Failure mechanisms in encapsulated copper wire-bonded devices

The cost of gold has increased drastically in the last few years forcing IC manufacturers to switch to copper wire bonding. Copper is used in many plastic-encapsulated devices, but manufacturers are still using the same qualification tests for copper wire-bonded devices as for gold wire-bonded devices, even though damage accumulates differently in these two interconnections, and the failure mechanisms and models are significantly different. To study the damage to the wire, bond shear and pull tests are performed. However, due to the absence of a decapsulation procedure to reveal the ball bond and the wedge bond, these tests are usually performed on non-molded compounds, which does not account for the effect of the molding compound in the failure. To study critical failure mechanisms such as corrosion, wire axial fatigue, and wire bond fatigue, a novel method for wire bond decapsulation was developed, that uses a sequential process, resulting in minimum degradationor, in some cases, no degradation of wire bonds. Previous studies presented decapsulation methods that either revealed the ball bond or the wedge bond separately, but these methods cannot be used to perform wire pull tests. Exposing both the ball bond and the wedge bond is not easy, as the wire tends to break at the loop due to the long exposure time. This paper, describes - a successful decapsulation technique and presents bond shear results for devices with copper wire bonds, thereby permitting studies of encapsulated devices.