GB2164294A

GB2164294A - Electrical distribution boards

Info

Publication number: GB2164294A
Application number: GB08423142A
Authority: GB
Inventors: Harold E Mccarron; Ralph J Wenzel
Original assignee: CTS Corp
Current assignee: CTS Corp
Priority date: 1984-09-13
Filing date: 1984-09-13
Publication date: 1986-03-19
Also published as: GB8423142D0; GB2164294B

Abstract

An insulated metal substrate (12) having an organic laminated electrical circuit of conductive foil material (14) is provided as a low cost, multipurpose application for power distribution boards. The construction (11) comprises an metal substrate (12) having disposed thereon a three-ply multilayer insulative material (20) for bonding the electrical circuit of conductive foil (14) to the underlying metal substrate. The multilayer insulative material (20) comprises two layers of a thermosetting dry film acrylic adhesive (21) sandwiched around a highly puncture resistant insulative layer (22) consisting of either a polyimide film or a polyetherimide film. <IMAGE>

Description

SPECIFICATION Organic laminated electrical distribution device This invention relates to an organic laminated electrical distribution device which may be utilised to provide power supplies for electrical equipment.

There has long been a need for an easily manufacturable, low cost power distribution board for utilisation with electrical equipment. Many prior art devices have attempted to provide such a construction, such as a metal substrate having an organic coating with a conductive paste screened onto the organic coating. However, the prior art has not been able to provide an electrical power distribution board which has a high power capability, is fire retardant, has no limitation of substrate thickness or type of metal used as a substrate, utilises various conductor metals, provides multiple conductive layer capability, provides a construction with irregular surfaces, provides EMI and RFI shielding, and provides a construction which is bendable after initial fabrication.

Related prior art U.S. Patent No. 3,940,534 discloses an electrically conductive metallic layer laminated to a substrate of woven glass cloth interposed between a pair of outer substrate layers of a polyester mat. The use of metal foil to provide thin foil resistors secured to an insulative glass or ceramic substrate is disclosed in U.S. Patent No.

4,306,217. However, none of these prior art references provide a construction comprising an insulated metal substrate having a metal foil conductive circuit bonded to and insulated from the underlying metal substrate.

The present invention comprises an insulated metal substrate having an organic laminated electrical circuit of conductive foil material bonded to and insulated from the subjacent metal substrate.

The construction comprises an aluminum metal substrate having disposed thereon a three-ply multilayer insulative material for bonding the electrical circuit of conductive foil thereto. The multilayer insulative material comprises two layers of a thermosetting dry film acrylic adhesive having sandwiched therebetween a highly puncture resistant insulative layer consisting of either a polyimide film or a polyetherimide film. First, the multilayer insulative material is heat tacked to the metal substrate and then the metal foil is placed upon the multilayer insulative material prior to placing the sub-assembly in a laminating press. The conductive foil may be die cut to form an electrical circuit prior to its application to the multilayer insulative material, or after lamination the metal foil may be chem-milled.The metal foil is laminated to the underlying metal substrate, with the foil material remaining in electrical isolation from the underlying metal substrate. When a continuous sheet of conductive metal foil has been laminated to the substrate, the foil material is chem-milled to produce an electrical circuit of conductive foil. Prior to the application of the multilayer insulative material to the substrate, holes may be punched in the insulative material so that conductive foil laminated thereto and bridging the holes can be spot welded to the underlying metal substrate. After lamination and chem-milling, protective insulative overcoats are applied over the front and back sides of the construction.

The present invention is thereby able to provide a power distribution board which has high power capability, is fire retardant, and for which numerous configurations may be constructed. Various conductor metals may be used for the metal foil, as well as any metal utilised for the substrate. If a metal substrate is used, after or during fabrication, the substrate can be bent, and flat, curved, or irregular surfaces can be supplied. There is no limitation as to the substrate thickness and the resulting configuration will provide EMI and RFI shielding except when alumina substrates are utilised.

In addition to the above advantages, the organic laminated electrical distribution device can easily provide a construction having multiple layers of the conductive foil material, and provide substrates having conductive layers disposed on both sides and the conductive layers interconnected via openings in the substrate.

In the accompanying drawings: Figure 1 is a flow chart illustrating the steps for fabricating the electrical distribution device; Figure 2 is an exploded view illustrating the underlying metal substrate, three-ply multilayer insulative material, and conductive foil material prior to lamination; Figure 3 illustrates the subassembly of the construction after lamination (the conductive foil material having been die cut previous to lamination) and with the spot welding of plating tie bars completed; Figure 4 illustrates the subassembly with a protective cover coat applied thereto prior to the electroplating step of fabrication; and Figure 5 illustrates the organic laminated electrical distribution device after electroplating and removal of the plating tie bars and with insulative cover coats applied to the front and back sides of the metal substrate.

Referring to the flow chart illustrated in Figure 1 and the exploded view of Figure 2, the organic laminated electrical distribution device of the present invention is designated generally by reference numeral 11. Device 11 includes a metal substrate 12 which may comprise any metal desired for use in the assembly of a power distribution board. An example is an aluminum substrate of about 0.2 mm (0.050 inch) thickness and with numerous holes 15 located in the board, for reasons to be explained hereinafter. It should be clearly understood that any metal substrate may be utilised for the present construction and that cleaning and preparation procedures appropriate for the particular type of metal substrate should be used in the fabrication process.

Metal substrate 12 may comprise an aluminum metal substrate which is initially cleaned by a degreasing process in order to remove excess grease and oils on the surface of the substrate (STEP 1 of Figure 1). A chemical such as acetone, or other well known degreasing agents, may be used. In STEP 1, aluminum substrate 12 is immersed in a heated alkaline cleaner solution for several minutes, or it may be immersed in a acid cleaner solution heated to an elevated temperature, for several minutes. The purpose for immersing substrate 12 in the solution is to remove any remaining contaminants from the surface of the substrate 12. An alkaline cleaner such as Allied-Kelite Chemidize 740 or acid cleaner such as Allied-Kelite Isoprep 161 may be utilised, as well as other well known acid and alkaline cleaners. After immersion in the cleaner solution, substrate 12 is rinsed with water.

Next, aluminum substrate 12 is immersed in a sodium-dichromate-sulphuric acid solution for 10-15 minutes and then oven dried at 60 C. The sodium dichromate etch provides a controlled and porous layer of aluminum oxide on the surface of aluminum substrate 12, thereby providing improved adhesive and cohesive characteristics for the organic layer adhesive material to be applied thereto. Finally, a silane solution is applied to the surface of the substrate and allowed to air dry. The silane helps to further increase the adhesive strength of the organic material to be bonded to the aluminum substrate 12. A silane coupling agent such as Union Carbide A-1100 (gamma-aminopropyltriethroxy-silane) may be used in this last substrate cleaning procedure of STEP 1.

The conductive foil material 14 (see Figure 2) may be any foil material suitable for carrying electric current in a high power application, or capable of carrying electric current for the particular application desired. In the present embodiment, the foil material comprises aluminum foil of 0.02 mm (0.006 inch) thickness, and should be cleaned prior to use in the assembly process. Conductive foil 14, in this embodiment an aluminum foil, is cleaned in STEP A of Figure 1 by utilising two of the methods of cleaning the aluminum substrate, i.e. the foil is cleaned with acetone to effect degreasing and then immersed in either an alkaline cleaner solution or acid cleaner solution as described above, thereafter followed by rinse with water.

The multilayer insulative material 20 illustrated in Figure 2, consists of an acrylic dry film adhesive which flows only under heat and pressure and a layer of material that is highly puncture resistant, to provide an insulative layer between the overlying conductive foil material 14 and the subjacent metal substrate 12. A multilayer insulative material suitable for use in the present invention is a threeply sandwich construction comprising two layers of a thermosetting acrylic adhesive sandwiched around a layer of either a polyimide film or a po lyetherimide film. The thermosetting acrylic adhesive, in the form of a dry film adhesive, is designated by reference numeral 21 in Figure 2.It is a standard dry film acrylic adhesive, and the interior layer 22 is a polyimide film sold by E.I. du Pont de Nemours Corporation of Wilmington, Delaware, under the name "Kapton" (Trade Mark) or a polyetherimide film sold by General Electric Corporation and designated as "Ultem" (Trade Mark). Either material for intermediate layer 22 provides a highly puncture resistant layer suitable for insulating metal foil 14 from metal substrate 12.

The three-ply multilayer insulative material 20 may be obtained from the Sheldahl Company, Northfield, Minnesota, and is designated as Sheldahl MKC-111 which is adhesive coated on one or two sides of Kapton (Trade Mark). MKC-111 is typically one mil of acrylic clad on both sides of one mil of the polyimide Kapton (Trade Mark). MKC-111 has found prior uses as a cover sheet on polyimide circuitry, and also has been used as a bond ply or core material for multilayer circuitry. However, MKC-111 has been found particularly useful in the present invention wherein an insulated metal substrate supports overlying metal foil circuitry to provide a power distribu--ion board.Three-ply multilayer insulative material 20 is particularly advantageous because the dry film acrylic adhesive makes it easy to heat tack circuitry onto the underlying metal substrate with the application of only localised heat, provides a high bond strength (1.07 - 1.42 kg/cm 6 - 8 pounds/inch), has good chemical/ solvent resistance, and during lamination only a minimal amount of the resin is squeezed out. Because of its puncture resistance, the interior layer 22 comprised of either a polyimide such as Kapton (Trade Mark) or the polyetherimide Ultem (Trade Mark) prevents the overlying circuitry from pushing through the acrylic adhesive.

In accordance with STEP 2 of Figure 1, insulative layer 20 is heat tacked to metal substrate 12. Prior to heat tacking, ground holes 23 are cut in insulative layer 20 so that the overlying metal foil 14 may, after lamination, be spot welded to the underlying metal substrate to provide a ground connection 17, or may be bonded to a metal foil strip on the other side of substrate 12 and accessible by means of an opening 15. Insulative material 20 and metal substrate 12 are placed in a lamination press heated to 55 to 65 degrees C (130 - 150 degrees F) under approximately 7 kg/sq.cm (100 psi). Tacking of the insulative material is necessary prior to lamination in order to prevent air bubbles from being trapped between adhesive 21 and substrate 12.

Next, as indicated by STEP 3 of Figure 1, metal foil 14 is applied to the surface of material 20 which has been heat tacked to the underlying aluminum substrate 12. The metal foil may be held in place by tape or other means to secure it temporarily during the lamination process, and may be applied to material 20 as either a continuous sheet of metal foil or as a die cut circuit pattern (OPTION 1 of Figure 1). If the circuit pattern is fabricated by die cutting of foil 14 prior to application to insulative material 20, then the metal foil circuit pattern is applied to the insulative material 20 prior to the lamination STEP 4. However, if a continuous sheet of metal foil is applied to insulative material 20 in STEP 3, then OPTION II of Figure 1 is performed wherein the metal foil is chem-milled to provide the electrical circuit pattern 26. The lamination STEP 4 comprises placing the metal substrate 12, insulative material 20 heat tacked thereto, and overlying conductive metal foil 14 or metal foil cir cuit pattern 26, into a lamination press for approximately one hour at approximately 180 degrees C (360 degrees F) and 10.5 - 35 kg/sq.cm (150 - 500 psi), with 28 kg/sq.cm (400 psi) producing excellent results. After lamination STEP 4 of Figure 1, the organic laminated construction is chem-milled in accordance with OPTION II if circuit pattern 26 was not provided by die cutting of the metal foil (OP TION I) as illustrated by construction 16 (Figure 3).

Circuit pattern 26 is formed from foil 14 by standard photographic art work-etching techniques (OPTION II). Several patterns are formed on the subassembly construction and it is unnecessary to specifically delineate all of the details of this process, as it is well known to those reasonably skilled in the art. Briefly, the particular circuit pattern or a plurality of such patterns are photographed and reduced to the desired size. Foil 14 is coated with a photo sensitive masking medium and exposed to the photographic circuit pattern. The photo sensitive masking medium is retained only on the foil material not to be removed, this being a negative process wherein the exposed material remains after etching. The sub-assembly is then subjected to an acid etching bath which removes only the minimal amount of foil that need be removed.

Thus, large areas of aluminum foil which are not to be used as part of the circuit pattern remain but are effectively isolated from the chemically etched circuit pattern. In the alternative, a positive process can be used whereby only the circuit foil material remains and other portions of the foil are removed after photo exposure to produce a circuit such as that illustrated in Figure 3. After chemical milling in ferric chloride solutions, the panel construction is oven dried at approximately 135 degrees C (275 degrees F). STEP 5 of Figure 1 provides for the spot welding of metal foil runs which cover or bridge openings 23 in the insulative material 20.

The openings 23, illustrated in Figures 2 and 3, allow the overlying metal foil 14 of circuit pattern 26 to make contact with underlying metal substrate 12, and these contact areas are spot welded to produce a strong mechanical and electrical bond between the foil run and the metal substrate.

In the manufacture of an electrical power distribution board, it is desirable to have selected terminals of the circuit pattern plated with gold, solder, or other metal which will provide a termination having not only improved electrical conductivity, but which resists oxidation. In Figure 3, there is illustrated a number of such terminals 28. These terminals may be plated by standard plating methods such as immersion plating and electrolytic plating.

To effect electrolytic plating, it is necessary (1) to provide interconnections or plating tie bars so that an electric current can be applied to appropriate portions of the circuit pattern during the electroplating process, and (2) to provide a protective masking for coating areas not to be electrolytically plated. The interconnections are accomplished by spot welding narrow strips of aluminum 32 to selected areas of circuit pattern 26 in order to provide contacts for applying an electric current to terminals 28. The interconnections or plating tie bars 32 are essentially shunt contacts for applying to the entire circuit, or selected portions of the circuit, a current for the electroplating step. Selected runs of the circuit 26 bridge or cover openings 23 and are spot welded or bonded to the underlying metal substrate 12 to provide ground connection 17.By attaching a lead to substrate 12, electric current is applied through the ground connections 17 to the selected runs for the electroplating STEP 7.

Figure 3 illustrates the subassembly 16 with tie bars 32 applied thereto.

A protective coating or masking is applied to areas not to be plated. After application of plating tie bars 32, a protective overcoat comprising a solder resist is screen printed over the conductive circuit 26 STEP 6) by screen printing methods well known in the art. As illustrated by subassembly 18 in Figure 4, the solder resist mask 34 is screened onto circuit pattern 26 and organic adhesive 21, but the plating tie bars 32 and terminals 28 are protected by the emulsion of the screen and are not coated. A suitable protective overcoat material is MacDermid Macu (Trade Mark) Mask solder resist No. 9446 which is screened onto the front side of subassembly 18 and cured for approximately one hour at 65 degrees C (150 degrees F).This particular solder resist mask flows well between the runs of circuit pattern 26 and provides an effective coating during the electrolytic plating process of STEP 7.

Next, subassembly 18 has plating resist tape placed over the backside of metal substrate 12 and over the plating tie bars 32. Plating resist tape prevents electrolytic plating of the backside of the substrate and tie bars which are used to provide a current to circuit pattern 26. The subassembly is electroplated (STEP 7) by any standard selective plating process well known in the art, whereby the areas not masked by either the solder resist mask or the plating resist tape, receive metallic plating thereto. The preparatory steps and electroplating (STEPS 6 and 7) are well known and standard in the art, and therefore have not been described in great detail. After the gold plating or solder plating of selected terminals 28, subassembly panel 18 is oven dried.

STEPS 8 and 9 of Figure 1 illustrate that the next steps are the removal of plating tie bars 32 and the application of a protective insulative overcoating.

The subassembly now comprises metal substrate 12 with multilayer insulative material 20 and circuit pattern 26 laminated thereto, a solder resist mask adhered to the front surface over most of circuit pattern 26, and plated terminals 28. In STEP 9, an insulative overcoat 40 (Figure 5) is screen printed onto the front surface of the subassembly with plated terminals 28 being masked from the insulative coating so that these will remain available for use as terminations. Any standard electronic cover coat material may be utilised, such as PC-401 which is a two component epoxy overcoat used throughout the electrDnics industry. One or more applications provide an insulative overcoating and improve cosmetic appearance. The uncovered tie bars areas are covered by the overcoat 40. Any rough spots may be sanded down and cover coated.Cover coat 40 is cured for approximately one hour at 150 degrees C.

Finally, an insulative coating is applied to the backside of metal substrate 12. Again, any electronic cover coat material may be utilized with particular coatings used for particular applications and depending on the metal utilised for the substrate.

For an aluminum substrate 12, an acrylic enamel may be screened onto the backside of the panel and cured for approximately one hour at 150 degrees C. Finally, power distribution board 38 (Figure 5) is completed by cutting the three-ply insulative material 20 from holes 15 located in metal substrate 12. Also, it should be clearly understood that the protective cover coatings applied to the front and back sides of board 38 may be applied prior to plating of selected terminals 28.

Various electrical components and structures may be attached to board 38 by inserting legs or stanchions through holes 15.

The organic laminated electrical distribution construction of the present invention provides an insulated metal substrate having many advantages over prior art power distribution boards. The construction provides a distribution board having high power capability, fire retardant construction, EMI and RFI shielding (except for alumina substrates), can be bent to other configurations, may be utilised for the provision of flat, curved and irregular surfaces, has a maximum recommended operating temperature of 150 degrees C, and provides terminations through solder pads or pressure connectors. Additionally, the electrical distribution construction does not have any restrictions on the metal utilised for the substrate or on substrate thickness.Multiple conductive layers are accomplished by placing another three-ply insulative layer over the surface of the board and laminating another metal foil sheet or circuit pattern thereto.

Various conductor metals other than aluminum foil may be used and the construction may be manufactured in sizes as large as 0.8361 square metre (9 square foot) panels. The organic laminated electrical distribution construction disclosed herein solves many of the disadvantages of the prior art in that it has not only low cost but an almost unlimited number of applications, uses and configurations.

In general, the invention provides an organic laminated electrical distribution device which may be utilised to provide electrical distribution for electronic equipment.

Although the present invention has been illustrated and described in connection with the example embodiment, it will be understood that this is illustrative of the invention, and by no means restrictive thereof. Numerous revisions and additions to the embodiment are possible within the scope of the invention as defined by the appended

Claims

claims. CLAI MS

1. An electrical laminate device for electrically isolating and supporting an electrically conductive metallic layer on a metal substrate surface, the laminate device comprising a metal substrate on which the electrically conductive metallic layer comprises an electrical circuit of conductive foil material, and a multilayer insulative material bonding said foil material to the surface of said metal substrate, said multilayer insulative material comprising two layers of thermosetting dry acrylic adhesive having disposed therebetween a layer of highly puncture resistant material comprising one of a polyimide material and a polyetherimide material, said multilayer insulative material adhering said conductive foil material to said surface of the metal substrate while supporting and insulating said electrical circuit on said metal substrate to form the electrical laminate device.

2. The electrical laminate device in accordance with claim 1, further comprising openings in said multilayer insulative material with conductive foil upon said openings and bonded to the surface of the metal substrate.

3. The electrical laminate device in accordance with claim 1 or claim 2, further comprising metal plated upon selected portions of said conductive foil material.

4. The electrical laminate device in accordance with claim 3, further comprising an electrically insulative overcoat applied to said construction.

5. The electrical laminate device in accordance with any of claims 1 to 4, further comprising an insulative coating applied to other surfaces of said metal substrate.

6. A method of producing an insulated metal substrate, comprising the steps of (1) cleaning a metal substrate and an electrically conductive foil material comprising electrical conduction means to be bonded to said metal substrate, (2) disposing a three-ply multilayer insulation material on a surface of said metal substrate, the three-ply multilayer insulation material comprising two layers of a thermosetting acrylic adhesive and disposed there- between a layer of material comprising one of a polyimide film and a polyetherimide film, (3) placing the electrically conductive foil material comprising electrical conduction means over said three-ply multilayer insulation material, and (4) applying heat and pressure to said construction to laminate said conductive foil material and insulation material to said metal substrate to provide electrical conduction means bonded by said threeply multilayer insulative material to said metal substrate with the conduction means effectively insulated to preclude electrical contact with the underlying metal substrate.

7. The method in accordance with claim 6, further comprising the step of providing openings in said three-ply multilayer material so that overlying conductive foil material is bondable to the underlying metal substrate.

8. The method in accordance with claim 6 or claim 7, further comprising the steps of placing electrically conductive strips of material upon selected sections of the electrical circuit, placing a masking material over said conductive foil material except for said strips and selected portions of said electrical circuit, and electrolytically plating said lamination to provide metal plating of the selected portions of said electrical circuit of conductive foil not covered by said masking material.

9. The method in accordance with any of claims 6 to 8, further comprising the step of applying an insulative coating to the other surfaces of said insulated metal substrate.

10. The method in accordance with any of claims 6 to 9, further comprising the steps of chemically etching the electrical conduction means laminated to said substrate to provide an electrical circuit bonded to and insulated from said metal substrate.

11. The method in accordance with any of claims 6 to 10, wherein the metal substrate and foil material are aluminum.

12. The method in accordance with any of claims 6 to 11, further comprising the step of die cutting said conductive foil material whereby the electrical conduction means comprises an electrical circuit pattern.

13. An electrical laminate device for electrically isolating and supporting an electrically conductive metallic layer on a subjacent metal substrate surface, the laminant construction comprising an aluminum substrate, an electrically conductive metallic layer comprising an electrical circuit of aluminum foil adhered to and isolated from the underlying aluminum substrate surface, and a multilayer insulative material adhering said aluminum foil to the surface of said aluminum substrate, the multilayer insulative material comprising two layers of a dry film acrylic adhesive having disposed therebetween a layer of material comprising one of a polyimide material and a polyetherimide material, the multilayer insulative material bonding said aluminum foil to the surface of the aluminum substrate while supporting and insulating said metallic layer on said aluminum substrate.

14. An electrical laminate device substantially as hereinbefore described with reference to the accompanying drawings.

15. A method of producing an electrical laminate device substantially as hereinbefore described with reference to the accompanying drawings.