A NASA engineering bulletin examines two cases of thick-film chip resistors cracking during environmental testing of printed circuit board assemblies. In both cases, the failures appeared during thermal cycling after vibration testing, causing intermittent open circuits at hot temperatures that cleared when the boards returned to room temperature.
The fractures crossed the short axis of each resistor, consistent with bending or tensile stress. The components were mounted in similar positions near the edges of two identical boards. Independent analyses found no clear crack-initiation defect, and the damage patterns were not consistent with vibration-induced fatigue. Tests also provided no direct evidence that the resistors, which came from the same production lot, were defective.
The bulletin does not identify a single root cause. It lists several possible contributing factors, including the resistors’ proximity to the board edge and orientation, excessive solder reaching the tops of their terminations, and stresses from board bowing or twisting. Rigid mounting and fastener tightening patterns may also concentrate stress in component locations.
To reduce the risk of cracking in ceramic components, the bulletin recommends avoiding placement that aligns a large component’s long axis parallel to a nearby board edge. Where edge placement is necessary, it advises using finite element analysis to assess stress and displacement. It also recommends keeping solder off the tops of terminations, checking solder fillets against manufacturer guidance, and conducting one to three thermal cycles with a functional check during the hot dwell before vibration testing.
The bulletin is titled “TB 26-10 Printed Circuit Board Assembly Chip Resistor Body Cracking.” For more information, it directs readers to contact Joel W. Sills at NASA’s Langley Research Center.
Sources: NASA Breaking News.