Which Frequency is Best for Wirebonding?

Abstract

In industrial practice, frequencies from 40 kHz up to 160 kHz are currently employed for wire-bonding while for experimental purposes, lower and higher frequencies from around 25 kHz up to 300 kHz are discussed. Typically, heavy aluminium wire or ribbon is preferably bonded at lower frequencies between 40 and 80 kHz, while thin aluminium or gold wires are wedge- or ball-bonded at frequencies between 100 and 140 kHz. We present a discussion of the pros and cons of different ultrasonic frequencies by looking at the mechanics during the bonding process. 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 which can be employed for higher frequencies. 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 begin to take place, while running a higher risk of damaging the bond after forming it. Resonance effects can seriously damage bond formation, and ways are discussed on how to deal with this problem.