CA2240235A1 - Multilayer electrical interconnection device and method of making same - Google Patents

Abstract

A method of bonding a thermally sprayed coating to a non-roughened aluminum substrate includes thermal spraying aluminum onto a fluxed and thermally activated aluminum substrate to provide a substrate capable of bonding to ceramic and metal multilayers.

Description

CA 0224023~ 1998-06-10 MULTILAYER ELECTRICAL INTERCONNECTION
DEVICE AND METHOD OF MAKING SAME

Technical Field This invention generally relates to multilayer electrical interconnection devices. More specifically, the present invention relates to a multilayer electrical interconnection device which is capable of bonding ceramics and metal multilayers to non-roughened aluminum substrates with the use of thermal spray technology.

Backqround Of The Invention The use of circuit boards in manufacturing electronic equipment provides many advantages, including a reduction in the space and weight required, increased reliability, and suitability for automated production. A circuit board comprises an insulating layer, carrying conductive metal traces, and bonding locations for electrical components. Due to advances in electronics, particularly in the area of miniaturization of integrated circuits, the need for multilayer boards has increased to facilitate the increased number of circuit interconnections per unit of surface area on a circuit board.
These multilayer circuit boards are designed to replace the conventional printed circuit board modules within which electrical connections are made.
Conventional printed boards are housed in cast aluminum to provide the heat evacuation needed during service. Multilayer circuit boards utilize separate trace patterns on various layers in three dimensions and layer-to-layer interconnects (i.e., vias or plated CA 0224023~ 1998-06-10 throughholes) to implement complex interconnections in a small space.
The creation of such a multilayer circuit board, however, depends on adequate bonding of each layer to the adjacent one, and the electrical and mechanical properties of the layers. Until now, the principal bonding technique has required roughening of the cast aluminum surface to effectuate bonding. Such roughening has been carried out by mechanical means such as grit blasting, high pressure water, electric discharge machining or chemical etchants. However, such techniques are problematic due to the cost and extent of disruption of the substrate and/or the environment required. Accordingly, there exists a need for a method of bonding multilayers to a substrate without mechanical roughening.
From a bonding standpoint, aluminum and aluminum alloys are generally very reactive and readily form intermetallic alloys with nickel, titanium, copper and iron at moderate temperatures.
To offset such reactivity, aluminum or aluminum alloys form a passivating surface oxide film when exposed to the atmosphere at ambient temperatures. This oxide film inhibits adherence of metals and ceramics to non-roughened aluminum. Thus, to effect a metallurgical,chemical or intermetallic bond between aluminum or aluminum alloy and other metals and ceramics, it is often necessary to remove, dissolve or disrupt the oxide film. Once stripped of the oxide, aluminum or an aluminum alloy will readily form an alloyed bond at temperatures as low as 500~ C.
To this end, fluxes are readily used to achieve oxide film stripping. One such example involves brazing two pieces of aluminum alloy sheet CA 0224023~ 1998-06-10 metal which are joined by assembling the pieces in a jointed relationship and then flooding the joint area with a flux applied at room temperature. When heated aggressively, the flux melts and strips the surface oxide, thereby allowing the layer to form an interfacial alloy joint with the aluminum (see U.S.
Patent 4,911,351). Such fluxing techniques while effective with rolled aluminum sheet, fail to function with cast aluminum alloys. Since cast aluminum is porous, non-homogenous and has a melting temperature which may overlap with the melting temperature of the flux, fluxing is not a suitable technique for oxide stripping of cast aluminum alloys, except when additions are made to the flux that reduces its activation temperature.
Accordingly, there is a need for a method of bonding each layer of a multilayer circuit board to the adjacent one, including a method of bonding multilayers to a metal substrate without roughening.

Summar~ Of The Invention An object of the present invention is to provide a method of bonding ceramic and metal multilayers to non-roughened aluminum surfaces.
Another object of the present invention is to provide a method of bonding which prevents the formation of galvanic couples between the substrate and any intermediary coatings. Yet another object of this invention is to create a rough aluminum substrate surface for bonding ceramic coatings to metals.
In carrying out the above objects, a method is disclosed for bonding a thermally sprayed coating to a non-roughened aluminum substrate including the CA 0224023~ 1998-06-10 following steps: a) cleaning the substrate such that the substrate is substantially devoid of grease and oil; b) depositing a flux material on the substrate to provide a dry flux coated substrate, wherein the flux is capable of removing metal oxides on the metal substrate; c) thermally activating the flux on the flux coated substrate to melt and dissolve any metal oxide residing on the coated aluminum substrate; and d) subsequent to the step of thermally activating, thermally spraying aluminum onto the flux coated substrate by spraying to form a coating that is chemically bonded to the aluminum substrate.
This invention also discloses a method of bonding a thermally sprayed coating to a non-roughened aluminum substrate, which includes the step of thermally spraying aluminum onto a flux coated substrate with an aluminum powder to form a coating that is mechanically bonded to the substrate.

Brief Description Of The Drawinqs Figure 1 is a generalized schematic diagram illustrating the thermal spray apparatus used in accordance with the invention;
Figure 2 is a table which illustrates the roughness of various layers sprayed to produce a circuit board, such as a) an uncoated and unfluxed substrate, b) the aluminum spray on a fluxed and activated substrate, c) the first ceramic layer, d) the copper trace, e) the second ceramic layer, f) the second copper trace, and g) the third ceramic layer;
Figure 3 is a highly enlarged sectional view of a portion of a spray gun on an aluminum substrate;

CA 0224023~ 1998-06-10 Figure 4 is a microphotograph depicting an aluminum fluxed substrate and a three-layer coating structure; and Figure 5 is a microphotograph depicting a seven layer circuit board.

Detailed Description Of The Invention A multilayer electrical interconnection can be achieved through thermal spray methods. Figure 1 shows a thermal spray apparatus for generating a thermal spray to selectably deposit insulating and conducting material to manufacture an interconnect device. A thermal spray comprised of carrier gases and particles of selected materials is formed by heating particulates using an electric arc or chemical combustion as a heat source. Layered structures are formed by alternately spraying insulating and conducting particles in predetermined patterns.
As depicted in Figure 1, a thermal spray nozzle 10 receives material to be deposited from supply bins 11 and 16. The flow of material into nozzle 10 is selectably controlled using feeder valves 13 and 14, respectively. One bin may contain particles of a conducting material while the other contains particles of an insulating material.
Particles are heated, propelled by a carrier gas and directed out of nozzle 10 as a thermal spray 15 for deposition. Thermal spray lS is directed toward the substrate 12 where the desired interconnect device is to be fabricated.
Three-dimensional circuity is formed by sprayed multilayers which are connected via the use of positive and negative masks that produce localized and CA 0224023~ 1998-06-10 well defined points of conductive and insulating junctions in the board. Creation of a such a successful multilayer structure depends on adequate bonding of each coating layer to the adjacent layer, and ensuring that the coatings are reliably bonded to the substrate.
One such method to reliably bond the coatings to the substrate involves roughening the substrate by water jet or grit blasting. However, both of these techniques are undesirable in a high volume manufacturing environment. In answer to this problem, Ford has developed a method of bonding thermal spray coatings to non-roughened aluminum surfaces involving the use of metallic coatings in combination with bond coats such as nickel, aluminides, silicon bronze or aluminum bronze to induce metallurgical bonding of the coatings to the substrate. However, with certain applications, the method of bonding thermal spray coatings to non-roughened surfaces, using metallic coatings, createsthe possible formation of galvanic couples between the substrates and the coatings. Moreover, with respect to ceramic coatings, since ceramic cannot metallurgically bond to metals, an alternate boding technique is required. The present invention thus describes the additional steps which need to be taken to apply a spray method to bond ceramic and metal multilayers to metal substrates without the need for mechanical roughening and to prevent the formation of galvanic couples.
The present invention thus describes a technique for roughening an aluminum substrate such that ceramic coatings can bond to metals by intercalating an intermediate, metallurgically bonded, CA 0224023~ 1998-06-10 metallic coating by spraying an aluminum coating that bonds to a fluxed aluminum substrate and does not form a galvanic couple with aluminum. While previous thermal spray techniques are appropriate for metal to metal multilayers, for ceramic to metal multilayers, roughening of the metal substrate is required. In addition, depending on the desired application, the absence of galvanic couples may be important, such as for creation of multilayer circuit boards.
To prevent the formation of galvanic couples and to provide a surface suitable for ceramic bonding, aluminum is thermally sprayed onto an aluminum substrate. Since aluminum is thermally sprayed on an underlying aluminum substrate, the formation of galvanic couples is prevented -- as compared to the deposition of other metals such as nickel and the like to the aluminum substrate. In addition, the sprayed aluminum on the aluminum substrate creates a surface with sufficient roughness for bonding to a ceramic layer.
The metal substrate is preferably prepared by the following steps: (1) cleaning and degreasing of the metal substrate such that the metal substrate is substantially devoid of grease and oils. The second step involves fluxing and drying of the metal substrate.
This invention is preferably concerned with successfully fluxing cast aluminum alloys, such as 319, 356, 380 and 390, that contain silicon, copper, manganese or iron ingredients in amounts ranging from 0.5-5~ by weight, the latter aluminum alloys possess a melting temperature of about 580-600~ C. The surface roughness of these cast alloys is usually about 1-3 ~m Ra which is insufficient by itself to provide a CA 0224023~ 1998-06-10 mechanical interlock with thermally sprayed coatings thereover. Since the reactivity of aluminum is diminished due to the formation of oxide films on their surface, the aluminum substrate is fluxed to remove any oxide film therefrom which facilitates adherence of ceramics to the fluxed aluminum substrate.
Figure 2 is a table which provides a comparison of the roughness values for the various layers of a seven layer circuit board. Of particular importance is the difference in roughness between the unfluxed and uncoated aluminum substrate, having a roughness of 0.4746 ~m, and the fluxed, activated and aluminum-sprayed substrate, having a roughness of 22.2367 ~m. Following application and activation of the flux material and subsequent aluminum spray, the aluminum substrate has been altered to create a surface with a roughness sufficient to facilitate bonding of ceramic and metal layers thereon. A flux material is thus applied to the aluminum substrate to strip the substrate of any oxide films.
A flux material having a melting temperature well below the melting temperature of the cast aluminum alloy, i.e., about 60-80~ C below, is deposited thereon and dried. The flux is selected preferably to be eutectic comprising a double fluoride salt having the phase formula ~. K3AlF6+KAlF4. Such a eutectic contains AlF3 at about 45 mol percent of the double fluoride salts, with KF being about 55 mol percent. The eutectic has a melting temperature of about 560~ C which is about 40~ C below that of the melting temperature of the cast alloy of the substrate. If the double fluoride salt has a substantially different mol or percentage of AlF3, thus CA 0224023~ 1998-06-10 _ g_ not being a eutectic, the melting temperature will rapidly rise. Other double fluoride salts, and for that matter other alkaline metal fluoride or chloride salts, can be used as long as they have a melting temperature that can be heat activated without disturbing the cast aluminum alloy. Fluoride salts are useful, but undesirable because they fail to provide corrosion resistance on the aluminum product, and may attack aluminum alloy grain boundaries.
To deposit the flux, the salt is dissolved or suspended in a sprayable medium, such as water or alcohol, in a concentration of about 0.5-5.0 ~ by volume or minimum of 5 grams of flux per square meter of surface. The solution may contain a mild alkaline wash, such as the commercial chemical product 5896, permitting the flux to spread more evenly by reducing surface tension. This solution may also contain other additional ingredients, such as up to 50 wt. ~ of LiF, or CsF which facilitate working with other substrates such as magnesium oxide films.
The double fluoride salt is added to the sprayable medium and closely control particle size to minimize the need for stirring and to retain at least 25~ by volume of the salt and suspension at all times.
To this end, the salt particle size is equal to or less than 10 microns with about 70~ being 2-4 microns.
The salt is spray deposited in a density of about 3-7 grams per square meter, preferably about 5 grams per square meter; too much salt will inhibit flux melting and too little will fail to achieve the fluxing effect.
Deposition is carried out preferably by use of a liquid spray gun which applies the flux solution in a controlled manner to achieve the desired coverage ' CA 0224023~ l998-06-lO
.

and coating uniformity. After deposition, the flux is dried preferably by placing the flux coated substrate in a dehumidifier and removing the solvent. This leaves a fine talc-like powder on the substrate.
The next step involves thermal activation of the flux and dried surface to its eutectic melting temperature, i.e., 500-580~ C. In contrast to the application of other metals, since molten aluminum has a temperature of about 660~ C, which is less than the melting temperature of the flux, a separate thermal activation step of the flux is required. Thermal activation of the flux can optimally be brought about by independent means such as flame, induction, and resistance heating. Thermal activation of the flux is achieved by heating the flux to its melting point, rendering it colorless.
The next step involves intercalating an intermediate, metallurgically bonded metal coating by thermally spraying pure aluminum onto the activated surface using wire or powder feedstock. With respect to wire thermal spray, the wire preferably has a diameter of at least 2 mm, O. 062 inches, and the spray gun parameters are preferably adjusted to produce spray at a current of 100-150 amperes, with a voltage of 20-25 volts, and projecting a gas pressure of 50-60 psi. With respect to powder spray systems, the starting powder particle size distribution can be judiciously chosen to produce the required droplet size irrespective of the wire parameters. Under these conditions, the absolute roughness Ra of the sprayed surface is between 12 microns and 30 microns or, more preferably, between 15 and 25 microns, a roughness sufficient to chemically lock in place subsequent ceramic and copper layers. When aluminum powder is CA 0224023~ l998-06-lO

deposited, in the preferred embodiment, a single plasma or flame thermospray gun is utilized to activate the flux and also spray the aluminum powder onto the aluminum substrate. Under this preferred 5 embodiment, the plasma gun operates with a voltage of approximately 78 volts, at a current of 525 amps, and with an argon gas flow rate of 27 cubic feet per hour.
The preferred aluminum particle size ranges from 10-75 ~m. Accordingly, an intermediate layer is created, having sufficient roughness to facilitate bonding the ceramic and metal layers.
Thermal spraying of aluminum droplets or particles can be carried out by use of an apparatus as shown in Figure 3. A metallic wire feedstock 18 is 15 fed into the plasma or arc 19 of a thermal gun 20 such that the tip 21 of the feedstock 18 melts and is atomized into droplets by high velocity gas jets 23 and 24. The gas jets project a spray 15 onto the substrate 12 and thereby deposit a coating 26. The 20 gun 20 may be comprised of an inner nozzle 27 which focuses a heat source, such as the flame or plasma plume 19. The plasma plume 19 is generated by stripping off electrons from the primary gas 23 as it passes between the anode 2 8 and the cathode 29 25 resulting in highly heated ionic discharge or plume 19. The heat source melts the wire tip 21 and the resulting droplets 22 are carried by the primary gas 23 at great velocity to the target. A pressurized secondary gas 24 may be used to further control the 30 spray pattern 15. Such secondary gas is introduced through channel 30 formed between the cathode 29 and the housing 31. The secondary gas 24 iS directed radially inwardly with respect to the axis 32 of the plume. Melting of the wire 18 is made possible by CA 0224023~ 1998-06-10 .

connecting the wire as an anode when striking an arc with cathode 29. The resulting coating 26 will constitute splat layers or particles 33. While use of the wire feedstock is described in detail, powder fed thermal spray devices can also be used to produce the same bonding effect.
Figure 4 shows a scanning electron micrograph of a fluxed aluminum substrate. Figure 4(b) illustrates one application of this thermal spray bonding method, to create a three layer circuit board, composed of an aluminum, alumina and copper layer.
While we do not wish to be bound by any theoretical reason, the bonding achieved in this invention can be attributed to intermetallic alloy formation and/or pairing of oxygen atoms located at the hot droplet surfaces with the oxide free aluminum surface.
Figure 5 is a scanning electron micrograph of yet another application of the bonding method to create a seven layer circuit board. As depicted, the seven layer circuit board is composed of several layers bonded to one another with the following layer structure: aluminum, alumina, copper, alumina, copper, alumina and copper.
The present invention thus facilitates bonding of ceramics and metals to non-roughened aluminum substrates to form multilayer structures.
While the best mode and viable alternate embodiments for carrying out the invention have been described in detail now shown on the drawings, those familiar to the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.

Claims (11)

1. A method of bonding a thermally sprayed coating to a non-roughened aluminum substrate, comprising:
a) cleaning the substrate such that the substrate is substantially devoid of grease and oils;
b) depositing a flux material on the substrate to provide a dry flux coated substrate, said flux being capable of removing aluminum oxides;
c) thermally activating said flux on said flux coated substrate to melt and dissolve any aluminum oxide residing on said coated aluminum substrate; and d) subsequent to step c, thermally spraying aluminum onto said flux coated and activated substrate by wire spraying to form a coating that is chemically bonded to the aluminum substrate, wherein said coating has a surface roughness between 12 and 30 µm.

2. The method of claim 1, wherein said wire spraying is achieved by a wire having a diameter of at least 2mm.

3. The method of claim 1, wherein said wire spraying is performed under currents ranging between 100-150 amperes and voltages of 25-35 volts at a gas pressure 50-100 psi.

4. The method of claim 1, in which said flux is comprised essentially of a potassium aluminum fluoride and containing less than 50 molar percent of other ingredients.

5. The method of claim 1, in which said flux is applied as a solution sprayed onto the light metal surface, said solution having a water or alcohol solvent base.

6. The method of claim 5, in which said flux is comprised essentially of potassium aluminum fluoride salt having a particle size of less than 10 microns and having about 20% of such particles of a size between 2-4 microns, causing 20-30% by volume of said particles to remain in suspension in the solution at all times without stirring.

7. The method of claim 5, in which said solution is sprayed in a volume of 3-10 grams per m2.

8. The method of claim 5, in which said sprayed solution is dried after disposition to remove the solvent of said solution.

9. The method of claim 4, in which said deposited flux is thermally activated at a temperature of 500-580° C.

10. The method of claim 1, wherein said deposited flux is thermally activated by one of the following selected methods: direct flame, resistance and induction heating.

11. A method of bonding a thermally sprayed coating to a non-roughened aluminum substrate, comprising:
a) cleaning the substrate such that the substrate is substantially devoid of grease and oils;

b) depositing a flux material to provide a dry flux coated substrate, said flux being capable of removing aluminum oxides;
c) thermally activating said flux of said flux coated substrate to melt and dissolve any aluminum oxide residing on the aluminum substrate; and d) subsequent to step c, thermally spraying aluminum onto said flux coated and activated substrate with an aluminum powder to form a coating that is chemically bonded to the substrate, and which has an aluminum particle size of between 10 and 75 µm.