Printed wiring board

A printed wiring board comprising:

• a wiring substrate;
• a build-up wiring layer formed by laminating conductor circuits and at least one interlaminar resin insulating layer alternately on the wiring substrate;
• at least one viahole which electrically connects first and second conductor circuits situated in mutually different layers;
• a solder resist layer provided on the outermost interlaminar resin insulating layer and the outermost conductor circuit;
• a plurality of openings formed in the solder resist layer to expose a plurality of parts of a conductor circuit which forms a plurality of conductor pads for mounting electronic parts, respectively;
• a plurality of solder bumps formed on the plurality of conductor pads, respectively,
• at least one electric part mounted on the conductor pads through the solder bumps;
• and
• an under fill resin sealing between the electronic part and the solder resist layer,
• wherein the conductor pads are aligned at a pitch of about 200 μm or less, and a ratio W/D of a diameter W of the solder bumps to an opening diameter D of the openings formed in the solder resist layer is about 1.05 to about 1.26.