Patent ID: 8309856

Claim:
A circuit board comprising: a substrate; a conductor layer formed on the substrate; an insulation layer formed on the substrate and the conductor layer, the insulating layer having an opening with an undercut therein, the opening reaching the conductor layer; a metal layer formed in the opening of the insulation layer and connected to the conductor layer; a solder layer formed in the opening of the insulation layer and outside of the opening; and an alloy layer formed in a boundary region between the metal layer and the solder layer in the opening, the alloy layer comprising a metal of the metal layer and a composition of the solder layer, the alloy layer being more fragile than the metal layer and being formed in a position misaligned from an edge of the undercut of the opening formed on the insulation layer such that the alloy layer is positioned on a side of the edge which includes the solder layer, wherein a distance from the edge of the undercut to the alloy layer is 6-30 μm.