Process for producing a circuit substrate

A process for producing a circuit substrate comprising the steps of:

• preparing a sintered ceramic substrate;
• applying an intermediate layer containing glass and sinterable at a low temperature for forming said circuit substrate on said sintered ceramic substrate;
• printing a wiring pattern, or a resistor on the unsintered transfer sheet unsinterable at a sintering temperature of said intermediate layer;
• stacking said unsintered transfer sheet on said intermediate layer over said sintered ceramic substrate to form a laminated body and thermocompressing said laminated body to form a compressed body;
• firing said compressed body at a sintering temperature of said intermediate layer to form one body of said intermediate layer and said sintered ceramic substrate, thereby preparing a fired body by transferring said wiring pattern on said unsintered transfer sheet to said intermediate layer;
• and
• removing said unsintered transfer sheet from said fired body thereby preparing a circuit substrate.

A process for producing a circuit substrate comprising the steps of:

• preparing at least one ceramic greensheet having a via hole filled with a conductor, a wiring pattern or a combination thereof, and an unsinterable transfer greensheet which is not sintered at a sintering temperature of the ceramic greensheet;
• printing a wiring pattern on said unsintered transfer greensheets;
• stacking said unsinterable transfer greensheets on both surfaces of said ceramic greensheet, which stack is to be formed into a laminated body;
• placing said laminated body in a rigid frame having the shape of the laminated body and providing films of an organic substance selected from the group consisting of polyester, polyphenylene sulfide, polyether imide, polyether sulfone, polyether ketone, polyimide, methylpentene copolymer and paper having a surface roughness of 0.4-0.75 .mu.m Ra, between the laminated body and each of the upper and lower parallel plates of the rigid frame;
• applying upward and downward pressure by said upper and lower parallel plates which sandwich said laminated body, thereby preparing a compressed body while maintaining its planar elongation rate under 0.05%;
• taking out said compressed body from said rigid frame;
• firing said compressed body at a sintering temperature of said ceramic greensheet without compressing said compressed body by a compressing mean;
• and
• removing said unsinterable transfer greensheet from said compressed body, thereby preparing a circuit substrate.

A process for producing a circuit substrate comprising the steps of:

• preparing at least one ceramic greensheet and at least two unsinterable transfer greensheets which are not sintered at a sintering temperature of the ceramic greensheet;
• printing wiring patterns on said at least two unsinterable transfer greensheets;
• stacking said unsinterable transfer greensheets on both surfaces of said ceramic greensheet, which stack is to be formed into a laminated body;
• compressing said laminated body in a liquid by isodirectional pressurization thereby preparing a compressed body while maintaining its planar elongation rate under 0.05%;
• removing said compressed body from said rigid frame;
• firing said compressed body at a sintering temperature of said ceramic greensheet without compressing said compressed body in a compressing means, thereby preparing a fired body by transferring said wiring patterns on said at least two unsinterable transfer greensheets to said ceramic substrate;
• and
• removing said unsinterable transfer greensheets from said compressed body thereby preparing a sintered ceramic substrate.