US Patent 7402760 Multi-layer printed wiring board and manufacturing method thereof

A multi-layer printed wiring board has a core substrate, a throughhole structure, a first interlayer insulation layer, a first via, a second interlayer insulation layer and a second via. The core substrate has a throughhole opening, and the throughhole structure is formed in the throughhole opening. The first interlayer insulation layer is formed over the core substrate. The first via is formed in the first interlayer insulation layer and has a bottom portion having a first radius. The second interlayer insulation layer is formed over the first interlayer insulation layer and the first via. The second via is formed in the second interlayer insulation layer and has a bottom portion having a second radius greater than the first radius. The first via is positioned inside a circle having a radius (D1) from a gravity center of the throughhole opening, and the radius (D1) of the circle satisfies a formula, (D1)=(R)+(r)/3, where (R) represents a radius of the throughhole opening and (r) represents the first radius of the first via.