HK1074277A - The use of conductor compositions in electronic circuits - Google Patents

Description

Use of conductor compositions in electrical circuits

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Technical Field

The present invention relates to a conductor composition with aluminium present in the composition and to the use of the composition in the manufacture of electronic components. These compositions are used in particular in the manufacture of defogging elements in heating windows, for example, in automotive glazings, and in particular in automotive rear windows (backlights).

Background

It is well known in the electronics art to use thick film conductors as components in hybrid microcircuits. Typically, the compositions used to make such components employ paste-like solid-liquid dispersions (dispersions) in which the solid phase comprises finely divided particles of noble metal or noble metal alloy or mixtures thereof and an inorganic binder. The liquid carrier of the dispersion is typically an organic liquid medium, but may also be an aqueous based liquid medium. Minor amounts (typically less than about 3% by weight of the composition) of additional materials may be added to modify the properties of the composition, these including colorants, rheology modifiers, cohesion enhancers, and burn modifiers.

The metal used in the formulation of the thick film conductor composition is typically selected from silver, gold, platinum and palladium. The metals may be used alone or as a mixture that forms an alloy upon firing. Common metal mixtures include platinum/gold, palladium/silver, platinum/palladium/gold, and platinum/palladium/silver. The most common systems used in the manufacture of heating elements are silver and silver/palladium. The inorganic binder is typically glass or a glass-forming material such as lead silicate and functions as a binder both in the composition and between the composition and the substrate to which the composition is applied. The use of lead-containing adhesives is becoming less common due to environmental concerns, and lead-free adhesives such as zinc or bismuth borosilicate are now often used. The organic medium functions to disperse the particulate component to facilitate delivery of the composition to the substrate.

The consistency and rheology of the composition is adjusted according to the particular method of application, which may include screen printing, brushing, dipping, extruding, spraying, and the like. Typically, screen printing is used to apply the composition. The paste is typically applied to an inert substrate such as an alumina, glass, ceramic, enamel, enameled glass or metal substrate to form a patterned layer. The thick film conductor layer is typically dried and then fired, typically at a temperature between about 600 and 900 c, to volatilize or burn off the liquid vehicle and to fire or melt the inorganic binder and metal components. Direct wet firing is also used to create patterned layers, i.e., the thick film layer is not dried prior to firing.

Of course, it is necessary to connect the conductive pattern to other components of the circuit, such as power supplies, resistor and capacitor networks, resistors, trimming potentiometers, chip resistors, and chip carriers. This is typically accomplished by using metal studs comprising copper that are soldered directly to adjacent or overlying conductive layers. In the case of soldering the studs to the conductive layer, they are fixed directly to the conductive pattern itself or to a solderable composition additionally printed on top of the pattern ("overprint"). Additional printing is typically applied only to the area of the conductive pattern where the metal studs are secured by solder, this area being commonly referred to as the "stud field". The ability to solder to the conductive layer is an important parameter in the manufacture of the heating element, as it obviates the need for additional printing. However, inorganic adhesives, which are important for bonding paste to a substrate, can interfere with the solder wetting and result in poor adhesion of the soldered metal stud to the conductive layer. It is often difficult to simultaneously meet the requirements of high adhesion of the substrate and high solderability (or adhesion of the metal studs to the conductive pattern). U.S. patent No. 5,518,663 provides a solution to this problem by binding crystalline materials of the feldspar family into the composite.

An important application of patterned conductive layers is in the automotive industry, particularly in the manufacture of windows that can be defrosted and/or defogged by a conductive mesh permanently affixed to the window and capable of generating heat when energized by a voltage source. In order to defrost the window quickly, the circuit must be able to provide a large amount of power from a low voltage power supply, typically 12 volts, for which the resistivity requirements of the conductive pattern typically range from about 2 to about 5 μ Ω (5 m Ω/□ at 10 μm after ignition). This requirement is easily met by conductors comprising noble metals, in particular silver, which is the most commonly used material in this application.

In certain applications, a conductive composition having a relatively high resistivity is desired. In particular, the electrical resistance requirements of window-heating units in automobiles are expected to change in the short term, as the automotive industry is expected to employ 42 and 48 volt power supplies in the near future. As a result, the conductive composition used to make the window-heating element will be required to exhibit relatively high resistivity values, typically greater than about 10 μ Ω cm, preferably greater than about 12 μ Ω cm, and particularly in the range of about 20 to about 70 μ Ω cm.

Many different materials may be added to adjust the specific resistivity of the conductive composition. For example, metal resinates such as rhodium and manganese resinates have been used to increase resistivity, as disclosed in US 5162062 and US 5378404. In addition, increasing the composition of precious metals, particularly platinum group metals such as platinum and palladium, have also been used to increase specific resistivities. Silver/palladium and silver/platinum compositions can give resistivity values from about 2 μ Ω cm (resistivity for compositions comprising only silver and binder) to about 100 μ Ω cm (for a 70: 30 Pd: Ag mixture). However, systems comprising platinum and/or palladium are significantly more expensive and their use will be prohibitive in applications requiring coverage of large areas, such as window-heating units used in the automotive industry. Furthermore, for certain metal mixtures, such as compositions comprising a high palladium content, overprinting of compositions comprising a large amount of silver is generally required in order to obtain a suitable degree of solder adhesion. Conventional conductive compositions, which typically operate at 2 to 5 μ Ω cm resistivity and include predominantly silver, do not require overprinting because acceptable levels of solder adhesion can be obtained by adjusting the composition of the inorganic binder.

Other lower cost methods of achieving high resistivity include incorporating large amounts of fillers into the silver-containing conductive composition to block the conductive path. Typically, the filler is an inorganic material, commonly used being glass (which may be the same or different from the glass used for the binder) and alumina (or other metal oxide). However, this approach tends to result in a loss of weld acceptability and weld adhesion. For example, a suitable weld adhesion level may be maintained up to a level of about 10% alumina by weight of the composition, but generally this level is too low for an appreciable increase in resistivity. For glass type fillers, loss of weld adhesion occurs even at lower levels, which again is too low for an appreciable increase in resistivity. Furthermore, this problem is generally not improved by using silver overprinting, due to glass flow between layers during firing, especially from the conductive coating layer into the overprint.

Yet another advantageous property of the conductive composition is chemical durability and restoring force upon exposure to changing environmental conditions such as temperature, humidity, acids and salts. In general, compositions comprising large amounts of glass fillers (especially lead-free glass fillers) are rather unstable to these factors.

An additional consideration is that it is desirable that the resistance of the coating composition be substantially independent of the firing temperature used in fabricating the patterned conductive layer. For example, in the case of applying a conductive composition to a glass substrate, the properties of the composition should remain substantially constant at temperatures between about 620 and 680 ℃ under sintering and melting. However, a resistance change of about 10% between these two temperatures is generally tolerated, which corresponds to the properties of the pure silver composition. The result of using a large amount of filler to significantly increase the resistivity is that the composition generally does not meet this requirement.

Yet another additional consideration is: it is desirable that the relationship between resistivity and the amount of resistivity modifier added to the composition be fairly predictable and/or substantially linear over the target range of resistivity required. In general, the resistivity of a composition comprising a large amount of filler increases in a nearly linear fashion until a critical concentration is reached. At this critical concentration, the resistivity may often rise by orders of magnitude very quickly when the content of resistivity modifier is increased by only a small percentage by weight. As a result, it is difficult to target a particular value of the resistivity of such a composition.

It is an object of the present invention to provide a higher resistivity conductive composition that does not have the above-mentioned disadvantages. In particular, it is an object of the present invention to provide an economical conductive coating composition having increased electrical resistivity while exhibiting excellent solderability.

Summary of The Invention

According to the present invention, there is provided a use of a composition comprising (a) finely divided particles of an electrically conductive material; (b) finely divided particles of one or more inorganic binders; and (c) finely divided particles of aluminum, wherein the components (a), (b) and (c) are dispersed in a liquid carrier, preferably an organic medium, in order to increase the resistivity of the conductive pattern on the substrate.

According to yet another aspect of the present invention, there is provided a use of finely divided particles of aluminum in a composition further comprising, dispersed in a liquid carrier, (a) finely divided particles of a conductive material and (b) finely divided particles of one or more inorganic binders, for the purpose of increasing the resistivity of a conductive pattern manufactured from the composition.

According to still another aspect of the present invention, there is provided a method for increasing the resistivity of a conductive pattern manufactured from a composition comprising (a) finely divided particles of a conductive material and (b) finely divided particles of one or more inorganic binders dispersed in a liquid-phase carrier, the method comprising incorporating (c) finely divided particles of aluminum into the composition.

According to yet another aspect of the present invention, there is provided a process for manufacturing a conductive pattern, the process comprising applying a composition to a substrate, the composition comprising (a) finely divided particles of a conductive material dispersed in a liquid carrier; (b) finely divided particles of one or more inorganic binders; and (c) finely divided particles of aluminum, the components (a), (b) and (c) preferably being dispersed in a liquid carrier of an organic medium, and firing the coated substrate to effect sintering of the finely divided particles onto the substrate. Preferably, the process is a screen printing process.

According to a further aspect of the present invention there is provided a substrate, typically a rigid substrate such as a glass (including tough and laminated glasses), enamel, enamelled glass, ceramic, alumina or metal substrate, having an electrically conductive pattern on one or more surfaces of the substrate, the electrically conductive pattern comprising (a) an electrically conductive material; (b) one or more inorganic binders; and (c) aluminum.

Detailed description of the invention

For example, the composition is suitable for use as a paste composition for forming a thick film conductive pattern on a substrate by a screen printing process. In particular, in applications in the automotive industry, composites are used in particular as a component for the manufacture of windows which can be defrosted and/or demisted by means of an electrically conductive mesh attached to the window.

The composition preferably exhibits a resistivity value greater than about 10 μ Ω cm, preferably greater than about 12 μ Ω cm, more preferably in the range of from about 20 to about 70 μ Ω cm, and even more preferably in the range of from about 20 to about 50 μ Ω cm. Thus, as used herein, the term "increasing resistivity" means preferably increasing the resistivity to a value greater than about 10 μ Ω cm, preferably greater than about 12 μ Ω cm, more preferably in the range of from about 20 to about 70 μ Ω cm, and even more preferably in the range of from about 20 to about 50 μ Ω cm. In one embodiment, the resistivity ranges from about 30 to about 40 μ Ω cm.

It is intended that the term "finely divided" as used herein means that the particles are fine enough to pass through a 400-mesh screen (U.S. standard sieve scale). Preferably at least 50%, more preferably at least 90%, and most preferably substantially all of the particles have a size in the range of 0.01 to 20 μm. Preferably, substantially all of the particles have a size no greater than about 10 μm, desirably no greater than about 5 μm.

Preferably, the ingredients are provided in amounts such that the total amount of ingredients (a), (b), and (c) is from about 50 to about 95% by weight of the composition, and the liquid carrier is provided in an amount from about 5 to about 50% by weight of the composition. In a preferred embodiment, the total amount of components (a), (b) and (c) is in the range of about 60 to about 90% by weight of the composition, preferably in the range of about 70 to about 85% by weight of the composition.

In general, mixtures (a), (b) and (c) include substantially all of the solid phase materials used in formulating the compositions used in the present invention.

Preferably, ingredient (a) is provided in an amount of from about 30 to about 99.4%, preferably from about 50 to about 98%, more preferably from about 60 to about 90%, and even more preferably from about 65 to about 75% by weight of the total solids present in the composition.

Preferably, component (b) is provided in an amount of from about 0.5 to about 40%, preferably from about 1 to about 25%, preferably from about 2 to about 20%, and most preferably from about 4 to about 20% by weight of the total solids present in the composition. In one embodiment, ingredient (b) is provided in an amount of from about 2 to about 15%, preferably from about 4 to about 15%, by weight of the total solids present in the composition.

Preferably, component (c) is provided in an amount of from about 0.1 to about 30%, preferably from about 2 to about 15%, and most preferably from about 7 to about 15% by weight of the total solids present in the composition. In one embodiment, ingredient (c) is provided in an amount of from about 4 to about 10%, more preferably from about 5 to about 9%, by weight of the total solids present in the composition. In yet another embodiment, ingredient (c) is provided in an amount of from about 10 to about 15 percent by weight of the total solids present in the composition.

The form of the conductive particles of component (a) may be any form suitable for use in making the composition used in the present invention. For example, the conductive metal particles may be in the form of metal powders or metal flakes or mixtures thereof. In one embodiment of the invention, the metal particles are a mixture of powder and flakes. The particle size of the metal powder or flake is not itself precise and critical in terms of technical effectiveness. However, the particle size, which does affect the sintering characteristics of the metal, is that large particles sinter at a slower rate than small particles. As is well known in the art, mixtures of powders and/or flakes of different sizes and/or proportions may be used to tailor the sintering characteristics of the conductor composition during sintering. However, the metal particles should have a size suitable for their application method, which is usually screen printing. Thus, the size of the metal particles should generally not be greater than about 20 μm, and preferably less than about 40 μm. The minimum particle size is typically about 0.1 μm.

The preferred metal for the conductive component (a) of the conductor composition is silver. Silver particles greater than about 1.0 μm impart more color to the composition. Preferably, the composition comprises at least 50% by weight of silver particles larger than 1.0 μm. Typically, silver is of high purity, typically greater than 99% purity. However, less pure materials may be used depending on the electrical requirements of the conductive layer or pattern. In one embodiment of the invention, component (a) comprises a mixture of silver and nickel and/or suitable derivatives. A preferred nickel derivative suitable for use in this embodiment of the invention is nickel boride (Ni)₃B) In that respect Typically, the ratio of Ag to Ni is from about 1: 1 to about 25: 1, preferably at least about 1.5: 1, and more preferably from about 1.5: 1 to about 3: 1. It will be understood by those skilled in the art that references herein to the relative amounts of conductive component (a) and conductive component (a) do not include references to component (c) or its relative amounts, even if the particles of component (c) are themselves conductive. Likewise, references herein to particles of component (c) and their relative amounts do not include references to conductive particles of component (a) and their relative amounts, even though the particles of component (c) are themselves conductive.

Component (c) of the composition used in the present invention comprises aluminum in the form of one or more of the following:

(i) metallic aluminum particles;

(ii) aluminum-containing alloy particles;

(iii) substantially converted to metallic aluminum derivatives under the action of heat.

Preferably, the particles of component (c) are metallic aluminum particles and/or particles of an aluminum-containing alloy. More preferably, the particles of component (c) are metallic aluminum particles.

The size of the particles should generally not be greater than about 20 μm, preferably less than 10 μm. The minimum particle size is typically about 0.1 μm. The shape of the particles may be in the form of small flakes or powders, spherical or oblate spheroidal or irregularly shaped, or any other suitable morphology.

The composition provided using component (c) as an additive exhibits (i) high resistivity; and (ii) high weld adhesion; preferably (iii) the resistivity rises more uniformly with increasing concentration of additive relative to the composition in which the bulk of the filler is used to increase the resistivity; and also preferably (iv) the resistance varies less with firing temperature. In addition, aluminum is a relatively inexpensive material and is an economical method of increasing resistivity.

Suitable inorganic binders for use in the present invention are materials that, when sintered, bind a metal to a substrate such as a glass (including tough and laminated glass), enamel, enamelled glass, ceramic, alumina or metal substrate. Inorganic binders, also known as glass raw materials, comprise finely divided particles and are a key component of the compositions described herein. For metal powders/flakes and substrates, the softening point and consistency of the glass raw material during firing as well as its moisture characteristics are important factors. The particle size of the glass raw materials is not critical and the glass raw materials useful in the present invention generally have an average particle size of from about 0.5 to about 4.5 μm, preferably from about 1 to about 3 μm.

Preferably, the inorganic binder is a glass frit having a softening point between about 350 and 620 c so that the composition can be fired at the required temperature (typically 300 to 700 c, especially 580 to 680 c) to effect the proper sintering, wetting and bonding of the substrates, especially glass substrates. It is known that mixtures of high and low melting glass raw materials can be used to control the sintering characteristics of the conductive particles. In particular, it is believed that the high temperature glass frit dissolves in the lower temperature glass frit and together they slow the sintering rate of the conductive particles compared to a paste containing only low melting glass frit. This control of the sintering characteristics is particularly advantageous when printing and firing the composition on decorative enamels. (decorative enamels are generally pastes comprising one or more pigment oxides and opacifying agents dispersed in an organic medium and a glass frit). The high-melting glass raw material is considered to be a glass raw material having a softening point of 500 ℃ or higher, and the low-melting glass raw material is considered to be a glass raw material having a softening point of 500 ℃ or lower. The difference between the melting temperatures of the high and low melting glass raw materials should be at least 100 c, preferably at least 150 c. Three or more glass raw materials having different melting temperatures may also be used. When mixtures of high and low melting glass raw materials are used in the present invention, they are generally used in a weight ratio of from 4: 1 to 1: 4.

As used herein, the term "softening point" refers to the softening temperature obtained by the fine wire drawing method of ASTM C338-57.

Suitable binders include lead borate, lead silicate, lead borosilicate, cadmium borate, lead cadmium borosilicate, zinc borosilicate, sodium cadmium borosilicate, bismuth silicate, bismuth borosilicate, bismuth lead silicate, and bismuth lead borosilicate. Generally, any glass having a high bismuth oxide content is preferred, preferably at least 50% by weight bismuth oxide, more preferably at least 70% by weight bismuth oxide. Lead oxide may also be added as a separate phase (separatephase) if desired. However, lead-free adhesives are preferred due to environmental concerns. Examples of glass compositions (compositions a to I) are given in table 1; the oxide composition is given in weight percent.

TABLE 1 glass compositions

	A	B	C	D	E	F	G	H	I
										Bi2O3	75.1	82.7			78.1	94.8	73.3	73.7	69.82
PbO	10.9	1.83	43.6	0.7
										B2O3	1.2	1.34	4.8	26.7					8.38
SiO2	9.3	10.3	37.5	21.7	8.6	5.2	4.7	4.8	7.11
										CaO	2.4	2.68	9.7	4.0					0.53
BaO				0.9
										ZnO				27.6	3.9			5.0	12.03
CuO					7.6		5.5
										CoO					1.8
Al2O3	1.1	1.22	4.3	5.7					2.13
										Na2O				8.7
ZrO2				4.0
										GeO2							16.5	16.6

The glass binder is formulated by conventional glass-making techniques by mixing the required ingredients (or their precursors, e.g. for B) in the required proportions₂O₃H of (A) to (B)₃BO₃) And heating the mixture to form a melt. As is well known in the art, heating is carried out to a peak temperature for a period of time such that the melt becomes entirely liquid and the escape of gas has also ceased. The peak temperature is generally in the range 1100 ℃ to 1500 ℃, usually 1200 ℃ to 1400 ℃. The melt is then quenched by cooling the melt, typically by pouring onto a cold belt or cold running water. The particle size may then be reduced by grinding as needed.

Other transition metal oxides may also be used as part of the inorganic binder, as will be appreciated by those skilled in the art. In particular, when a substrate other than a glass substrate such as an alumina substrate is used, an oxide or an oxide precursor of zinc, cobalt, copper, nickel, manganese, and iron is generally used. These additives are known to improve weld adhesion.

The inorganic binder may also include a paste of about 4 parts pyrochlore-related oxide by weight having the general formula:

(M_XM’_2-X)M”₂O_7-Z

wherein the content of the first and second substances,

m is selected from at least one of Pb, Bi, Cd, Cu, Ir, Ag, Y and rare earth metals having an atomic number of 57-71 and mixtures thereof,

m' is selected from Pb, Bi and mixtures thereof,

m' is selected from the group consisting of Ru, Ir, Rh and mixtures thereof,

x is 0-0.5, and

Z＝0-1。

pyrochlore material has been disclosed in detail in U.S. patent No. 3,583,931, the disclosure of which is incorporated herein by reference. For the compositions of the present invention, the pyrochlore material acts as an adhesion promoter. Preferably copper bismuth ruthenate (Cu)_0.5Bi_1.5Ru₂O_6.75)。

Traditionally, conductive compositions have been based on lead glass raw materials. The removal of lead from glass compositions to meet current toxicity and environmental regulations may limit the types of adhesives that can be used to achieve the desired softening and flow characteristics, while meeting wetting ability, thermal expansion, cosmetic (cosmetic) and performance requirements. U.S. Pat. No. 5,378,406, incorporated herein by reference, describes a composition Bi₂O₃、Al₂O₃、SiO₂、CaO、ZnO、B₂O₃All of which can be used in the compositions described herein.

In the preferred embodiment of the present invention, the glass raw material is composition I in table 1 herein.

Ingredients (a) to (c) described hereinbefore are typically dispersed in a liquid carrier to form a semi-fluid paste capable of printing the desired circuit pattern. The liquid carrier may be an organic medium or may be water-based. The liquid carrier is preferably an organic medium. The liquid carrier should provide acceptable wettability of the solid and substrate, fairly stable dispersion of the particles in the paste, good printing properties, strength of the dried film sufficient to withstand rough handling, and good firing characteristics. Various organic liquids, stabilizers and/or other conventional additives, with or without thickeners, are suitable for use in formulating the compositions of the present invention. Examples of organic liquids that may be suitable are alcohols (including ethylene glycol), esters of such alcohols such as acetate, propionate and phthalate (e.g. dibutyl phthalate), terpenes such as pine oil, terpineol and the like, solutions of resins such as polymethacrylates of lower alcohol, or solutions of ethyl cellulose in solvents such as pine oil and titanyl ether of diethylene glycol. The carrier may also include a volatile liquid to promote rapid solidification after application to the substrate.

Preferred organic media are based on a combination of thickeners comprising ethyl cellulose in terpineol (generally in a ratio of 1 to 9), for example, with dibutyl phthalate or with a titanate ether of diethylene glycol (as butyl Carbitol)^TMAnd sold). Further preferred organic media are solvent mixtures based on ethylcellulose resin and alpha-, beta-, and gamma-terpineol (typically, 85-92% alpha-terpineol comprises 8-15% beta-and gamma-terpineol).

The ratio of liquid carrier to solid in the dispersion can vary considerably and is determined by the ultimate requirement for chemical consistency, which in turn is determined by the printing requirements of the system. Generally, to achieve good coverage, the dispersion will comprise from about 50 to about 95%, preferably from about 60 to 90%, by weight of solids, and from about 5 to about 50%, preferably from about 10 to 40%, by weight of liquid carrier, as described above.

The compositions described herein may additionally include other additives known in the art, such as colorants and colorants, rheology modifiers, bond enhancers, sintering inhibitors, green-state modifiers, surfactants, and the like.

In formulating the composition, the particulate organic solid is mixed with a liquid carrier and dispersed using suitable equipment, such as a three-roll mill or a power-mixer, to form a suspension, according to conventional techniques well known in the art. For example, at 25 ℃ on a Brookfield HBT viscometer using a #5 spindle motor, at 4 seconds^-1The resulting composition generally has a viscosity in the range of about 10 to about 500, preferably in the range of about 10 to about 200, and more preferably in the range of about 15 to about 100pa.s at the shear rate of (a). The general procedure for formulating the compositions described herein will be described below.

The paste was weighed in a container for all ingredients. These ingredients are then vigorously mixed by a mechanical mixer to form a homogeneous mixture; the mixture is then passed through a dispersing apparatus, such as a three-roll mill, to obtain well-dispersed particles to produce a paste composition of suitable consistency and rheology for application to a substrate (e.g., by screen printing). Hegman gauges were used to determine the state of dispersion of the particles in the paste. The instrument included a channel in a steel block that was 25 μm deep (1mil) at one end and tilted to zero depth at the other end. A blade is used to pull down the batter along the length of the channel. The diameter of the agglomerate where the scratch occurs in the channel is greater than the channel depth. A satisfactory dispersion will give a fourth scratch point of typically 10-18 μm. Typically, the point at which half of the channels are not covered by well-dispersed paste is between 3 and 8 μm. A fourth scratch measurement > 20 μm and a "half-channel" measurement > 10 μm indicate a poorly dispersed suspension.

The composition is then applied to the substrate using conventional techniques known in the art (typically by screen printing) to a wet thickness of about 20-60 μm, preferably about 35-50 μm. The composition may be printed onto the substrate in a conventional manner by using an automatic printer or a manual printer. Preferably, an automatic screen printing technique is used that applies 200 to 300 mesh per inch of screen. The printed pattern is optionally baked at a temperature below 200 c, preferably at about 150 c, for a time period of about 20 seconds to about 15 seconds prior to firing. Firing to achieve sintering of both the inorganic binder and the finely divided particles is preferably accomplished in a well-ventilated conveyor furnace having a temperature profile of about 200-1000 c that allows for the carrier to be burned off, followed by a maximum temperature period of about 500-1000 c for about 30 seconds to about 15 minutes, preferably about 600-850 c. Followed by a cooling period that may be optionally controlled to prevent over-sintering, undesirable chemical reactions at intermediate temperatures, or substrate chipping due to excessive cooling. Alumina substrates are particularly susceptible to chipping caused by excessive cooling. The entire firing process preferably continues for a period of about 2 to 60 minutes, with about 1 to 25 minutes to reach the firing temperature, about 10 seconds to 10 minutes at the firing temperature, and about 5 seconds to about 25 minutes of cooling. For the manufacture of tough glass substrates, a controlled cooling cycle is generally used, wherein the entire firing process generally continues for a period of about 2 to 5 minutes, about 1 to 4 minutes to reach the firing temperature, followed by rapid cooling.

After firing, a thick film is formed with a typical thickness of from about 3 μm to about 40 μm, preferably from about 8 μm to about 20 μm.

The compositions described herein are primarily intended for use in the manufacture of heating elements in windows such as defogging or defrosting elements in automotive windows, particularly in the rear windows of automobiles. Composites may also be used to incorporate other conductive functions into the window, such as printed antennas. However, the coating composition may be used in various other applications, typically including printed circuits and heating elements. For example, the composition may be used as a substrate in a hot water heater. There is a general need in the electronics and electrical industries for low cost heating elements, particularly screen printable heating elements.

The compositions described herein were evaluated using the following test procedures.

Test procedure

Binder

Copper studs (available from Quality Product gen. eng. (Wickwar), UK) were soldered to the fired conductive pattern (dimensions 10.2cm x 5.1cm x 3mm) on the glass substrate using 70/27/3 Pb/Sn/Ag alloy solder at a soldering iron temperature of 350 to 380 ℃. A small amount of mild reactive rosin flux, such as ALPHA Metals Limited, Croydon, u.k., may be used to increase the moisture content of the solder and hold the solder and terminals in place during assembly of the part, in which case the flux is applied to the solder using a tray containing a thin film of fresh flux. The degree of adhesion was measured on a Chattillon  model USTM pull-out tester at a pull-out rate of 0.75 ± 0.1 inches per minute (1.91 ± 0.25cm per minute) and the pull-out strength recorded at the bond failure. The average of the bond failures for the 8 samples was determined. Preferably, the degree of adhesion is greater than 10 kg, more preferably greater than 15 kg, and most preferably greater than 20 kg. The primary failure modes of the bond are as follows:

(a) the posts are separated from the conductive pattern (i.e., poor substrate adhesion).

(b) The conductive pattern is separated from the substrate (i.e., poor substrate adhesion).

(c) Glass pull-out/fracture (i.e., the bond strength between the stud and the conductive layer and between the conductive layer and the substrate is greater than the strength of the substrate).

(d) Failure in the weld.

Resistance and resistivity

The resistance of the fired conductive pattern on the glass substrate was measured using an RLC bridge of the GenRad1657 type calibrated for between 1 and 900 omega or equivalent. The thickness of the conductive layer is measured using a thickness measuring device such as a surf-analyzer (e.g., TALYSURF, a contact measuring device that analyzes the surface of the substrate in 2 dimensions using a spring-loaded stylus, any change in height that deflects the stylus and then records this change on a recorder such as a chart recorder, the difference between the baseline and average height giving the print thickness). The resistance of the pattern is determined by placing the probe tip at the point where the conductive trace meets the pad. The bulk resistivity (normalized thickness) of the layer is determined by dividing the measured pattern resistance by the square number therein (where the square number is the length of the conductive trace divided by the width of the conductive trace). The resistance value in terms of normalized thickness (here 10 μm) was obtained as m Ω/□, here expressed in units of μ Ω cm.

Particle size

The particle size in the composition was measured according to ASTM D1201-79 using a large Hegman type fine grid gauge.

Chemical resistance

A 1% very cold solution of acetic acid in deionized water was used in this test. A glass substrate (50 x 100mm) with a conductive pattern fired on it was inserted into a plastic container half-filled with the test solution. The container was then sealed and left at ambient temperature. The test substrates were removed after 96, 168 and 336 hours, dried and then analyzed by lift test. The lifting test involved holding a 0.75 inch (1.91mm) wide shielding tape (Niceday)^TM) Applied to the substrate and then rapidly removed in about 1/2 seconds. The results of the lift-off test are given as an approximate percentage of the area of film removed by the tape.

The invention will now be described with reference to the following examples. It will be understood that the examples are not intended to be limiting and that modifications in detail can be made without departing from the scope of the invention.

Examples of the present invention

Examples 1 to 13

The conductive pattern is formulated using the methods described herein above. The aluminum particles used were atomized aluminum particles (Cotronic Corporation). The silver particles are 50% spherical silver particles (0.80-1.40 m)²g^-1Surface area) and 50% of small flake silver particles (0.60-0.90 m)²g^-1Surface area of). Glass usedGlass is composition I in table 1 herein. The liquid carrier is ethyl cellulose (generally in a ratio of 1 to 9) in terpineol with a titanyl ether of diethylene glycol as butyl Carbitol^TMAnd sold). The substrate is a float glass (untempered) substrate. The fired film thickness was 8 to 20 μm. Unless otherwise noted, all parts were fired through a conveyor oven with a peak firing temperature of 660 ℃, at which the samples spent about 72 seconds. The total door-to-door transition time in the furnace is about 21 minutes.

The resistivity and weld adhesion of the pattern were measured as a function of composition according to the procedure described above, and the results are given in table 2 below.

Table 2-bond strength (W) and resistivity (p) as a function of composition

Examples of the invention	Solid silver%	Solid glass%	Solid aluminum%	ρ/μΩcm	W/kg
						1	87.56	4.90	7.54	18.50	＞20
2	85.97	4.93	9.10	36.00	＞20
						3	84.35	4.95	10.70	90.00	＞20
4	82.54	4.76	12.70	22.18	18.81
						5	80.95	4.76	14.29	38.69	15.88
6	79.85	6.06	14.09	33.98	18.43
						7	78.77	7.34	13.90	32.62	＞20
8	77.72	8.57	13.72	38.86	18.09
						9	76.69	9.78	13.53	30.35	18.33
10	75.69	10.95	13.36	36.29	18.78
						11	72.86	14.28	12.86	43.35	17.93
12	71.08	16.38	12.55	39.89	17.25
						13	69.39	18.37	12.24	42.51	16.30

The data show that compositions comprising aluminum allow for the formulation of conductive patterns that increase resistivity while maintaining solder adhesion.

Claims (15)

1. Use of a composition comprising (a) finely divided particles of an electrically conductive material; (b) finely divided particles of one or more inorganic binders; and (c) finely divided particles of aluminum, wherein the components (a), (b) and (c) are dispersed in a liquid carrier in order to increase the resistivity of the conductive pattern on the production substrate.

2. A method of increasing the resistivity of a conductive pattern comprising using a composition comprising (a) finely divided particles of a conductive material, (b) finely divided particles of one or more inorganic binders, and (c) finely divided particles of aluminum in the manufacture of the conductive pattern, wherein the components (a), (b), and (c) are dispersed in a liquid carrier.

3. Use or method according to claim 1 or 2, wherein the liquid carrier is an organic medium.

4. Use or method according to claim 1 or 2, wherein component (c) comprises metallic aluminium particles.

5. The use or method according to claim 1 or 2, wherein component (c) comprises particles of an aluminium-containing alloy.

6. The use or method according to claim 1 or 2, wherein the conductive particles are silver particles.

7. Use or a method as claimed in claim 1 or claim 2 wherein substantially all particles are in the range 0.01 to 20 μm.

8. The use or method as claimed in claim 1 or claim 2, wherein the total amount of components (a), (b) and (c) is from about 50 to about 95% by weight of the composition.

9. The use or method as claimed in claim 1 or claim 2 wherein component (a) is present in an amount of from about 50 to about 98% by weight of the total solids present in the composition.

10. The use or method as claimed in claim 1 or claim 2 wherein component (b) is present in an amount of from about 2 to about 15% by weight of the total solids present in the composition.

11. The use or method as claimed in claim 1 or claim 2 wherein component (c) is present in an amount of from about 2 to about 15% by weight of the total solids present in the composition.

12. The use or method according to any preceding claim wherein the manufacture of the conductive pattern comprises applying to a substrate a composition comprising (a) finely divided particles of a conductive material, (b) finely divided particles of one or more inorganic binders and (c) finely divided particles of aluminium, the components (a), (b) and (c) being dispersed in a liquid carrier, and firing the coated substrate to effect sintering of the finely divided particles to the substrate.

13. The use or method according to claim 12, wherein said manufacturing comprises a screen printing brush process.

14. The use of the finely divided particles of aluminum in the composition further includes (a) finely divided particles of a conductive material and (b) finely divided particles of one or more inorganic binders dispersed in a liquid carrier in order to increase the resistivity of the conductive pattern produced from the composition.

15. A method for increasing the resistivity of a conductive pattern manufactured from the composition comprising (a) finely divided particles of a conductive material and (b) finely divided particles of one or more inorganic binders dispersed in a liquid carrier, the method comprising incorporating (c) finely divided particles of aluminum into the composition.