A circuit board including conductive layers bonded to both surfaces of an insulating ceramic substrate, with a brazing material disposed therebetween. The conductive layers comprise at least 99.98% by mass of aluminum, and display an average crystal grain diameter within a range from 0.5 mm to 5 mm and a standard deviation .sigma. for that crystal grain diameter of no more than 2 mm. Each conductive layer comprises at least 20 ppm of Cu, Fe and Si. The surface area of the crystal with the maximum crystal grain diameter within the conductive layers accounts for no more than 15% of the surface area of the insulating ceramic substrate.