The topic
Electronic components are constructed of a variety of materials and layers that conduct electricity or insulate, are ductile or brittle and are made of metals, glasses and polymers. All of them are required for the function of devices but from materials science point of view they do not necessarily match together. They might not bind well to each other and they expand to different extents when heated. The different properties lead to high levels of thermomechanical stresses, in particular in switching operations. How long a component can withstand these stress is a critical characteristic of the final product. The ability to test this quickly and reliably would put companies a significant step ahead of the competition.
The research question
Electrical function is at the forefront in the development of new components, which are carefully assembled layer by layer. But how much stress can be applied before the layers no longer reliably bind to one another? And how does the component react to low-level but frequently repeated stresses? A reliable prediction of the robustness used to rely on complex electrical stress tests: over several months the power is turned on and off, thereby subjecting the component to tens of thousands of “on/off” or “hot/cold” cycles, simulating the operation of the final product. This naturally takes time – and is expensive. Infineon is thus looking for quicker and simpler tests to speed up the development of new and reliable products. A test of this kind would also open new markets for the producers of ultrasonic wire-bonding systems that can be developed into testing equipment. This explains why F&S Bondtec joined the collaboration – an ideal partner as it is the only producer worldwide with experience in both ultrasonic wire-bonding and bond testing.
Collaboration in the CD Laboratory
Ultrasonic methods are an established way for conventional fatigue testing of materials. They enable tens of thousands of stress cycles per second, so it would make sense to apply them to electronic components. Tests that previously took months could be carried out in a matter of hours. In Dr Khatibi, a materials scientist at the Vienna University of Technology, Infineon has found a partner with internationally recognized expertise. She is addressing the many questions raised by the method. For example, the ultrasonic test represents a vibration test at a fixed temperature: is the induced stress really the same as that caused by temperature cycles? And can the method be applied to miniaturized components and their layers? A transfer from the centimetre scale to the micro- and nanometre range is not self-evident as it must be scientifically validated and the method adapted. This requires detailed fundamental knowledge of the stresses in various types of component: high power semiconductors used in high-speed trains and wind power stations; smaller switches that permit high frequency power conversion in electronic equipment; and miniaturized intelligent power switches for complex automobile applications.
Results
The work in the CD Laboratory has already confirmed that stresses induced by ultrasonic vibrations can be equated to thermomechanical stress in certain areas. Based on this finding, F&S Bondtec has developed a prototype for quick tests of wire bonds in the high-performance applications (locomotives, wind power plants) and Infineon is already using it. There have also been promising preliminary studies of the transfer to smaller-scale applications at lower currents, smaller chips and thinner metal compounds (electronic equipment, automobile applications). The issue of whether the testing method is also appropriate for the cyclic testing of layered assemblies is still very much in the realm of fundamental research. Infineon expects the CD Laboratory to determine whether such a use will eventually become possible.
