Patent GB-1501940-A (GB)
Title: Programmable memory device comprising conductive film elements
IPC: G11C17/16
Filed: 1976-06-14
Abstract: 1501940 Printed circuit memories WELWYN ELECTRIC Ltd 14 June 1976 [10 July 1975] 28997/75 Heading H1R [Also in Division G4] A memory, e.g. for pattern recognition apparatus (Fig. 1) is formed on a substrate 1 of, e.g. epoxy glass laminate on which plural first copper film conductors 2 are disposed parallelly on a first face and plural second copper film conductors 3 are disposed parallelly on a second face orthogonal to those on the first. Apertures 4 extend through the substrate and conductors 3 and also through copper pads 5 on the first face formed simultaneously with the conductors; the aperture walls being coated with, e.g. copper, to interconnect conductors 3 and pads 5; the copper conductive elements and coatings being protected by plated tin lead alloy. Meandered nickel phosphorus resistances 6 on the first face are connected between a conductor 2 and a conductor 3 over pad 5 and the walls of apertures 4, so as to form a plurality of cells each comprising a pair of conductors 2, a pair of conductors 3, four resistances 6, and two coated apertures 4; the conductors on one face terminating in broad gold plated elements at an edge of the substrate which mate with known printed circuit board edge connectors (Fig. 3, not shown). Alternatively (Fig. 4, not shown) corresponding pairs of elements may be provided on opposed faces connected directly and over plated through holes to alternate conductors on the one face. The conductors of the other face may be connected similarly to elements at the opposite edge. The memory is programmed by open circuiting selected resistances 6 by mechanical severance, e.g. by a laser beam or by electrical burn out, e.g. at 10. In a modification (Fig. 2) a substrate has plural paired parallel elements 2 with one on one face at 2a and the other on an opposed face at 2b. Plural second paired parallel elements 3 are located in part on one and in part on the other orthogonal to the elements 2 arranged to be on the opposite face to elements 2a, 2b where they traverse these. The ends of elements 3 overlap on opposed faces of the substrate and are interconnected for continuity by apertures 4 whose walls are, e.g. copper coated. The elements are plated with lead tin alloy, and plural nickel phosphorous resistors 6a, 6b are provided on the first and the second faces each to connect an element 2a, 2b with an element 3. A plurality of cells, e.g. 7, are thus formed comprising each a pair of elements 2a, 2b and a pair of elements 3 with four resistors 6a therebetween so that each element 2a, 2b is connected respectively to each element 3 by a separate resistor. Edge terminations are provided at the substrate edges for the elements 2a, 2b, 3 (Fig. 5, not shown) to engage printed circuit board connectors, and the memory is programmed as before by selectively severing the resistors at, e.g. 10 (Fig. 2). In production the conductive elements may be formed by photo etching a copper clad laminate, the through holes by drilling, electroless plating and electroplating, and the resistors by electroless deposition from a buffered hypophosphate and nickel salt over a surface activation by a solution of stannous and palladium ions. Alternatively an epoxy glass laminate may be used coated with nickel phosphorus overlain with copper on one face and plain copper on the other; a pattern of photoresist defining resistors 6, pads 5, conductors 2 and terminations being imposed on one face and a similar pattern defining conductors 3 and terminations being imposed on the other. The exposed copper is etched out by acid ferric chloride, the exposed nickel phosphorus is etched out using acid copper sulphate, and the photoresist is removed. Apertures 4 are drilled, walls are electrolessly copper plated together with the exposed copper element pattern, and a further photoresist defining the required elements 2, 3 is deposited. The apertures and exposed elements are then electroplated with copper and with tin lead alloy; after which the photoresist is removed and the copper overlying the nickel phosphorus resistors is etched out with acid ferric chloride. In producing the modification of Fig. 2, a substrate with nickel phosphorus overlain with copper on both faces is used.