Stress vs. Strength: The Foundation of Component Reliability in FMEDA

The So What:

Your product's safety and reliability depend on the constant battle between component stress and strength throughout its useful lifetime. This dynamic is influenced by everything from design and manufacturing to usage and environment. Crucially, without accurately factoring in real-world stress and strength, your reliability predictions will be flawed, leading to under or overestimation. This prevents effective design optimization. An effective FMEDA tool and robust component reliability database are therefore essential, enabling you to tailor predictions to actual operating conditions for truly safe and reliable products.

Introduction:

An FMEDA (Failure Modes, Effects, and Diagnostics Analysis) predicts product safety and reliability for dependable / safety-critical products. It sums the failure rate contributions per failure mode for each individual component in a design. This article will explain how stress and strength drive individual component failure rates used in FMEDA and identify the key factors influencing them.

How Stress and Strength Impact Component Failure Rates

Component failure occurs when stress exceeds strength. Both strength and stress are random variables, representable by a normal probability distribution (as shown in the figure). Given the random nature of strength and stress, there's always some probability of stress exceeding strength, illustrated by the shaded area in the graph.

Figure. Component Stress vs Strength

The shaded area in the figure represents the probability of stress exceeding strength, which is proportional to the failure rate. A larger shaded area indicates a higher failure rate. Increasing product strength (by shifting the blue curve to the right) or reducing stressors (shifting the red curve to the left)—or both—reduces the shaded area and, consequently, the failure rate.

Stress and Strength Influencers

Because stress and strength are shaped by numerous players—from the component manufacturer to the product designer to the end-user—a comprehensive understanding is non-negotiable. Overlook any factor, and your product's reliability in actual conditions will be different from predictions.

Component strength is determined by the manufacturer through design and manufacturing processes (i.e., component quality), varying from lot to lot and manufacturer to manufacturer. Component stress depends on both its operating environment (e.g., temperature & humidity, shock & vibration, electrical overstress, etc.) and application. Stress magnitude also depends on the forces encountered during manufacturing, packaging, transport, and operation.

The table below summarizes factors influencing component stress/strength levels. These should all be considered when performing an FMEDA.

Factor	Description	Determined By	Examples of Influence
Component Design	Inherent characteristics and specifications of the component.	Component Manufacturer	Material properties, dimensions, tolerances, manufacturing processes, internal structure (e.g., die size in ICs), rated voltage/current.
Component Usage	How the component is applied within the specific product.	Product Designer & Manufacturer	Circuit configuration, electrical connections, mounting techniques, heat sinking, derating, applied voltage/current levels (within ratings).
Overall Product Design	The broader electrical and mechanical context in which the component operates.	Product Designer & Manufacturer	Enclosure design, vibration isolation, thermal management system, power supply stability, signal integrity, mechanical stress distribution.
Product Usage Profile	The operational demands and patterns placed on the product.	End User of Product	Duty cycles, frequency of operation, load variations, switching rates, operational duration, maintenance schedules.
Product Environment	The external conditions to which the product and its components are exposed.	End User of Product	Ambient temperature, humidity, vibration levels, shock, electromagnetic interference (EMI), radiation, corrosive atmospheres.

Table. Factors Influencing Component Stress / Strength Levels

Download an Infographic version of the table to use as a reference.

Conclusion

FMEDA aims to enhance design reliability by helping you optimize the balance between stress and strength. A good FMEDA tool and component reliability database should allow you to consider component stress vs strength factors based on actual conditions.

exida’s Component Reliability Database™ (CRD) allows you to model reliability based on application and environmental stress factors, such as:

Average Internal Product Temperature during Operation
Temperature Cycles – Magnitude and Frequency
Average Altitude – Meters (Radiation Effects)
Electrical Overstress (EOS)
Humidity
Salt / Chemical Corrosion
Mechanical Shock / Vibration

exida’s FMEDAx™ tool allows designers to take credit for good design decisions including EMC/surge protection, de-rating of component parameters, heat sinks, PC board coating, physical protection, surface finish on moving parts, etc.

Reach out to request a demo of our OEMx tool (incl. FMEDA and CRD).