The flow characteristics of a number of underfills were evaluated with quartz dies of different patterns and pitches bonded on different substrate surfaces. Perimeter, mixed array, and full array patterns were tested. Observations on the flow front uniformity, streaking, voiding, and filler segregation were collected. The information was compared with the results predicted by a new simulation code, plastic integrated circuit encapsulationcomputer aided design (PLICE-CAD) under DARPA-funded development. The two-phase model of the combined resin and air takes into account geometrical factors such as bumps and die edges, together with boundary conditions in order to track accurately the propagation of the flow fronts. The two-phase flow field is based on the volume-of-fluid (VOF) methodology embedded in a general-purpose three-dimensional (3-D) flow solver. Index Terms-Capillary flow, filler settling, flip chip, flow simulation, flow streaking, full array pattern, mixed array pattern, organic laminates, peripheral pattern, quartz dies, underfill flow.

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A series of novel underfill materials has been developed. These non-epoxy, thermoplastic-based materials satisfy all the basic requirements for underfill materials: fast flow, fast cure, and reworkable. In addition, these new materials show high modulus, low thermal expansion coefficient, and glass transition temperature (T,) comparable to epoxies. The new materials exhibit excellent flow behavior, taking only 10 seconds for a quarter inch full array die. Fast cure can be achieved within 5 minutes at 150°C. Another distinct feature of these underfill materials is the reworkability. With current commercial epoxy-based underfills, reworking is impractical due to the high crosslinking density. Rework of these underfills was achieved by thermal removal of the malfunctioned die, followed by solvent cleaning of the residue. Laboratory work demonstrated that the chip removal and underfill cleanup process can he completed within 5 minutes.