Acoustic Micro Imaging and X-Ray Analysis for More Thorough Evaluation of Microelectronic Devices

Acoustic micro imaging (AMI) uses high frequency ultrasound (5 to 500 MHz) to image the internal features of samples. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps. X-Ray uses short wavelength electromagnetic radiation capable of penetrating most materials to look for discontinuities such as voids in solder bonds. One method may be better for the detection of specific defect types. For instance very thin gaps (delaminations) in a plastic encapsulated package are readily detected using AMI. However, they can be overlooked using X-Ray unless the device is viewed at the correct angle. Another method may be better for accessing the area of interest in the sample through certain types of materials. In plastic encapsulated parts, evaluation of wire bonds is limited using AMI as lower frequencies that have lower resolution are required to penetrate the molding compound. X-Ray however readily penetrates the molding material to provide high resolution images of the bond wires. X-Ray also has the advantage of allowing for rotation of the viewing angle. Although there is overlap of applications between AMI and X-Ray imaging, in some cases one technology can provide information that the other cannot. The examples in this paper will demonstrate how both analysis methods, used together, can provide a more comprehensive evaluation of devices or materials.