Circuit board and method of manufacturing the circuit board

A method of manufacturing 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 formed of a cured product of a prepreg;
• and
• a via formed of a cured product of a via paste, filled in a hole formed on the prepreg to be 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 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., and
• 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,
• the method comprising:
• a preparing step of preparing the prepreg;
• a first step of covering a surface of the prepreg with a protective film;
• a second step of forming the hole on the prepreg through the protective film;
• a third step of filling the hole with the via paste including the first latent curing agent and the second latent curing agent which have different curing start temperatures from each other, the uncured resin mixture to be cured by the first latent curing agent and the second latent curing agent, and the conductive particle;
• a fourth step of peeling the protective film for exposing the protruded portion, which is formed by protruding a part of the via paste, from the through hole after the third step;
• a fifth step of disposing a copper foil all over the prepreg in order to cover the protruded portion;
• a sixth step of press bonding the copper foil to a surface of the prepreg to compress the via paste through the protruded portion for providing a face contact portion formed by face contacts between the conductive particles;
• then
• a heating step including a first heating step of heating the via paste up to 40° C. or more and 200° C. or less for starting a thermal curing reaction of the uncured resin mixture and the first latent curing agent;
• a second heating step of carrying out heating the via paste up to a higher temperature than that in the first heating step and 200° C. or less for softening a prepreg resin in the prepreg;
• and
• a third heating step of carrying out heating the via paste up to a higher temperature than that in the second heating step and 200° C. or less for starting a heat curing reaction of the residual uncured resin mixture and the second latent curing agent,
• wherein the core substrate portion is formed by the first step through the fourth step;
• the second wiring is formed through the fifth step;
• and
• the via is formed through the sixth step and the subsequent heating step.