Patent ID: 7936569

Claim:
A method of manufacturing a circuit device, comprising: forming a conductive pattern on a surface of a substrate, the conductive pattern including a first pad covered with a plated film and a second pad smaller than the first pad, wherein the substrate is comprised of metal and an upper surface of the metal substrate is covered by a insulating layer and the conductive pattern is formed on the insulating layer, and wherein the plated film is comprised of nickel and the conductive pattern is comprised of Cu; applying solder paste, said solder paste comprising a mixture of flux and solder powder, to the plated film formed on the surface of the first pad directly and heating and melting the solder paste to form solder directly on a nickel-comprising surface of the plated film in such a manner that at the start of the step of applying solder paste there is no intermediate layer between the solder paste and the nickel-comprising surface of the plated film, wherein an alloy layer of an intermetallic compound that includes Ni and Sn is formed on a boundary between the solder and the plated film, and wherein said solder paste is Pb-free and contains Sn, Ag and Cu; fixing a small signal transistor or a chip component to the second pad; fixing a heat sink on which a power transistor is mounted to the first pad by remelting the solder formed on the plated film which covers the surface of the first pad; wherein said solder paste includes sulfur that is provided in a proportion of 20 to 80 PPM within said flux, and wherein said solder paste is made by first dissolving a granular sulfur in a solvent, then after the solvent containing sulfur and flux are mixed, the flux and solder powder are mixed, and wherein a proportion of the flux contained in the solder paste is about 5 to 15 wt %.