Degradation and failure mechanisms in thermally exposed Au–Al ball bonds

Abstract

During the manufacturing and the service life of Au-Al wire bonded electronic packages, the ball bonds experience elevated temperatures and hence accelerated interdiffusion reactions that promote the transformation of the Au-Al phases and the growth of creep cavities. In the current study, these service conditions were simulated by thermally exposing Au-Al ball bonds at 175 and 250°C for up to 1000 h. The Au-Al phase transformations and the growth of cavities were characterized by scanning electron microscopy. The volume changes associated with the transformation of the intermetallic phases were theoretically calculated, and the effect of the phase transformations on the growth of cavities was studied. The as-bonded microstructure of a Au-Al ball bond typically consisted of an alloyed zone and a line of discontinuous voids (void line) between the Au bump and the bonded Al metallization. Thermal exposure resulted in the nucleation, growth, and the transformation of the Au-Al phases and the growth of cavities along the void line. Theoretical analysis showed that the phase transformations across and lateral to the ball bond result in significant volumetric shrinkage. The volumetric shrinkage results in tensile stresses and promotes the growth of creep cavities at the void line. Cavity growth is higher at the crack front due to stress concentration, which was initially at the edge of the void line. The crack propagation occurs laterally by the coalescence of sufficiently grown cavities at the void line resulting in the failure of the Au-Al ball bonds.

Assessing Au-Al wire bond reliability using integrated stress sensors

“…to the bond expand and push the Au ball upwards [13]. Alternatively, evidence of these dark regions being oxidized Au 4 Al is given by the fact that all dark regions occur on the yellow line of Au 4 Al in Fig.…”

“…In [13], Noolu observes a row of discontinuous voids between the 4N Au and the small discontinuous regions of intermetallics which form between the Au ball and Al pad metal in unaged bonds.…”

“…As the bond ages further, these cracks propagate towards the center. While it is proposed that the cracking is due to Kirkendall porosity [1,5,13] these two layers is credited to either the Kirkendall effect due to Au being transported to the lower Au 4 Al layer [2] or by oxidation of the Au 4 Al layers [3].…”

“…In several published studies [2,11,13,17] wire bond microstructures are examined for bonds made on single layer metallizations. While studies exist with bonds made on multi-layer metalli- zations with diffusion barriers [16,19,4] there is no in depth analysis on the yellow intermetallic which forms near the diffusion barrier when intermetallic growth consumes the lower Al layer of pad.…”

Formation of Al/Au Bimetallic Interface Studied by Angle-Resolved X-Ray Photoelectron Spectroscopy (ARXPS)

“…The Au-Al system has been extensively studied especially due to its application in the microelectronic industry [1][2][3][4][5][6][7][8][9]. Miniaturization of device dimensions provides an increase of signal propagation speed and lower power dissipation.…”