In addition to the traditional methods of experimental design used in ultrasonic wire bonding, such as Design of Experiments (DoE), the performance of shear and pull tests, and the optical assessment of the bond joint using microscopic images, numerous supplementary experiments and measurements were carried out as part of the research work at the Chair to investigate the influence of bonding process parameters on joint quality:

• Experimental vibration and modal analysis using laser vibrometry,
• low-frequency bonding tests (<1 kHz) to measure dynamic process forces and high-speed recordings of the bonding process for motion-tracking analyses,
• Analysis of ultrasonic material softening for the parameterisation of simulation models.

Experimental vibration and modal analysis is an important tool for investigating the influence of substrate vibrations on bond quality during the bonding process. In order to prevent damage such as delamination of the substrate or damage to existing bonds, and to direct the ultrasonic power into the joint zone as effectively as possible, vibration excitation of the substrate should be kept to a minimum. Bonding tests carried out on a copper rod enabled the influence of substrate resonances on the vibration behaviour during bonding to be investigated. It was found that ‘super-resonant’ bonding is beneficial for reducing substrate vibrations.

In addition to vibration analysis during bonding, the measurement of process forces is also of interest for analysing the formation of the bond during wire bonding. Dynamic force measurement at ultrasonic frequencies involves a great deal of effort due to the necessary calibration of the force sensors in this frequency range. For this reason, the department has developed a test rig for investigating the bonding phases in low-frequency wire bonding, which is based on a modified tensile-compression testing machine: an electrodynamic shaker is used here instead of an ultrasonic transducer. The basic processes involved in low-frequency wire bonding are the same as those in conventional ultrasonic wire bonding. The low excitation frequency enables the measurement of process forces using conventional piezoelectric force sensors, which have been dynamically calibrated at the department for the frequency range up to 20 kHz. In addition to measuring the process forces, the test rig is used for high-speed imaging, which enables the process sequences to be tracked at high resolution.