Dual side solder resist layers for coreless packages and packages with an embedded interconnect bridge and their methods of fabrication

A method of forming a package substrate, comprising:

• providing a temporary substrate;
• forming a first plurality of contact pads on a top surface of the temporary substrate;
• forming at least one insulating layer and at least one conductive layer on the first plurality of contact pads, wherein a top conductive layer forms a second plurality of contact pads on a top surface of a topmost insulating layer;
• forming a first layer of solder resist on the top surface of the topmost insulating layer, exposing at least a portion of the second plurality of contact pads;
• removing the temporary substrate to expose a bottom surface of the bottommost insulating layer;
• temporarily planarizing the intermediate structure;
• and
• forming a second layer of solder resist on the bottom surface of the bottommost insulating layer, exposing at least a portion of the first plurality of contact pads.

A method of forming surface finishes on a package structure, comprising:

• providing a coreless package substrate having a first side and a second side opposite of the first side, the coreless package substrate comprising a single build-up structure comprising at least one insulating layer, at least one via, and at least one conductive layer, a first plurality of contact pads on the first side, and a second plurality of contact pads on the second side, a first solder resist layer on the first side, and a second solder resist layer on the second side;
• forming a first protection layer on the first side of the coreless package substrate and depositing a bottom surface finish on the second plurality of contact pads, the bottom surface finish having a first thickness;
• removing the first protection layer from the first side of the coreless package substrate to expose one or more contact pads disposed on the first side;
• forming a second protection layer on the second side of the coreless package substrate and depositing a top surface finish on the first plurality of contact pads, the top surface finish having a second thickness different than the first thickness;
• and
• removing the second protection layer from the second side of the coreless package substrate.