Abstract:
A method and an arrangement for connecting a component, such as a chip ( 6 ), on a substrate ( 7 ) to a conductive surface of a carrier. The conductive surface can be an earth plane ( 10 ) and the carrier can be a printed circuit board. The method and arrangement allow the component to thermally conduct and/or electrically conduct onto the conductive surface.

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9900962-3 filed in Sweden on Mar. 17. 1999; the entire content of which is hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a method for connecting a component, such as a chip or like component, to a conductive carrier surface, such as an earth plane on a printed circuit board or the like, in a compact design, and also to an arrangement for connecting a component so as to achieve conduction between said component and a carrier, such as between a chip and a printed circuit board, in a compact design. 
     DESCRIPTION OF THE BACKGROUND ART 
     It is known to mount substrate.  1  with chip  2  on printed circuit board  3 . In compact designs, with a multi-layer ceramic carrier substrates, the discrete components/the chips are mounted on the underside of the substrate between said substrate and the printed circuit board, wherewith heat generated in the components can be transmitted solely to the substrate and from there to the circuit board and its various connections, such as contact pins or beads  4  of solder that melt at high temperatures located between the substrate and the printed circuit board. The connections function as electrical connections and as mechanical spacing means, this latter by virtue of being slightly larger than the remaining discrete components on the underside of the substrate. The components can be shielded by providing on the circuit board, immediately beneath the components, an earth plane  5  which, together with the earth plane of respective components, shields said components without the discrete components being in contact with the earth plane. The substrate-mounted components are liable to generate heat in operation, which in certain cases may be quite considerable. It is therefore important that the thermal contact achieved between the components/the chips and the printed circuit board with its wide copper surfaces is as effective as possible. Because none of the discrete components have direct contact with the circuit board, the major part of the heat generated in said components must be transferred to the circuit board via the substrate and the circuit board connections. In the case of ceramic substrates, the greatest temperature gradient occurs in the longitudinal direction of the substrate in these instances, because ceramics are relatively poor conductors of heat. Transistors in the components may well become damaged at excessively high temperatures, as a result of the poor thermal conductivity of the ceramic substrate. 
     SUMMARY OF THE INVENTION 
     With the intention of protecting a component, such as a chip, that is mounted on a ceramic substrate connected to a carrier, such as a printed circuit board, from damage as a result of the limited thermal conductivity of the ceramic, the chip has been connected directly to the circuit board with a thin layer that provides the absolute shortest path to the circuit board. Direct connection of the chip with the circuit board can be achieved by soldering the chip so that its height, which is critical, will be essentially the same as the height of the connections, and by mounting the rear side of the chip directly to the circuit board with a thin layer of adhesive or solder. An acceptable substrate earth can be obtained, by metallizing the rear side of the chip and making the connection between the rear side of the chip and the earth plane on the printed circuit board electrically conductive. 
    
    
     The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawing. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a chip connected to a carrier-mounted substrate in accordance with known technology. 
     FIG. 2 shows a chip connected to a substrate which is mounted on and connected to a carrier in accordance with the invention. 
     FIG. 3 shows a chip connected to a substrate mounted on and connected to a carrier-mounted cooling element in accordance with the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In response to requirements for a compact design, such as a design which in respect of components is particularly created for embodiment in telephones or other similar applications, there has been created a new type of module in which discrete components/chips  6  have been mounted on the underside of a carrier substrate  7  that is comprised of several ceramic layers, between said substrate and a printed circuit board  8 , so as to be in contact with said board. In this case, the connections between substrate and printed circuit board are also comprised of beads  9  of solder that melts at high temperatures. The beads function as electrical connections and also as mechanical spacing means, by virtue of being slightly larger than all other discrete components on the underside of the substrate. The printed circuit board may include immediately beneath the substrate-mounted chip an earth plane  10  which, together with the earth plane of the chip, electrically shields sensitive parts of the chip. With the intention of improving the thermal and electrical properties of the chip, the chip has been connected directly to the circuit board with the aid of a thin conductive layer  11 , because of the limited thermal conductivity of the substrate ceramic. This provides the absolutely shortest thermal path to the circuit board. Direct connection of the chip can be achieved by adapting chip height, which may be critical, so that the height of the soldered chip will be almost the same as the height of the beads, or balls, and then fastening the rear side of the chip directly to the circuit board with a thin conductive layer of adhesive or solder. When the rear side of the chip is metallized and the connection between the rear side of the chip and the earth plane on the circuit board are made electrically conductive, there can be obtained, at the same time, effective substrate earthing of the chip. When the rear side of the chip/chips  6  is metallized and a conductive connection  11  to the circuit board  6  is provided, specific chip cooling elements  12  provided on or in the circuit board can be connected directly to the chip/chips. 
     Contact between chip and circuit board can conceivably be effected in chiefly two ways, either by gluing or soldering. In conjunction with soldering the module, the chip can be glued firmly in position by introducing some additional process steps, subsequent to having applied solder paste to the circuit board., by pressing for instance and after having dispensed adhesive to the circuit board where the chip is to be placed. The adhesive or glue used may conveniently be of a kind which will cure in conjunction with the re-melting process. No electrically conductive contact on the rear side of the chip is required in order to obtain solely thermal contact between chip and circuit board. However, a rear-side contact is required in contact with an electrically conductive adhesive, when effective substrate-earthing of the chip is required. 
     The chip can be soldered to the circuit board without requiring any additional process steps. It is necessary, however, that the chip has a solderable rear-side contact, and that the chip is fixed to the ceramic substrate with a bottom filling, so that there is no danger of the chip being moved out of its position when beads or balls of low-melting solder, so-called flipchip bumps, are re-melted. The soldering process per se is very simple, since an earth plane is already present beneath the chip/chips. All that may be required is to make an opening in the protective varnish present beneath the chip, and that a solder paste is also applied in this region. 
     It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof, and that modifications can be made within the scope of the accompanying Claims.