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Process advantages of thermosonic wedge-wedge bonding using dosed tool heating
Keywords: thermosonic bonding, wire bonding, tool heating
This paper presents the first broad analysis of heavy wire bonds created using a novel thermosonic bonding process. By adding an additional heat source to the well-established ultrasonic heavy wire bonding process, its capabilities and performance are improved. This improvement is expected to be most significant on materials which react sensitive during purely ultrasonic bonding like battery caps, copper alloys, coated caps/clips or which are fragile like dies or sensors. There is a strongly increasing demand for such applications to serve the rapidly growing market of electric vehicles (EVs), with the electrification of the power train and the introduction of 48 V technology, and the markets for renewable energy and internet of things (IoT). It is well known from ultrasonic bonding that the bondability of many materials can be improved by increasing the temperature at the bonding interface. The presented novel thermosonic bonding process is using ultrasonic and thermal energy in a user-definable composition to achieve optimal connections. The additional thermal energy provides another degree of freedom which allows for optimally matched processes for diverse applications. The substitution of vibration energy by thermal energy allows ultrasonic vibration amplitude and/or normal force to be reduced. This lowers the stress to the substrate and thus makes to process more viable for sensitive substrates. Alternatively, if substrate sensitivity is not an issue, the additional thermal energy can be used to reduce bonding time and therefore increase throughput and efficiency for high volume production. In conventional thermosonic bonding, the whole package is heated in order to obtain the desired increased bond pad temperature. Today, such conventional thermosonic bonding processes are only used to bond thin wires, mostly in ball-wedge technology, to contact integrated circuits (ICs) in microelectronics. These processes use relatively low ultrasonic power, normal force and bonding time. This is necessary to not expose the fragile and costly silicon IC chip to critical mechanical stress and to avoid damaging it while deforming the wire and building the required intermetallic contact. While heating the whole electronic package for this process is state of the art, it may cause problems, as the maximum operating temperature is limited by the part with the lowest critical exposition or oxidation temperature. In order to overcome these restrictions and to allow utilization of thermosonic bonding also for heavy wire and ribbon bonding, a flexible and powerful heat source for rapid heating has been developed, acting directly on the bonding tool. This heat source heats the tool tip to a selected temperature within some milliseconds. Heat is transferred when the tool is pressed onto the wire/ribbon and affects the bonding process positively. Compared to a conventional thermosonic process, the temperature acts only on a very limited volume, avoiding negative effects to the surrounding material. During the process, it is necessary to ensure appropriate heating of the bonding zone with different tool geometries and combinations of wire/ribbon and substrate. By integrating a temperature measurement system and a closed-loop controller, it is possible to control the desired temperature at the tool-tip very fast and precisely. This contribution presents the results of a comparative study in which heavy aluminium and copper wire was bonded to test substrates, applying different levels of heating to the bonding tool. Besides tool heating, the input parameters of bonding time, ultrasonic power, and normal force, known from conventional heatless bonding, were modified in the study. The produced bonds were analysed using microscopy and shear tests. The results confirm that compared to heatless ultrasonic bonding, thermosonic bonding using dosed tool heating allows to reach the same bond strength at shorter bonding time, higher bond strength at the same bonding time, or a combination of both, all at reduced mechanical stress to the substrate.
Matthias Hunstig,
Hesse GmbH
Paderborn, NRW
Germany


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