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Frequency Interactions with Semiconductor PackagingUsing Thermosonic Wire Bond Interconnects
Keywords: wire bond, ultrasonic, resonance
Wire bonding has been and continues to be the method of choice for interconnection of semiconductor die through a package to the outside world. Early thermo-compression wire bonding processes involved high temperatures (300C+), forces and sometimes low frequency vibrations (60Hz) to form an electrical connection between bond pads on the silicon die with gold wire. Early development of ultrasonic transducers provided the ability to form interconnects at lower temperatures, forces and time paving the way for development of todays high-performance thermosonic wire bonders. The first ultrasonic wire bonders utilized transducers that operated at frequencies in the 60kHz region. The choice of this specific frequency had more to do with mechanical envelopes, available materials and control capabilities at the time then actual studies of the bonding process itself. Fortunately 60kHz worked, and so remained the dominant frequency until the mid-1990s. At this point, higher frequency transducers in the range of 100-150kHz were developed. Temperatures, which dropped dramatically with initial use of ultrasonic energy, could be further reduced, increasing flexibility for packaging materials. Additionally, bonding time reduced increasing productivity at a time when the I/O count of the high-end ICs was rapidly increasing. Even with lower temperatures and shorter bond times, improved inter-metallic formation was achieved enhancing reliability of the interconnection. Without question, wire bonding would not be where it is today without the advent of ultrasonic energy. However, the same energy can in some cases interact with the package in a negative fashion during bond formations. The interaction is sometimes pervasive at bond time, having dramatic affects on standard bonding responses. More insidious are conditions where the effects are subtle, only showing up as failures after the part is built into end-user consumer devices. This paper is intended to help the reader recognize symptoms of such interactions. Case examples of the problem are presented with detailed descriptions and data related to the effect on the wire bonding process. Instrumentation of the phenomenon happening in real time provides clear evidence of the underlying causes for the problem. Structural modeling will be presented to support the instrumented findings. Finally, suggestions to deal with the problem are presented. These include both package design recommendations along with bonding process parameter optimizations.
Jon Brunner, Senior Staff Engineer
Kulicke & Soffa Industries Incorporated
Fort Washinton, PA

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