Fabrication method of multilayer printed wiring board

Claim 1

A fabrication method of a multilayer printed wiring board, said method comprising the steps of: (a) preparing an insulating substrate having a first patterned conductive layer serving as an inner circuit; said first patterned conductive layer being adhered on a surface of said substrate; (b) forming a first insulating layer serving as an undercoating layer on said first conductive layer and said uncovered surface of said substrate; said first insulating layer being made of a material having a photo-setting property that cures due to exposure to light, a softening property that softens at a first temperature higher than a room temperature, and a thermosetting property that cures at a second temperature or higher; said second temperature being higher than said first temperature; (c) exposing said first insulating layer to light, thereby partially curing said first insulating layer; (d) forming a second insulating layer on a surface of a second conductive layer to be coupled therewith; said second insulating layer being made of a material having a thermosetting property that cures at a third temperature or higher; said third temperature being higher than said first temperature; (e) laminating said combination of said second insulating layer and said second conductive layer under heat on said partially-cured first insulating layer so that said second insulating layer is contacted with said first insulating layer; during this step, said first insulating layer being heated up to said first temperature, thereby softening said first insulating layer to reduce its surface irregularity; (f) heating said first and second insulating layers at a temperature equal to or higher than said second and third temperatures, thereby entirely curing said first and second insulating layers and combining said substrate, said first conductive layer, said first insulating layer, said second insulating layer, and said second conductive layer with each other; (g) forming holes to vertically penetrate said substrate; and (h) patterning said second conductive layer to form an outer circuit.