TB 26-10 Printed Circuit Board Assembly Chip Resistor Body Cracking

Meta Title: NASA Technical Bulletin Examines Chip Resistor Cracking on PCB Assemblies

Meta Description: NASA’s TB 26-10 describes rare cases of thick-film chip resistor body cracking during environmental testing. Learn what is known, possible mechanisms, and implications for PCB reliability.

URL Slug: nasa-chip-resistor-body-cracking-pcb-assembly

Excerpt: NASA’s Technical Bulletin TB 26-10 reports two instances of thick-film chip resistor cracking following vibration and thermal-cycling tests. The cases highlight why small passive components deserve careful attention in high-reliability electronics design, assembly, and qualification testing.

# NASA Report Highlights Chip Resistor Cracking During PCB Environmental Tests

Surface-mount chip resistors are among the most common components on modern printed circuit board assemblies. They are small, comparatively inexpensive, and usually highly reliable. Yet their size can make them vulnerable to mechanical stresses transferred through a circuit board during manufacturing, handling, vibration, and temperature changes.

A NASA technical bulletin, **TB 26-10: Printed Circuit Board Assembly Chip Resistor Body Cracking**, describes two observed cases of cracking in thick-film chip resistors during environmental testing. According to the source summary, the cracks occurred in alumina resistor substrates after thermal cycling that followed vibration testing.

The report is notable not because chip resistor cracking is necessarily common, but because the failure mode was rare enough in NASA project experience to merit focused documentation. For engineers working on aerospace, industrial, automotive, medical, or other high-reliability hardware, the cases offer a useful reminder: passive components can be affected by system-level mechanical and thermal conditions.

## What NASA’s Bulletin Says

The available NASA summary identifies several key facts:

– The affected parts were **thick-film chip resistors**.
– Their bodies used an **alumina ceramic substrate**.
– Two cases of cracking were observed.
– The cracking was detected during **thermal cycling after vibration testing**.
– Fractures occurred across the component’s **short axis**.
– NASA characterizes such cracking as rare in its experience with chip resistors used in projects.

These details matter because the test sequence may be as important as either environmental exposure considered alone. Vibration can introduce or worsen damage, while repeated temperature changes can impose additional stresses that make a pre-existing weakness propagate into a visible fracture.

The source summary does not establish that all chip resistor cracks arise from one mechanism, nor does it provide enough information to assign a root cause for every application. Important details—including the precise resistor package sizes, board stack-up, mounting geometry, vibration levels, thermal range, solder-joint condition, and failure-analysis findings—should be verified in the complete bulletin PDF before being used as a design rule.

## Why Ceramic Chip Resistors Can Crack

A typical thick-film chip resistor consists of a ceramic body, resistive film, protective coatings, and conductive end terminations. Alumina is a useful electronic substrate because it is electrically insulating, thermally stable, and dimensionally consistent. It is also a ceramic, which means it is strong in compression but can be susceptible to fracture when tensile stress, bending, or localized mechanical loading exceeds its tolerance.

On an assembled board, the resistor is not isolated. It is soldered between copper pads and becomes part of a larger mechanical structure. Any board bending, twisting, or differential expansion can transfer stress into the component through the solder joints and terminations.

Cracks across the resistor’s short axis can be consistent with flex-related loading, but crack direction alone should not be treated as conclusive proof of a particular cause. Failure analysts generally combine visual inspection, microscopy, cross-sectioning, solder-joint evaluation, and a review of test and manufacturing histories to determine how damage likely developed.

## The Role of Vibration and Thermal Cycling

Vibration testing is intended to demonstrate that an assembly can withstand expected launch, transport, operational, or handling environments. It can excite circuit-board bending modes, stress solder connections, and expose weaknesses in mechanical support or component placement.

Thermal cycling adds another type of load. Circuit-board materials, copper conductors, solder alloys, ceramic components, and surrounding hardware do not all expand and contract at the same rate. As temperatures repeatedly change, these differences can create cyclic stresses at interfaces.

In a combined test history, several scenarios are possible:

1. Vibration creates a small, initially undetected crack or weakens a component or solder interface.
2. Thermal cycling expands that defect through repeated mechanical strain.
3. The component, board, mounting configuration, or test fixture experiences a combination of stresses that is not apparent from any single part specification.

This does not mean vibration and thermal cycling are inherently problematic. They are essential qualification tools. Rather, NASA’s observations reinforce the value of considering sequence effects, cumulative exposure, and the mechanical behavior of the complete assembly.

## Board Flexure Is a Key Reliability Concern

One recurring concern in surface-mount assembly reliability is printed circuit board flexure. A board can bend during depanelization, connector mating, screw installation, fixture loading, manual handling, conformal-coating operations, and environmental testing.

Large components, board edges, mounting holes, connectors, and stiffeners can all influence where strain concentrates. Small ceramic capacitors receive substantial attention for flex cracking, but ceramic-body resistors can also be affected when stresses are high enough or the placement is unfavorable.

Good design practice may include locating vulnerable ceramic components away from areas expected to bend, especially near board edges, mounting points, and heavy connectors. The orientation of rectangular chip components relative to expected bending direction can also influence the stress they experience.

However, no universal placement rule can replace assembly-specific analysis. Board thickness, laminate material, component package, pad geometry, solder volume, mechanical constraints, and service environment all matter.

## Implications for High-Reliability Electronics

For aerospace programs, a crack in a resistor can be more consequential than its small size suggests. Depending on the crack location and severity, the part may remain electrically functional for a time, drift in resistance, become intermittent, or eventually fail open. A crack can also compromise protective layers and change the component’s long-term resistance to moisture, contamination, or additional mechanical stress.

The practical lesson is not that engineers should avoid thick-film resistors. These components remain widely used because they are compact, capable, and generally dependable. Instead, teams should evaluate them as mechanical as well as electrical elements of the PCB assembly.

A robust reliability approach can include:

– Selecting resistor packages appropriate for the expected environment.
– Reviewing component orientation in areas of potential board strain.
– Controlling PCB handling and depanelization procedures.
– Evaluating board support, mounting, and fixture designs.
– Reviewing vibration and thermal test sequences, not just individual test conditions.
– Inspecting suspect assemblies with suitable optical and analytical methods.
– Maintaining traceability for component lots, assembly profiles, and test histories.

## What Is Known—and What Remains Unclear

NASA’s bulletin provides valuable evidence that chip resistor body cracking was observed in two cases after a vibration-and-thermal-cycling test history. It also identifies the affected devices as thick-film resistors with alumina substrates and notes the crack orientation.

What is not clear from the supplied summary is the complete root-cause determination, the degree to which vibration initiated the damage, whether board flexure was measured directly, or whether factors such as soldering conditions, component design, handling, or fixture constraints contributed.

That distinction is important. Engineers should avoid treating a limited number of observed failures as proof that every resistor in a comparable package or test program is at risk. The strongest use of the bulletin is as a prompt to review mechanical strain pathways and to investigate failures with evidence rather than assumption.

## A Small Component With a System-Level Lesson

NASA’s TB 26-10 illustrates a broader truth about electronic reliability: failures that appear at the component level often originate in the interaction among component materials, PCB design, assembly processes, mounting hardware, and environmental loads.

Chip resistors may be electrically simple, but their survival in demanding environments depends on the mechanical behavior of the entire assembly. Careful placement, controlled manufacturing, representative test setups, and disciplined failure analysis can help turn a rare observed defect into a practical reliability lesson.

## Authority sources (external links):

– [NASA Technical Bulletin TB 26-10: Printed Circuit Board Assembly Chip Resistor Body Cracking](https://www.nasa.gov/centers-and-facilities/nesc/tb-26-10-printed-circuit-board-assembly-chip-resistor-body-cracking/)
– [NASA Engineering and Safety Center technical resources](https://www.nasa.gov/nesc/)
– [NASA lessons learned database](https://llis.nasa.gov/)
– [NASA Electronic Parts and Packaging Program](https://nepp.nasa.gov/)
– [ESA space engineering standards and resources](https://www.esa.int/Enabling_Support/Space_Engineering_Technology