Adhesion of polymer/inorganic interfaces: Effects of filler addition, environment and chemistry

Miniaturization of microelectronic devices is increasingly limited by the ability to mechanically and electrically connect the chip to the substrate. Structures required for high-density packaging must be fabricated with layer thicknesses and feature sizes approaching micron size scales. Second phase inorganic particles are often added to the underfill epoxy that surrounds a ball grid array (BGA) to achieve the mechanical and thermal properties necessary to protect the electrical solder connections. These filler additions cause a variation in mechanical properties throughout the underfill layer and introduce a distribution of potentially weak interfaces, both of which can affect the behavior of the underfill/passivation interface and overall reliability of the package during thermal cycling of the device.; A fracture mechanics approach was used to study the adhesion of SiN _x to two silica-filled model underfills based on either a cycloaliphatic or bisphenol F epoxy. The influence of filler content and distribution on critical adhesion and debond path selection was examined. As the filler content increased, the elastic particles constrained the plastic deformation in the epoxy, causing the critical adhesion of the underfill/SiN_x interface to decrease. For the bisphenol F system, however, this embrittling effect was countered by weak adhesion between the epoxy matrix and the silica, resulting in microcrack toughening and higher interfacial adhesion values. At high filler contents, the dense concentration of microcracks directed the debond away from the interface and into the underfill layer.; Over the lifetime of the device, interfaces are susceptible to moisture-assisted debonding, progressive debond growth at loads well below the critical interface fracture energies. This effect is often seen for interface systems such as aliphatic/SiN_x and is similar to the stress corrosion behavior observed for bulk glasses. The bisphenol F/SiN_x interface surprisingly only exhibited subcritical debond growth at extremely low growth rates. This low growth rate behavior was examined with respect to variations in temperature and humidity as well as changes in the epoxy and interface chemistry. Potential mechanisms are proposed.