Integrated circuit structure having dies with connectors of different sizes

A structure comprising:

• an interposer having a first surface, the first surface comprising a first die-attach area and a second die-attach area;
• a first die on the interposer in the first die-attach area, the first die being electrically and mechanically coupled by first connectors to the interposer, the first connectors each having a first diameter along the first die-attach area and having a first pitch between respective adjacent ones of the first connectors along the first die-attach area;
• a first underfill disposed between the first die-attach area and the first die;
• a second die on the interposer in the second die-attach area, the second die being electrically and mechanically coupled by second connectors to the interposer, the second connectors each having a second diameter along the second die-attach area and having a second pitch between respective adjacent ones of the second connectors along the second die-attach area, the first diameter being greater than the second diameter, and the first pitch being greater than the second pitch, wherein each of the second connectors comprises:
• a first under bump metal (UBM) structure on a lower side of the second die facing the interposer;
• a first metal pillar electrically and mechanically coupled to the first UBM structure;
• a first electrically conductive layer on a lower surface of the first metal pillar facing the interposer;
• a second UBM structure on the first surface of the interposer;
• a second metal pillar electrically and mechanically coupled to the second UBM structure;
• a solder material between and electrically coupling the first electrically conductive layer and the second metal pillar, wherein sidewalls of the first electrically conductive layer and sidewalls of the first metal pillar are free of the solder material;
• a second underfill disposed between the second die-attach area and the second die, wherein a thickness of the first underfill and second underfill is thinner between the first and second die-attach areas than under their respective dies;
• a substrate electrically and mechanically coupled by third connectors to the interposer, wherein the interposer is between the substrate and the first die;
• and
• a third underfill disposed between the interposer and the substrate, wherein the first underfill, the second underfill and the third underfill are a same material.

A method for forming a structure, the method comprising:

• attaching a first die to a first surface of a substrate, the first die being attached by first electrical connectors, each of the first electrical connectors comprising a solder bump extending from a first under bump metal (UBM) structure of the first die to a first conductive feature on the first surface of the substrate, the substrate having a plurality of connectors attached to a second surface of the substrate opposing the first surface;
• and
• attaching a second die to the first surface of the substrate, the second die being attached by second electrical connectors, each of the second electrical connectors comprising a first metal pillar, a second conductive feature on the first surface of the substrate, and a solder material between the first metal pillar and the second conductive feature, the first metal pillar being attached to a second UBM structure of the second die and having straight sidewalls extending from the second UBM structure to a top surface of the first metal pillar distal the second die,
• wherein the second conductive feature is a second metal pillar attached to a third UBM structure of the substrate, wherein each of the second electrical connectors further comprises a conductive layer between the first metal pillar and the solder material, wherein the solder material fills a space between the conductive layer and the second metal pillar, wherein the solder material extends continuously along sidewalls of the second metal pillar and along sidewalls of the third UBM structure from an upper surface of the second metal pillar facing the second die to the first surface of the substrate, and wherein there is no solder material on sidewalls of the conductive layer or sidewalls of the first metal pillar.