Experimental and Numerical Simulation Study of Pre-Deformed Heavy Copper Wire Wedge Bonds (bibtex)
by , , , , , , , , ,
Abstract:
Power semiconductor modules are used to control and switch high electrical currents and voltages. Within the power module package wire bonding is used as an interconnection technology. In recent years, aluminum wire has been used preferably, but an ever-growing market of powerful and efficient power modules requires a material with better mechanical and electrical properties. For this reason, a technology change from aluminum to copper is indispensable. However, the copper wire bonding process reacts more sensitive to parameter changes. This makes manufacturing reliable copper bond connections a challenging task. The aim of the BMBF funded project Itsowl-InCuB is the development of self-optimizing techniques to enable the reliable production of copper bond connections under varying conditions. A model of the process is essential to achieve this aim. This model needs to include the dynamic elasto-plastic deformation, the ultrasonic softening effect and the proceeding adhesion between wire and substrate. This paper focusses on the pre-deformation process. In the touchdown phase, the wire is pressed into the V-groove of the tool and a small initial contact area between wire and substrate arise. The local characteristics of the material change abruptly because of the cold forming. Consequently, the pre-deformation has a strong effect on the joining process. In [1], a pre-cleaning effect during the touchdown process of aluminum wires by cracking of oxide layers was presented. These interactions of the process parameters are still largely unknown for copper. In a first step, this paper validates the importance of modeling the pre-deformation by showing its impact on the wire deformation characteristic experimentally. Creating cross-section views of pre-deformed copper wires has shown a low deformation degree compared to aluminum. By using a digital microscope and a scanning confocal microscope an analysis about the contact areas and penetration depths after touchdown has been made. Additionally, it has to be taken into account that the dynamical touchdown force depends on the touchdown speed and the touchdown force set in the bonding machine. In order to measure the overshoot in the force signals, a strain gauge sensor has been used. Subsequently, the affecting factors have been interpreted independently Furthermore, the material properties of copper wire have been investigated with tensile tests and hardness measurements. In a second step, the paper presents finite element models of the touchdown process for source and destination bonds. These models take the measured overshoot in the touchdown forces into account. A multi-linear, isotropic material model has been selected to map the material properties of the copper. A validation of the model with the experimental determined contact areas, normal pressures and penetration depths reveals the high model quality. Thus, the simulation is able to calculate and visualize the three dimensional pre-deformation with an integrated material parameter of the wire if the touchdown parameters of the bonding machine are known. Based on the calculated deformation degrees of wire and substrate, it is probably possible to investigate the effect of the pre-deformation on the pre-cleaning phase in the copper wire bonding.
Reference:
Unger, A.; Sextro, W.; Althoff, S.; Eichwald, P.; Meyer, T.; Eacock, F.; Brökelmann, M.; Hunstig, M.; Bolowski, D.; Guth, K.: Experimental and Numerical Simulation Study of Pre-Deformed Heavy Copper Wire Wedge Bonds. Proceedings of the 47th International Symposium on Microelectronics (IMAPS), 2014.
Bibtex Entry:
@INPROCEEDINGS{Unger2014a,
  howpublished = {Conference Proceedings},
  author = {Andreas Unger AND Walter Sextro AND Simon Althoff AND Paul Eichwald
	AND Tobias Meyer AND Florian Eacock AND Michael Br{\"o}kelmann AND
	Matthias Hunstig AND Daniel Bolowski AND Karsten Guth},
  title = {Experimental and Numerical Simulation Study of Pre-Deformed Heavy
	Copper Wire Wedge Bonds},
  booktitle = {Proceedings of the 47th International Symposium on Microelectronics
	(IMAPS)},
  year = {2014},
  pages = {289-294},
  address = {San Diego, CA, US},
  abstract = {Power semiconductor modules are used to control and switch high electrical
	currents and voltages. Within the power module package wire bonding
	is used as an interconnection technology. In recent years, aluminum
	wire has been used preferably, but an ever-growing market of powerful
	and efficient power modules requires a material with better mechanical
	and electrical properties. For this reason, a technology change from
	aluminum to copper is indispensable. However, the copper wire bonding
	process reacts more sensitive to parameter changes. This makes manufacturing
	reliable copper bond connections a challenging task. The aim of the
	BMBF funded project Itsowl-InCuB is the development of self-optimizing
	techniques to enable the reliable production of copper bond connections
	under varying conditions. A model of the process is essential to
	achieve this aim. This model needs to include the dynamic elasto-plastic
	deformation, the ultrasonic softening effect and the proceeding adhesion
	between wire and substrate. This paper focusses on the pre-deformation
	process. In the touchdown phase, the wire is pressed into the V-groove
	of the tool and a small initial contact area between wire and substrate
	arise. The local characteristics of the material change abruptly
	because of the cold forming. Consequently, the pre-deformation has
	a strong effect on the joining process. In [1], a pre-cleaning effect
	during the touchdown process of aluminum wires by cracking of oxide
	layers was presented. These interactions of the process parameters
	are still largely unknown for copper. In a first step, this paper
	validates the importance of modeling the pre-deformation by showing
	its impact on the wire deformation characteristic experimentally.
	Creating cross-section views of pre-deformed copper wires has shown
	a low deformation degree compared to aluminum. By using a digital
	microscope and a scanning confocal microscope an analysis about the
	contact areas and penetration depths after touchdown has been made.
	Additionally, it has to be taken into account that the dynamical
	touchdown force depends on the touchdown speed and the touchdown
	force set in the bonding machine. In order to measure the overshoot
	in the force signals, a strain gauge sensor has been used. Subsequently,
	the affecting factors have been interpreted independently Furthermore,
	the material properties of copper wire have been investigated with
	tensile tests and hardness measurements. In a second step, the paper
	presents finite element models of the touchdown process for source
	and destination bonds. These models take the measured overshoot in
	the touchdown forces into account. A multi-linear, isotropic material
	model has been selected to map the material properties of the copper.
	A validation of the model with the experimental determined contact
	areas, normal pressures and penetration depths reveals the high model
	quality. Thus, the simulation is able to calculate and visualize
	the three dimensional pre-deformation with an integrated material
	parameter of the wire if the touchdown parameters of the bonding
	machine are known. Based on the calculated deformation degrees of
	wire and substrate, it is probably possible to investigate the effect
	of the pre-deformation on the pre-cleaning phase in the copper wire
	bonding.},
  keywords = {pre-deformation, copper wire bonding, finite element model}
}
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