High-performance epoxy/binary spherical alumina composite as underfill material for electronic packaging

In this study, epoxy (EP)/binary spherical alumina (S-Al₂O₃) composites with a high loading of 50 vol% were fabricated by incorporating different sizes of S-Al₂O₃ into EP to increase the thermal conductivity and yet retain the flowability of the composites. Notably, a distinctly reduced viscosity (21.8 Pa·s) and a highly increased thermal conductivity (1.364 W/m·K) were achieved by a binary S-Al₂O₃ mixture with 80% 30 μm and 20% 5 μm alumina that would have a theoretical maximum packing volume, thus permitting larger available free volume for the motion of particles and hence reducing the friction between them. Also, these EP/S-Al₂O₃ binary composites possessed superior electrical insulation, high thermal stability, significantly reduced thermal expansion coefficient and good mechanical properties. These combined desirable properties indicate that binary S-Al₂O₃ mixtures with an optimized size distribution and maximum packing volume are best candidates to develop high-performance epoxy-based underfill materials which would improve the flip-chip reliability.