Resultant Load
Internal force exerted by a polymer encapsulant on the solder joints and silicon surfaces within a flip chip assembly. Mismatch between the expansion coefficients of the organic underfill and the metallic solder bumps generates underfill stress during temperature changes. The material shrinks more than the silicon chip during cooling after the curing process.
Compressive force places the solder joints in a state that helps prevent fatigue cracks.
Thermal Mismatch
Differential expansion rates between the die and the substrate drive the magnitude of the underfill stress across the bond line. High filler content in the epoxy resin helps lower the thermal expansion coefficient to better match the copper and silicon components. These stresses are most severe during the thermal cycling required for reliability qualification.
Interface Integrity
Elevated levels of underfill stress can lead to the delamination of the polymer from the surface of the die or the substrate. Such failures often start at the corners of the chip where the strain is highest. Ensuring a clean surface and proper wetting is necessary to maintain the bond against these internal loads.
Package Warpage
Bending of the entire electronic package occurs when the underfill stress is not balanced across the assembly. Warpage can make it difficult to mount the component onto a printed circuit board or cause fractures in the silicon itself. Adjusting the filler content in the epoxy allows manufacturers to tune the mechanical properties to reduce these effects.
