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NASA bulletin flags chip resistor cracking on circuit boards
NASA's Engineering and Safety Center said two thick-film chip resistors cracked during thermal cycling after vibration, and issued assembly guidelines to reduce flexure-induced failures on printed circuit boards.
NASA's Engineering and Safety Center published technical bulletin TB 26-10 on Nov. 3 describing two cases of thick-film chip resistor body cracking on printed circuit board assemblies during environmental acceptance testing1.
Both failures occurred during thermal cycling after vibration testing, with intermittent open-circuit behavior at hot dwell temperatures that recovered at room temperature1. Transverse fractures ran across the mid-span of each resistor, parallel to the terminations, along the short axis, consistent with bending1. The two resistors were installed in similar locations near the edge of two identical boards and showed no signs of pre-existing defects1.
Chip resistors are widely used on NASA projects, and NESC said cases of alumina substrate cracking during vibration and thermal cycling have historically been rare1. Early theories pointed to vibration damage in a potentially defective production lot, but the lot met MIL-PRF-55342 and extra tests did not show an elevated defect risk1.
Independent failure analyses concluded the cracks were bending and tensile failures, with a single straight crack across the short axis and no clear initiation site1. Damage signatures were not consistent with cyclic fatigue, and optical and scanning electron microscopy did not identify fatigue initiation sites, making a defective lot or vibration fatigue unlikely as the root cause1.
NESC did not identify a single direct root cause1. It listed contributing factors for large-footprint ceramic parts near board edges: case-size 2512 resistors with the long axis parallel to the card edge; excess solder extending to top pads; board bow or twist; hard-mounting the board to a rigid frame; and fastener torque patterns that may concentrate mounting stress at the resistor locations1.
The bulletin's guidelines include avoiding that orientation near board edges, using finite element analysis when large parts must sit near an edge, keeping solder off the top of terminations, inspecting fillets against manufacturer limits, and running one to three thermal cycles with a hot-dwell functional test before vibration1. Point of contact is Joel W. Sills at Langley Research Center1.
Sources
NASAOfficial source