GB2080630A - Printed circuit panels - Google Patents

Abstract

A printed-circuit structure is printed upon an anodised surface of a panel of metal, suitably aluminium. The metal panel may itself serve as earth plane and/or as heat sink. It may be sealed or encapsulated. If the panel is to be perforated, perforation should precede anodisation. If the panel is to be chemically or electrochemically polished, such polishing should be subsequent to perforation, but prior to anodisation. The anodised panel may advantageously be sealed by immersion in boiling water prior to application of the printed circuit.

Description

SPECIFICATION Improvements relating to circuit panels This invention concerns improvements relating to circuit panels and methods for their production, whereby significant advantages can be achieved, in comparison with known so-called printed-circuit boards, with respect to manufacture and/or use.

Customarlily, printed-circuit boards are made from phenolic or glass-fibre/epoxy-type sheet material clad with copper on one or both faces. In the event of errors occurring in the course of production, boards have commonly to be discarded with loss of all the materials and production labour and resultant financial loss. In the case of boards with through connections from one face to the other, the connections may prove to be frail or exhibit discontinuities. This also involves financial loss or additional expenditure for mechanical or electro-chemical repair. Conventional boards in themselves afford no earth or ground plane and such a plane can be provided only by recourse to an expensive and technically difficu It form of multi-layer circuit board.Similar observations apply to heat transference and dissipation, for which additional provision may commonly be necessary and, despite such provision, heat distribution and dissipation possibilities may be limited.

According to the present invention, instead of such a board, use is made of a structure in which the circuit is printed upon a panel of anodised metal. For most requirements, an anodisd aluminium or aluminium-alloy panel will be preferred, but other anodisable metals such as tantalum, titanium and magnesium, or alloys of these respective metals, may be employed for some applications.

Anodised aluminium can be plated with a strongly adherent metallic deposit of any thickness required in practice. By the use, for example, of standard selective resist or masking techniques, metallic circuit patterns can be applied in a completely predetermined and repeatable manner.

Advantageously, anodising of the aluminium panel is preceded by chemical or electrolytic "polishing", which is performed after drilling or punching in the case of a perforated panel.

Anodised-metal circuit panels are advantageous in several respects: Not only are they strong and durable, but, because of the inherent high heat-dissipation properties of the metal panels, the life of sensitive circuit components can be enhanced and their operational stability improved, so that designers are subject to less constraints with respect to thermal limits.

Improved conditions for through-hole, side-to-side connections are achieved, particularly if chemical or electro-chemical polishing is performed before the anodising. Clean holes with well-radiused edges can be produced. Additional through metallisation, such as is required with circuit boards to bridge gaps between dielectric and conductors, is not necessary.

The metal core of the panel can itself furnish an effective earth plane, so that a panel equivalent to a multi-layer board with an earth plane can be provided without additional cost or at less cost than a multi-layer board.

Because the metal panel is robust, additional control of heat dissipation can, if required, readily be furnished by an additional metallic heat sink and/or by fins formed or provided on the panel.

The panel lends itself to the provision of characteristics imposed by particular requirements. Thus the dielectric constant, and/or breakdown voltage, of the anodised film can be simply varied in dependence upon the nature of the anodisation and the thickness of the film produced. Because of the robust nature of the panel, it will also lend itself to additional sealing techniques such as resin impregnation or encapsulation.

The anodised film can be modified if required for identification or other purposes, for example by incorporation of organic or inorganic materials.

In the event of a mistake at any stage of production after drilling or punching, the panel does not have to be discarded, but can be re-processed.

The use of an anodised metal plate is also advantageous with respect to possible health hazards in production and handling as compared, say, with boards comprising glass-fibre/epoxy resin layers. Ways of carrying out the invention will now be more fully described by way of example: For one form of method in accordance with the invention, reference will be made to the accompanying diagrammatic drawing illustrating stages in the production of a printed-circuit panel. The drawing is for purely explanatory purposes and is not to scale.

Sections of drilled aluminium panel are degreased with trichorethylene and then chemically polished.

Chemical polishing, when used, should be subsequent to drilling of any holes in the panel, but before anodising of the latter. It may be carried out by treatment of the panel in a mixture of very strong acids such, for instance, as a mixture of phosphoric, acetic and nitric acids or a mixture of phosphoric, sulphuric and nitric acids. It is important that the dissolution products of the treatment should be viscous so that they cling to the surface being polished. Under magnification, the surface appears to comprise hills and valleys. The viscous layer of dissolution products initially lies on the metal surface, leaving the tops of the hills exposed, so that they are dissolved by reaction with fresh acid ions, whereby levelling of the surface is achieved.

Other chemical polishing solutions, such as that known by the trade mark "Erftwerk", although not themselves viscous, do produce viscous dissolution products, with the effect just described.

Alternatively electrolytic polishing may be employed using similar acid mixtures or solutions or mixtures of phosphoric and chromic acids, phosphoric and sulphuric acids or perchloric and acetic acids. In this case, the dissolution of asperities is assisted by making the metal surfaces anodic.

The polishing step not only produces a smooth surface, but also removes burrs and raggs from the drilled holes, leaving them smooth and bright and with radisued edges. Also subsequent growth of electrodeposits without risk of edge weakness is facilitated.

The polished panels are then anodised to Specification DEF 151 Type 1, using the sulphuric acid to give an anodised film thickness of, say, 10 microns. After rinsing, the panels are "sealed", for example by immersion in boiling pure water, whereby the anodic film is fully hydrated, rendering it less porous, harder, more resistant to contamination and less prone to exhibit electrical conductivity.

The stage thus reached is illustrated by Figure 1, in which the aluminium sheet 1 has been drilled wih holes 2, chemically polished and anodised.

To areas of each circuit panel not to be metallised, as hereinafter described, a photo resist 3 is applied (Figure 2). This may be performed in known manner, for example by use of commercially available polymer materials such as those known by the trade names "Riston" and "Dynachem". Because of the heat-capacity of the aluminium, it may be desirable to preheat the panel to ensure complete curing of the polymer resist.

The anodised panels are then sensitized in a manner known for non metallic boards. This involves standard activation/sensitisation techniques such as immersion in a stannous chloride solution and, after a brief water rinse, immersion in a typical palladous chloride solution. Other sensitization techniques which may be used involve colloidally protected catalysts or catalysts dissolved in organic solvents. Next, the panels are metallised in the aforesaid areas and on the surfaces within the drilled holes. This may be achieved by brief immersion in an electroless nickel bath until coverage with nickel is obtained. Finally, copper is chemically deposited to a depth of 1 to 2 microns. Adhesion of the deposited metals to the anodised aluminium is excellent and reliable through-hole connections are obtained.

Alternatively, metallisation can be achieved in other ways, in particular by vacuum deposition, sputtering, controlled-piasma type metal spraying, metal peening, micro welding and selective metal cladding.

The metalised stage is shown in Figure 3 in which the circuit tracks 6 not covered with resist, and which originally appeared as aluminium in Figure 2 now appear as copper.

The resist 3 is removed, which may be performed by standard known methods. The tracks 6 of copper over nickel then stand upon a backround of anodised aluminium (Figure 4).

Another form of method in accordance with the invention comprised the following stages: 1) An aluminium panel 1 was cut to size and 2) degreased, using trichlorethylene vapour.

3) Circuit holes required by the intended circuit were drilled in the panel.

4) Optionally the panel was vapour-blasted, followed by 5) cleaning off of the vapour-blast medium, suitably by treatment with an alkaline-soak cleaner at 60 C for 3 minutes, whereafter the panel was water washed and dried. Additionally this stage may be carried out with the application of ultrasonic agitation.

6) The panel was anodised to provide an anodised film of required thickness (cf. Tables I and II below, for example). For the anodisation, use ws made of sulphuric-acid electrolyte to Specification DEF 151. The anodised panel was water rinsed, "sealed" by immersion in boiling water and dried. The anodic film is fully hydrated by this sealing, rendering it less porous, harder, more resistant to contamination and less prone to exhibit electrical conductivity.

7) Optionally the panel was subjected to vapour blasting.

8) The anodic film was sensitized, using a) treatment with stannous chloride (10g/l) and concentrated hydrochloric acid (10ml/l) for 1 - 2 minutes at room temperature, followed, after b) a brief water rinse, by activation in palladium chloride (1 .0g/1) and concentrated hydrochloric acid (10ml/l) for 1 - 2 minutes at room temperature, followed by 9) a water rinse for 30 seconds at room temperature 10) Electroless nickel plating of the whole panel, including the suface within the holes, was effected, using a hypophosphite reduced bath for 1 - 4 minutes at 80 C, followed by 11) a water rinse.

12) Copper electro-plating of the panel was performed, using a copper sulphate bath with brightening agents at a current density of 2 amps/dm2 for 5 to 10 minutes to give a copper deposit of 2.5 microns, whereafterthe panel was 13) water rinsed and dried.

14) The panel was then passed through a planishing machine, specifically a "Somaco" machine, to produce a smooth flat surface.

15) A dry-film resist was applied to the panel, using a laminator machine of conventional type.

16) The applied resist was exposed under photo artwork to give a negative circuit after development.

17) The panel was washed in solvent to ensure that the exposed copper was free from residual resist.

18) The panel was subjected to cleaning treatment comprising: a) soak cleaning for 3 minutes at 60"C.

b) cathodic cyanide cleaning with a current density of 2 amps/dm2 for 2 - 3 minutes, followed by, c) a 10% sulphuric acid rinse for 30 seconds at room temperature. A water rinse was interposed between steps a and b and between steps hand c.

19) The circuit tracks of copper were built up to a deposit thickness of 37 to 40 microns using an acid copper electrolyte and a current density of 2 amperes/dm2 for 80 minutes.

20) A water rinse was followed by 21) tin/lead (60:40) electroplating, to a deposit thickness of 5.0 - 7.5 microns, at a current density of 3 amperes/dm2 for 4 - 5 minutes at a temperature of 25 C.

22) The dry film resist was removed with solvent, whereafter the panel was alkaline-soak cleaned for 3 minutes at 60 C, rinsed with water, rinsed with 10% sulphuric acid and finally with water.

23) Unwanted copper was etched away, using ammoniacal copper-chloride solution.

24) Unwanted nickel was removed, using vapour honing.

In the panel obtained, the circuit track of copper overlies nickel on the exposed anodised surface of the panel.

In carrying out the above method, adhesion of deposited coatings may be improved by vapour blasting, the grade of abrasive used being determined to suit the widths of tracks and gaps.

The sensitizing and activating solutions may be made more concentrated and effective by solution in organic solvents, for instance acetone or an acetone/ethyl acetate mixture. Sensitization/activation may also be enhanced by the use of ultrasonic agitation.

The copper deposition could alernatively be performed from pyrophosphate, cyanide or fluoborate baths.

As alternative etches, use could be made, for instance, of ferric chloride, ammonium persulphate or chromic acid/sulphuric acid etches.

For the above final step of nickel removal, other methods known per se and utilizing chemical, electrochemical or mechanical methods may be employed. Ion bombardment methods may also be practicable.

Chemical polishing may be used subsequent to the drilling of holes in the panel, but before anodising.

Afurtherform of method in accordance with the invention is on lines similar to what has become known as an "additive" process. In a typical example of this form of method, the first seven steps would be the steps 1 to 7 listed above. After these steps, however: 8) A dry film resist is applied as in step 15 above.

9) Which resist is exposed under photo artwork and 10) is developed to reveal the required circuit pattern.

11) The panel is washed in solvent to ensure that the exposed aluminium is free from residual resist and 12) subjected to cleaning treatment comprising: a) soak cleaning (alkaline) for 3 minutes at 60 C and b) rinsing with 10% sulphuric acid for 30 seconds at room temperature. Each of steps a and b is followed by a water rinse.

13) The anodicfilm is sensitized using a) treatment with stannous chloride (1 Og/l) and concentrated hydrochloric acid (40ml/l) for 1 - 2 minutes at room temperature, followed, after b) a brief water rinse, by c) activation in palladium chloride (1.Og/l) and concentrated hydrochloric acid (1 Oml/l) for 1 - 2 minutes at room temperature. After 14) a water rinse at room temperature.

15) Electroless nickel plating of the panel is effected, using a hypophosphite reduced bath for 1 - 4 minutes at 80 C, followed by 16) a water rinse.

17) Electroless copper plating of the panel is performed to give a deposit thickness of 35 to 70 microns, or thinner deposits, if required.

18) The copper is planished to produce a smooth flat surface.

19) The resist is removed by an appropriate solvent.

20) At this stage, further plating processes, for example with gold or lead/tin, may be carried out.

As an alternative for wet chemical and electrochemical procedures in both of the forms of method described above, metalisation could be achieved in various other ways. The circuit pattern may, for example, be sputtered or vacuum deposited on an anodised aluminium panel through a mask or metal cladding may be followed by etching or metal spraying may be employed.

Anodised films on aluminium produced by the sulphuric acid process to a thickness of 10 microns will produce a breakdown voltage in excess of 200 volts and more usually 400 volts. However, the breakdown voltage of an anodic film on aluminiun will depend upon the homogeneity of the aluminium substrate, the presence of impurities in the anodised film, the thickness of the film, the electrolyte in which it is formed and the nature of any sealing process to which the film is subjected.Typical Figures to be expected are illustrated in Table l: TABLE I Breakdown Voltage Thickness ofanodic Unsealed Sealed in Sealed in boiling water followed by film film boiling water paraffin wax Microns .0005" 12.5 - 200-400 .001" 25.0 250 250 550 .002" 50.0 950 1200 1500 .004" 100.0 1850 1400 2000 As may be inferred, breakdown voltages more than adequate for metallised circuit panels can be provided.

The breakdown voltage is increased by increasing the thickness of the film, by sealing in boiling water and, yet further, by additional sealing by paraffin wax, resins or the like.

The specific resistance of the anodic film can also be made more than adequate, as is illustrated by the following comparison with a hard-rubber layer.

TABLE II Material Specific resistance Temperature Hard rubber layer 2 x 1015 20C Dry anodic film (50 microns thick) 4 x 1015 200C Dryanodicfilm (50 microns thick) 8 x 1014 1000C The anodic film can thus have a specific resistance comparable with or better than hard rubber. Moreover the specific resistance is largely maintained with temperature, which offers a significant advantage for metallised circuit panels, particularly such required to meet modern demands. Its specific resistance is to a large extent maintained at relatively high temperatures, say up to 500"C, at which layers of most organic materials would already have degraded.

Anodic films can also afford values of dielectric constant requisite for metallised circuit panels. The dielectric constant will increase with increased thickness of the film and with sealing. The dielectric constant obtainable will also depend, inter alia, upon the type of electrolyte used for the anodisation process. The use of the sulphuric acid process is advantageous, but, depending upon requirements, other known electrolytes may be employed such as boric acid/ammonium borate electrolytes or those based on oxalic acid or chromic acid.

The robust nature of anodised-metal plated circuit panels in accordance with this invention and the electrical and physical properties which can be simply provided, make it possible to satisfy not only existing but also foreseeable demands of electronic circuitry, including such arising from higher propagation speeds, by more economic and functionally reliable high quality circuit panels offering ease of component mounting, close spacing of tracks and reliable inter-connections from one side to the other of the panel.

Additionally, it is feasible to create circuit patterns on both regular and irregularly shaped anodised metal, for example spheres, foil, wire and tubes.

Furthermore the production of metallised circuits on anodisable metal is expected to be possible without the necessity for recourse to "clean-room"techniques.

If it is desired to add a separate finned heat sink to a circuit-panel produced as described above, this may be done by letting an end portion of the aluminium panel, for example in the form of an end plug, into a closely fitting slot in the heat sink, the anodised film having first been abraded from the surfaces of the said end portion to be entered into the slot.

Provision for the establishment of external connections to the circuit of the panel may be made by means of a grooved terminal strip which is engaged over an end extension of the aluminium panel provided with terminal connections complementary to respective terminal means with which the strip is fumished.

Claims (14)

1. A printed-circuit structure, wherein the circuit is printed upon an anodised suface of a metal panel.

2. A structure according to claim 1, wherein the panel is of anodised aluminium or aluminium alloy.

3. A structure according to claim 1 or 2, wherein the metal panel is itself utilised to afford an effective earth plane.

4. A structure according to any one of the preceding claims, wherein the metal panel itself serves as a heat sink.

5. A structure according to any one of the preceding claims, wherein an additional heat sink is formed or provided on the metal panel.

6. A structure according to any one of the preceding claims, wherein the circuit is of copper printed upon the anodised suface over nickel.

7. A structure according to any one of the preceding claims, wherein the panel is additionally sealed or encapsulated.

8. A printed-circuit structure substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of producing a printed-circuit structure, wherein the circuit is printed upon a panel of anodised metal.

10. A method according to claim 9, wherein anodising of the metal panel is preceded by chemical or electrolytic polishing of the panel.

11. A method according to claim 9 or 10, wherein, in the case of a perforated panel, the said polishing is performed after the perforation of the panel.

12. A method according to any one of claims 9 to 11, wherein the anodised film is modified by incorporation of organic or inorganic material.

13. A method according to any one of the preceding claims 9 to 12, wherein the anodised panel is sealed by immersion in boiling water to hydrate the anodised surface before application thereto of the printed circuit.

14. A method of producing a printed circuit panel substantially as herein before described.