Stressed to the Max: Why Failure Rates Tell Only Part of the Story

Introduction to Stress Factors

As a consultancy working on a wide variety of safety and reliability projects, we spend a lot of our time looking at failure rates and failure probabilities. In the world of electronic hardware, there are numerous excellent sources defining failure rates, e.g. FIDES, 217Plus, IEC 61709, and the now–retired (or rather integrated into ISO 26262) IEC TR 62380. For software, there are also sources that can be used to estimate reliability, although it is a difficult subject. Regarding reliability, there are complex strategies in software reliability growth models that can be applied, e.g. those described by Neufelder: How reliable is your software growth? » Lorit Consultancy

However, an area that is often given less focus than it perhaps should is the impact of mechanical and environmental stress. In this blog, we look at how these topics are handled in existing standards and how practical strategies can be used to include these topics in analyses.

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Key Reliability Factors

The typical stress sources that impact devices during normal operational use, as listed in FIDES 2022, are:

• Temperature

• Electrical stress

• Thermal cycling

• Mechanical stress

• Humidity

• Chemical exposure

Additional factors that induce stress include:

• Placement – the item‘s position in the equipment.

• Application – the operating environment in which the item is used.

• Ruggedizing – the level of overstress sensitivity built into the item.

• Sensitivity – the overstress sensitivity inherent to the technology of the item.

Typical Analysis Criteria

For most analyses, only a subset of the factors listed above is utilized. Thermal stress based on a mission profile is the most common approach, yielding the Arrhenius relationship over the full temperature range, as indicated below for a Schottky diode.

Figure 1. HRC.1913 – Diode, Schottky, 60V, 1A, SOD-123W (Isograph)

Thermal cycling is used in assessing materials such as conformal coatings to ensure that the coating is still effective after this preconditioning. This testing is often performed over a wide temperature range, for example from −40°C to +125°C, with a transition time of 30 seconds between temperature extremes.

Sweating it out over Humidity

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Relative humidity is a factor that is dependent on knowing 3 key characteristics: the humidity outside the equipment, the temperature outside the equipment, and the temperature rise inside the equipment.

Humidity can play a role in impacting other stress factors. For example, low humidity can significantly increase the charge build up and hence the electorstatic discharge (ESD) voltage magnitude. As relative humidity increases from 10% to 75%, an order-of-magnitude reduction in ESD voltage is possible, with all other reference conditions remaining constant.