The invention relates to an electrical circuit comprising electrical conductors formed on a surface of a substrate and to a method of manufacturing such electrical circuits.
Hybrid circuits typically use a substrate of alumina onto which various pastes are printed and fired to form conductors, resistors and insulating layers. The disadvantage of the alumina substrate is that it is brittle and becomes difficult to handle in sizes above about 4 inches square.
In printed circuit board technology there is a trend towards using "lead-less" components as these are directly soldered to the conductor tracks and do not require holes to be drilled in the boards into which component leads are inserted. The cost of drilling of printed circuit boards is a significant proportion of the total cost of the assembly. An article by P. R. Jones entitlted "Leadless carriers, components increase board density by 6:1" which was published in the issue of Electronics International dated 25th Aug. 1981 at pages 137 to 140 describes such a technique and is hereby incorporated by reference. The substrates used in the equipment described in this article were the usual epoxy-glass or polyimide-glass substrates which have significantly different temperature co-efficients of expansion from that of the components which are soldered to the printed tracks. The author of this article states that the differences in temperature co-efficients of expansion of the substrate and the ceramic leadless components did not effect the reliability of the circuits. However other articles suggest that this effect may cause broken joints between the components and the conductor tracks when the circuit is temperature cycled.
It has been proposed to use various metal alloys such as Invar or the Alloy 42 nickel-iron material as a substrate for printed circuit boards. These materials have a temperature coefficient of expansion which is more nearly compatible with that of the leadless components. Such substrates have been described in articles in the issues Electronics International dated 13th Jan. 1981 (pages 48, 53 and 54), 10th Feb. 1981 (pages 44 to 46) and 16th June 1981 (page 46). In all these cases the metal alloys are used as a backing material to give structural strength and are bonded to epoxy-glass layers on which the circuit is printed.