PRINTED CIRCUIT BOARD SUBSTRATES

Abstract: A printed circuit board substrate is provided wherein a composite metal layer (4, 5) is affixed to at least one surface of a flexible dielectric substrate (2, 3), the composite metal layer comprising an inner aluminium layer (4) and an outer copper layer (5).

Description: The present invention relates to improved substrates for ted circuit boards according to claim 1 and to printed circuit boards according to claim 2. In Japanese Patent Application No. 59-72,472, the applicant has proposed a substrate for a printed circuit board for high-frequency application which is characterized by small coefficients of thermal deformation, such as thermal expansion or shrinkage. This substrate is provided with an insulating layer which comprises a porous polytetrafluoroethylene resin element affixed to a polytetrafluoroethylene layer. It is known from Japanese Patent Publication (Kokai) No. 60-225,750 that electrical characteristics of substrates for printed circuit boards can be improved by arranging expanded porous polytetrafluoroethylene resin layers along conductive circuits. Such substrates for printed circuit boards, however, had conductive layers made of copper on both sides of a dielectric substrate, and, when they were subjected to bending, it became impossible to relieve stress caused by bending of the printed circuit board itself. As a result, cracks appeared in the conductive copper layer. According to the invention there is provided a substrate for a printed circuit board comprising a composite metal layer affixed to at least one surface of a flexible dielectric substrate, the composite metal layer comprising an inner aluminium layer and outer copper layer. The invention also includes within its scope a printed circuit board formed from the aforesaid substrate. The following is a more detailed description of some embodiments of the invention by way of example, reference being made to the accompanying drawings, in which:- Figure 1 is a cross-section view of one embodiment of a printed circuit board substrate, Figure 2 is a cross-sectional view of the printed circuit board substrate of Figure 1 in a bent state. Figure 3 is a perspective view, partly in cross-section, of a printed circuit board incorporating a printed circuit board substrate of the kind shown in Figures 1 and 2. A printed circuit board is provided wherein a composite metal layer is affixed to at least one surface of a flexible dielectric substrate, the composite metal layer comprising an inner aluminium layer and an outer copper layer. Owing to the relatively high elongation of the aluminium layer when the printed circuit board is flexed the circuit thereon does not crack. The problem inherent in the prior art techniques of forming printed circuit boards is solved in the printed circuit boards now to be described with reference to the drawings by means of the substrate for a printed circuit board formed of a flexible dielectric layer and a metal layer attached to at least one side of the above mentioned flexible dielectric layer, the metal layer having an aluminium layer on the inner side and a copper layer attached to the outer surface of the aluminium layer. Because a conductive layer which is installed at least on one side of a flexible dielectric layer is formed of a layer of aluminium on the side of the dielectric and a layer of copper on the outer surface of the aluminium layer, stress which is generated by bending the printed circuit board does not form cracks because this stress is absorbed by the aluminium, when is characterized by high relative elongation. In addition, although aluminium is a metal which cannot be easily attached by soldering, because one side of the aluminium layer is coated with an easily soldered copper layer and because this copper layer forms the outermost surface of the substrate for the printed circuit board, it provides improved conditions for attachment of various circuit elements to the printed circuit board substrate. Referring first to Figure 1, the printed circuit board substrate shown in this Figure comprises a flexible dielectric layer 2 preferably made of expanded, porous polytetrafluoroethylene, which is sandwiched via adhesive layers, 3, 3 such as film of ethylene tetrafluoroethylene between conductive layers 6, 6 located on both its side and formed each from a sequentially arranged aluminium layer 4, 4 and a copper layer 5, 5. As shown in Figure 2, when printed circuit board substrate 7 is bent, strains indicated by the arrows are generated. These strains cause stresses in metal layers 12, but because the stresses have greater value on the sides of flexible dielectric layer 8 which, through adhesive layers 9 contacts aluminium layers 10, the stresses which work n the direction of the arrows are substantially absorbed by deformation of aluminium layers 10. As copper layers 11 are thin, they are easily adjusted to the deformation of the aluminium layers and are not subject to cracking. Even though local cracks can occur, the printed circuit board substrate will not lose its performance characteristics since conductivity will still be maintained through the aluminium layers. Figure 3 is a perspective view of a printed circuit board 13 of the kind described above with refrence to Figures 1 and 2 with a circuit pattern 14 installed in the printed circuit board substrate and with an element 15 soldered to the above mentioned pattern. In the case when an element such as element 15 is attached by soldering to circuit 14 of printed circuit board 13 of the type shown in Figure 3, metal layers 18 which are attached to both sides of a flexible dielectric layer 16 via adhesive layers 18 are formed of aluminium layers 19 on the side which is closer to the flexible dielectric layer 16, while their outermost layers 20 are formed of copper. Because the outer layers are made of copper, various elements can easily be soldered through solders 21 to the printed circuit board with the same ease as in the case of conventional printed circuit boards. Because, as has been shown above, the structure has metal layers installed on one or both sides of a flexible dielectric layer with aluminium layers located closer to the above mentioned dielectric layer and with copper layers located on the outer sides of the aluminium layers, the stresses which are formed in the printed circuit board in the case of its bending are effectively absorbed by the aluminium layers due to their specific properties. As a result, the stresses are relieved and the metal layer is protected from the formation of cracks. As is known, aluminium is a material which is unsuitable for soldering. Therefore, it is very difficult to connect various elements directly to the aluminium surface. Because, in the device described above with reference to the drawings, the aluminium surfaces are coated by copper layers which form the outer surfaces of the printed circuit board substrate, various elements can be easily attached to the board by soldering in the same manner as in the case of conventional printed circuit board substrates. As has been mentioned above, these printed circuit board substrates are free of disadvantages inherent in conventional printed circuit board substrates, possesses flexibility, improves compactness in mounting wiring elements, and increases durability and reliability of operation. It will be appreciated that there are various methods of connection between the flexible dielectric layer and metal layers, as well as different structures of the metal layers. Therefore, various modifications are possible.

Claims: A substrate for a printed circuit board of the kind comprising a metal layer (4, 5) affixed to at least one surface of a flexible dielectric substrate, (2, 3) characterised in that said metal layer is a composite metal layer comprising an inner aluminium layer (4) and an outer copper layer (5). A printed circuit board formed from the substrate according to claim 1.

Inventors: SUZUKI, MASAHIRO
Applicants: JUNKOSHA CO. LTD.
Application year: 1987
Main classification: H05K1
IPC codes: H05K1