Process Control Inspection Of Surface Mounted Components Using Acoustic Micro Imaging – A Review
Author: Janet E. Semmens Company: Sonoscan, Inc. Date Published: 10/16/2011
Abstract: Acoustic micro imaging has been used in the microelectronics industry for years to evaluate the quality of devices and in some instances to evaluate the quality of their attachments to substrates. As this method is non-destructive components can be evaluated at various stages of the manufacturing process, screened for possible internal defects before mounting to the substrates, and/or tested before and after various stages of life cycle testing. AMI (Acoustic Micro Imaging) is a non-destructive test method that utilizes high frequency ultrasound in the range of 5 MHz to 500 MHz. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations, cracks and voids). There is a direct relationship between frequency and resolution in AMI. Higher frequencies have shorter wavelengths and, therefore, provide higher resolution. Lower frequencies, which have longer wavelengths, provide better penetration of the ultrasound energy through attenuating materials, thicker materials or multiple layer assemblies. Generally a compromise is found between sufficient resolution and maintaining satisfactory penetration and working distance for a given application. For example un-encapsulated flip chip assemblies can make use of high frequency ultrasound to achieve the maximum resolution of the small features. Plastic encapsulated IC packages need lower frequencies to penetrate through the molding compound. However, currently there is a trend to miniaturize the plastic encapsulated devices and to use molded underfill methods to encapsulate flip chips prior to the inspection stage in the manufacturing process. This has necessitated changes in the frequencies used and the design of the transducers to keep current with the designs of the devices. This paper will present an overview of AMI applications pertaining to surface mount processes and discuss AMI developments to meet the challenges presented by new device designs and manufacturing processes.