SMTA International Conference Proceedings


EVALUATION OF LEAD-FREE SOLDER BONDS AND EFFECTS OF THE LEAD-FREE PROCESS ON DEVICES USING ACOUSTIC MICRO IMAGING

Author: Janet E. Semmens
Company: Sonoscan, Inc.
Date Published: 9/21/2003   Conference: SMTA International


Abstract: Solder attach has long been established as a method of bonding for various electronic applications. The applications range from the attach of heat sinks in relatively large power devices to the very small flip chip interconnects. The quality of the solder bonds is critical. For example large voids in the bond of a heat sink will prevent proper heat dissipation, and an open solder connection in a flip chip interrupts operation of the device.

Clearly a method is needed to assess the quality of the bonds. In addition lead free solder manufacturing process changes must be considered, specifically if higher temperatures are needed for reflow of the different materials. Higher temperatures may have a deleterious effect on the devices or packaging of the devices not directly related to the lead free solder bond integrity.

Acoustic micro imaging (AMI) is one technique that is used for non-destructive evaluation of solder bonds and packages. Acoustic micro imaging uses high frequency ultrasound (5 to 500 MHz) to image the internal features of samples. Unlike x-ray and visual inspection, which are also common methods for evaluating solder bonds, ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations and voids). This is unique to ultrasound.

Over the past years much experience has been gained concerning the acoustic detection of various defect types. Also, in working with the manufacturers of the devices, information has been gained concerning the causes of certain failures and phenomena. However, as new products emerge the manufacturing technology changes.

Presently there is an effort to convert to lead free soldering methods. There is a need to determine if and how the new materials will influence the quality and lifetime of the devices. Also the new materials may necessitate changes in the acoustic analysis method used to evaluate the devices. With the evolution of flip chip devices to smaller sizes and/or higher IO count the size of the bumps/bonds has become increasingly smaller. This will create the necessity of even higher resolution in AMI in order to visualize and evaluate the small bonds.

This paper will present examples of AMI solder bonding applications in microelectronics with particular attention to lead- free solder bond evaluation, and discuss AMI developments to meet the challenges presented by the design and manufacturing of various components and assemblies.

Key words: acoustic micro imaging, lead-free solder, flip chip, die attach.



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