Insulating material containing cycloolefin polymer

A high-density assembly board having interlayer-connecting via holes at most 200 μm in diameter, wherein an interlayer insulating film of the board comprises a curable resin composition comprising

• (A) a cycloolefin polymer containing polar groups, wherein the polar groups are selected from the group consisting of a carboxyl group and a hydroxyl group and the rate of introduction of the polar groups in the cycloolefin polymer is within a range of 0.1-100 mol % based on the total number of units in the polymer, and at least 50 mol % of a repeating unit derived from a cycloolefin monomer, and
• (B) a hardener selected from the group consisting of an organic peroxide and a hardener capable of exhibiting its effect by heat,
• and further wherein the thickness of the interlayer insulating film is from 20 to 80 μm, and the via holes are formed by a laser.