Molded reliability study for different Cu wire bonding configurations

With the transition from Au wire bonding to Cu wire bonding, both bare Cu wire and Pd coated Cu (PCC) wire are now commonly used. Humidity reliability is particularly challenging for Cu wire bonding. In this paper, 18 um (0.7 mil) bare Cu and PCC wire are used in the wire bonding and HAST reliability test. Various factors can influence the HAST reliability outcome. The key factors examined in this paper include: wire bonding parameters such as ultrasonic energy, force and time; wire type including bare Cu wire and PCC wire; cover gas type including forming gas and nitrogen for PCC wire; molding compound; and plasma cleaning. From our HAST test results, the most influential factor is the wire type. PCC wire exhibited much better HAST results than Cu wire. Using the recently developed Cu wire bonding processes, bonding parameters played a secondary role in the reliability outcome. For PCC wire, the lowest and highest ultrasonic energy settings both passed 336 hours with IMC at 71% and 94%. The IMC%, a key factor used in evaluating wire bonding quality, does not seem to correlate to the reliability outcome for PCC wire. A higher IMC% does help bare Cu wire HAST results. Our tests indicated that there was no difference between forming gas and Nitrogen in the HAST testing. A plasma cleaning time of 20 sec provided better HAST results than no plasma cleaning or a long plasma cleaning time. A time interval study was carried out to study the failure mechanism. Samples at different HAST time intervals were cross sectioned and analyzed using transmission electron microscopy (TEM). The different stages include before HAST but after molding and post molding cure (PMC), HAST test at 48hr, 96 hr, 168 hr and 336 hr. A typical failure starts with Cu nano-particles and Al oxides formation at the Cu rich IMC layer at 48hr. Over time, the Cu-Al IMC corrodes and more Cu and Al oxides form and leave small voids, and the voids can develop into continuous gaps (cracks). The gap normally starts at the edge of the bond and propagates inward. When the gap propagates all the way to the center of the bond, an open failure will occur. We found two primary factors that influence the HAST outcome. The first factor is the presence of the Pd elements. For the PCC wire samples, even though Pd is absent at the ball and pad interface immediately after wire bonding, it tends to diffuse into the interface during HAST testing. For a good interface with no cracks, a higher concentration of Pd is present and Pd-Cu-Al IMC is formed. At the failed bond, Pd elements are absent. This indicates that the Pd element has the effect of slowing or preventing the Cu-Al IMC corrosion or Pd-Cu-Al IMC is more corrosion resistant. That explains why PCC wire has a better HAST test outcome. The second factor is the concentration of chlorine. At failed bonds, a much higher concentration of chlorine was found, which implies the role of chlorine in the IMC corrosion. Controlling the chlorine level, and mobility, are key considerations in molding compound formulation.