Wiring substrate

A wiring substrate, comprising:

• a core substrate;
• a first build-up part formed on a first surface of the core substrate and comprising a plurality of insulating layers and a plurality of conductor layers;
• a second build-up part formed on a second surface of the core substrate on an opposite side with respect to the first surface of the core substrate and comprising a conductor layer;
• a first covering insulating layer formed on the first build-up part such that the first covering insulating layer is covering an outermost surface of the first build-up part;
• and
• a second covering insulating layer formed on the second build-up part such that the second covering insulating layer is covering an outermost surface of the second build-up part,
• wherein the first build-up part is formed such that the plurality of insulating layers includes a first insulating layer forming an outermost one of the insulating layers, that the plurality of conductor layers is laminated on the insulating layers respectively and includes a first conductor layer formed on the first insulating layer and including a plurality of first conductor pads, and that a tensile strength of the first insulating layer is higher than a tensile strength of each of the insulating layers other than the first insulating layer in the first build-up part, the first covering insulating layer is formed such that the first covering insulating layer has a plurality of openings entirely exposing upper surfaces and side surfaces of the first conductor pads, respectively, and the second covering insulating layer has a plurality of openings exposing portions of the conductor layer in the second build-up part respectively such that the openings of the second covering insulating layer define a plurality of second conductor pads, respectively.