No flow underfill with novel fluxes

Self-fluxing capability is one of the key property requirements for no flow underfill. Small molecule carboxylic acids or anhydrides are typically used as fluxing agents in no flow underfill formulations. Various performance related issues currently exist, especially for lead-free assemblies. This paper describes no flow underfill formulations with novel fluxing agents. These fluxing agents demonstrate superior fluxing capability, both in the neat form and in formulation. They eliminate voiding issues associated with traditional fluxes, particularly in cyanate ester resin formulations. A very important benefit of the new fluxing agents is that many of them react with the base resin and accelerate curing at relatively low temperatures. This results in improved properties of the cured underfill. For example, the curing temperature of certain formulations is decreased by over 30 degC; while the T_g of the cured materials is enhanced by up to 30degC. This is a dramatic improvement over traditional fluxes, which often decrease the T_g due to the plasticization effect. Such additional benefits make these fluxing agents especially suitable for use in no flow underfill formulations. The robust fluxing capability of these fluxes has been demonstrated in a variety of resin platforms, including epoxy, cyanate ester and acrylate