Functional polymer composite encapsulants for electronic packaging

Abstract

Polymer-based materials have attracted more and more interests in recent years for fundamental studies and for practical applications, for they combine material benefits of both the polymer matrix and the inorganic filler. In electronic packages, polymer composites are commonly used for the applications of encapsulants, underfills, and molding compounds using their mechanical, thermomechanical, and optical properties. This thesis is mainly focused on the understanding and applications of nanocomposite materials in electronic packaging. First, high refractive index, silicone-based LED encapsulants were fabricated by incorporating TiO2 nanoparticles. The surfaces of nanoparticles were modified with silane surfactants during and after nanoparticle syntheses, and the method of surface modification significantly affected the particle dispersion and size control, both of which were shown to be correlated to the optical performance of nanocomposite encapsulants. Encapsulant with refractive index > 1.7 and relative transmittance > 90% was demonstrated, and the nanocomposite also showed resistance to thermal cycling degradation under high humidity conditions. Expanding from the study of filler dispersion, the interface between filler and polymeric matrix was further investigated in silica-epoxy nanocomposites for underfill application. A two- layer silica surface modification method was employed, where the inner layer served as coupling agent and the outer polysiloxane layer served to absorb stress and toughen the nanocomposite. Compared to unmodified or silane-modified silica, the two-layer modified silica fillers also showed improved interphase properties as shown in thermomechanical and mechanical properties, including higher glass transition temperatures, lower thermal expansion in the underfills, and stronger silica-epoxy adhesion. With the understanding of underfill composition and properties, we further explored methods to control the flow of nanocomposite underfill and to reduce filler entrapment in solders for 3D IC packaging. Fluid control on hydrophobic/hydrophilic patterned surfaces were simulated to determine the critical contact angles the surface. Superhydrophobic Cu bond pads and hydrophilic Si3N4 were fabricated according to the computational analyses. Self-patterning of underfill was demonstrated, as well as the interconnection bonding using the superhydrophobic Cu. Filler entrapment is shown to be reduced using this technology for enhanced interconnect reliability.