The invention relates to a resistor of SMD construction (SMD is the abbreviation for Surface Mounted Devices).
To an increasing extent, SMD technology is finding application in the industrial manufacture of flat assemblies for utilization in electronic and electrical devices and systems. In this surface mounting technique, the components are not connected to a printed circuit board with wire-like connecting pins. Rather, they are soldered on with flat contact elements. The soldering can be effected by a Reflow method (remelting soldering) or by other known methods, such as flow or wave soldering using a tin bath. A substantial advantage of surface mounting is the reduction and better usage of the space required on the printed circuit board. The smaller the components the greater this advantage.
In typical SMD film resistors, which may also be termed chip resistors, the resistive film or layer is connected, by means of conductive connecting layers situated at the edge of a substrate, to two solderable outer caps which engage around the edge of the substrate and which are soldered as contact elements to the printed circuit board. See German Patent No. 37 05 279. In order to manufacture these resistors, a number of resistive layer elements can be produced between slit-shaped recesses parallel to one another, using a thick film technique, on a common substrate plate of insulating material such as aluminum oxide. Thereafter, electrode layers are applied, extending on the rear surface of the substrate from the edges of the resistive layer elements through the slit-shaped recesses, by a thin film technique to form the outer caps. Subsequently, the substrate is split transverse to the slits and the resistors produced are thereby separated. Instead of this procedure, a resistive layer can be applied by a thin film technique onto a strip-shaped insulating substrate which is provided at its edges with an electrode film to form the outer caps. Thereafter, the resistive layer is etched to form a predetermined pattern of resistive elements and finally the substrate is split to separate the resistors. With this method the resistors may not be manufactured in large numbers without difficulty. Moreover, the resistors may not be suitable for high currents and large power losses, particularly due to relatively poor thermal dissipation through the substrate's insulating material and its thickness, necessary for strength reasons.
Precision resistors of SMD construction, suitable amongst other things for measuring purposes, with relatively low resistance values in the milliohm range, are already known; the actual resistive element or resistive track may be formed by photoetching from a thin film, for instance of a CuNi alloy. See Europe Patent Publication No. 0 484 756. The resistive metal film of this known component is provided on its side with connector pads of soldering metal with which the resistor is soldered to leads on a printed circuit board, whilst it is secured on its other side to a thermally conductive metal plate, for instance of aluminum, which, however, is not connected to the printed circuit board.