NASA identifies flexure risks in chip resistor cracking

NASA said in its Tuesday 03 November 2026 bulletin that two instances of thick-film chip resistor body cracking occurred during environmental acceptance testing on printed circuit board assemblies. Both failures appeared during thermal cycling after vibration testing. The resistors showed intermittent open circuit failures at hot dwell temperatures, but the failures recovered at room temperature.

Each resistor had a single, straight crack across its short axis at the mid-span. The fractures ran parallel to the terminations and indicated bending or tensile failure without twisting. The resistors were case size 2512 and were installed in similar locations near the edges of two identical boards, with their long axes parallel to the card edges. Neither showed signs of pre-existing defects.

NESC failure analyses found no clear crack initiation site and no evidence of fatigue damage. Independent optical and scanning electron microscopy did not identify flaws that would indicate fatigue initiation or progression. Both resistors came from the same production lot, but the lot met the requirements of procurement specification MIL-PRF-55342. Additional tests found no direct evidence of a defective resistor lot or an elevated risk of defective resistors.

NASA identified several contributing factors, including board flexure, component orientation, excessive solder, and mounting stresses. Solder extended to the top pads on the resistor terminations. Board bow or twist created excessive bending moments or loads. Hard mounting to a rigid frame could store and amplify those stresses, while suboptimal fastener torquing patterns could concentrate mounting stress at the resistor locations.

The bulletin recommends avoiding orientation of the long axis of large-footprint ceramic components parallel to nearby printed circuit board edges. NASA also recommends sufficient finite element analysis to demonstrate adequate stress and displacement margins when large components must be placed near a board edge. These measures are intended to reduce susceptibility to global and local flexure damage.

NASA advises keeping solder off the top surface of terminations because a rigid joint can transfer printed circuit board flexure directly into the ceramic. Physical solder joints or photographic images should be inspected to verify sufficient but not excessive solder fillets under manufacturer limits. The bulletin also recommends one to three thermal cycles, including a hot-dwell functional test, before vibration testing to document as-built component integrity.