GB1054797A - - Google Patents

Abstract

1,054,797. Printed circuits. NORTH AMERICAN AVIATION Inc. Sept. 12, 1963 [Sept. 24, 1962], No. 36017/63. Heading H1R. A multilayer printed circuit board having internal interconnections between printed wiring at different levels is made thus, starting with an insulating substrate 12 of epoxy resin impregnated glass cloth clad on both sides with copper sheets 10, 11. Holes 16<SP>1</SP> ... 19<SP>1</SP> are formed in copper sheet 11 by a photo-etch process using FeCl 3 solution as etchant; then registering holes 1611, 17<SP>11</SP> &c. are etched in substrate 12 using an etchant composed of a solution of H 2 SO 4 and HF. The walls of the holes and the exposed surfaces of sheets 10, 11 are electroless-plated with Cu, which coating is oxidized and then reduced with formaldehyde solution to form a rough surface on the walls of the holes; the Cu coating is removed from sheet 11, as by abrading. Next the whole is electroplated with Cu; then the holes are filled with conductive material to connect plates 10 and 11 together. The conductive filler may be a mixture of Cu, Ag, or Au and resin, or an alloy of Ga, In, Hg &c. with Cu, Au, Ag, &c. A preferred filler comprises a eutectic of Ga and In mixed with Au, and may be applied to the holes by squeezing in the plastic state; alternatively, equal weights of powdered In and Au are placed in the holes and heated to alloy; or a sphere of a mixture of powdered metals such as Au, Pb and Sn is placed over each hole and heated. Preferably the assembly is again electroplated with Cu. A wiring pattern is formed in Cu sheet 11 by photo-etching; if desired a pattern may be formed in sheet 10 simultaneously or subsequently. A further sheet of insulating material clad with a third conductive sheet is laminated over the pattern formed in sheet 11 and the process is repeated; and so on, for all subsequent layers.