Formulation of Percolating Thermal Underfill by Sequential Convective Gap Filling

Abstract

In 3D chip stacks, heat dissipation through wiring layers and the bonding interface contributes to the total temperature gradient. The effective thermal impedance of micro solder-ball arrays filled with a poorly-conducting silica underfill can be as high as 30 K*mm2/W, three times the value of a thermal grease interface. Efforts to improve the underfill conductivity to 5 W/(m*K) are underway, which would translate into in a significant interface-resistance reduction. To achieve thermal conductivities >1 W/(m*K), alumina particles were introduced in capillary underfills at particle loadings above the percolation threshold, but at these loading levels the high viscosity of the resulting underfill no longer permits capillary filling. We propose a novel sequential gap-filling method. Particles are suspended in a carrier fluid at a low concentration (0.1 vol%). Using forced convection, the suspension is injected into the cavity formed between the IC dies by the C4 array. A filter element at the cavity outlet triggers particle accumulation in the cavity. The particles form a percolation network with an effective thermal conductivity of >1 W/(m*K). Next an evaporation step removes the carrier fluid, and the exposed pores between the particles are refilled with a particle-free adhesive using capillary forces. Finally, the matrix is cured at 65 °C. 10x10 mm2 standard and micro-C4 cavities (>30 μm) can be completely filled in 2 min at 0.2 bar, resulting in a homogeneous volumetric fill of 36%. Percolation was identified by SEM inspection. For the micro-C4 arrays filler particles of < 10 μm were used. Uniform particle filling is precluded because of the longer filling time due to the small pore sizes. Particle trapping sites are introduced to form local stacks that provide an additional drainage network to guarantee acceptable filling times. Effective thermal-conductivity values of the percolating thermal underfill method proposed here are reported.

Investigation of novel solder patterns for power delivery and heat removal support

“…The electrical joint is a major thermal bottleneck due to the low area fill fraction of approximately 21% in case of fully populated solder ball arrays (dummy solder balls are inserted to yield full population) [ 13,14].…”

Formulation of Percolating Thermal Underfills Using Hierarchical Self-Assembly of Microparticles and Nanoparticles by Centrifugal Forces and Capillary Bridging

“…The self-assembly of filler particles by viscous drag has already been demonstrated [7]. The micron-sized particles were suspended in water and pumped through the cavities with a filter placed at their outlet.…”

“…3) Backfilling: A particle-free capillary underfill is finally dispensed along the chip etch to penetrate the particle bed including the necks, completing the sequential underfill composite (Fig. 2, panel 3) as discussed in [7].…”

“…Figure 17b shows a percolation path with enhanced thermal contacts (two collars and one neck) between the two surfaces of the cavity. The achieved neck reinforcement allows the completion of the enhanced thermal underfill by capillary injection of the unfilled adhesive matrix as the final process step (as reported in [7]).…”

“…1b). In both applications, a major heat-removal bottle-neck is the solder ball array interface between die and laminate or individual dies, when using traditional packaging materials [6,7]. The space between the solder balls (denoted here as cavity) is traditionally filled with a capillary underfill, composed of silica spheres dispersed in an adhesive matrix, to mitigate mechanical stress concentrations and therefore improve mechanical reliability [8].…”