An integrated circuit package assembly formed by stacking flip-chip mounted substrates interleaved with precisely dimensioned spacers and then bonded by injection molding the stack. The sides of the stack are sawed off to expose vias in the substrates, and multilevel-interconnect substrates are precisely aligned on the sides of the stack. Solder pads on the interconnect substrates are reflowed to form a solder connection to the exposed vias, allowing complex interconnection between diverse points along the edge connectors of each substrate. In one embodiment, solder balls are reflowed on ball-grid-array pads at the top of the stack to provide external electrical connections.