TCXO failure analysis

A detailed failure analysis of a TCXO (Temperature Compensated Crystal Oscillator) from a ThunderScope open hardware oscilloscope prototype. After the oscilloscope exhibited a 6.6% frequency error and the ADC PLL failed to lock, the author depackaged the hermetically sealed ceramic TCXO using a mill and scalpel, then systematically inspected the quartz crystal (intact and electrically resonant at 20 MHz), the controller die (identified as AKM-made, ~250nm process node with three aluminum metal layers), and finally the bond wires. The root cause was a fractured crescent bond on the long gold wire connecting the controller die to the substrate trace for the quartz crystal. Both ultrasonic cleaning of the PCB and potentially poor wire bonding process control (evidenced by unusually deep indentation at a nearby good bond) were identified as likely contributing factors. A second identical failure mode was subsequently reported after ultrasonic cleaning, corroborating the finding.