows a section of a hybrid with various connections.
In a preferred embodiment, a thick-film hybrid is produced using the following steps, illustrated in Figs 1 to 3:
Screen printing of metal films based on silver compositions (AgPt, AgPd, Ag)
Initially, a ceramic substrate 1, for example Al2O3, has a thick-film conductor applied to it. The conductor pattern to be printed is produced with a standard process as described above, using silver-based thick-film inks. After degassing of the inks in air for half an hour, the substrates are passed through a belt dryer at around 15O0C for about lOmin and subsequently through a multizone belt furnace with maximum temperature of about 8500C and with air or N2-atmosphere. In the hottest zone the substrates should be present for about lOmin in order to completely burn out all organic constituents and to melt the glass frit of the ink, which then tends to flow into the interface between the substrate and the thick film layer thus acting as glue. Typical layer thicknesses are in the order of about 8 to 18 μm. If larger thicknesses are required, multiple printing and burning of layers on tυp of each other is possible. Fine line patterns down to track widths of about 75μm are feasible. Complex wiring can therefore be accomplished. To maintain low levels of silver migration the distance d, see Fig.l, between two tracks 2 should be about 1 mm / 100V or a minimum of about 0.5 mm.
Screen printing of Resistor films
The resistor ink family used has to be compatible with the AgPd or AgPt termination metallurgies as specified by the suppliers. The production of resistors with different resistivity ranges is accomplished by printing and drying for about 10-15 min at around 1500C the different pastes successively, which are then simultaneously co-fired at about 8500C in air for about 10 min at peak temperature.
Overglaze
The overglaze 3 is a multifunctional layer. Apart from protecting most of the silver layer printed on the alumina substrate (from migration) it simultaneously serves as a mask for the subsequent metallisation of the I/O-pads and as a solderstop during the reflow soldering process of the final assembly. In both processes it prevents the additional material, for example nickel/gold plated onto the silver and, respectively, solder wetting the nickel/gold surface, from contacting the regions of the hybrid where it is not supposed to be deposited. The overglaze on the conductors has only a small overlap (a) to the ceramic substrate as shown in Fig. 1 of about a=0.25 mm, which serves to prevent leakage current flow between different conductor tracks through the glass. This also works as a barrier to silver ions, which cannot easily pass the surface of the glass. The limitation of covering only the silver tracks and resistors minimizes possible leakage currents in the glass.
Resistor Trimming
The printed resistors may be laser-trimmed to adjust the electrical characteristics with an accuracy of ~ 1%. The resistor material and the overglaze are cut (e.g. L-cut) with a resulting open cut that is very susceptible to any contamination by the ambient or subsequent processes.
Overglaze on top of the laser cut resistors
This step is necessary to close the open cuts to protect the resistors against an intrusion of gas or bath solutions during the following steps or long-term operation. Only the resistor pattern is screen printed again and the same overglaze may be used as before.
Metallisation of pads
To facilitate the simultaneous use of different mounting and interconnection technologies (e.g. by soldering, aluminium wire bonding, welding) which are required for the connections between components or different hybrids and to reach a long-time stable contact behaviour without the formation of intermetallic phases, a nickel-gold interface 4 is introduced between the contact areas and the silver-based conductor tracks 2 below. A suitable electroless metallization process is described for example in UK Patent Application No. 0417917.2. A layer of nickel is applied to the pads, followed by a thin layer of gold. Equipping the substrates with components
Surface-mountable components 5, such as semiconductor 7 may be affixed by applying solder 6 to the substrate contacts, by using screen printing as mentioned above. The solder is ideally a high temperature solder with a high lead content (Pb95Sn5) and a solidus temperature above 30O0C. In this case, only bare chips and components with non-organic housings can be used. The components are placed into the wet solder and then soldered in a batch oven with an oxygen-reducing atmosphere. The peak temperature is about 3500C for around 2 minutes. Ideally, soft solders like lead are used which can act as an elastic mechanical connection, to minimize the mechanical stress on the components during the cool-down process, due to thermal mismatch between the substrate and the components and the shrinkage of the solid solder itself.
Gluing with an electrical conductive material is an alternative or additional process, which does not need the high process temperature compared to soldering. Therefore additional devices can be used which would not withstand the solder process. The composition may be for example a metal filled epoxy that concurrently provides the mechanical fixation and electrical connection of the component. The epoxy is applied to the corresponding contacts after any soldering has been done. The component is then placed on the substrate and then cured at elevated temperatures (below 2000C).
Wire Bonding
An ultrasonic bonding process with aluminium wire 8 is used to make electrical contacts between bare semiconductor chips and the nickel gold pads. An advantage of the aluminium wire is the monometallic connection to the chip contacts and the proven and known long-time reliability with the nickel connection on top of the silver tracks.
Because of the potential formation of voids between aluminium and gold leading to mechanical and electrical degradation after short term operation at high temperature, the gold layer on top of the nickel has to be very thin. Welding
The interconnections between different substrates as shown in Fig. 3, to connectors or to flying leads are made by thin (low current) or thick (high current) nickel strips 9, which are micro-welded on the nickel-Au pads. The welding process may be achieved by ohmic heating on the tip of the weld tool, with the welding voltage and time selected as necessary.
Although the invention has been described with reference to the embodiment above, there are many other variations possible within the scope of the claims. In particular, any materials may be used which function adequately within the hybrid structure.

Claims

Claims
1. A process for producing a thick-film hybrid, comprising the steps of:
providing a ceramic substrate;
printing at least one thick-film conducting layer onto the substrate to form a conductive pattern on the substrate, the pattern comprising at least one conductive pad located for subsequent connection to an electrical component;
covering at least a portion of the thick-film conductor with an insulating overglaze, leaving the or each pad uncovered by the overglaze;
metallising the pad(s); and
electrically connecting an electrical component to the or each metallised pad.
2. A process according to Claim 1, wherein the thick-film conductor comprises silver.
3. A process according to any preceding claim, wherein after the at least one thick- film conductor layer is printed onto the substrate, a resistor film is printed onto the substrate, to form at least one resistor.
4. A process according to Claim 3, wherein the step of covering the thick-film conductor with an overglaze includes covering the resistor film with the overglaze.
5. A process according to Claim 4, wherein the at least one overglazed resistor is trimmed to adjust its electrical characteristics.
6. A process according to Claim 5, wherein the trimming is carried out using a laser.
7. A process according to Claim 6, wherein the trimmed resistor is re-covered by overglaze.
8. A process according to any preceding claim, wherein the step of metallising the pads comprises applying a layer of nickel to the pads.
9. A process according to Claim 8, wherein a layer of gold is applied to the layer of nickel.
10. A process according to any preceding claim, wherein the step of electrically connecting electrical components to the metallised pads comprises affixing components to the pads using solder.
1 1. A process according to any preceding claim, wherein the step of electrically connecting electrical components to the metallised pads comprises affixing components to the pads using electrically conductive glue.
12. A process according to any preceding claim, wherein the step of electrically connecting electrical components to the metallised pads comprises affixing wire bonds to the pads.
13. A process according to Claim 12, wherein the wire bonds comprise aluminium.
14. A process according to either of Claims 12 and 13, wherein the wire bonds are affixed ultrasonically.
15. A process according to any preceding claim, wherein the step of electrically connecting electrical components to the metallised pads comprises affixing metallic strips to the pads by welding.
16. A process according to Claim 15, wherein the metallic strips comprise nickel.
17. A thick-film hybrid produced by the process according to any preceding claim.
18. A thick-film hybrid comprising a ceramic substrate, at least one thick-film conducting layer forming a conductive pattern arranged on the substrate, the pattern comprising at least one conductive pad, wherein at least a portion of the thick-film conductor is covered with an insulating overglaze leaving the or each pad uncovered by said overglaze, said at least one pad is metallised, and an electrical component is electrically connected to the or each metallised pad.
19. A process for producing a thick-film hybrid substantially as herein described with reference, to the accompanying drawings.
PCT/GB2005/003290 2004-09-22 2005-08-24 Thick-film hybrid production process WO2006032836A1 (en)

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NO20071954A NO20071954L (en) 2004-09-22 2007-04-17 Process of producing thick film hybrid

Applications Claiming Priority (2)

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GB0420992A GB2418538A (en) 2004-09-22 2004-09-22 Thick-film printed circuit
GB0420992.0 2004-09-22

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JP7032042B2 (en) * 2015-12-22 2022-03-08 ヘレウス ドイチェラント ゲーエムベーハー ウント カンパニー カーゲー Direct-bonded copper substrate improved with thick film paste
EP3301082B1 (en) * 2016-09-30 2024-09-18 Infineon Technologies AG Method for producing a metal-ceramic substrate

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NO20071954L (en) 2007-06-22
GB0420992D0 (en) 2004-10-20
GB0706875D0 (en) 2007-05-16
GB2418538A (en) 2006-03-29

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