NASA Bulletin Ties Chip Resistor Cracking to Board Flexure
NASA's NESC reports two thick-film chip resistor substrate fractures during acceptance testing, ruling out defective lots and fatigue and citing board flexure.
The story in focus
NASA's NESC reported two cases of thick-film chip resistor alumina substrate cracking during environmental acceptance testing of printed circuit board assemblies. Both failures appeared during thermal cycling after vibration, with transverse fractures across the resistors' short axis. Analyses found no defective lot or fatigue evidence, citing board flexure, component orientation, excessive solder and mounting stress instead.
- Two failures occurred during thermal cycling after vibration testing on two identical board assemblies.
- Failed resistors were installed near board edges in similar locations; both showed intermittent open circuits at hot dwell that recovered at room temperature.
- Lot testing met MIL-PRF-55342 requirements plus additional tests, with no direct evidence of a defective resistor lot.
- Independent microscopy found no clear fatigue initiation sites, making fatigue and defective resistors unlikely.
- No single root cause was identified; board bow or twist and component placement are listed as contributing factors.
What failed and when
The bulletin describes two thick-film chip resistors whose ceramic bodies cracked during environmental acceptance testing of printed circuit board assemblies. Each failure surfaced during thermal cycling after vibration, showing intermittent open circuits at hot dwell temperatures that recovered at room temperature. Both resistors sat in similar positions near board edges on two identical boards, and each fracture ran across the short axis, parallel to the terminations, consistent with bending.
Testing and failure analysis
An initial theory blamed vibration damage to defective parts, since both resistors came from one production lot. That lot met the MIL-PRF-55342 procurement specification and passed additional tests, with no indication of elevated defect risk. Independent failure analyses attributed the cracks to bending and tensile stress. The fracture patterns did not match cyclic fatigue, and optical and scanning electron microscopy found no clear flaw sites for fatigue initiation, so defective parts and vibration fatigue are considered unlikely.
Recommended controls
No definitive root cause was identified. The bulletin lists contributing factors for large-footprint ceramic components near board edges, including manufacturing or installation bow and twist, placement near edges, component orientation, excessive solder and mounting stress. NASA's recommendations include careful component placement, finite element analysis, controlled solder fillet height, thorough inspection and functional thermal shock testing. The bulletin does not specify how widely it expects the guidance to be applied.
What did the failure analyses rule out as causes of the resistor cracks?
NASA's NESC ruled out a defective production lot, noting the lot met MIL-PRF-55342 plus additional tests, and found no clear evidence of fatigue initiation, making vibration fatigue unlikely. Bending and tensile failure from board flexure best fit the cracks.
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