Handbook for Robustness Validation of Automotive Electrical/Electronic Modules

In late 2006 Members of the SAE International Automotive Electronic Systems Reliability Standards Committee and ZVEI (German Electrical and Electronic Manufacturers’ Association) formed a joint task force to update SAE Recommended Practice J1211 NOV1978 “Recommended Environmental Practices for Electronic Equipment Design.” The 1978 of version of SAE J1211¹ was written in an era when electronics were first being introduced to the automobile. There was a high level of concern that the harsh environmental conditions experienced in locations in the vehicle could have a serious negative affect on the reliability of electronic components and systems. Some early engine control modules (ECMs) had failure rates in the 350 failures per million hours (f/10⁶ h) range, or expressed in the customer’s terms, a 25% probability of failure in the first 12 months of vehicle ownership. At that time, warranty data was presented in R/100 (repairs per 100 vehicles) units, for example, 25 R/100 at 12 months.

In these early years, when the automotive electronics industry was in it’s infancy, a large percentage of these were “hard” catastrophic and intermittent failures exacerbated by exposure to environmental extremes of temperature (–40 °C to +85 °C); high mechanical loads from rough road vibration and rail shipment; mechanical shocks of up to 100 g from handling and crash impact; severe electrical transients, electrostatic discharge and electromagnetic interference; large swings in electrical supply voltage; reverse electrical supply voltage; and exposure to highly corrosive chemicals (e.g., road salt and battery acid). The focus of the 1978 version of SAE J1211 was on characterizing these harsh vehicle environment for areas of the vehicle (engine compartment, instrument panel, passenger compartment, truck, under body, etc.) and suggesting lab test methods which design engineers could use to evaluate the performance of their components and systems at or near the worst-case conditions expected in the area of the vehicle where their electrical/electronic components would be mounted. By testing their prototypes at the worst case conditions (i.e., at the product’s specification limits) described in the 1978 version of SAE J1211 designers were able to detect and design out weaknesses and thereby reduce the likelihood of failure due to environmental factors.

By the mid-1980s, it became common practice to specify “test-to-pass” (zero failures allowed) environmental conditions-based reliability demonstration life tests with acceptance levels in the 90% to 95% reliability range (with confidence levels of 70% to 90%). This translates to approximately 5 to 20 f/10⁶ h. The sample size for these tests was determined using binomial distribution statistical tables and this would result in a requirement to test 6 to 24 test units without experiencing a failure. If a failure occurred, the sample size would have to be increased and the testing continued without another failure till the “bogie” was reached. The environmental conditions during the test were typically defined such that the units under test were operated at specification limits based on SAE J1211 recommended practices (e.g., –40 °C and +85 °C) for at least some portion of the total test time. The “goal” of passing such a demonstration test was often very challenging and the “test-analyze-fix” programs that resulted, although very time-consuming and expensive, produced much-needed reliability growth. Reliability improved significantly in the late 1980s and early 1990s and vehicle manufactures and their suppliers began expressing warranty data in R/1000 units instead of R/100 units.

By the turn of the century automobile warranty periods had increased from 12 months to 3, 4, 5 (and even 10 years for some systems) and most manufacturers had started specifying life expectancies for vehicle components of 10, 15 and sometimes 20 years. And by this time several vehicle manufacturers and their best electrical/electronic component suppliers had improved reliability to the point where warranty data was being expressed in parts-per-million (ppm) in the triple, double and even single-digit range. This translates to failure rates in the 0.05 f/10⁶ h range and better! The achievement of such high reliability is not the result of test-to-pass reliability demonstration testing based on binomial distribution statistical tables. With this method, reliability demonstration in the 99.99% to 99.9999% range would require thousands of test units! On the contrary, the methods and techniques used by engineering teams achieving such reliability excellence did not require increasingly large sample sizes, more expensive and lengthy testing, or more engineers. It is about working smarter, not harder; and about systems-level robust design and robustness validation thinking rather than component-level “test-to-pass” thinking.

The task force leaders and members were of the strong opinion that the 2008 version of SAE J1211 should document the state-of-the-art methods and techniques being used by leading companies and engineering teams to achieve ultra-high reliability while at the same time reducing overall cost life-cycle and shortening time-to-market. The SAE International Automotive Electronic Systems Reliability Standards Committee and ZVEI (German Electrical and Electronic Manufacturers’ Association) are hopeful that this Handbook for Robustness Validation of Automotive Electrical/Electronic Modules will help many companies and engineering teams make the transition from the 1980s “cookbook” reliability demonstration approach to a more effective, economically feasible knowledge-based Robustness Validation approach.