During the first prototype stage of product development, there is no one-size-fits-all answer for the right sample size. The appropriate sample size depends on various factors, including the objectives of your research, the nature of the prototype, the variability of the measurements, and the constraints of your project such as budget and timeline.
At the prototype stages of product development, especially for electromechanical devices, understanding the safety factor in your design is crucial. The safety factor, often referred to as the Factor of Safety (FoS), is a measure of the load-carrying capacity of a system beyond the expected or actual loads. Essentially, it indicates how much stronger the system is than it usually needs to be under normal conditions. A common approach to validate the safety factor in design, particularly for new and untested devices, is through step-stress accelerated life testing (SSALT). This method involves subjecting the prototype to increasing stress levels until failure occurs, providing insights into the product’s durability and reliability under various conditions.
The step-stress approach is advantageous because it simulates real-life usage conditions that can lead to failure but does so in a compressed timeframe. This method allows engineers to identify potential weaknesses in the design and materials, ensuring that the final product will withstand the stresses it will encounter during its lifespan. For electromechanical devices, stresses can include mechanical loads, thermal cycles, and electrical overloads, among others.
Please see next example of step stress graph, where maximum application pressure is 0.5 bar:
However, it’s important to note that while step-stress testing can provide valuable insights into the product’s behavior under increased stress levels, it does not directly yield information about the product’s long-term durability or life expectancy under normal operating conditions. This is because the test accelerates the failure process by applying stress levels that are higher than what the product would typically experience during regular use.
Highly Accelerated Life Testing (HALT) is another critical test similar to step-stress testing, primarily used in the electronics portion of electromechanical products. While both HALT and step-stress testing aim to expose weaknesses in a product, there are distinct differences in their objectives and methodologies.
HALT is a rigorous testing method designed to expose product weaknesses and potential failure modes early in the development process. It subjects the product to extreme stress conditions well beyond its operational specifications to identify the limits of its design. The primary goal of HALT is not to determine product life or durability under normal use conditions but to quickly find design weaknesses that could lead to failures, allowing for improvements before the product reaches the market.
Usually for HALT test it is sufficient 3 samples as well.
Assuming early stages of design 6 prototypes can be considering as sufficient and of course it depends on complexity and cost. 3 samples run for stress step test and 3 for HALT test.
So what can be done to estimate life and reliability at early stage of design?
To perform life analysis based on material properties, including activation energy and yield strength, simulations can be conducted using various software tools that incorporate both thermal and mechanical analysis. These simulations typically involve the following steps:
- Material Characterization: Gather detailed material properties, such as thermal conductivity, coefficient of thermal expansion, modulus of elasticity, and yield strength.
- Thermal Analysis: Use thermal simulation to determine the temperature distribution within the electronic component during operation and under accelerated life testing conditions.
- Mechanical Analysis: Apply mechanical simulation to assess the stresses and strains on the component, particularly focusing on areas where yield strength may be approached or exceeded.
- Failure Rate Prediction: Combine the thermal and mechanical analysis results with the activation energy to estimate the failure rates using the Arrhenius equation or other life prediction models.
In summary, the integration of material properties, activation energy, and yield strength into life analysis simulations, combined with empirical testing like HALT and step-stress testing, provides a robust framework for evaluating the design, identifying weak components, and enhancing the overall resilience of electronic components in electromechanical devices. This multi-faceted approach ensures that the final product meets the required reliability standards and performs as expected throughout its intended lifespan.
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