NASA Issues Technical Bulletin Regarding Chip Resistor Failures in Printed Circuit Boards

Overview of the Technical Bulletin

NASA has issued a technical bulletin, designated TB 26-10, to address the rare occurrence of alumina substrate cracking in thick-film chip resistors used in printed circuit board assemblies. While these components are widely utilized across various NASA projects, the agency noted that instances of body cracking during environmental testing are uncommon. The bulletin, authored by Joel W. Sills of the Langley Research Center, provides guidance on mitigating risks associated with flexure-induced damage in ceramic body components.

Observations of Component Failure

The investigation into these failures was prompted by two specific instances of thick-film chip resistor cracking observed during environmental acceptance testing. According to NASA, both failures occurred during thermal cycling that followed vibration testing. The resistors exhibited intermittent open circuit failures when subjected to hot dwell temperatures, though they recovered once returned to room temperature. Physical inspection revealed that both resistors had experienced a transverse fracture at their mid-span, with the cracks running parallel to the terminations along the short axis of the components.

Analysis of Potential Causes

Initial assessments considered the possibility that the resistors were damaged by vibration or that the components themselves were defective, as both failed units originated from the same production lot. However, NASA reported that the lot met the requirements of the procurement specification MIL-PRF-55342. Further testing and independent failure analyses, including optical and scanning electron microscopy, found no evidence of a defective lot or pre-existing flaws. The damage signatures were inconsistent with cyclic fatigue, leading investigators to rule out vibration-induced fatigue as the root cause.

Identifying Contributing Factors

The failure analysis concluded that the resistors suffered from bending or tensile failures. While no single root cause was identified, the NESC highlighted several contributing factors related to the design and installation of large footprint ceramic components. These factors include the placement of large resistors near the edge of a printed circuit board, particularly when the long axis is parallel to the board edge, which increases susceptibility to flexure. Additionally, excessive solder extending to the top pads of the resistor terminations can create a rigid joint that transfers board stress directly into the ceramic.