An insulation sheet for use in producing a wiring substrate comprises, as
via bole conductors, conductive paste filled in via holes formed through
the insulation sheet, and a curing-starting temperature of the conductive
paste is lower than a melting-starting temperature of the insulation
sheet. A wiring substrate is produced by laminating such insulation
sheets, that have conductive paste in via holes, and subjecting this
laminate to thermo-compression bonding, wherein deformation of via holes
and dislocation of the via holes, because of a molten insulation sheet,
does not occur.
