PRINTED CIRCUIT BOARD

Abstract: A printed circuit board having a supporting base element (2) of anodized aluminum. A glue film (4) is interposed between the supporting base element and a laminate layer (7) which has a printed circuit (6a, 6b), the glue film sealing any pores in the anodic film of the supporting base element.

Description: Printed circuit board TECHNICAL FIELD The present invention relates to a printed circuit board, including a supporting base element in the shape of a heat removing metal plate, one side of which having a laminate layer with at least one printed circuit'. BACKG_RO_UND_ART The manufacture of such printed circuit boards have caused problems because of the requirement that the heat removing plate must be electrically insulated. Efforts have been made inter alia to use circuit boards having a core of anodized aluminium, i. e. with an electrically insulating metal oxide film. Aluminium or steel plates covered with different types of resins have also been tested. These tests have not been very successful, for instance because of the formation of pores in the insulating film, thereby causing short-circuits between the metal plate and the electric circuit. If, on the other hand, the insulating film is made sufficiently thick in order to ensure that no pores are present, the ability of leading away the heat from the circuit components will decrease. DISCLOSUREJOF THE INVENTION The object of the present invention is, therefore, to pro¬ pose a circuit board of the type desribed by way of intro- duction, in which the insulating material is a metal oxide film, which, can be made relatively thin without any incres- ing risk of short-circuit due to pore formation. This ob¬ ject is achieved by the circuit board in accordance with the invention having been given the characteristic fea- tures indicated in Claim 1. Further developments of the invention are evident from the sub claims. The invention is based on the surprising discovery that metal oxide films - contrary to the prevailing experience - can be used as an electrically insulating film on heat removing plates in circuit boards without any risk of - ά pore formation and short-circuit, if a glue film is pro¬ vided between the metal oxide film and the laminate layer of the circuit board, the glue film suitably being of a nature resin or a synthetic resin. The invention will be described below in greater detail with reference to the accompanying drawing, which illu¬ strates a preferred embodiment of the invention. JESCJRIPTIOJ _gF_THE_FIGORES_ Figure 1 shows a sectioned perspective view of a printed circuit board in accordance with the invention. Figure 2 shows schematically a cross section of a portion of a printed circuit board in accordance with the invention, illustrating the way in which the metal plate can be used as a screening of the circuit board. J?REFERRED MBO_piME_NT_ The printed circuit board includes a supporting base element 1 in the shape of a metal plate or metal sheet 2 being covered by an electrically insulating metal oxide film 3. The metal plate is, in the embodiment shown, of anodized aluminium, the anodic film of which constituting said metal oxide film. The metal plate 2 has on one side thereof, that is the lower one in Figures 1 and 2, a glue film 4, an insulating varnish film 5a, an electric printed circuit 6a, an elec¬ trically insulating glass fiber laminate layer 7, an elec¬ tric printed circuit 6b (shown in Figure 2 only), and an insulating varnish film 5b, the layers and films being arranged in said order. The laminate layer 7 and the printed circuits 6a and 6b and the varnish films as well are of known nature, for which reason they need not be further described here. The electric components of the printed circuit board, which are for instance in the shape of IC-circuits 8, of which only one is shown in Figure 1, are mounted on the free side of the plate 2, that is the upper side in Fi¬ gures 1 and 2. It is important that the electric compo¬ nents are in close contact with the upper side of the anodized plate in order to ensure a thermally satisfac- tory contact between the components 8 and the plate 2, and a satisfactory heat conducting and heat spreading effect. The components 8 are electrically connected to the cir- cuits 6a and 6b via contact members 9. The contact mem¬ bers 9 extend through apertures 10 which are formed in the plate 2. The cross section area of the apertures 10 is conically widening as seen in the direction towards the opening at the component side of the plate 2 in order to enable the contact members 9 to be completely lowered therein. By this way the components 8 can much easier be mounted in close contact with the plate 2. The anodized plate 2 can be connected to earth, and it will then function as an electric screening, in spite of the fact that the outer film 3 of the aluminium plate is non-conductive. Said connection is made through a non- -anodized hole 11 in the circuit board, see Figure 2. A pin or rivet 12 is in close contact with the conductive wall of the hole 11, and is electrically connected to one or several earthings 13 in the circuits 6a and 6b. The free ends of the pin 12 are insulated by any known insu¬ lating varnish 14a and 14b, respectively. The glue film is suitably made of a nature or a synthetic resin glue, preferably a 2-component glue, for example a polyurethane glue on a polyether/polyester base and with polyisocyanide as hardener. Other similar glues are, how¬ ever, conceivable. The thickness of the glue film should preferably be at least 10 um in order to give a satisfac¬ tory pore sealing. The thickness should also preferably be in the order of 10 to 50 u , and more preferably in the order of 20 to 40 u . It has been manifested that a pore sealing of the anodic film 3 is obtained by means of said glue film, the sealing being so effective that any risk of short-circuit between the plate 2 and the circuits 6a and 6b will be eliminated. Accordingly, the anodic film 3 can be made very thin, for example in the order of 10 to 30 μ , without any risk of troublesome pore formation. The anodized plate should have a thickness of about 3 to 10 times the thickness of the laminate layer 7. By this way, the circuit board will not be deformed when exposed to high temperatures. The anodic film 3 may be coloured in a suitable colour, or it may be in its natural colour. By joining together the different layers and films of the circuit board, it is suitable to proceed in such a manner that the anodized plate is manufactured as a separate unit, and that the laminate layer together with the circuits and the insulating varnishes are manufactured as a second separate unit, which forms the actual circuit board. The two units are then joined together by the glue film 4. It is of course conceivable within the scope of the in¬ vention to join the anodized plate with a plurality of such "actual" cicuit boards.

Claims: Claims 1. Printed circuit board, including a supporting base ele¬ ment (2) in the shape of a heat removing metal plate, one side of which having a laminate layer (7) with at least one printed circuit (6a, 6b), c h a r a c t e r i s e d in two electrically insulating films lying against one another in the area between the metal plate (2) and the laminate layer (7), one of the films being a metal oxide film (3), the other film being a glue film (4), suitably of a nature or a synthetic resin glue, the metal oxide film lying against the metal plate. 2. Circuit board in accordance with Claim 1, c h a r a c - t e r i s e d in that also the side of the metal plate (2) being opposite to the laminate layer (7) is covered with an electrically insulating metal oxide film (3). 3. Circuit board in accordance with any of the foregoing Claims, c h a r a c t e r i s e d in that the metal plate (2) is of aluminium and that the metal oxide film (3) is made by anodization of the metal plate. 4. Circuit board in accordance with Claim 3, c h a r a c - t e r i s e d in that the metal plate (2) has at least one non-anodized hole (11), the wall of which being in electric contact with at least one earthing (13) in the printed circuit (6a, 6b). 5. Circuit board in accordance with any of the Claims 1 - 4, c h a r a c t e r i s e d in that the thickness of the glue film (4) is at least 10 um. 6. Circuit board in accordance with Claim 5, c h a r a c ¬ t e r i s e d in that the thickness of the glue film (4) is about 10 - 50 um, preferably about 20 - 40 um. 7. Circuit board in accordance with any of the Claims 1 - 6, c h a r a c t e r i s e d in that the thickness of the metal plate (2) is about 3 - 10 times the thickness of the laminate layer (7). 8. Circuit board in accordance with any of the Claims 1 - 7, c h a r a c t e r i s e d in that the electric components (8) that are intended for the printed circuits (6a, 6b) are arranged on the metal plate (2) on the side thereof that is opposite to the laminate layer (7). 9. Circuit board in accordance with Claim 8, c h a r a c ¬ t e r i s e d in that contact members (9 ) which are ne¬ cessary for the printed circuits (6a, 6b) extend through electrically insulated holes (10) in the metal plate (2). 10. Circuit board in accordance with Claim 9, c h a r a c ¬ t e r i s e d in that the cross section area of said last- mentioned hole (10) is widening as seen in the direction towards the component side of the metal plate (2).

Inventors: DAHLBERG ROLF, DAHLBERG TOMAS, DAHLBERG, ROLF, DAHLBERG, TOMAS
Applicants: DAHLBERG ROLF, DAHLBERG TOMAS, DAHLBERG, ROLF, DAHLBERG, TOMAS
Application year: 1984
Main classification: H05K1
IPC codes: H05K1, H05K3, H05K7