Patent ID: 8563869

Claim:
A circuit board comprising, an insulating-ceramic substrate, a metal circuit plate, and a metal heat sink having a thickness which is equal to a thickness of said metal circuit plate, the circuit board having a structure in which, one face of said metal circuit plate is bonded to one face of said insulating-ceramic substrate, one face of said metal heat sink is bonded to another face of said insulating-ceramic substrate, and a ceramic substrate is bonded to neither another face of said metal circuit plate nor another face of said metal heat sink, such that only one insulating-ceramic substrate is used, wherein, an in-plane fracture toughness value of said insulating-ceramic substrate, estimated from a length of a horizontal crack formed by a Vickers indenter on a vertical cross section of said insulating-ceramic substrate alone, is 5.5 MPa·m 1/2 or higher, a thickness of said insulating-ceramic substrate is in the range of 0.2-1.0 mm, both said metal circuit plate and said metal heat sink are made of copper, and the thickness of said metal circuit plate and the thickness of said metal heat sink are in the range of 0.5-5 mm, a ratio of gross area of said metal circuit plate to gross area of said metal heat sink is 5/9 or more, said metal circuit plate is bonded to said insulating-ceramic substrate and said metal heat sink is bonded to said insulating-ceramic substrate via an active metal brazing filler, and a bonding temperature is set at 600° C.-900° C., an internal fracture toughness value of said insulating-ceramic substrate, estimated from lengths of both horizontal and vertical cracks formed by a Vickers indenter on a vertical cross section of said insulating-ceramic substrate alone, is in the range of 5.5-7.5 MPa·m 1/2 , an in-plane apparent fracture toughness value of said circuit board, estimated from a length of a horizontal crack formed by a Vickers indenter on a vertical cross section of said insulating-ceramic substrate on which both said metal circuit plate and said metal heat sink are bonded, is in the range of 3.0-6.5 MPa·m 1/2 , and an absolute value of an amount of camber of said circuit board is 80 μm/inch or less.