An unusual mechanical failure mode in gold ballbonds at 50 μm pitch due to degradation at the Au–Au4Al interface during ageing in air at 175 °C

While investigating the bake test reliability of gold ball bonds in air at 175 °C an unusual failure mode was encountered that can strictly be classified as a ball lift but which has pull strengths as high as normal neck breaks. The failure mode is termed a high strength ball lift (HSBL) and is distinguished by partial de-bonding between the gold ball and intermetallic compound. The partial de-bonding occurs as a ring or annulus that starts from the outside of the ball and grows inwards toward the ball centre, occurring within the Au₄Al compound layer and between the Au₄Al and Au ball while the remaining part of the gold ball is well-bonded to the Au₄Al at the centre of the bondpad. During pull testing, the de-bonded outer ring acts as a notch that propagates a blunt crack into the gold ball resulting in high pull strengths. As ageing time increases the de-bonded region grows larger and advances inwards towards the ball centre. EDX of both types of ball-lifts indicates higher levels of oxygen on the Au₄Al surface that suggests annulus growth may be partially due to oxidation of the intermetallic. The number of HSBL failures and the point in time at which they occur is observed to depend on the bonded ball squash height, a relatively easily controlled parameter in gold ballbonding. When devices with targeted squash heights (TSH) of 6 μm, 7 μm and 8 μm are isothermally aged, zero ball lifts are only obtained at 7 μm squash height. HSBL failures are encountered earlier at the 6 μm TSH than at 8 μm TSH and as the de-bonded region progresses further inwards during baking, the HSBL’s are observed to transition to low strength ball lifts (LSBL) as the load-bearing area of the ball is reduced. The dependence of failure mode on squash height is believed to be related to interactions between bonded ball stress, intrinsic intermetallic growth stresses and thermo-chemical degradation of the Au₄Al intermetallic compound.