An extremely thin copper foil is provided that enables formation of highly fine different wiring patterns with a line/space (L/S) of 10 μm or less/10 μm or less on two sides of the copper foil and is thus usable as an inexpensive and readily processable substitution for silicon and glass interposers. The extremely thin copper foil includes, in sequence, a first extremely thin copper layer, an etching stopper layer, and the second extremely thin copper layer. Two sides of the extremely thin copper foil each have an arithmetic average roughness Ra of 20 nm or less.

A wiring substrate includes an insulating layer including a projection and a wiring layer on the projection. The wiring layer includes a first metal layer on an end face of the projection and a second metal layer on the first metal layer. The width of the end face of the projection is different from at least one of the width of the first metal layer and the width of the second metal layer. An inner wall surface and a bottom surface of a depression around the projection are roughened surfaces.

A flexible electronic structure includes a first film, made of a first polymer or glass, and a second film, made of a second polymer, in which at least one electronic component is arranged. The second film covers the first film. The flexible electronic structure also includes at least one electrically conductive track arranged between the first film and the second film, and each electrically connected to one of the electronic components, by a respective interconnection element. Optionally, the flexible electronic structure includes a third film, made of a third polymer or glass, covering the second film. The interconnection element is arranged near the neutral plane of the structure, and the structure includes at least one compensation layer, so as to position the neutral plane at a desired location.

A printed circuit board includes: a first insulating layer; and a heat radiating circuit pattern disposed on a first surface of the first insulating layer and having a pad and a via. The heat radiating circuit pattern includes: a first metal layer disposed on the first insulating layer; a graphite layer disposed on the first metal layer; and a second metal layer disposed on the graphite layer.

A flexible electronic structure includes a first film, made of a first polymer or glass, and a second film, made of a second polymer, in which at least one electronic component is arranged. The second film covers the first film. The flexible electronic structure also includes at least one electrically conductive track arranged between the first film and the second film, and each electrically connected to one of the electronic components, by a respective interconnection element. Optionally, the flexible electronic structure includes a third film, made of a third polymer or glass, covering the second film. The interconnection element is arranged near the neutral plane of the structure, and the structure includes at least one compensation layer, so as to position the neutral plane at a desired location.

The method includes providing a rigid base plate; forming an etch-stop layer on one side of the rigid base plate; forming a first flexible substrate on another side of the etch-stop layer; fabricating a plurality of stretchable circuits on the surface of the first flexible substrate facing away from the etch-stop layer; etching a first flexible substrate between two adjacent stretchable circuits to form a through hole; forming a first elastic layer that covers the plurality of stretchable circuits and fills the through hole; separating the rigid base plate and the etch-stop layer; and forming a second elastic layer opposite to the first elastic layer such that the plurality of stretchable circuits is encircled by the first and second elastic layers.

An extremely thin copper foil is provided that enables formation of highly fine different wiring patterns with a line/space (L/S) of 10 μm or less/10 μm or less on two sides of the copper foil and is thus usable as an inexpensive and readily processable substitution for silicon and glass interposers. The extremely thin copper foil includes, in sequence, a first extremely thin copper layer, an etching stopper layer, and the second extremely thin copper layer. Two sides of the extremely thin copper foil each have an arithmetic average roughness Ra of 20 nm or less.

A circuit substrate includes a substrate, a wire build-up layer structure, and an insulating layer. The substrate has a first surface and a second surface opposites to the first surface. The substrate includes a plurality of patterned pads. The patterned pads are disposed on the first surface of the substrate, and having contact openings. The wire build-up layer structure is disposed on the first surface of the substrate. The wire build-up layer structure includes an interconnect build-up layer and a plurality of conductive pillars. The conductive pillars electrically connect to the interconnect build-up layer and the patterned pads. The insulating layer is disposed between the substrate and the wire build-up layer structure.

A printed circuit board includes a core layer having a first surface and a second surface opposing the first surface; a first built-up structure disposed on the first surface of the core layer; a second built-up structure disposed on the second surface of the core layer; and a first penetration portion penetrating the first built-up structure and the core layer and penetrating a portion of the second built-up structure. The first penetration portion has a step portion on at least a portion of a wall of the first penetration portion in the region of the first penetration portion penetrating the second built-up structure, and a region including the first penetration portion on a plane is configured as a flexible region.

A printed circuit board includes: a first insulating layer; and a heat radiating circuit pattern disposed on a first surface of the first insulating layer and having a pad and a via. The heat radiating circuit pattern includes: a first metal layer disposed on the first insulating layer; a graphite layer disposed on the first metal layer; and a second metal layer disposed on the graphite layer.