Trenches in wafer level packages for improvements in warpage reliability and thermals

A method of forming a composite die, comprising:

• attaching a plurality of dies to a surface of a base substrate, the plurality of dies comprising pairs of neighboring dies, each pair of neighboring dies comprising a first die having a first sidewall laterally opposite a second sidewall, and a second die having a first sidewall laterally opposite a second sidewall, wherein the first sidewall of the second die faces the second sidewall of the first die;
• disposing an underfill layer under and around the plurality of dies;
• forming a trench into the underfill layer, wherein the trench is between neighboring dies of the plurality of dies;
• disposing a mold layer on the plurality of dies, the mold layer in contact with the first sidewall of the first die and in contact with the second sidewall of the second die of each pair of neighboring dies, wherein the mold layer fills the trench;
• and
• singulating the plurality of dies into a plurality of composite dies.

An electronic package, comprising:

• a package substrate;
• and
• a composite die electrically coupled to the package substrate, wherein the composite die comprises:
• a base substrate;
• a plurality of dies over the package substrate, wherein the plurality of dies comprises a first die having a first sidewall laterally opposite a second sidewall, and a second die having a first sidewall laterally opposite a second sidewall, wherein the first sidewall of the second die faces the second sidewall of the first die;
• an underfill layer under and around the plurality of dies;
• and
• a trench into the underfill layer, wherein the trench is between a first die and a second die of the plurality of dies;
• and
• a mold layer on the plurality of dies, the mold layer in contact with the first sidewall of the first die and in contact with the second sidewall of the second die, wherein the mold layer fills the trench.

A composite die, comprising:

• a base substrate;
• a first die over the base substrate;
• a second die over the base substrate and adjacent to the first die;
• an underfill layer between the first die and the base substrate, between the second die and the base substrate, and between the first die and the second die, wherein a trench into the underfill layer is between the first die and the second die, and wherein the trench has a tapered profile;
• and
• a mold layer over the first die and the second die, wherein the mold layer fills the trench.

A composite die, comprising:

• a base substrate;
• a first die over the base substrate;
• a second die over the base substrate and adjacent to the first die;
• an underfill layer between the first die and the base substrate, between the second die and the base substrate, and between the first die and the second die, and wherein a trench into the underfill layer is between the first die and the second die;
• and
• a mold layer over the first die and the second die, wherein the mold layer fills the trench, wherein a coefficient of thermal expansion (CTE) of the mold layer is lower than the CTE of the underfill layer.