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The elastic properties of processing related inhomogeneities, such as filler settling and voids, are characterized using the acoustic microscope, A procedure to calculate the acoustic impedance of materials, and hence the elastic properties, at internal interfaces is proposed. The acoustic impedance of the silica filled polymer material under the die is measured at different locations corresponding to areas of different brightness in the acoustic image. Correlating the acoustic impedance measurements with the HS-model indicated that a) darker areas are regions where the underfill has a homogeneous distribution of filler b) lighter areas are regions characterized by filler settling. Destructive cross sectioning and microscopy confirmed the above predictions. Further, the elastic properties of the different areas adjacent to the die are estimated using the model. The relatively quick, nondestructive technique presented in this paper could be useful in advahced process control, rapid yield management and in providing input into package reliability studies (such as finite element analysis).

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The elastic properties of IC packaging materials (molding compounds and underfills) are measured nondestructively, over a microscopic area, in this study. The first application involves determining the engineering moduli of molding compounds (MP8000CH, MP190ML, 6300HA and 7320) from a nondestructive measurement of ultrasonic velocities. The second application involves the characterization of processing flaws in flip-chip packages to a) unambiguously identify delaminations (or voids) from resin rich areas, and b) quantitatively estimate the relative change in filler content. Concomitant changes in elastic properties of the underfill are also determined. Regions of filler segregation, exhibiting a lower impedance of about 3.9 MRayl (26% lower) are estimated to have a Young’s modulus of 7 GPa (50% lower) and a CTE of 41 x 10-6/°C (57% higher than the base material). Techniques presented in this paper have potential application in advanced process control, yield management and in analysis of package reliability.