Vibration-induced fatigue failures in bonding wires used in stacked chip modules

An instrument recently constructed at Goddard Space Flight Center involved stacked chip memory modules with about six thousand 1 mil gold wires, each between 4.6 mm and 6.1 mm long. Four bond wires fractured during the final box-level random vibration test, which had spectral power to 850 Hz. We determined that these wires have fundamental resonant frequencies in the range from 1.4 kHz to 2.3 kHz, with quality factors up to 300, when the bonds at each end are rigid. However, a nearly severed top bond decreases the lead´s fundamental frequency into the range from 600 Hz to 750 Hz. The lowered mode is strongly excited by the box-level vibration test, leading to fatigue-fracture of that already weakened bond during the several minutes of the vibration test. However, such a lowered mode would not be excited by previous shock and vibration tests, since these happened to have no spectral power in the region from 600 Hz to 750 Hz. In separate tests, an unfractured 1 mil gold wire was still intact after 12 days (1.4 billion oscillations) at 1.4 kHz at a root-strain of 0.2%, while several (deliberately) nearly fractured wires failed after less than a minute (less than 41,000 oscillations) at 680 Hz at the same drive stress