Enhancement of underfill encapsulants for flip-chip technology

This paper presents results on adding silane coupling agents to the underfi ll encapsulant to enhance the rheology and wetting of the underfill. These results include rheology measurements, contact angle measurements, and in s itu flows using a simulated test chip on an FR4 with solder mask substrate. Three properties of the underfill encapsulant that can affect the mechanic al reliability of the die and substrate assembly are: CTE; elastic modulus; and adhesion to the die and substrate surfaces. The approach taken in this paper is to add silane coupling agents with different chemistries to the u nderfill encapsulant to provide interfacial coupling of the underfill mater ial to different die and substrate materials. This paper presents results o n the enhancement of the adhesion of underfill encapsulant to silicon (Si), silicon nitride (SIN) die passivation, benzocyclobutene (BCB) die passivat ion, and solder mask surfaces. The adhesion strength was measured by die sh ear testing.