Prior to choosing a calculation model, you must decide whether you want to perform a parts count or parts stress prediction. A parts count prediction is generally used in the early design stage of a project, when parts and part parameters have not been exactly identified. It uses generic failure rates for various part types given an operating environment and temperature. These part failure rates are multiplied by quality factors and then summed up to obtain a system failure rate. This methodology is specifically defined in MIL-HDBK-217, Telcordia, and GJB/z 299B. The rough failure rate estimates generated by parts count analysis often help drive design efforts.

Parts stress prediction is normally used later in the development stage when most of the components and operating conditions have been identified. In a parts stress prediction, temperature and electrical stress become important factors in predicting the part failure rate. Temperature can be set at the system level, the assembly level, and the component level. A junction temperature rise per component may also be considered, depending on the depth of the parts stress prediction. The electrical stress usually assumes the form of a ratio of operating value to rated value. For instance, the defining stress factor for capacitors is voltage. Consequently, operating voltage and rated voltage are used in the failure calculation model. These factors are generally consistent across the different reliability prediction standards.

As more information becomes available during the design process, the failure rate estimates obtained from the part stress prediction become more accurate. You can modify system level parameters and default values to perform “what-if” analysis. For example, you might want to see how changes to any or all of the following parameters affect predicted part, assembly, and system, assembly failure rates: