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Ultrasonic welding is often used to weld fine wires to thick or thin film contact pads in applications such as integrated circuit interconnection and sensor manufacture. It is very successful when applied to aluminium, gold and copper wires.

Platinum wire work-hardens significantly during cold ultrasonic bonding therefore thermosonic (ultrasonic bonding with the addition of heat) bonding with the substrate heated to >300°C is possible.

The most common method used for this application is thermocompression bonding. This process uses heat, force and time as the key variables to produce a welded joint without melting the material, thus the weld achieved is defined as a solid phase weld with interdiffusion of the wire and pad material forming the weld.

Thermocompression bonding of fine wires (<100 µm diameter) is best achieved by using a light force resistance welding head fitted with a one-piece bonded electrode. Either DC or high frequency inverter types power supplies should be used as they offer fine adjustment of weld energy and time with excellent repeatability control via closed loop monitoring systems. The figure below shows the set schematically.

The power supply delivers a pre-set pulse of energy for a pre-set duration (e.g. 15 amps for 50 msec) to the electrode via the connecting cables and the weld head. The weld head applies a downward force, which maintains intimate contact between the electrode, the wire and the substrate. The current passes from the positive electrode to the negative one via the wire, which generates heat both within the electrode tip and the wire. The weld is formed when the heat generated is sufficient to promote diffusion of the wire material into the pad and vice versa.

In order to maintain a consistent process the location of the wire relative to the electrode and substrate material condition must be controlled. Variations in the surface condition, e.g. scratches or contamination can lead to wire burnout and electrode damage.

For further information and help with development of a welding process solution please contact Chris Otter (chris.otter@twi.co.uk) or Roger Wise (roger.wise@twi.co.uk) at TWI (The Welding Institute)

Answer posted December 2004