Patent ID: 8658910

Claim:
A circuit board comprising: a core substrate portion having a first wiring provided on a surface thereof; a second wiring laminated on the core substrate portion through an insulating layer; and a via filled in a hole formed on the insulating layer, such that the via has a protruded portion in the hole, for electrically connecting the first wiring to the second wiring, wherein the insulating layer is formed of a cured product of a prepreg, the via is formed of a cured product of a via paste, the via paste has a first latent curing agent and a second latent curing agent, an uncured resin mixture to be cured by the first latent curing agent and the second latent curing agent, and a conductive particle, both a softening temperature of the first latent curing agent and that of the second latent curing agent are equal to or higher than 40° C. and are equal to or lower than 200° C., and a difference between the softening temperature of the first latent curing agent and that of the second latent curing agent is equal to or higher than 10° C. and is equal to or lower than 140° C., the prepreg is softened between the softening temperature of the first latent curing agent and that of the second latent curing agent, and the via paste is cured by increase in a viscosity and is thus changed into the cured product of the via paste in the hole of the prepreg.