阅读说明：本技术 用于氮化硅和其他基底的导电厚膜浆料 (Conductive thick film pastes for silicon nitride and other substrates ) 是由 U·库马尔 约翰·J·马洛尼 布拉德福德·J·史密斯 庞萨米·帕拉尼萨姆 斯里尼瓦桑·斯里德 于 2019-09-06 设计创作，主要内容包括：用于微电子电路应用的与氮化铝、氧化铝和氮化硅基底兼容的导电厚膜组合物。该导电厚膜组合物包括第一铜粉、第二铜粉和玻璃组分。导电厚膜组合物还包括Cu-2O、Ag和至少一种选自Ti、V、Zr、Mn、Cr、Co和Sn的金属元素。在烧制之后,导电厚膜组合物呈现出改进的薄层电阻率和改进的与下面的基底的粘附力。(Conductive thick film compositions compatible with aluminum nitride, aluminum oxide, and silicon nitride substrates for microelectronic circuit applications. The conductive thick film composition includes a first copper powder, a second copper powder, and a glass component. The conductive thick film composition further comprises Cu 2 O, Ag and at least one metal element selected from Ti, V, Zr, Mn, Cr, Co and Sn. After firing, the conductive thick film composition exhibits improved sheet resistivity and improved adhesion to the underlying substrate.)

1. A thick film paste comprising in wt.%:

(a) from about 20% to about 49% of the first Cu,

(b) from about 20% to about 34% of a second Cu,

(c) from about 3% to about 12% Cu₂O，

(d) From about 8% to about 25% Ag,

(e) from about 8% to about 25% of at least one metallic element selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn, and

(f) does not contain lead and cadmium, and does not contain lead,

wherein the first Cu has a D50 of about 0.1 to 8 microns, preferably 0.5 to 5 microns, and the second Cu has a D50 of about 10 to 20 microns, preferably 12 to 20 microns.

2. The thick film paste of claim 1, wherein the thick film paste comprises:

(a) from about 21% to about 36% of the first Cu,

(b) from about 20% to about 30% of the second Cu,

(c) from about 3% to about 12% Cu₂O，

(d) From about 8% to about 20% Ag, and

(e) from about 8% to about 20% of at least one metallic element selected from the group consisting of Ti, V, Zr, Cr, and Sn.

3. The thick film paste of claim 1, wherein the thick film paste comprises:

(a) from about 23% to about 29% of the first Cu,

(b) from about 20% to about 25% of a second Cu,

(c) from about 4% to about 9% Cu₂O，

(d) From about 8% to about 17% Ag, and

(e) from about 10% to about 16% of at least one metallic element selected from the group consisting of Ti and Sn.

4. The thick film paste of claim 1, further comprising from about 10% to about 20% of an organic component fraction.

5. A lead-free and cadmium-free conductive thick film paste includes a glass component and a copper component,

the glass component comprises in wt.%:

(a) from about 10% to about 70%, preferably from about 15% to about 65%, more preferably from about 20% to about 60% of at least one alkaline earth metal oxide,

(b) from about 0.01% to about 10%, preferably from about 0.1% to about 8%, more preferably from about 1% to about 8% of at least one alkali metal oxide,

(c) from about 22% to about 70%, preferably about 25% to about 65%, more preferably about 25% to about 65% (B)₂O₃+SiO₂) And an

(d) From about 0.01% to about 15%, preferably about 0.1% to about 13%, more preferably about 0.5% to about 12% of Al₂O₃，

Wherein the alkaline earth metal oxide is selected from the group consisting of MgO, CaO, SrO, BaO and ZnO; and the alkali metal oxide is selected from the group consisting of Li₂O、Na₂O、K₂O and Rb₂O; and is

The Cu component comprises in wt.%:

(a) from about 14% to about 23% of the first Cu,

(b) from about 20% to about 28% of a second Cu,

(c) from about 15% to about 24% of third Cu, and

(d) from about 5% to about 11% Cu₂O，

Wherein the first Cu has a D50 of about 5 microns, the second Cu has a D50 of about 1.5 microns, the third Cu has a D50 of about 10 microns, and

wherein the glass component and the Cu component are present in a weight ratio of about 1:15 to about 1: 30.

6. The thick film paste of claim 5, wherein the alkaline earth metal oxide is selected from MgO, CaO and ZnO.

7. The thick film paste of claim 5, wherein the alkaline earth metal oxide is selected from CaO and ZnO.

8. The thick film paste of claim 5, wherein the glass component has a D50 of from about 0.5 microns to about 20 microns.

9. The thick film paste of claim 5, wherein the Cu comprises in wt.%:

(a) from about 15% to about 23% of the first Cu,

(b) from about 22% to about 28% of the second Cu,

(c) from about 17% to about 24% of the third Cu, and

(d) from about 6% to about 10% Cu₂O。

10. The thick film paste of claim 5, wherein the Cu comprises in wt.%:

(a) from about 17% to about 22% of the first Cu,

(b) from about 23% to about 26% of the second Cu,

(c) from about 18% to about 24% of the third Cu, and

(d) from about 7% to about 10% Cu₂O。

11. The thick film paste of claim 5, further comprising from about 10% to about 30% by weight of an organic component fraction.

12. A method of forming a fired conductive thick film on a substrate comprising:

(a) applying at least one thick film paste according to claim 1 to said substrate to form a thick film thereon,

(b) drying the thick film to form a dried conductive thick film, an

(c) Firing the dried conductive thick film to form a fired conductive thick film on the substrate, wherein the firing is on N₂The method is carried out in the atmosphere,

wherein the substrate is selected from the group consisting of aluminum oxide, aluminum nitride, and silicon nitride.

13. The method of claim 12, wherein at least one of the conductive thick film paste of claim 1 and the conductive thick film paste of claim 5 is applied by one selected from the group consisting of additive manufacturing, screen printing, syringe deposition, and digital printing techniques.

14. The method of claim 12, wherein the firing is performed at a temperature from about 850 ℃ to about 1050 ℃.

15. The method of claim 12, wherein the firing is performed at a temperature from about 850 ℃ to about 900 ℃.

16. The method of claim 12, wherein the atmosphere comprises less than 10ppm O₂。

17. The method of claim 12, wherein the firing is performed in a reducing atmosphere.

18. The method of claim 12, wherein the reducing atmosphere comprises N₂。

19. An electronic device, comprising:

(a) a substrate; and

(b) a lead-free and cadmium-free conductive thick film disposed on at least a portion of the substrate, the lead-free and cadmium-free conductive thick film comprising at least a first conductive thick film and a second conductive thick film,

wherein the substrate is selected from the group consisting of aluminum oxide, aluminum nitride and silicon nitride, and

wherein the first conductive thick film comprises the conductive thick film paste of claim 1 and the second conductive thick film comprises the conductive thick film paste of claim 1.

20. The electronic device of claim 19, wherein a nitrogen-containing interface is formed between the substrate and the lead-free and cadmium-free conductive thick film.

Technical Field

The present invention relates to lead-and cadmium-free conductive copper thick film pastes for the production of circuits and electronic devices (electronic devices) on aluminum nitride, aluminum oxide and silicon nitride substrates.

Background

Thick film circuits are a well known form of monolithically integrated microelectronic circuits. This type of circuit is particularly useful in situations where a large number of passive components are required or where moderately high power losses are required. Thick film circuits are less expensive to produce and, depending on composition and dimensional characteristics, can produce a wider range of resistance values than thin film circuits.

The fabrication of thick film circuits is an improvement over the well-known screen printing techniques. Thick film circuits consist of a pattern of conductors, resistors, and other passive circuit components printed on a particular substrate. In most known processes, a variety of pastes are applied to a substrate or continuous layer of circuitry through a screen or stencil of a particular printed pattern. The successive layers are dried after printing and fired in a belt furnace to sinter the material.

In a typical thick film circuit, the substrate is usually a ceramic material, such as alumina. However, for demanding applications, such as in automotive electronics where extreme temperature variations are typical, other ceramic materials with improved mechanical properties may be considered. The coefficient of thermal expansion may also be important in applications where thick film circuits are formed in conjunction with silicon devices. In these applications, there is much room for improvement in thick film pastes formed on substrates. Thick film pastes are typically compositions of glass particles, metal and/or metal oxide particles, along with organic solvents, resins, and viscosity control agents known as thixotropic agents. The composition of these thick film pastes depends on the type of passive electrical component being printed.

A variety of metal-containing thick film compositions (i.e., pastes, inks, tapes, etc.) that can be used to form resistors, dielectrics, and conductors employed in hybrid microelectronic components have been developed in the field of hybrid microelectronics. Generally, such compositions, and particularly paste or ink compositions, include a conductor (e.g., silver, palladium, copper, aluminum, gold, platinum, and the like, as well as alloys of each of these various metals), a resistive or dielectric component (e.g., a glass or inorganic oxide), an adhesive or inorganic fluxing material (e.g., glass or inorganic oxide), and an organic component or vehicle (vehicle) that typically includes a solvent with a resin and a thixotropic and/or wetting agent.

The paste or ink compositions described above are applied to a suitable substrate in a desired configuration or pattern to form the desired circuit for use as a hybrid microelectronic component.

Summary of The Invention

Conductive copper thick film pastes for the production of circuits and electronic devices on aluminum nitride, aluminum oxide and silicon nitride substrates are described. A paste is a raw, unfired (green) or "wet") mixture of glass and/or metal components along with organic components (typically a binder or vehicle, and an organic solvent).

Silicon nitride has high strength and fracture toughness even at high temperatures. Therefore, it is used as a high-temperature structural member for automobile engines, gas turbines, and combustor parts.

In addition, because the low coefficient of thermal expansion of silicon nitride is matched to that of silicon, silicon nitride is ideally suited for direct bonding of silicon-based circuit devices, such as Insulated Gate Bipolar Transistors (IGBTs) used as electronic switches in high power circuits. The packaging of these devices typically experiences extreme temperature variations due to sudden changes in high electrical power. The low coefficient of thermal expansion of silicon nitride combined with high flexural strength and relatively high thermal conductivity makes it suitable for applications requiring high thermal shock resistance.

Since silicon nitride is a very inert material, suitable metal conductive thick film paste compositions that can be used to form circuits on silicon nitride are not known.

The present disclosure provides details of screen printable metal thick film paste compositions comprising copper, silver and titanium metal powders suitable for forming such circuits. The fired film showed very good conductivity and adhesion to the substrate. Inkjet printing, digital printing, and additive manufacturing (3D printing) application methods are also contemplated. Other processing techniques and conditions include: screen printing, stencil printing, syringe deposition; depositing one or two layers by digital printing techniques; including ambient air (ambient), helium, argon, xenon, neon, krypton, and nitrogen (N)₂) And combinations thereof. The layers may be fired separately or together. Multiple firings may be performed to build up the thickness of the final fired part. The layers may be co-fired (1, 2, 3 or more layers).

Except for silicon nitride (SiNx or Si)₃N₄) In addition to the substrate, the present subject matter is also applicable to AlN. Other substrates of interest include AlN, SiON, BN, and alumina.

The products of the invention may be used in MLCCs, LTCCs, capacitors, resistors, cellular phones, computers, computer components, stereos, home appliances, automotive components, televisions and other electronic products.

Alloys containing Ag, Cu and Ti metal powders have been identified as suitable candidates for bonding silicon nitride to other materials such as silicon, stainless steel, etc. These compositions flow well just above the melting point. Furthermore, they are used only in vacuum or in an argon (inert) atmosphere.

To form thick film circuits, dry screen printed patterns are in air or in N in a belt oven₂And firing in the atmosphere. Any composition comprising Cu as part of a conductor is in N₂And firing in the atmosphere. The industry standard for thick film processing is 850 ℃ -900 ℃. The pattern dimensions in the x-y direction should not change after firing. The fired film should be dense and very well bonded to the substrate. If the formulation (formulation) is intended for good conductivity, the resistivity should be as low as possible, typically a few milliohms per square.

In view of these goals, several compositions with Ag, Cu and Ti powders were tested. The individual metal powder concentrations away from the eutectic region are selected to control the amount of liquid formation at 875-900 ℃. Identification of suitability for use in N₂Compositions fired in an atmosphere at 875 ℃ to 900 ℃ having good adhesion and low resistivity.

After firing, the fired surface did not show very good solder wetting. To improve the soldering, another copper paste with a suitable glass is used as top layer. Resistivity and adhesion data were collected with a two layer structure.

Within the knowledge of the inventors, the bonding operation is carried out in vacuum or in an inert atmosphere such as argon. Typical active metal braze alloy compositions are made of Ag-Cu eutectic with small amounts of active metal such as Ti.

In one aspect, the present subject matter provides a lead-free and cadmium-free conductive thick film paste comprising 20-40 wt.% of a first Cu (copper metal), 20-23 wt.% of a second Cu (copper metal), 3-12 wt.% of Cu₂O, 0.01-25 wt.% Ag, 0.01-25 wt.% of at least one metallic element selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn, and 5-20 wt.% of an organic component. The first Cu has a D50 particle size in microns of about 2-12, 3-11, 4-10, 5-9, 6-8, and unspecified ranges between values selected from the ranges in the sentence. In other embodiments, the first Cu hasA D50 particle size on the micrometer of about 0.01-8, 0.01-5, or 0.5 to 3 or an unspecified range between values selected from the ranges in the sentence. The second Cu has a D50 particle size in microns in the range of about 12-30, 14-27, 16-25, 17-24, 18-23, 19-22, 20-21 and unspecified ranges between values selected from the ranges in the sentence. In other embodiments, the second Cu has a D50 particle size in microns of about 0.01-8, 0.01-5, or 0.5 to 3, or an unspecified range between values selected from the ranges in the sentence.

The second Cu has a D50 particle size in microns in the range of about 12-30, 14-27, 16-25, 17-24, 18-23, 19-22, 20-21 and unspecified ranges between values selected from the ranges in the sentence. The slurry is an unfired composition.

In another aspect, the present subject matter provides a lead-free and cadmium-free conductive thick film paste including a glass component and a copper (Cu) component. The glass component comprises 15-65 wt.% alkaline earth metal oxide, 0.01-10 wt.% alkali metal oxide, 22-70 wt.% (B)₂O₃+SiO₂) And 0.01-15 wt.% of Al₂O₃. The alkaline earth metal oxide is selected from the group consisting of MgO, CaO, SrO, BaO and ZnO. The alkali metal oxide is selected from Li₂O、Na₂O、K₂O and Rb₂O.

In yet another aspect, the present subject matter provides a method of forming a conductive thick film on a substrate. The method includes applying at least one layer of a first conductive thick film paste and a second conductive thick film to form a conductive thick film on a substrate. The first conductive thick film paste comprises, in wt.%, 20-40 wt.% of first Cu, 20-23 wt.% of second Cu, 3-12 wt.% of Cu₂O, 0.01-25 wt.% Ag, 0.01-25 wt.% of at least one metallic element selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn, and 12-20 wt.% of an organic component. The first Cu has a D50 particle size in microns of about 2 up to 12, 3-11, 4-10, 5-9, 6-8, and unspecified ranges between values selected from the ranges in the sentence. The second Cu has a Cu content of about 12-30, 14-27, in microns,16-25, 17-24, 18-23, 19-22, 20-21, and unspecified ranges of D50 particle sizes between values selected from the ranges in the sentence. The second conductive thick film paste includes a glass component and a copper (Cu) component. The glass component comprises 15-65 wt.% alkaline earth metal oxide, 0.01-10 wt.% alkali metal oxide, 22-70 wt.% (B)₂O₃+SiO₂) And 0.01-15 wt.% of Al₂O₃. The alkaline earth metal oxide is selected from the group consisting of MgO, CaO, SrO and BaO. ZnO may be included in the alkaline earth metal oxide component. Ca. Oxides of Ba and Zn are preferred, and oxides of Ca and Zn are most preferred. The alkali metal oxide is selected from Li₂O、Na₂O、K₂O and Rb₂O and combinations thereof. The method further includes drying the first and second conductive thick film pastes on the substrate. The method further includes firing the dried first conductive thick film paste and the dried second conductive thick film paste to form a fired conductive thick film on the substrate. The substrate is selected from the group consisting of aluminum oxide, aluminum nitride, silicon nitride, and boron nitride.

In yet another aspect, the present subject matter provides an electronic device comprising a substrate and a fired conductive thick film disposed on at least a portion of the substrate. The fired conductive thick film includes at least a first fired conductive thick film and a second fired conductive thick film. The substrate is selected from the group consisting of aluminum oxide, aluminum nitride, and silicon nitride. The first fired conductive thick film comprises a conductive thick film paste comprising 20-40 wt.% first Cu, 20-23 wt.% second Cu, 3-12 wt.% Cu₂O, 0.01-25 wt.% Ag, 0.01-25 wt.% of at least one metallic element selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn, and 12-20 wt.% of an organic component. The first Cu has a D50 particle size in microns of about 2-12, 3-11, 4-10, 5-9, 6-8, and unspecified ranges between values selected from the ranges in the sentence. The second Cu has a D50 particle size in microns of about 12-30, 14-27, 16-25, 17-24, 18-23, 19-22, 20-21 and unspecified ranges between values selected from the ranges in the sentence. First, theThe dual fired conductive thick film includes a conductive thick film paste including a glass component and a copper (Cu) component. The glass component comprises 15-65 wt.% alkaline earth metal oxide, 0.01-10 wt.% alkali metal oxide, 22-70 wt.% (B)₂O₃+SiO₂) And 0.01-15 wt.% of Al₂O₃. The alkaline earth metal oxide is selected from the group consisting of MgO, CaO, SrO, BaO and ZnO. The alkali metal oxide is selected from Li₂O、Na₂O、K₂O and Rb₂O.

The foregoing and other features of the subject invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the subject invention, these being indicative, however, of but a few of the various ways in which the principles of the subject invention may be employed.

Brief Description of Drawings

Fig. 1 is an SEM photograph and EDS plot of slurry a fired on silicon nitride.

Fig. 2 is an SEM photograph and EDS plot of slurry a fired on aluminum nitride.

Detailed Description

The present invention provides a lead-free and cadmium-free conductive thick film paste containing copper for producing hybrid microelectronic components over a wide temperature range. Such copper-containing thick film pastes are formed on alumina, aluminum nitride and silicon nitride without undergoing sintering and include a glass component that flows at relatively low firing temperatures.

The automotive industry, the optoelectronic industry, or the military require more than one silicon-based circuit device in an electronic system. One type of silicon-based circuit device includes, for example, an Insulated Gate Bipolar Transistor (IGBT) used as an electronic switch in high power circuits. Electronic systems including IGBTs are often required to experience extreme temperature variations due to sudden changes in high electrical power. IGBTs are fabricated on ceramic substrates such as aluminum oxide (alumina) and aluminum nitride (ain). A thin copper layer is metallized on an alumina substrate.

In addition to the ceramic substrates currently used, it is preferred to fire these new conductors at lower temperatures, such as about 750 ℃ or less, about 700 ℃ or less, or most preferably about 650 ℃ or less, in order to further provide reliability that minimizes interaction with resistors fired or pre-fired at 900 ℃ or more, thus minimizing changes in the Thermal Coefficient of Resistance (TCR) and resistivity. Practical lower limits for such firing temperatures are known to the skilled person. Other applications will require firing at about 800 c or about 850 c. Thus, the thick film of the present invention with a wide process window (650 ℃ -850 ℃) has advantages over the prior art. The thick films of the present invention have additional desirable characteristics such as good solderability (i.e., excellent solder wetting), good wire bondability, low resistivity, and provide excellent adhesion to a variety of substrates including 96% alumina and glass clad stainless steel substrates, as well as low resistivity and a microstructure that is dense and substantially free of pores after firing.

Copper is an ideal conductor material for thick film and power electronics applications because it has high electrical conductivity, high thermal conductivity, resistance to solder leaching, and much better electromigration resistance than other conductors such as silver, and can handle high current densities. The prior art low temperature fired copper thick film systems exhibit minimal adhesion to common substrates, poor solderability, and often contain undesirable metals such as lead or cadmium.

As already stated, the paste composition of the present invention is electrically conductive. Although the line between the conductor and the resistor is often unclear, the paste composition of the present invention has a sheet resistivity (sheet resistivity) of about 2.1-3.1 milliohms/square/mil. And the maximum resistivity is about 20 milliohms per square (mOhm/square).

The present subject matter also provides an electronic device having a lead-free and cadmium-free conductive thick film applied thereto and fired to form a circuit. The electronic devices to which the lead-free and cadmium-free conductive thick films can be applied and fired include direct bonding of silicon-based circuit devices such as Insulated Gate Bipolar Transistors (IGBTs) used as electronic switches in high power circuits. Throughout this specification and in the appended claims, the term "electronic device" means any electronic device that includes a thick film and/or hybrid thick film circuit that will at least withstand the firing temperatures disclosed herein and benefit from the conductive circuitry provided by the lead-free and cadmium-free thick film paste composition.

In general, the present subject matter provides a lead-free and cadmium-free conductive thick film paste having at least a Cu metal component and a glass component. The metal component includes copper. The glass composition includes a first glass and a second glass and is free of lead, cadmium, and compounds of lead and cadmium.

The lead-free and cadmium-free conductive thick film paste of the present subject matter is typically applied to the surface of an electronic device on which one or more circuits or other electronic components (e.g., transistors, capacitors, and resistors) have been formed. The conductive thick film paste is preferably dried and fired, as described more fully below, to form a lead-free and cadmium-free circuit on the substrate. As used throughout this specification and the appended claims, the phrase "lead-free and cadmium-free" means that no lead or PbO, cadmium or CdO is intentionally added to the composition, and that the composition, after firing, contains less than about 0.1% by weight of Pb or Cd.

In particular, the conductive thick film paste of the present invention can be applied to a substrate by means of screen printing, syringe deposition and digital printing techniques. The slurry may contain an organic component or vehicle to provide an appropriate viscosity for passing through the screen. The paste may also contain thixotropic materials so as to set quickly (set up) after being screened to give good resolution. While rheological properties are of paramount importance, the organic component is preferably also formulated to give adequate wetting of the solid and substrate, good drying rates, dry film strength sufficient to withstand rough handling, and good firing properties. A satisfactory appearance of the fired composition is also important.

In view of all the foregoing criteria, a variety of inert liquids may be used in the organic component. The organic component for most conductive thick film pastes is typically a resin solution dissolved in a solvent, and often a solvent solution containing both a resin and a thixotropic agent. The solvent typically boils in the range of about 130 ℃ to about 350 ℃. The most commonly used resin for this purpose is ethyl cellulose. However, resins such as ethyl hydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and monobutyl ether of ethylene glycol monoacetate can also be used. The most widely used solvents for thick film applications are terpenes such as alpha-or beta-terpineol or mixtures thereof with other solvents such as kerosene, dibutyl phthalate, butyl carbitol acetate, hexylene glycol, 2, 4-trimethyl-1, 3-pentanediol monoisobutyl ester (texanol) and high boiling alcohols and alcohol esters. Various combinations of these solvents and other solvents are formulated to achieve the viscosity and volatility requirements desired for each application.

Commonly used thixotropic agents are organic based thixotropic agents such as, for example, hydrogenated castor oil and derivatives thereof. Of course, it is not always necessary to incorporate a thixotropic agent, as the solvent/resin properties plus any shear thinning inherent in the suspension may alone be suitable in this regard. In addition, wetting agents such as fatty acid esters, for example, N-tallow-1, 3-diaminopropane dioleate, N-tallow trimethylene diamine diacetate, N-coco trimethylene diamine, beta diamine, N-oleyl trimethylene diamine, N-tallow trimethylene diamine, and/or N-tallow trimethylene diamine dioleate may be employed.

The ratio of organic component to solid in the conductive composition of the present invention can vary considerably and depends on the manner in which the conductive composition is to be applied and the type of organic component used. Generally to achieve good coverage, the conductive composition may comprise 60% -90% solids and 40% -10% liquid organic components by weight. Such conductive compositions are typically of a semi-fluid consistency and are commonly referred to as "pastes".

For the purposes of the present subject matter, the first conductive thick film paste preferably comprises from about 75% to about 94% by weight solids and from about 6% to about 25% by weight liquid organic components. Further, a first guide according to the present subject matterPreferred ranges of components in the solid portion of the electro-thick film paste are as follows (table 1): a) from about 20.0% to about 40.0%, more preferably from about 20.0% to about 36.0%, most preferably from about 23.0% to about 29.0% of the first copper, preferably the first conductive thick film paste; b) from about 20.0% to about 36.0%, more preferably from about 20.0% to about 30.0%, most preferably from about 20.0% to about 25.0% of the second copper, preferably the first conductive thick film paste; c) cuprous oxide (Cu)₂O) comprising preferably from about 0.01% to about 12.0%, more preferably from about 4.0% to about 12.0%, most preferably from about 4.0% to about 9.0% of the first conductive thick film paste; d) silver (Ag), preferably from about 0.01% to about 25.0%, more preferably from about 2.0% to about 20.0%, most preferably from about 8.0% to about 17.0% of the first conductive thick film paste; e) at least one metallic element, preferably from about 0.01% to about 25.0%, more preferably from about 1.0% to about 20.0%, most preferably from about 10.0% to about 16.0% of the first conductive thick film paste. At least one metal element is selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn. In a preferred embodiment, the at least one metal element is selected from the group selected from Ti, V, Zr, Cr and Sn. In a more preferred embodiment, the at least one metallic element is selected from the group selected from Ti and Sn. In a most preferred embodiment, at least one of the metallic elements is Ti.

TABLE 1 solid portion of first conductive thick film paste

Composition comprising a metal oxide and a metal oxide	It is preferable that	More preferred	Most preferred
				First Cu	20-40	20-36	23-29
Second Cu	20-36	20-30	20-25
				Cuprous oxide (Cu)2O)	0.01-12	4-12	4-9
Silver (Ag)	0.01-25	2-20	8-17
				Metallic element	0.01-25	1-20	10-16

With regard to the organic component, preferred compositions according to the invention were found to be as follows: 1) at least about 90 percent by weight of an organic solvent; 2) up to about 15 percent by weight of a resin; 3) up to about 4 percent by weight of a thixotropic agent; and 4) up to about 2 percent by weight of a wetting agent. Exemplary vehicles are EV2803, 235 and No flow m7, all available from Ferro Corporation, which consist of ethyl cellulose dissolved in terpineol and 2,2, 4-trimethyl-1, 3-pentanediol monoisobutyl ester. The copper metal is advantageously provided in the form of powder and/or flakes.

For the purposes of the present subject matter, a second conductive thick filmThe slurry preferably comprises from about 70% to about 94% by weight solids and from about 8% to about 30% by weight liquid organic components. Further, preferred ranges of components in the solid portion of the second conductive thick film paste according to the present subject matter are as follows (table 2): a) from about 14% to about 23%, more preferably from about 15% to about 23%, most preferably from about 17% to about 22% of the first copper, preferably the second conductive thick film paste; b) from about 20% to about 28%, more preferably from about 22% to about 28%, most preferably from about 23% to about 26% of the second copper, preferably the second conductive thick film paste; c) third copper, preferably from about 15% to about 24%, more preferably from about 15% to about 24%, most preferably from about 18% to about 24% of the second conductive thick film paste; and d) cuprous oxide (Cu)₂O) comprising preferably from about 0.1% to about 11%, more preferably from about 6% to about 10%, most preferably from about 7% to about 10% of the second conductive thick film paste.

TABLE 2 solid portion of second conductive thick film paste

Composition comprising a metal oxide and a metal oxide	It is preferable that	More preferred	Most preferred
				First Cu	14-23	15-23	17-22
Second Cu	20-28	22-28	23-26
				Third Cu	15-24	15-24	18-24
Cuprous oxide (Cu)2O)	0.1-11	6-10	7-10

The solid portion of the second conductive thick film paste also includes a glass component, typically initially provided in the form of one or more glass frits. In one embodiment, the present subject matter provides an electrically conductive thick film paste comprising a Pb-free and Cd-free glass component comprising, in mole%: about 15% to about 65% of an alkaline earth metal oxide, about 0.01% to about 10% of an alkali metal oxide, about 22% to about 70% (B)₂O₃+SiO₂) And about 0.01% to about 15% Al₂O₃. The at least one alkaline earth metal oxide is selected from MgO, CaO, SrO, BaO and ZnO. In a preferred embodiment, the at least one alkaline earth metal oxide is selected from MgO, CaO and ZnO. In a most preferred embodiment, the at least one alkaline earth metal oxide is selected from CaO and ZnO. At least one alkali metal oxide is selected from Li₂O、Na₂O、K₂O and Rb₂O。

It should be kept in mind that the foregoing compositional ranges are preferred and are not intended to be limiting in that ranges, where one of ordinary skill in the art will recognize that these ranges may vary depending on the particular application, the particular components, and the conditions used to process and form the final product. For each disclosure of a range of values bounded at the lower end by zero, the disclosure is also considered to have a value of 0.01 or 0.1 at the lower end.

The slurry according to the invention can be conveniently prepared on a three-roll mill. The amount and type of organic components used is determined primarily by the final desired formulation viscosity, fineness of grind of the slurry, and wet print thickness. In preparing the compositions according to the present subject matter, the particulate inorganic solid is mixed with the organic component and dispersed with a suitable apparatus, such as a three-roll mill, to form a suspension, resulting in a time of 9.6 seconds^-1A viscosity, as determined on a brookfield viscometer 2HBT, spindle CP-51, measured at 25 ℃, of a composition in the range of about 50 to about 200kcps, preferably about 100 to about 150 kcps.

The circuit substrate according to the present subject matter is preferably produced by applying the first conductive thick film paste of the present subject matter to a substrate to a desired wet thickness, for example from about 60 microns to about 80 microns, typically via a screen printing process. An automatic screen printing technique using an 80-325 mesh screen may be employed. The printed pattern is then dried at less than 200 ℃, for example preferably at about 175 ℃ for about 20 minutes to 40 minutes. Subsequently, the second conductive thick film paste of the present subject matter is applied on top of the dried first conductive thick film paste. The printed pattern is then dried at less than 200 ℃, for example preferably at about 175 ℃ for about 20 minutes to 40 minutes before firing. The glass is melted and the metal is sintered in a controlled, non-oxidizing atmosphere belt conveyor furnace. Firing is typically carried out according to a temperature profile that will allow the organic material to burn off at about 300 ℃ to about 550 ℃, with a peak temperature time of about 850 ℃ to about 1050 ℃ for about 5 minutes to 15 minutes, followed by a controlled cooling cycle to prevent over-sintering, unwanted chemical reactions at intermediate temperatures, or substrate cracking, which can occur when the substrate cools too quickly. A non-oxidizing atmosphere such as nitrogen, argon or mixtures thereof is used to prevent oxidation of metals, particularly copper, which tends to oxidize in air even at room temperature. For the purposes of the present invention, a nitrogen atmosphere is preferred. For example, the amount of oxygen in nitrogen may preferably be controlled to be below 10 ppm. The overall firing schedule will preferably last (extended over) for a period of about 60 to 85 minutes, with firing temperatures of about 25 to 30 minutes being reached, about 5 to 15 minutes at firing temperatures and about 30 to 40 minutes in cooling.

An exemplary firing cycle is to ramp up to a peak temperature of 900 ℃ at about 30 ℃/min assuming a room temperature of 20 ℃, hold at 900 ℃ for 12 minutes, and cool at 60 ℃/min for about 15 minutes to exit the furnace at about 60 ℃.

The following examples are intended only to illustrate the present invention and should not be construed as imposing limitations upon the claims. The following experimental methods, conditions and equipment were employed in the preparation of the exemplary slurries detailed below.

Conductive thick film paste

The conductive thick film paste includes a first conductive thick film paste and a second conductive thick film paste. In one embodiment, the first conductive thick film paste is formed directly on top of the substrate and the second conductive thick film paste is formed directly on top of the first conductive thick film paste, wherein the first conductive thick film paste is in a dry state.

The first conductive thick film paste is prepared by: copper powder, silver powder and cuprous oxide (Cu) powder with two (2) kinds of different particle sizes are mixed in proper amount₂O) powder, at least one metallic element, boric acid, Triton X-100, Ethanox 702 and an organic vehicle, first homogenized in a planetary mixer and then homogenized in a three-roll mill to achieve less than 50 μm, preferably less than<40 μm, more preferably<30 μm, even more preferably<20 μm, even more preferably<A fineness of grind of 10 μm.

The second conductive thick film paste is prepared by: copper powder and Cu with three (3) kinds of different particle sizes₂O powder and glass powder, boric acid, Triton X-100, Ethanox 702 were mixed with an organic vehicle, first homogenized in a planetary mixer and then homogenized in a three-roll mill to achieve a grind fineness of less than 20 μm. The viscosities of the first and second conductive thick film pastes were measured using a brookfield HBT viscometer 2HB, 2.5rpm at 25 ℃ using spindle CP-51. The shelf life of the green slurry was good.

Glass

The second conductive thick film paste includes Pb-free and Cd-free glass frit. The glass frit formulation comprises about 15% to 65% alkaline earth metal oxide, about 0.01 wt.% to 10 wt.% alkali metal oxide, about 22 wt.% to 70 wt.% (B)₂O₃+SiO₂) And about 0.01 wt.% to 15 wt.% Al₂O₃. The alkaline earth metal oxide is one or more selected from the group consisting of MgO, CaO, SrO, BaO, and ZnO. The alkali metal oxide is selected from Li₂O、Na₂O、K₂O and Rb₂O. Suitable commercially available glasses from Ferro Corporation include EG0026, EG0028, EG2807, EG3046, LF256, EG2810, BBS 2.

Copper powder

The metal component includes copper metal. Copper metal is typically provided in the form of at least one of a powder and/or a flake. The copper powder may have more than one copper powder with different particle size distributions and/or particle morphologies. In particular, more than one size range of copper particles may be used for the first and second conductive thick film pastes, respectively. For example, for the first conductive thick film paste, the copper powder may comprise a first copper powder having a D50 and a Specific Surface Area (SSA) of about 5 microns. The first copper powder is sold under the designation ICP UF-5-AC and is available from Makin-UK. The second copper powder had a D50 of between about 0.3 microns and 0.5 microns. The second copper powder is sold under the product name CU-F400 available from CNPC.

For the second conductive thick film paste, the copper powder may include a first copper powder having a D50 of about 5 microns. The first copper powder is sold under the designation ICP UF-5-AC and is available from Makin-UK. The second copper powder had a D50 of about 1.5 microns and a D of 0.65m²Specific surface area per gram (SSA). A second copper powder is sold as Cu22-201 and is available from technical Corporation. The third copper powder had a D50 of about 10 microns. A third copper powder is sold as Cu-APC0412000CX00 and is available from Safina Corporation.

Other metals and oxides

The first conductive thick film includes a flaky silver (Ag) powder having D50 of less than 9 microns. Specific Surface Area (SSA) in the range of 0.65m²/g-1.35m²(ii) in terms of/g. Ag powder is sold as Silver Flake SF-4, available from Ames Corporation. Titanium (Ti) powder has a D100 of less than 20 microns and is sold as Ti-101, available from AEE Corporation. Commercially available cuprous oxide (Cu)₂O) is used to form both the first conductive thick film and the second conductive thick film.

Method

Substrate: the substrates used were: (1) silicon nitride (Maruwa, Japan), (2) aluminum nitride (Maruwa, Japan), and (3) 96% alumina (CoorsTek, USA).

Screen printing: the first conductive thick film paste was screen printed on the substrate using an 80 mesh screen and a pattern suitable for testing resistivity and adhesion. The printed paste was dried in a box oven at 175 ℃ for 30 minutes. Subsequently, a second conductive thick film paste was screen printed on the dried first conductive thick film paste using an 80 mesh screen and a pattern suitable for testing resistivity and adhesion. The printed second conductive thick film paste was dried in a box oven at 175 ℃ for 30 minutes. The green screen-printed paste had a thickness of about 80-120 μm and was reduced to 50-65 μm after drying. Other thicknesses are possible in various embodiments, such as green body thicknesses in the range of 10-50 (with 325 mesh), 70-130, 75-125, 85-115, 90-110, 95-105, and unspecified ranges between values selected from the ranges in the sentence, in microns. Other possible fired thicknesses in various embodiments, such as 5-25, 40-90, 45-85, 50-80, 55-75, 60-70 in microns, and ranges having upper and lower limits selected from the ranges in this sentence.

Firing profile and conditions: a belt furnace was used with a belt speed of 1.88 inches (4.78cm) per minute. The sample was heated to the peak temperature over the course of 28 minutes. The sample is held at the peak temperature for 10 minutes to 30 minutes. The sample was cooled at a controlled rate to about 60 c, which took about 38 minutes. The sample had less than 10ppm O₂And firing in a nitrogen atmosphere. The peak temperature is 650 ℃ or 1050 ℃ or other values in between.

The tests performed included electrical characteristics, initial adhesion to the substrate, and aged adhesion to the substrate. The electrical testing included determination of resistivity, expressed in mOhm/square, calculated from the measured resistance of a serpentine pattern 500 microns wide with 200 squares and a fired thickness of about 30 μm, then normalized to 25.4 μm.

Adhesion was measured by dip soldering, where a 22AWG Cu-Sn wire uses a 62Sn/36Pb/2Ag solder andRMA flux 197 was soldered to a 0.080 "x 0.080" square pad.Is a registered trademark of Kester Solder, Des Plaines, Ill. -2675. The wire was then pulled at 90 ° to failure using Shepard Crook method. The adhesion strength is expressed as pounds force required to break the line (peaks of force). The aged adhesion was measured after subjecting the welded joint to a temperature of 150 ℃ for 48 hours.

After firing at the temperatures set forth above, the interface between the substrate and the fired conductive thick film was examined by scanning electron microscopy (FEI-Quanta FEG 450) for determining the nature of the bonding. Chemical mapping of the substrate and conductive thick film was performed by energy dispersive spectroscopy (Oxford X-max 50). In particular, a chemical mapping of Ti and N is recorded for a cross section of a conductive thick film formed on a substrate.

Ti and N₂The interface SEM and chemical mapping of (a) are reproduced below.

FIG. 1 SEM/EDS observations at the silicon nitride/slurry A interface

Fig. 1 shows that Ti is detected as a continuous layer at the interface and indicates the interface reaction and bonding to the substrate.

In addition, the slurry combinations were tested on an aluminum nitride substrate. As reported in table 4, very good electrical conduction and adhesion was observed. The reaction zone is shown very clearly by the interface analysis of SEM/EDS.

FIG. 2 SEM/EDS observations at the aluminum nitride/slurry A interface

In each of fig. 1 and 2, the fired slurry is above the reaction zone and the substrate is below the reaction zone.

Example 1. the compositions of thick film paste, paste a, paste B and paste C of the present invention are described in table 3. Silicon nitride and aluminum nitride are used as substrates. Either paste a or paste B was formed directly on the substrate as an adhesive layer. After slurry a or slurry B is dried, slurry C is formed on slurry a or slurry B. Note that the detailed compositions of paste a and paste B are slightly different from each other to adjust the adhesion of the Cu thick film on the substrate. After the paste C is dried, the dried conductive thick film stack is at N₂Firing at 900 ℃ in an atmosphere. Selected electrical and mechanical properties of the stack including paste C on paste a after firing on silicon nitride and aluminum nitride are shown in table 4. Typically, an aged adhesion of at least 3 lbf is considered acceptable for thick films formed on a substrate. The aged adhesion force is above acceptable levels and in the range of 5-6 lbf, which is promising for extended periods of use. The fired conductive thick film has a sheet resistivity in the range of 2.1 mOhm/square to 3.1 mOhm/square. Considering that typically 1 mOhm/square to 5 mOhm/square is considered a good conductor, it is expected that a conductive thick film according to the present subject matter will work as a thick film circuit on silicon nitride and aluminum nitride.

TABLE 3 compositions of slurry A, slurry B and slurry C

TABLE 4 Electrical and mechanical Properties of the conductive Thick film Stack (paste C on paste A) after sintering

Example 2. inventive thick film pastes B and C in table 3 were selected to form a thick film stack. Silicon nitride, aluminum nitride, and aluminum oxide are used as the substrate. Thick film pastes B and C were formed on each substrate such that paste B was in direct contact with the substrate. After slurry B was dried at a similar temperature to slurry a in example 1, slurry C was formed on slurry B and dried. Thick film stack in N₂Firing at 900 ℃ in an atmosphere. Selected electrical and mechanical properties of the fired conductive thick film stack are shown in table 5. Regardless of the substrate used, the sheet resistivity of the fired conductive thick film is in the range of 3.0 mOhm/square to 3.1 mOhm/square and within an acceptable range as a conductive circuit. Both initial adhesion and aged adhesion were also measured to be well above the typical standard of at least 3 lbf. In particular, the conductive thick film stack according to the present subject matter shows improved adhesion (both initial adhesion and aged adhesion) on silicon nitride substrates over other substrates including aluminum nitride and aluminum oxide.

Various powders and components of the present invention are commercially available and have the following characteristics.

The above ingredients are available from the following suppliers:

Makin Metal Powder(UK)Ltd.Buckley Road,Rochdale Lancashire,UK OL12 9DT.

Technic Inc.1Spectacle St.,Creston,RI 02910USA

Safina,a.s.Videnska 104,252 50Vestec Czech republic

AEE：Atlantic Equipment Engineers 13Foster St.Bergenfield,NJ 07621

Ames Advanced Materials Corporation 3900South Clinton Ave.South Plainfield,NJ 07080

CNPC Powder Group Co.,Ltd.,Room 1211,No.8Office Bldg.,Wanda Plaza,58He Xuan Road,Shanghai,China 201803

substrate-cuprous oxide from JT Baker/Avantar Performance Materials; as received powder-coarse (d50), but the information is not available.

TABLE 5 Electrical and mechanical Properties of the conductive Thick film Stack (paste C on paste B) after sintering

The present invention is further defined by the following items.

Item a-1. a lead-free and cadmium-free conductive thick film paste comprising, in wt.%:

(a) from about 20% to about 49% of the first Cu,

(b) from about 20% to about 34% of a second Cu,

(c) from about 3% to about 12% Cu₂O，

(d) From about 8% to about 25% Ag, and

(e) from about 8% to about 25% of at least one metallic element selected from the group consisting of Ti, V, Zr, Mn, Cr, Co and Sn,

wherein the first Cu has a D50 of about 0.1 to 8 microns, preferably 0.5 to 5 microns, and the second Cu has a D50 of about 10 to 20 microns, preferably 12-20 microns.

Item a-2. the thick film paste of item a-1, wherein the thick film paste comprises:

(a) from about 21% to about 36% of the first Cu,

(b) from about 20% to about 30% of a second Cu,

(c) from about 3% to about 12% Cu₂O，

(d) From about 8% to about 20% Ag, and

(e) from about 8% to about 20% of at least one metallic element selected from the group consisting of Ti, V, Zr, Cr, and Sn.

Item A-3. the thick film paste of item A-1 or item A-2, wherein the thick film paste comprises:

(a) from about 23% to about 29% of the first Cu,

(b) from about 20% to about 25% of a second Cu,

(c) from about 4% to about 9% Cu₂O，

(d) From about 8% to about 17% Ag, and

(e) from about 10% to about 16% of at least one metallic element selected from the group consisting of Ti and Sn.

Item a-4. the thick film paste of any one of items a-1 or a-2 or a-3, further comprising an organic component portion, the paste comprising from about 10% to about 20% by weight of the organic component portion.

Item B-1. a lead-free and cadmium-free conductive thick film paste comprising a glass component and a copper (Cu) component, the glass component comprising in wt.%:

(a) from about 10% to about 70%, preferably from about 15% to about 65%, more preferably from about 20% to about 60% of at least one alkaline earth metal oxide,

(b) from about 0.01% to about 10%, preferably from about 0.1% to about 8%, more preferably from about 1% to about 8% of at least one alkali metal oxide,

(c) from about 22% to about 70%, preferably about 25% to about 65%, more preferably about 25% to about 65% (B)₂O₃+SiO₂) And an

(d) From about 0.01% to about 15%, preferably about 0.1% to about 13%, more preferably about 0.5% to about 12% of Al₂O₃，

Wherein the alkaline earth metal oxide is selected from the group consisting of MgO, CaO, SrO, BaO and ZnO; and the alkali metal oxide is selected from the group consisting of Li₂O、Na₂O、K₂O and Rb₂O; and is

The Cu component comprises in wt.%:

(a) from about 14% to about 23% of the first Cu,

(b) from about 20% to about 28% of a second Cu,

(c) from about 15% to about 24% of third Cu, and

(d) from about 5% to about 11% Cu₂O，

Wherein the first Cu has a D50 of about 5 microns, the second Cu has a D50 of about 1.5 microns, the third Cu has a D50 of about 10 microns, and

wherein the glass component and the Cu component are present in a weight ratio of about 1:15 to about 1: 30.

Item B-2 the thick film paste of item B-1, wherein the alkaline earth metal oxide is selected from the group consisting of MgO, CaO, and ZnO.

Item B-3. the thick film paste of item B-2 or item B-2, wherein the alkaline earth metal oxide is selected from CaO and ZnO.

Item B-4. the thick film paste of any one of items B-1, B-2, or B-3, wherein D50 of the glass component is from about 0.5 microns to about 20 microns.

Item B-5. the thick film paste of any one of items B-1, B-2, B-3, or B-4, wherein the copper comprises, in wt.%:

(a) from about 15% to about 23% of a first Cu,

(b) from about 22% to about 28% of a second Cu,

(c) from about 17% to about 24% of third Cu, and

(d) from about 6% to about 10% Cu₂O。

Item B-6. the thick film paste of item B-1, wherein Cu comprises, in wt.%:

(a) from about 17% to about 22% of a first Cu,

(b) from about 23% to about 26% second Cu,

(c) from about 18% to about 24% of third Cu, and

(d) from about 7% to about 10% Cu₂O。

Item B-7. the thick film paste of item B-1, further comprising an organic component portion, the paste comprising from about 10% to about 30% by weight of the organic component portion.

Item C-1. a method of forming a fired conductive thick film on a substrate comprising:

(a) applying at least one of the conductive thick film paste of any one of items A-1 to A-4 and the conductive thick film paste of any one of items B-1 to B-7 to form a conductive thick film on a substrate,

(b) drying the conductive thick film paste of any one of items a-1 to a-4 and the conductive thick film paste of any one of items B-1 to B-7, and

(c) firing the dried conductive thick film paste of any one of items A-1 to A-4 and the conductive thick film paste of any one of items B-1 to B-7 to form a fired conductive thick film on a substrate, wherein firing is on N₂The method is carried out in the atmosphere,

wherein the substrate is selected from the group consisting of aluminum oxide, aluminum nitride, and silicon nitride.

Item C-2. the method of item C-1, wherein at least one of the conductive thick film paste of any one of items a-1 to a-4 and the conductive thick film paste of any one of items B-1 to B-7 is applied by one selected from the group consisting of additive manufacturing, screen printing, syringe deposition, and digital printing techniques.

Item C-3. the method of item C-1 or C-2, wherein firing is performed at a temperature from about 850 ℃ to about 1050 ℃.

Item C-4. the method of any one of items C-1 to C-3, wherein firing is performed at a temperature of from about 850 ℃ to about 900 ℃.

Item C-5. the method of any one of items C-1 to C-4, wherein the atmosphere comprises less than 10 parts per million (ppm) oxygen.

Item C-6. the method of any one of items C-1 to C-4, wherein firing is performed in a reducing atmosphere.

Item C-7. the method of any one of items C-1 to C-6, wherein the reducing atmosphere comprises nitrogen (N)₂)。

Item D-1. an electronic device, comprising:

(a) a substrate; and

(b) a lead-free and cadmium-free conductive thick film disposed on at least a portion of the substrate, the lead-free and cadmium-free conductive thick film comprising at least a first conductive thick film and a second conductive thick film,

wherein the substrate is selected from the group consisting of aluminum oxide, aluminum nitride and silicon nitride, and

wherein the first conductive thick film comprises the conductive thick film paste of item a-1 and the second conductive thick film comprises the conductive thick film paste of any one of items a-1 to a-4 or items B-1 to B-7.

Item D-2 the electronic device of item D-1, wherein an interface comprising nitrogen (interface composing reagent) is formed between the substrate and the lead-free and cadmium-free conductive thick film.

Item D-3. the electronic device of item D-1 or item D-2, wherein the interface further comprises Ti.

Item D-4 the electronic device of any one of items D-1 to D-3, wherein the first conductive thick film is disposed directly on the substrate.

Item D-5 the electronic device of any one of items D-1 to D-4, wherein the second conductive thick film is disposed on the first conductive thick film.

Item D-6 the electronic device of any one of items D-1 to D-5, wherein the lead-free and cadmium-free conductive thick film is formed by firing at a temperature from about 850 ℃ to about 1050 ℃.

It should be understood that although in the foregoing embodiments, the substrates employed were limited to silicon nitride, aluminum nitride and aluminum oxide, the thick film paste of the present invention may be used in conjunction with a variety of substrates including, but not limited to, enamel-coated steel, beryllium oxide substrates, glass substrates, barium titanate substrates, silicon substrates, boron nitride substrates and silicon carbide substrates. Additionally, it should be understood that in addition to the screen printing techniques used in the previous examples, the thick film paste of the present invention may be applied using a variety of additional techniques known in the art, including spraying, brushing, dipping, ink jetting, or doctor blading.

It should also be understood that the term "glass" as used herein is intended to provide a broad interpretation, and thus it encompasses both glasses and glass-ceramics that exhibit some degree of crystallinity.

Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.