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Underfill materials play a key role in the reliability of flip-chip assemblies on organic circuit boards. Underfill selection has great impact on the interfacial integrity, stresses in the die, and strains in the solder bump. Therefore, it is critical to optimize underfill properties for enhanced reliability of flip-chip assemblies. In this study, effect of underfill properties on interfacial integrity, stresses in the die, and strains in solder bumps is investigated using analytical and experimental methods. Interfacial fracture toughness for die/underfill and underfill/circuit board interfaces are obtained using four-point-bend specimens. Effect of underfill properties on die cracking is investigated by an analytical solution for a three-layered structure. The impact of underfill properties on maximum strains in solder joints and their implications on solder fatigue are evaluated using the moire interferometry method. Results show that a good underfill can be characterized as (a) having adequate interfacial fracture toughness at chip/underfill and underfill/circuit board interfaces, (b) minimizing strains in solder bumps, (c) not causing excessive tensile stress in the die, and (d) having fracture toughness higher or comparable to that of the interfaces. In addition to the above essential requirements, underfill should have a high glass transition temperature, good flow properties, and high moisture resistance.

Cracked glass beads embedded in the plastic housing material of fuel vapor pressure sensors have been observed after a few hundreds of cycles of thermal shock test. In order to diagnose the root cause of the cracked glass bead, the speckle correlation method is used to in-situ measure the thermal deformation field on the surface of the glass bead. Measurements are taken at 130°C, 26°C and −40°C. The speckle patterns at different temperature are recorded digitally by a CCD camera with a resolution of 1024×1024 pixels. Every two sequential speckle patterns are compared to yield the displacement field of the sample. Then the strain fields are calculated by a finite deformation formulation. Results reveal that the material made of the glass bead is highly non-uniform, and there exists a highly tensile strain zone on the surface of the glass bead. This is correlated with the location of cracking in the bead. Moreover, there is a sliding zone along the boundary of the glass bead, which may result in leakage. As a replacement for glass bead, a metal bead embedded in housing material is examined. Experimental results show that strain level in the metal bead is lower than that in the glass bead, which will reduce disturbance on the sensor part. In addition to that, there are no potential cracking and leakage problems in the pressure sensor applications. Therefore, the metal bead is recommended for future pressure sensor applications.