Understanding IEC61709: A New Standard for Failure Rates in Safety Applications

The automotive safety standard ISO 26262 provides several catalogs for determining failure rates of electrical components. However, while standards such as SN29500 and IEC62380 have been widely used, they have not been updated in years, often leading to conservative failure rate predictions.

IEC61709 takes a different approach.

This standard doesn’t provide fixed reference failure rates. Instead, it offers formulas for calculating failure rates based on physical stress factors like temperature and voltage.

This enables an IC semiconductor company to determine their own failure rates and use the stress factors of IEC61709 to determine the failure in time (FIT) rates for a specific automotive mission profile. FIT rate assesses a device’s reliability over time by measuring how often a given component will fail over one billion device-hours.
 

Using IEC61709 to Determine Failure Rates for Electrical Components

The key distinction of IEC61709 is that it doesn't provide failure rates directly, as other standards do, but rather offers stress factors to adjust failure rates based on environmental conditions. For example, the failure rate for an IC using CMOS technology is calculated with the following formula:

Where:

Calculating the Reference Failure Rate

Reference failure rates for ICs can be derived from existing reliability tests, such as HTOL testing. If enough components have undergone testing, then you can calculate the failure rate of the IC using the upper confidence limit of the X² distribution as follows.

Where:

• X² = Inverse chi-squared distribution
• A = Temperature acceleration factor
• N = Sample size
• a = Confidence level
• tA = Stress time

According to this equation, a particular IC at a reference temperature of 55°C might give the following failure rates:

Figure 1:  Reference FIT for an IC at various confidence levels

A confidence level of 60% is typically used for reliability estimates. However, for the IEC61709 calculations, we recommend a 90% confidence level. In our example, that would give us a reference FIT rate of 3.03 at 55°C.
 

Applying IEC61709 to Automotive Mission Profiles

With the reference FIT rate determined, we can update the first equation thus:

IEC61709 stress factors for temperature and voltage an be applied to calculate the failure rate for a product based on its specific automotive mission profile. This profile is a histogram of assumed ambient temperature over operating hours, which the customer provides.

The FIT rate is calculated for each temperature point, and a weighted average is computed across the automotive mission profile. For instance, with a mission profile featuring an average ambient temperature of 29°C (typical for an in-cabin applications) and a reference FIT rate of 3.03, the integrated failure rate would be 1.38 FIT. This is significantly lower than the ~20 FIT estimate using the SN29500 standard.

Figure 2: FIT calculation for automotive mission profile using IEC61709
 

Conclusion

Failure rate standards are crucial for assessing the residual risk of products developed according to specific Automotive Safety Integrity Levels (ASIL). These metrics rely on a minimum level of diagnostic coverage for both single-point and latent faults. For a system to comply with ASIL D, the highest Safety Level, the typical requirements include:

• Single point fault metric (SPFM) > 99%
• Latent fault metric (LFM) > 90%
• Probabilistic metric for random hardware failures (PMHF) < 10 FIT.

FIT values are used safety analyses such as the Failure Modes, Effects, and Diagnostic Analysis (FMEDA) evaluation. At ADI, we have developed to perform these calculations according to the  IEC61709  standard, supporting our reliability engineers in functional safety efforts.

Read more from the Automotive FuSa blog series