The present invention relates to a burnt conductive paste used for a printed circuit board for an internal wiring material of electronic equipment and the like and a method of manufacturing a printed circuit board using the same.
A conventional conductive paste of this type consists of a metal powder, a glass frit, and an organic vehicle. As the metal powder, metals such as Au, Ag, Ag-Pd, Cu, and the like are used. These conductive pastes are printed on insulating boards and burnt at a temperature of about 900.degree. C. to form thick films. The resultant structures are used as conductor circuits having a specific resistance of about 5.times.10.sup.-5 .OMEGA..multidot.cm or less.
As the insulating boards used for these conductive pastes, ceramic boards, in particular, alumina boards, which do not change at the burning temperature, which have a sufficient adhesive strength with the conductive pastes, and which have good electrical insulating properties, are used.
Since Au, Ag, and Pd are expensive, Cu paste has recently received a great deal of attention. Copper (Cu) is inexpensive and has good characteristics, in that it has a good electrical conductivity and soldering properties, and it does not easily cause migration, and so on.
However, since copper is easily oxidized, it cannot be burnt in air, and must be burnt in a nitrogen atmosphere having a specific oxygen partial pressure in which an organic vehicle can be evaporated and copper is not oxidized, resulting in cumbersome operations. For this reason, a current Cu paste is expensive.
Recently, a conductive paste using copper oxide has been proposed. In this paste, a small amount of a Pt powder, a Pd powder, Ni powder, or a manganese oxide powder is added to a copper oxide powder, the resultant mixture is burnt at a temperature of about 700.degree. C. in air and is then reduced in a reducing atmosphere at a temperature of about 1,000.degree. C., thereby preparing a conductor. In this method, strict atmospheric control is not necessary. However, burning at this temperature aims at the same applications and effects as those of the Cu paste, and the resultant paste is assumed to be applied to a ceramic board such as an alumina board, as the board material. However, since the conductive pastes use expensive noble metals, they are still expensive (Japanese Patent Disclosure (Kokai) Nos. 61-289691, 61-292394, and 61-292393).
Recently, another conductive paste which can be burnt at a temperature of about 500.degree. to 675.degree. C. has been proposed. The conductive paste consists of a metal powder such as a powder of Ag, Cu, Ag alloy, Cu alloy, or the like, a glass frit, and an organic vehicle. The mixture is burnt while an amount of the glass frit is increased in order to improve the adhesive strength to the board and the mechanical strength of the resultant thick film (Japanese Patent Disclosure (Kokai) Nos. 58-71507 and 58-10302).
In addition, still another conductive paste containing a copper oxide powder as a base material is proposed. In this paste, 7 to 27% by weight of boron, 0 to 35% by weight of a glass frit, and the balance of the copper oxide powder are dissolved in the organic vehicle, and the resultant paste is heated in air at a temperature of 500.degree. to 675.degree. C. to prepare a thick film conductor (Japanese Patent Disclosure (Kokai) No. 57-55005).
However, the resultant conductive film obtained by heating the conductive paste in air has an unsatisfactory conductivity of 10 .OMEGA./.quadrature. or lower and poor soldering properties. Therefore, this paste cannot be used for a printed circuit board.
As described above, since the conventional copper oxide based conductive pastes aim at only decreasing a burning temperature of the paste in view of energy cost, the pastes having a low conductivity can only be obtained.