Curing Kinetics and Chemorheological Behavior of No‐flow Underfill for Sn/In/Bi Solder in Flexible Packaging Applications

A chemorheological analysis of a no‐flow underfill was conducted using curing kinetics through isothermal and dynamic differential scanning calorimetry, viscosity measurement, and solder (Sn/27In/54Bi, melting temperature of 86 °C) wetting observations. The analysis used an epoxy system with an anhydride curing agent and carboxyl fluxing capability to remove oxide on the surface of a metal filler. A curing kinetic of the no‐flow underfill with a processing temperature of 130 °C was successfully completed using phenomenological models such as autocatalytic and nth‐order models. Temperature‐dependent kinetic parameters were identified within a temperature range of 125 °C to 135 °C. The phenomenon of solder wetting was visually observed using an optical microscope, and the conversion and viscosity at the moment of solder wetting were quantitatively investigated. It is expected that the curing kinetics and rheological property of a no‐flow underfill can be adopted in arbitrary processing applications.