Pan Pacific Symposium Conference Proceedings


EVALUATION OF UNDERFILL IN FLIP CHIP AND BGA ON PC BOARDS USING 3V RECONSTRUCTION AND THROUGH- TRANSMISSION IMAGING

Author: Janet Semmens
Company: Sonoscan, Inc
Date Published: 1/25/2000   Conference: Pan Pacific Symposium


Abstract: Solder bump technology is being used increasingly as a method of interconnection of chips or packages to the substrate. Underfill is used to encapsulate the solder joints commonly in Flip Chips and is starting to be used between BGA packages and the boards. The condition of the underfill layer is critical to the operation of the device as this layer serves to redistribute the mechanical stresses in the sample and thereby reduce the stresses on the solder interconnections[1]. Voids and delaminations in the underfill can compromise the reliability of the device and seriously reduce its lifetime. For reliability assessment of the underfill a nondestructive method is needed. X-ray imaging has been used to examine the condition of the solder bumps in flip chip and BGA packages; however, the contrast provided in the x-ray images is insufficient for analysis of the underfill. Acoustic Micro Imaging (AMI) is a method that has been used over the past several years for evaluation of underfill in flip chips with successful results. More recently the application of AMI to study underfill has been extended to underfill evaluation of BGA packages mounted on PC boards.

Acoustic microscopes utilize high frequency ultrasound to examine the internal features in materials and components. The analysis methods presented in this study include through- transmission and three-dimensional reconstruction of the devices. Both techniques render the entire volume of the component visible. Through- transmission can image the entire thickness of the part/board assembly in one scan but the information is not level specific. The 3V method reconstructs a virtual part from individual, focused reflection mode scans taken at specific depths in the device or assembly. The part can be repeatedly sectioned and reassembled to gain a three- dimensional perspective of the features and flaws within a device. The combination of the analysis techniques allows for accurate detection and detailed characterization of any flaws that are present in the underfill. The information can then be used to help determine optimum process parameters for underfill dispensing. AMI can be used as well for inspection to monitor the process after set up and for quality control applications.

This paper will discuss the 3V reconstruction and through-transmission techniques and how they are applied to analyzing underfill in flip chip and BGA attach.



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