NASA Investigates Cracking in Ceramic Chip Resistors on Printed Circuit Boards

NASA’s National Aeronautics and Space Administration Engineering and Safety Center (NESC) has identified rare but notable failures of thick-film chip resistors on printed circuit board (PCB) assemblies during rigorous environmental testing. Two instances of alumina substrate cracking occurred during thermal cycling following vibration testing, manifesting as fractures across the resistor’s short axis caused by bending stresses.

Chip resistors, widely used in NASA projects for their reliability, showed intermittent open circuit failures at hot dwell temperatures which self-healed at room temperature. Both failures happened in similar resistor types located near the edge of identical PCBs, suggesting a positional influence on the component’s susceptibility to damage.

Comprehensive failure analyses, including optical and scanning electron microscopy, found no defects or fatigue damage that could explain the cracking. Instead, the damage was attributed to bending or tensile stress from PCB flexure rather than vibration-induced fatigue or manufacturing defects in the resistor lot, which met stringent MIL-PRF-55342 standards.

Key contributing factors include mounting the large size (2512 case style) ceramic resistors with their long axis parallel to the PCB edge, excessive solder extending onto resistor terminations creating rigid joints, and excessive bending moments from board bowing or twisting during assembly or operation. Additionally, rigid mounting to frames and uneven fastener torquing can concentrate stresses at resistor locations.

In response, NASA recommends careful design and assembly practices to mitigate flexure-induced cracking risks. These include avoiding component orientation parallel to PCB edges near board edges, performing finite element analysis to assess stress margins, controlling solder fillet height to prevent excessive rigidity, and thorough inspection of solder joints for compliance with manufacturer limits.

Further, NASA advocates incorporating functional thermal shock testing before vibration tests to verify component integrity under operational thermal stress conditions. This proactive approach aims to detect potential failures early in acceptance testing, preventing mission-critical reliability issues later.

These findings underscore the importance of PCB mechanical design and assembly controls, especially for ceramic components vulnerable to flexure-induced stress in aerospace and high-reliability environments. While chip resistor body cracking remains rare, the updated guidelines provide crucial steps to maintain electrical and structural integrity.

Limitations remain since no direct root cause of cracking was pinpointed beyond contributing factors, and the recommendations focus on prevention rather than redesign. Ongoing monitoring and analysis are needed to refine material and assembly processes to fully eliminate such failures.