1. ALT test - Design.

Prior to ALT, it’s important to establish a baseline for the product.

This involves:

- Functional Testing: Confirming all units perform within specifications at standard conditions.

- Initial Screening: Removing early-life failures (infant mortality) through burn-in or screening, so only representative units proceed to ALT.

- Material and Process Review: Evaluating materials, manufacturing, and assembly quality to eliminate defects unrelated to the intended stress factors.

- Failure Mode Analysis: Reviewing historical data and performing Failure Mode and Effects Analysis (FMEA) to anticipate which failure mechanisms could be accelerated during ALT.

 Defining Stress Levels: The Role of Step-Stress Testing

 A critical part of ALT design is selecting the right stress levels. Step-stress testing is a common method for this. In a step-stress ALT, the stress applied to test units is increased in discrete steps at set intervals. This approach allows observation of failure behavior at various stress levels and helps define the appropriate range for the actual ALT.

Step-stress testing is useful for: 

- Identifying the threshold where new failure mechanisms may appear.

- Avoiding overstressing, which could cause unrealistic or non-representative failures.

- Optimizing the duration and severity of each stress level to balance acceleration with data validity.

Example: Step-Stress ALT Design: Power-Only Variation at High Constant Temperature

 In this approach, power is incrementally increased while maintaining a high, constant ambient temperature. The goal is to accelerate failure by raising the component’s internal temperature through power dissipation, simulating worst-case operational stress. At least three samples are run to failure, and their failure points are used to define the stress levels for the Accelerated Life Test (ALT).

Failure and Duty Cycle Distributions

•        Failure Distribution: Three samples are subjected to step-stress power increases until failure, yielding failure points at 1.8 W, 2.0 W, and 2.2 W.

•        Duty Cycle Distribution: The operational (duty cycle) power distribution is typically much lower, with a maximum observed value of 1.2 W during normal use.

Interpretation:

The greater the gap between the operational (duty cycle) distribution and the failure distribution, the more conservative and robust the design. This margin indicates a higher safety factor.

ALT Stress Level Definition

Using the median of the three failure samples (2.0 W), the ALT stress levels are set as follows:

 Ensuring Validity and Relevance

The main goal of pre-ALT testing and step-stress design is to ensure that the accelerated conditions are relevant to real-world use and that observed failures represent field conditions. Overstressing can lead to non-representative failure mechanisms, while understressing may not accelerate failures enough. Careful planning—including step-stress testing—is essential for a successful ALT program.

By systematically increasing stress levels and closely monitoring responses, engineers can confidently define ALT parameters that yield actionable insights into product reliability.