Feature: Semiconductors


To help understand the differences between the first


three cited failure rate data sources for semiconductors - ADI component MTTF data focusing on Arrhenius high temperature operating life (HTOL), the Siemens Standard SN 29500, and the IEC TR 62380:2004 - the following sections will provide some insights on each of those methods and the associated databases.


What Is Arrhenius HTOL? HTOL is one of the most used accelerated life tests as defined in JEDEC standards to estimate component failure rate. HTOL testing aims to simulate device operation at elevated temperatures to provide sufficient acceleration to simulate many years of operation at ambient temperatures, typically at 55°C. Thus, HTOL estimates the long-term reliability of a semiconductor component - for example, MTTF - under accelerated stress conditions that compress the time to simulate the component’s lifespan while heating it and maintaining its operational voltages. Zooming into the details of reliability calculations,


the data generated at the accelerated testing conditions of HTOL (1,000hrs at 125°C or equivalent) is translated to lifetimes at the end user operating conditions (10 years at 55°C) by using the Arrhenius equation with an activation energy of 0.7eV. The chi-squared statistical distribution is used to calculate the confidence intervals (60 per cent and 90 per cent) on the failure rate data based on the number of units HTOL tested.


Figure 3: Wafer fabrication data tab from analog.com


What is Siemens Norm 29500? Te SN 29500 standard is a look-up table-based standard that was initiated by Siemens and is widely used as the basis for the reliability predictions in ISO 13849. With this, the reliability prediction is calculated through failure rates, where the failure rate is defined as the proportion of failures that can be expected on average under given environment and functional operation conditions in a time interval. Tis standard is recognised as representing a conservative


Where:


• x2 is the inverse chi-squared distribution whose value depends on the number of failures and confidence interval • N is the number of units HTOL tested • H is the duration of HTOL testing • At is the acceleration factor from test-to-use conditions calculated according to the Arrhenius equation Wafer fabrication data is one of the reliability data


and resources available at analog.com. Clicking it will give data that includes a product’s overall life-test data summary. This is composed of the overall sample size, quantity failing, the equivalent device hours at 55°C, the FIT values (based on HTOL data), and the MTTF data at 60 per cent and 90 per cent confidence levels. An example of this is shown in Figure 3. Functional safety often requires a confidence level of


70 per cent, so the 90 per cent level can be conservatively used or it could be converted using a process such as that shown in 'How to Change the Confidence Level of Your Reliability Predictions.'


30 June 2025 www.electronicsworld.co.uk


approach to determine the component failure rates. Te reference FIT values per device category have basically been determined from field returns of the specific component class. For this reason, they would include any kind of failure type seen in the application and not only intrinsic failures as induced by the HTOL method shown in the previous section. Tis includes failures due to electrical overstress (EOS),


which will not occur in the controlled lab environment used in HTOL testing. Equation 2 shows how the SN 29500-2 derives its failure


rate for integrated circuits. First, it provides a reference failure rate that corresponds to the component failure rate under the standard-defined reference conditions. Since the reference condition will not always be the same, the standard also provides conversion models to calculate failure rates depending on stress operating conditions such as voltage, temperature, and driſt sensitivity as shown in Equation 2.


Where: • λref is the failure rate under reference conditions, which scales with the number of transistors • πU is the voltage dependence factor • πT is the temperature dependence factor • πD is the driſt sensitivity factor


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