Lid Adhesive

In flip chip packages, the lid adhesive (also called lid attach adhesive ) is a specialized material used to bond the heat spreader lid (often a copper or nickel-plated lid) to the substrate. Its main purposes are 
(1) Mechanical bonding – fixes the lid firmly in place over the die and underfill region.
(2) Thermal management – provides a thermal path between the die/underfill and the heat spreader for better heat dissipation.
(3) Stress relief – accommodates coefficient of thermal expansion (CTE) mismatches between lid, substrate, and silicon.
(4) Environmental protection – seals against moisture and contamination.