You know how a ceramic tile on your kitchen floor will survive someone jumping on it dead center, but if the subfloor underneath warps even slightly, the tile cracks clean across? That is exactly what happened to two chip resistors on NASA printed circuit board assemblies — and the failure mechanism is just as mundane and just as consequential. The NESC investigated two thick-film chip resistors (case size 2512) that failed during environmental acceptance testing. Both cracked during thermal cycling after vibration, producing intermittent open circuits at hot dwell temperatures that recovered at room temperature. The fractures ran parallel to the terminations across the short axis of each resistor — textbook bending failure, not fatigue, not defect. Both resistors sat in similar edge-of-board locations on two identical boards. The obvious first theory — a defective production lot — was systematically eliminated. The resistor lot met all requirements of MIL-PRF-55342, and additional testing found no elevated defect risk within the batch. Independent optical and scanning electron microscopy identified no clear flaw initiation site on either resistor. The damage signatures were inconsistent with cyclic fatigue, ruling out vibration as a standalone root cause. This is a case where the component was fine; the system around it was not. The contributing factors form a clear causal chain: large-footprint ceramic components placed near the board edge with their long axis parallel to the card edge, excessive solder extending to top pads creating overly rigid joints, board bow and twist from hard mounting to a rigid frame, and suboptimal fastener torquing patterns that concentrated stress at the resistor locations. Each factor alone might be tolerable. Together, they turned a compliant resistor into a cracked one. NASA's guidelines are refreshingly specific. Orient large ceramic components so their long axis is not parallel to nearby board edges. Run finite element analysis on large components near board edges to demonstrate adequate stress and displacement margins. Keep solder off the top surface of terminations — solder on top creates a rigid bridge that transfers board flexure directly into the ceramic body. Inspect solder fillets against manufacturer limits. And critically, perform one to three thermal cycles with hot-dwell functional testing before vibration testing, to establish baseline component integrity. The bulletin's real value is diagnostic, not prescriptive. It names a failure mode that the aerospace electronics community had essentially no reported history of — chip resistor alumina cracking during environmental testing — and demonstrates that it emerges not from component defects but from the interaction of mounting, placement, and solder decisions that are each individually within spec. The lesson generalizes: ceramic components on flexible substrates are a system-level reliability problem, not a parts problem. This is the kind of engineering bulletin that prevents the next failure by making a previously invisible mechanism visible. No new physics, no novel materials — just careful forensics that turns a pair of cracked resistors into institutional knowledge.
