Wiring substrate and method of manufacturing the same

A wiring substrate, comprising:

• a first insulating resin layer;
• a first coupling agent layer formed on the first insulating resin layer;
• a first wiring layer including a first copper tin alloy layer formed on the first coupling agent layer, a copper layer formed on the first copper tin alloy layer, a seed layer formed on the copper layer, and a metal plating layer formed on the seed layer;
• and
• an underlayer wiring layer formed under the first insulating resin layer, a via hole being formed in the first insulating resin layer to reach the underlayer wiring layer,
• wherein the first coupling agent layer is formed on the first insulating resin layer except the via hole, and
• wherein the seed layer is formed to extend from an inner surface of the via hole onto the copper layer, and
• the first wiring layer is connected to the underlayer wiring layer via the via hole.