NZ209104A

NZ209104A - Thick-film circuit with a substrate containing molybdenum

Info

Publication number: NZ209104A
Application number: NZ209104A
Authority: NZ
Inventors: P M Turnbridge; A J Stanley
Original assignee: Plessey Overseas
Priority date: 1983-08-06
Filing date: 1984-08-03
Publication date: 1987-09-30
Also published as: AU3073484A; GB8321282D0; GB2144922A; GB2144922B

Description

New Zealand Paient Spedficaiion for Paient Number £09104 209104 Priority D8te(s): C. *• e3 Complete Specification Filed: Class: Publication Date P.O. Journal, No: 3.0 SEP 1987 * •»•••••»•••••••••••••• / 3oo DRAWIH6S N.2. PATENT -3 AUG 1984 RECEIVED Patents Form Do. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "STRUCTURE FOR THICK FILM ELECTRICAL CIRCUITS" 9 f7 WE PLESSEY OVERSEAS LIMITED, a British Company of Vicarage Lane, Ilford, Essex, England, hereby declare the invention, for which-fr/we pray that a patent may be granted to m»/uB, and the method by which it is to be performed, to be particularly described in and by the following statement:- . ' lunemi by P»e«1M 2 09 f 04 -1*, STRUCTURE FOR THICK FILM ELECTRICAL CIRCUITS This invention relates to a thick film circuit structure and to a method of manufacturing such a structure.

The use of thick film technology to provide high density interconnections and passive circuit functions is well established in electronics. As normal practice, alumina substrates are used. Porcelain enamelled steel is also used as a substrate. The thermal conductivity of such substrates is not high so problems with heat dissipation can arise particularly in the case of high power devices.

The present invention seeks to enable the manufacture of thick film circuits having improved heat dissipation capabilities.

According to one aspect of the invention there is provided a thick film circuit in which the substrate comprises molybdenum.

The substrate nay be pure molybdenum or a molybdenum based alloy.

The substrate nay be provided with a coating on to which there is deposited one or more layers of dielectric material which is adhesively compatible therewith.

The invention also includes the method of fabricating a thick film circuit including the steps of starting with a molybdenum based substrate and providing thereon a coating which permits adhesion of one or more dielectric layers. 2- 209104 The invention enables high density electronic circuitry to be built up which has a very high power dissipation capability.

The invention may also provide a substrate whose surface is compatible with existing thick film inks developed for use on alumina with firing temperatures around 850°C. With the thick film inks, printed and fired multilayer interconnection patterns with resistors can be fabricated• In order that the invention and its various other preferred features may be understood more easily, embodiments thereof, and their method of construction, will now be described The substrate consists of a Molybdenum or Molybdenum-based alloy, normally in the form of a flat sheet.

A barrier coating may be produced on the sheet to provide adhesion for subsequently applied dielectric layers. This barrier layer, in conjunction with the dielectric layers, serves to control the oxydation of the metal at high temperatures and provide an insulated surface on which an interconnection pattern can be printed.

A barrier coating, of for example Molybdenum-di-silicide, is applied by sintering the components in silicon powder at 700*C in a hydrogen atmosphere* or by electrolysis using a silicon anode and a molten alkali fluoride electrolyte at 700°C. A dielectric layer, comprising for 209104 example a ceramic loaded or devitrified glass, is applied to the initial barrier layer by screen printing. The dielectric may also be applied by electro-phoresis or powdered glass from an acqueous or organic medium, or the layer may be applied by flame spraying of such a powder.

Where the layer has been applied by thick film printing or electro-phoresis, a high temperature process at a temperature in the region of 600-1000°C is required to provide for the glass to fuse together to provide a dense film with adhesion to the barrier layer.

Subsequent to the deposition of the dielectric layer, a thick film interconnection layer is built up. This is achieved by printing such combinations of thick film conductor, dielectric and resistor pastes as are necessary to achieve the desired interconnection pattern, together with any printed resistors. The process uses proprietary pastes designed for firing in the temperature range 600-1000°C.

If nitrogen fireable thick film conductors and dielectric are used then a barrier layer is not required. The base dielectric layer may be air fired at around 500°C to burn out the organic vehicle before firing in nitrogen at the higher temperatures.

Following the production of the interconnection pattern, semiconductor and passive chip components may be soldered in place using standard techniques. Bare form 209104 components may then be wire bonded to provide electrical contact. Where bare components are used, they may be given hermetic protection by either glassing a wall on to the thick film structure, and then soldering or welding a flat lid in place, or by applying a shaped lid using solder or epoxy• The electronic circuit thus produced may then be incorporated in an equipment in a manner similar to a printed circuit board.

Molybdenum coated with a proprietary barrier layer has been coated with a proprietary thick film dielectric layer and fired at 900°C.

The technology is being developed to provide a means of manufacturing high power dissipation hybrid thick film circuits where alumina and coated steel substrates are inadequate. A Molybdenum-based substrate meets this criteria in that it has a thermal conductivity six times that of alumina.

The substrate may be any shape or size and carry boles enabling it to be bolted firmly to a heatsink. The heatsink may be the wall of an equipment or a heat pipe. Semiconductor and passive chip components may be soldered to the thick film interconnection pattern. Bare components can be protected by the addition of a soldered or glassed on wall/lid structure.

Claims

WHAT WE CLAIM IS:

1. A thick-film circuit including a substrate comprising molybdenum.

2. A thick-film circuit as claimed in claim 1# in which the substrate is composed of pure molybdenum or a molybdenum based alloy.

3. A thick-film circuit as claimed in claim 1 or claim 2, in which the substrate is provided with a coating on to which there is deposited one or more layers of dielectric material which is adhesively compatible therewith.

4. A method of fabricating a thick-film circuit, the method including providing a coating on a molybdenum based substrate, which coating permits adhesion of one or more dielectric layers.

5. A -thick-film circuit as claimed in claim 1 t substantially as hereinbefore described.

6. A method of fabricating a thick-film circuit as claimed in claim 4 substantially as hereinbefore described. v

7. Electrical equipment embodying a thick-film circuit as claimed in any one of claims 1 to 3, and 5, in which the substrate forms at least part of the equipment housing or er mechanical structure. BALDWIN, JSON & CAREY A ~ // ATTORNEYS FOB THE APPLICANTS 'A- . ■£ vy