Measurement of the temperature dependent constitutive behavior of underfill encapsulants

Reliable, consistent, and comprehensive material property data are needed for microelectronic encapsulants for the purpose of mechanical design, reliability assessment, and process optimization of electronic packages. In our research efforts, the mechanical responses of several different capillary flow snap cure underfill encapsulants are being characterized. A microscale tension-torsion testing machine has been used to evaluate the uniaxial tensile stress-strain behavior of underfill materials as a function of temperature and strain rate. A critical step to achieving accurate experimental results has been the development of a sample preparation procedure that produces mechanical test specimens that reflect the properties of true underfill encapsulant layers. In the developed method, 75-125μm (3-5 mil) thick underfill uniaxial tension specimens are dispensed and cured using production equipment and the same processing conditions as those used with actual flip chip assemblies. A three parameter hyperbolic tangent empirical model has been shown to provide accurate fits to the observed underfill nonlinear stress-strain behavior over a range of temperatures and strain rates. In addition, the first measurements of underfill mechanical behavior at cryogenic temperatures have been made.