Micross

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Which Frequency is Best for Wirebonding?
Keywords: Wirebonding, Ultrasonic Frequency, Optimizing
Perhaps surprisingly, in the impressively long history of wirebonding no consensus seems to have been reached as to which ultrasonic frequency is best used. In industrial practice, frequencies from 40 kHz up to 160 kHz are currently employed while for experimental purposes, lower and higher frequencies from around 25 kHz up to 300 kHz are discussed. More strangely yet, there appears to be not even a clear set of rules about which frequency to choose for a particular application. We present a discussion of the pros and cons of different ultrasonic frequencies ranging from 40 kHz to 160 kHz in which the majority of our wire bonders work. Very broadly speaking, one should use a frequency as high as possible and as low as necessary. Higher frequencies allow shorter bonding times and permit bonding on more sensitive surfaces, but suffer from a narrower parameter window. This is mainly due to the smaller vibration amplitudes at the tool tip. The major benefit for many applications is that higher bond quality is possible even at lower wire deformation which additionally creates a healthier bond heel. Lower frequencies, on the other hand, seem to have advantages for rougher surfaces where some planarizing and smoothing is required before bonding can start, while running a higher risk of damaging the bond after forming it. Recent high-speed microscope video also gives some insight into the mechanisms occurring at the interface between bond pad, wire surfaces and tool tip. They will be discussed in detail in this presentation, as will experimental work with some of the revealing data which can be collected from modern ultrasonic generators during the wire bonding process. The presentation will focus on wedge-wedge bonding for both thin and heavy aluminium wire and will also cover some aspects of gold-ball wire bonding.
Josef Sedlmair,
F&K Delvotec Bondtechnik
Ottobrunn 85521,
Germany


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