Insulating substrate and manufacturing method thereof

A manufacturing method of an insulating substrate, comprising:

• in the insulating substrate in which one principal surface of a heat-dissipation-side metal plate is bonded to one principal surface of a ceramic substrate via a brazing material layer provided therebetween that protrudes past an end portion of the heat-dissipation-side metal plate in a direction parallel to the one principal surface of the heat-dissipation-side metal plate and one principal surface of a circuit-side metal plate is bonded to another principal surface of the ceramic substrate via another brazing material layer provided therebetween,
• forming a plating resist on another principal surface of the heat-dissipation-side metal plate, wherein the another principal surface of the heat-dissipation-side metal plate is opposite from the one principal surface of the heat-dissipation-side metal plate in the direction parallel to the one principal surface of the heat-dissipation-side metal plate, and wherein the another principal surface of the heat-dissipation-side metal plate is directly bonded to a heat sink;
• and
• immersing the insulating substrate on which the plating resist is formed in a Ni plating solution to form a Ni plating layer on a surface of the brazing material layer that protrudes past the end portion of the heat-dissipation-side metal plate on which the plating resist is not formed and then peeling the plating resist.