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Active Power Cycling for End of Life Tests of Heavy Wire Bond Interfaces on Power Semiconductors

: Göhre, J.; Schneider-Ramelow, M.; Geißler, U.; Lang, K.-D.

PCIM Europe 2010 : International Exhibition & Conference for POWER ELECTRONICS INTELLIGENT MOTION POWER QUALITY, 4 - 6 May 2010, Exhibition Centre Nuremberg, Proceedings
Stuttgart: Mesago PCIM, 2010
ISBN: 978-3-8007-3229-6
PCIM Europe <2010, Nürnberg>
Fraunhofer IZM ()

Fatigue cracking in the inter-face between the heavy Al wire and the Al metallization of the power semiconductor is one of the major failure causes of power modules and discrete devices. These fatigue cracks are primarily caused by mismatch of the coefficients of thermal expansion (CTE) of the Al of the bonding wire and the Si of the chip. During thermal cycling periodic plastic and elastic strains occur in the interface region weakening the wire material with every cycle. Typically this leads to the initiation and growth of cracks in the interface between the wire and the metallization. The final result is a bond lift off and a failure of the device.
The main influencing factors for this degradation are the temperature swing and the maximum temperature of the cycle, the wire material and diameter and the initial wire bond quality. In the past, many publications in this field were concentrated on the reliability of complete power modules with many wire bonds. In these works the degradation process of a single wire was not monitored. Only the final result of the degradation - the bond wire lift off - was considered.
To gain more insight into the degradation process in the interface between the wire bond and the chip metallization, crack initiation and growth has to be monitored on a single wire basis. Since electrical measurements of cracks lengths in the micrometer range are not feasible and common inspection methods such as X-Ray and Acoustic Microscopes do not deliver high enough spatial resolution only metallographic cross sectioning followed by SEM or light microscope inspection can reveal the correct crack length and possibly the grain structure around the crack tip. Metallographic cross sectioning is a manual step and thus very time consuming. It is feasible only for a small number of samples but on a power semiconductor many wires (up to 20) are bonded in parallel to increase current capability and reliability of the module. In this case, cross sectioning of every wire is not possible.
Typically the interface quality of heavy wire bonds right after the bonding process is evaluated by the shear test. During the shear test, a shear tool is shearing through the wire material at a defined height above the pad metallization. The measured shear force and the coverage of the sheared bond site with wire material give a clue about the bonding quality. If during thermal cycling a crack is initiated and growing this must be detectable by decreasing shear forces and coverage of the shear site. For this reason the shear test is used for detecting the crack growth.
In this poster shear results after power cycling with different temperature swings and maximum temperatures are presented. In addition, cross sections and microstructure analyses reveal that the exact path of the crack is not directly in the interface between wire and metallization but in a layer above the interface. This layer is characterized by fine grains below and coarse grains above. Also, the analysis of the crack tip area shows mainly intercrystalline crack growth.