Reliability of a quad-flat-package (QFP) with a circuit-under-pad (CUP) structure is investigated for Cu interconnects with porous low dielectric constant (low-k) films in scaled-down ultralarge-scale integrations. The following experimental factors are discussed: 1) low-k material properties and their stacking structures; 2) CUP structure; and 3) mold compound material properties. The QFP characteristics are analyzed after chip dicing and Ag wire bonding, as well as after molding. Higher adhesion strength of porous low-k film to SiCN cap dielectrics and rigid Cu-anchored CUP structure can achieve highly reliable QFP packaging. A lower coefficient of thermal expansion (CTE) of the molding compound is also found to be effective in eliminating low-k delamination during thermal cycle test because it can reduce the stress at the cracking position. The adhesion-promoting porous SiOCH film with the Cu-anchored CUP in a low-CTE mold is a promising system to realize a reliable QFP with no low-k delamination, passing electrical tests after the pressure-cooker test and high-temperature storage test.