A method of attaching a flexible interconnection circuit assembly to an ink-jet pen cartridge. The cartridge includes a frame structure fabricated of a rigid plastic frame member formed of a first plastic material and a polymeric second material molded to the frame member. A headland region is defined at the tip of a snout region of the cartridge. An ink reservoir is connected through a standpipe defined by the rigid frame material with the headland region. The second plastic material coats the headland region, and portions of flap and tab sides of the snout regions. The interconnection circuit assembly includes a thermoplastic cover layer, a flexible tab circuit and a printhead die and orifice plate affixed to the tab circuit. The interconnection assembly is attached to the headland region after alignment by heatstaking the polymeric second plastic material covering cheek areas of the headland region and areas of the flap and tab sides of the snout region to corresponding areas of the cover layer of the assembly. This results in a strong chemical bond securing the flexible interconnection circuit to the cartridge.