Influences of flip chip layout on the underfilling process

摘要

: For improvement of flip chip reliability the use of underfill material is crucial. The underfill is not only protecting the contacts from environmental influences but is also compensating for the thermal mismatch of the materials used. This is achieved by distributing the mechanical stress concentration from the solder bumps and pads via an epoxy layer to the whole chip/substrate surface. The underfilling has proven to be very reliable for small chips, high standoffs and large pitches. With increasing chip sizes and smaller pitches it is important to optimize the flow properties of underfill materials for fast and void-free underfilling. Vias and leads located under the chip are additional challenges to the underfill process. These features often used in multilayer substrates bear the risk of air entrapment and thus a reduced reliability of the Flip Chip assemblies. In this paper the qualification of a recently developed material for Flip Chip underfilling is described. This qualification includes the determination of the thermomechanic properties and the reliability performance of underfilled Flip Chip modules in accelerated aging tests. Furthermore investigations of an improved version of this material are conducted concerning the rheological properties of the improved material compared to its predecessor and a commercially available material. It is shown that the modification yields a superior underfill material that allows the reliable underfilling of Flip Chips with gaps down to 20μm.