Power-module substrate unit and power module

A power-module substrate unit comprising:

• a ceramic substrate layer;
• a circuit layer structured from a plurality of small circuit layers joined on one surface of the ceramic substrate layer;
• a metal layer joined on other surface of the ceramic substrate layer;
• and
• one radiation plate joined on the metal layer, wherein
• each of the small circuit layers has a layered structure comprising a first aluminum layer joined on the one surface of the ceramic substrate layer and a first copper layer joined on the first aluminum layer by solid diffusion bonding;
• the metal layer is made from a same material as that of the first aluminum layer;
• the radiation plate is made from copper or copper alloy and joined on the metal layer by solid diffusion bonding;
• and
• a ratio (t1×A1×σ1)/(t2×A2×σ2) is not smaller than 0.80 and not larger than 1.20:
• where a thickness of the first copper layer is t1 (mm);
• a bonding area of the first copper layer is A1 (mm2);
• an yield stress of the first copper layer is σ1 (N/mm2);
• a thickness of the radiation plate at a bonding position to the metal layer is t2 (mm);
• a bonding area of the radiation plate is A2 (mm2);
• and an yield stress of the radiation plate is σ2 (N/mm2).