JPH0334676B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention is an electrical circuit comprising an electrical conductor formed on the surface of a substrate, the substrate comprising an alloy that forms a ceramic layer on the exposed surface of the substrate when heated in an oxidizing atmosphere. It concerns electrical circuits. Prior Art Most hybrid circuits use alumina substrates on which various pastes are printed and fired to form conductors, resistors, and insulating layers. However, the disadvantage of alumina substrates is that they are brittle and difficult to handle in sizes larger than approximately 10 cm (4 inches) square. There is a trend in printed circuit board technology to use "leadless" circuit elements. This is because these devices can be soldered directly to the conductor tracks and there is no need to drill holes in the substrate into which the device leads are inserted. The cost of drilling printed circuit boards represents a significant portion of the total cost of assembly. on August 25, 1981
The publication “Leadless carriers, components increase” published by Electronics International
board density by 6:1”, published by PRJones, discloses such a technique. - glass substrates, which have a coefficient of thermal expansion significantly different from that of the elements to be soldered to the printed track. states that the difference in rate had no effect on circuit reliability. However, other literature suggests that this effect can result in fracture seams between elements and conductors when the board is subjected to temperature cycling. It has been proposed to use various metal alloys as substrates for printed circuit boards, such as Invar or Alloy 42 nickel-iron materials. These materials have a coefficient of thermal expansion comparable to that of leadless devices. Such a substrate has a coefficient of thermal expansion of
January 13, 1981 edition of the literature published in Electronics International (pages 48, 53, 54) and
February 10, 1981 edition (pages 44-46) and January 1981
Disclosed in the 16th edition (page 46). In all of these cases, a metal alloy is used as the baking material to provide structural strength and is bonded to the epoxy glass on which the circuit is printed. OBJECTS AND SUMMARY OF THE INVENTION An object of the present invention is to provide a hybrid electric circuit board that is less brittle than conventionally used alumina substrates. Another object of the invention is that the invention is suitable for mounting leadless elements;
and does not require adhesion of an epoxy layer to the core.
An object of the present invention is to provide an electric circuit board in the form of a printed circuit board. The present invention is an electric circuit as described at the beginning, comprising:
An electric circuit is provided, wherein the alloy is an M-Cr-Al-Y alloy, and M is iron (Fe), nickel (Ni), or cobalt (Co). Addition of the elements yttrium or silicon to the alloy promotes the formation of an alumina layer and its adhesion to the alloy surface. Instead of forming the substrate entirely of an alloy, the alloy can be provided as a surface coating on substrates of different compositions. Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a substrate 1 on which thick film circuits 2, 3, 4 are formed. The substrate 1 is formed from an alloy having the property of forming a stable and highly adhesive ceramic layer on its exposed surfaces under oxidized conditions. A suitable alloy is an iron-chromium-aluminum alloy with an ittrinum additive. This alloy has the trademark FECRALLOY (United Kingdom
Such as the series offered by Resistalloy Limited under the trademark Atomic Energy Authority. The special alloy of this series (FECRALLOY A) is
Typical examples include carbon 0.03%, silicon 0.3%,
It has a composition of 15.8% chromium, 4.8% aluminum, 0.3% yttrium, and the rest iron. When the substrate 1 is formed from FECRALLOY A, the alloy is first heated to a temperature of 1100° C. in an oxidizing atmosphere. An alumina layer is then formed on the exposed surface of the substrate. The cross-sectional view in FIG. 2 shows the layers 5 and 6 of alumina formed on the surface of the substrate 1. When the substrate is used at temperatures below 1100°C, such as when used as a substrate for electrical circuits, it is recommended to hold the substrate at a temperature of 1100°C for 2 hours. Once the alumina layer 5 is formed on the substrate 1, a dielectric paste is printed on the layer 5 by a conventional screen printing process and baked at a temperature of 850° C. for 10 minutes. The total process time, including increasing the temperature to 850° C. and cooling the assembly to ambient temperature after a 10 minute baking time, is approximately 60 minutes. This step can be repeated to form a platform 9 of desired thickness. The conductor tracks 10, 11 and the resistor 7 are then printed and fired in a known manner. Many manufacturers offer dielectric, resistor, conductive pastes and inks for hybrid circuit fabrication that can be used sequentially with this substrate. An example is Pennsauken, New Jersey, 08110, United States.
Electro− located at 2211 Shearman Avenue
A system provided by Science Laboratories Inc., in which the dielectric is type 4903, the resistor is 3900 series, and the conductor is palladium-silver 9669. Next, solder is screen printed on the circuit, the circuit element 8 is attached, and it is melted and soldered to the conductor. Vapor phase soldering techniques are suitable for performing this operation. The FECRALLOY substrate can then be formed into a desired shape to form the structure of the device of which the circuitry forms a part. In this case, care must be taken not to deform the substrate in the region of the dielectric platform 9. For example, this substrate 1 can be formed as the base of a telephone device, which is fitted with a plastic cover. This makes it possible to reduce or eliminate the use of separately manufactured circuit boards. Alternatively, the substrate can be formed into a desired shape before printing and firing the dielectric, conductor, or resistive paste. FECRALLOY A is for high temperature (1000℃~1200℃)
It has important properties for use as a substrate for electrical circuits, forming a ceramic (alumina) coating that is stable, inert, and has strong adhesive strength when exposed to very small amounts of oxygen at temperatures (°C). There is. The processing conditions necessary to form this coating are not particularly stringent. Once formed, this coating can withstand repeated exposure to high temperatures (1000° C.) for thousands of hours in oxidizing and reducing atmospheres without degrading the properties of the alumina coating. Therefore, it is possible to support multilayer thick film structures, which are required for multiple firings at temperatures up to 950°C.
The final properties of these films are not significantly different from the high quality these films exhibit when the same films are conventionally deposited on ceramic substrates. That is,
The conductor can be multilayered, easily soldered with ordinary solder, and the conductor can be wire-bonded. Resistance is 10Ω/sq~
It can be manufactured with a sheet resistance in the range of 1MΩ/sq, can be formed between different conductor layers in the insulation layer, and has a withstand voltage of 400 volts.
A resistance of 10 ¹³ Ω can be obtained. This alumina coating can be applied to a uniform thickness over the entire surface of the previously specially shaped sheet. This is particularly important for providing good coverage of very difficult geometries, such as small holes, corners, edges, etc., which are almost impossible to achieve with other methods and materials. A large number of molded FECRALLOY
It can be formed on a single sheet of FECRALLOY, reducing the cost of processing each substrate. Each of these substrates can be separated from the array using common metal fabrication techniques, such as blanking, cutting, sawing, etc., without damaging the thick films on the substrate.
Avoid exposed oxidized edges resulting from cutting operations to prevent rust formation at these edges.
It is desirable to seal the edges of the FECRALLOY. FECRALLOY coated with multilayer thick film,
Especially when irregularly shaped substrates are required,
Alternatively, it can be used in place of alumina (Al ₂ O ₃ ) as a substrate for thick film circuits when alumina is expensive or a large-sized substrate that is easily damaged is required. The FECRALLOY may also be substituted for conventional printed circuit board materials to produce printed circuit boards having multilayer interconnect tracks for interconnecting circuit elements disposed on the surface. In this case, the important properties of FECRALLOY are: Its coefficient of thermal expansion (11×10 ^-6 ℃)
is closer to the thermal expansion coefficient of the circuit elements attached to the surface than that of the epoxy board (50 x 10 ^-6 °C). Therefore,
When using FECRALLOY as the substrate material, the assembled devices and plates can withstand temperature cycling over a wide temperature range, and more cycling between extreme temperatures. When manufacturing printed circuit boards,
The dielectric paste can be printed across one or more surfaces of the substrate. FECRALLOY, which is a particularly commercially available alloy
Although A was chosen as the most suitable for use at the time of this application, other alloys may be used which have the property of forming a ceramic layer on exposed surfaces when heated in an oxidizing atmosphere. Of course, when choosing an alloy to form the substrate, ensure that it can withstand the required processing temperatures, e.g. up to about 1000°C for firing dielectrics, conductors and resistive pastes, and It is necessary to take into account the correspondence between the coefficient of expansion and the coefficient of thermal expansion of the elements provided on the substrate. The table below shows the composition of the alloy according to the invention and the three known examples. A known example is as follows. Publicly known example - (1) - Japanese Patent Publication No. 17879/1982 Publicly known example - (2) - Japanese Patent Publication No. 61-49831 Publicly known example - (3) - Japanese Patent Publication No. 61-49827

[Table] The numbers in the above table indicate weight %. The present invention uses yttrium (Y) and silicon (Si).
is used as the composition in the alloy. These elements have the technical effect of significantly improving the adhesion of the formed alumina layer to the alloy surface. This alumina layer is formed when the alloy is heated in an oxidizing atmosphere. A range of iron-chromium-aluminum alloys that react with oxygen to form a protective layer of relatively pure alumina are described in the Ninth Edition of the Metals published by the American Society for Metals.
Disclosed in Volume 3 of the Handbook. Their use as electrical resistance alloys is discussed in 647 of this publication.
Discussed on the page. The composition of such alloys is listed in Table 5 and contains approximately 94.5%
An oxide is formed on these alloys consisting of Al ₂ O ₃ , 3.5% Cr ₂ O ₃ , and 2% Fe ₂ O ₃ at about 1200°C, and this dielectric film has an extremely high dielectric strength. Are listed. Iron-chromium-aluminum-yttrium alloys are disclosed in US Pat. GB Patent Application No. 2019886A discloses an iron-chromium-aluminum-yttrium alloy with certain additional properties of silicon.
FECRALLOY A has a proportion of iron, chromium, aluminium, yttrium and silicon within the range disclosed in the UK patent application. GB 1261261 describes an alloy consisting of 20-50% chromium, 10-20% aluminum, 0.03-2% yttrium or rare earth elements and the balance iron, a superalloy based on nickel and cobalt. Discloses a method of coating. Therefore,
The yttrium component of the alloy can be replaced with a rare earth metal. GB 1261261 and GB 1439628 further disclose methods for coating nickel and cobalt based superalloys. British Patent No. 1439628 specifies
The use of coating alloys consisting of chromium, aluminum, yttrium and nickel, cobalt or iron is discussed. All these alloys or coatings produce ceramic layers on their exposed surfaces under oxidizing conditions and can withstand the temperatures at which conventional pastes or inks are fired to form hybrid circuits.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a substrate of the present invention on which a plurality of thick film circuits are formed, and FIG. 2 is a partial cross-sectional view taken along line AA in FIG. 1. 1...Substrate, 2, 3, 4...Membrane circuit, 5, 6...
...Aluminum layer, 7...Resistance, 8...Element, 9
...Plateau, 10, 11...Conductor track.

Claims (1)

Claims: 1. An electrical circuit comprising an electrical conductor formed on the surface of a substrate, the substrate comprising an alloy that forms a ceramic layer on the exposed surface of the substrate when heated in an oxidizing atmosphere. An electric circuit characterized in that the alloy is an M-Cr-Al-Y alloy, and M is iron (Fe), nickel (Ni), or cobalt (Co). 2. Claim 1, characterized in that yttrium (Y) in the alloy is replaced with a rare earth element.
The electrical circuit described in section. 3. The electric circuit according to claim 1, wherein the alloy further contains silicon (Si). 4. An electrical circuit according to any one of claims 1 to 3, characterized in that the alloy forms a surface coating on a substrate of a different composition.