GB2206451A - Substrates for circuit panels - Google Patents

Abstract

Printed or hybrid circuits are formed on a metal substrate having an anodised surface layer, the substrate serving as a heat sink and the anodised layer forming a black body radiator. The circuit may be preformed or may be deposited directly on to the anodised layer. The substrate may be made of aluminium, titanium, tantalum, zirconium or magnesium.

Description

Circuit Panels This invention relates to anodisable metal substrates for use in circuit panels and particularly, but not exclusively to substrates for anodised hybrid circuit panels wherein an anodic film produced on the surface of the substrate serves as a heat sink, the circuit being deposited directly upon the anodised layer.

Known printed circuit board assemblies have a resin or plastic base that gets hot. To overcome this problem, it is known to interleave the printed circuit boards with metal panels, usually aluminium panels, to remove the heat. Alternatively the aluminium itself is used as the base the circuit being produced by electrodeposition onto the aluminium, so that the circuit and heat sink are one structure, an anodic layer on the metal providing the insulation previously supplied by a phenolic or epoxy board.

The object of the invention is to provide an inexpensive substrate which has good heat emissivity characteristics and superior properities to known substrates and whose properties can be modified according to the requirements of the particular circuit to be produced, using a method suitable for the required circuit.

A further object of this invention is to provide a drilled and anodised panel onto which adheres a preformed metal circuit. The preforming being achieved by punching, chemical milling or electroforming.

The important characteristics of a substrate are the mechanical and thermal properties. Good mechanical strength is required along with high thermal conductivity/emissivity1 for heat dissipation, and a low coefficient of thermal expansion for stability. The majority of thick film circuit manufacturers use an alumina substrate of 90-99% pure alumina. Aluminium loses heat by convection and conduction. However, under the same conditions anodised aluminium loses considerably more heat than bare aluminium because anodised aluminium also loses heat by radiation and can approach the theoretical ideal of a perfect radiator, for example: Heat loss Polished Aluminium 4.3-6.4% Anodised Aluminium 38-92% Radiation increases with the film thickness, thus the heat loss properties of the substrate can be changed to meet the required characteristics.

As anodised aluminium is a black body radiator it minimises the problem of the differing coefficient of linear expansion for aluminium and the anodic film.

The use of alumina as a substrate is also limited with respect size because of the problem of maintaining surface flatness over large areas without resorting to expensive treatments such as grinding and lapping.

The following is a summary of characteristics of the commonly used substrates and anodised aluminium details of which are shown in the attached table: Substrate Characteristics Alumina Limited by size, breakable Porcelanizied steel Expansion problems, heavy.

Ion migration from porcelain into circuit material.

Keralloy As above Beryllia Good heat characteristics, very expensive, poisonous Anodised Aluminium Heat loss characteristics better than alumina. Cheap. No limit to size.

Anodising is carried out by making the metal which is to form the substrate the anode in an electrolytic cell and passing an electric current until the desired film thickness is obtained. To obtain the required physical properties in the anodic film the parameters of the anodising electrolyte have to be controlled. These are composition, temperature, agitation, current density, current waveform and time.

The three commonly used electrolytes for producing anodic films are sulphuric acid, chromic acid and oxalic acid. They produce anodic films differing in appearance and physical properties. Many other electrolytes have been used, some commercially. Examples are sulphamic acid, phosphoric acid, saturated dicarboxylic acids (Glutaric, malonic, oxalic) unsaturated dicarboxylic acids (maleic), alpha hydroxy carboxylic acids (citric), sulpho derivatives such as sulpho-salicylic acid and hydroxy aromatic acids ie, protocatechuic acid.

The anodising process chosen will have an effect on the properties of the anodic film such as hardness, porosity, heat loss, di-electric constant and breakdown voltage. In general, the anodic film consists of an amorphous barrier layer in contact with the aluminium.

Onto this will grow a more structured layer. In the case of the sulphuric acid process the structure layer consists of a hexagonal columnar growth perpendicular to the barrier layer. It has a degree of porosity that allows dyeing during the product of a circuit to take place.

Other electrolytes produce varying degrees of porosity.

The choice of electrolyte has an effect on such properties as the ratio of barrier layer to columnar growth, hardness, porosity, dielectric constant, breakdown voltage, and heat emissivity etc.

The barrier layer prevents access of aggressive ions to the aluminium metal that is underneath; it also provides electrical insulation. The film can be 'tailored' to have a sufficient thickness of barrier layer to provide the required insulation value and an adequate amount of columnar growth to create adhesion of the circuit to the substrate.

The properties of the substrate can, therefore, be controlled to suit the product requirements.

In one embodiment of the present invention, the substrate is anodised using the chromic acid anodising process. The process gives thin flexible coatings of about 2.5 microns. The coating is compact and flexible and is useful where the requirement for heat emission is limited.

In another embodiment of the invention, the anodic film is produced using -the sulphuric acid process. This produces anodic films of thickness in the range of 1-100 microns and the ratio of outward growth to barrier layer can be altered by altering the operating conditions.

However, it can never be a 100% barrier layer. Coatings from the sulphuric acid process can be used for replacing ordinary printed circuit laminates or where a great amount of heat has to be dissipated. For a printed circuit panel replacement a 25 micron coating from the sulphuric acid process would suffice. For losing a lot of heat a 50-80 micron coating would be used. Thicker coatings generally give higher values for breakdown voltages and di-electric constants. However, the sealing method will influence these values.

In another embodiment of the invention, the anodic film is produced by a sulphuric acid bath of strength 10% vol/vol which contains aluminium, copper, and trace impurities and the anodising process is carried out at a temperature of 70'F and a current density of 10 amperes/sq. ft. Thicker coatings are produced, according to the invention, by using the same bath at a temperature of 0'C and with a higher current density. As the film is produced anodically it is also being dissolved chemically; lowering the temperature reduces the chemical dissolution so that more film is produced for the same amount of current. At the 2.50 micron range the chromic acid film is superior to the sulphuric.

In another embodiment, the oxalic acid process is used to produce the film as an alternative to the sulphuric acid process.

In another embodiment of the invention boric acid/borate solutions are used to produce the barrier layer coatings. This boric acid process is an "all barrier" process; the coatings are very thin.

In an other embodiment of the invention alkaline solutions, such as sodium carbonate or potassium pyrophosphate, are used to produce the barrier layer coatings.

In another embodiment of the invention a dense oxide film such as that produced by anodising, is produced on the substrate, by using a gas plasma in an atmosphere of oxygen or by using high pressure steam on the heat substrate.

For most ' requirements an anodised aluminium substrate will be preferred, but any other anodisable metal such as titanium, tantalum, zirconium or magnesium may be employed for some applications.

After anodising the substrate a circuit is then produced on this anodised substrate.

Thick film hybrid circuits are manufactured by screen printing conductor, resistor and protective materials in the form of special inks and glasses onto the anodised aluminium substrate. Each of the inks is then subjected to a firing process at a controlled temperature.

During the manufacture the resistors are trimmed to specific values, components such as integrated circuits are attached to the conductor material by microsoldering techniques. Leads are fitted to the terminal pads and the whole assembly is encapsulated in resin for protection.

The thin film circuits required by some electronic devices can be produced on the anodised substrate, the vacuum deposition techniques providing a variety of methods for achieving this. Sputtering is an example of a vacuum technique in which in vacuo metals are deposited onto the anodised aluminium. Patterns are produced by sputtering through a mask or sputtering all over the substrate and then etching defined areas by laser or ion gun.

Other vacuum techniques include magnetron Sputtering which is faster than sputtering and gives thicker coats, Vapour Deposition in which the metal is vaporised in a low vacuum to coat the substrate, Ion Plating which is a combination of vapour deposition and sputtering, Chemical Vapour Deposition which employs two reactive gases and a carrier gas to produce a non-volatile material which deposits onto the substrate, Metal Organic Chemical Vapour Deposition in which metals are made volatile by adding alkyl groups to them, Plasma deposition where an involatile material is produced by gas phase reactions in a glow discharge, the involatile material then depositing on the heated substrate.In Wet and Dry Combination processes either the anodised substrate is coated with resin, adhesive or lacquer and then plated with electroless nickel or copper, the circuit pattern is then being put down and any unwanted material etched away, or the circuit pattern is printed onto the anodised substrate with a special ink that initiates electroless plating. The processes initiated in the dry mode may be continued by electro/chemical plating. The circuit is resin coated with through connections and may be repeated to form a layered circuit. These processes also apply to circuits started by electroplating or electroless plating.

Properties of Anodised Aluminium compared with other materials.

Property Anodised Sapphire Sintered Sintered Keralloy aluminium (A1203) Alumina Beryllia (Al2 Os (A1203 Density 2.5-3.0 9 3.98 Z 3.6-3.82 2.85 z 7.22 Purity % 99 - 100 100 z 92-99.5 99.5 Linear 5g 4.5-5.3 6.0 6.5 11.0 expel x 10 6 in/ cbare x- 10 sinlo, 3 Electrical 132 z 1.33ohm resistivity 4101s 10 ohm/c#2/cm my2 /metre Surface finish full choice 0.6 .1 t Flexural - - - strength ASTM.F417.78 Volume Resistivity ASTM1829-66 8109 (3000 c) ohm/cm Dielectric (PERMITTIVITY constant ASTM D 150.74 # 11.8-15.4 8 9.5@25 c Dielectric strength < 3 ASTM D150-74 9500v-10 680v/ 9 lKV.D.C.

(.025" ) 0.001 Loss tangent ss004.0010 (lhz-100MHz) Size ANY 4" x 4" Thickness ANY .0015-.0030" Cost Impact resistance Good poor poor Good Thermal ~ I conductivity 810f.05 X .07 l .52 n.52 Cal.Cm-i.Sec-i2610f.15 .06 .40 deg-lc EMISSIVITY 10 coating =80% and increases with anodic thickness.

Approaches black body condition.

Other values: polished 10 aluminium 4 1 #film 30 3y #film 70 lop film 80 Perfect Black body = 100

Claims (19)

1. Claims: 1. A circuit structure including a metal substrate having an anodic surface layer such that the substrate and heat sink are one structure onto which the circuit is applied wherein the substrate has predetermined characteristics and properties.
2. 2. A circuit structure according to claim 1, wherein the substrate is an anodisable metal.
3. 3. A circuit structure according to claim 2, wherein the substrate is Aluminium, Titanium, Tantalum, Zirconium or Magnesium.
4. 4. A circuit structure according to claim 1, 2 or 3, wherein the metal substrate is perforated and anodised.
5. 5. A circuit structure according to claim 4, wherein the circuit is preformed.
6. 6. A method of producing a circuit structure including the steps of forming an anodised surface layer upon a metal substrate using a predetermined process and conditions and applying the circuit directly onto the anodised surface of the substrate.
7. 7. A method according to claim 6, wherein the metal substrate is anodised using sulphuric acid, chromic acid, oxalic acid, sulphamic acid, phosphoric acid, saturated dicarboxylic acid, unsaturated dicarboxylic acid, alpha hydroxy carboxylic acid, or sulpho derivative as an electrolyte.
8. 8. A method according to claim 7, wherein the saturated dicarboxylic acid is glutaric, malonic or oxalic acid.
9. 9. A method according to claim 7, wherein the unsaturated dicarboxylic acid is maleic acid.
10. 10. A method according to claim 7, wherein the alpha hydroxy carboxylic acid is citric acid.
11. 11. A method according to claim 7, wherein the sulpho derivative is sulpho-salicylic acid, hydroxy aromatic acid or protocatechuic acid.
12. 12. A method according to claim 6, wherein the metal substrate is anodised using a sulphuric acid bath of strength 10% vol/vol which contains aluminium, copper, and trace impurities, at a temperature of 700F and a current density of 10 amps/sq.ft.
13. 13. A method according to claim 6, wherein the surface layer is formed using a boric acid/borate solution.
14. 14. A method according to claim 6, wherein the surface layer is formed using an alkaline solution.
15. 15. A method according to claim 6, wherein the alkaline solution is sodium carbonate or potassium pyrophosphate.
16. 16. A method according to claim 6, wherein the surface layer is formed by using a gas plasma in an atmosphere of oxygen or by using high pressure steam on the substrate.
17. 17. A method according to any one of claims 6 to 16, wherein the circuit is a hybrid circuit which is produced by screen printing conductor, resistor and protective materials in the form of inks and glasses onto said anodised substrate.
18. 18. A method according to any one of claims 6 to 16, wherein the circuit is a thin film circuit which is produced by vacuum deposition techniques.
19. 19. A method according to claim 18, wherein said