Tailor Your System Assessment by Accounting for Process-Related Variability

When performing reliability prediction analyses, calculations are based on established reliability models. The Reliability Analysis Center (RAC) has developed the PRISM methodology to overcome inherent limitations in such widely accepted models as MIL-HDBK-217 (Reliability Prediction of Electronic Equipment), which is no longer being actively maintained or updated by the Department of Defense (DoD). In addition to using new component reliability prediction models, PRISM can account for the myriad of factors that influence system reliability by tailoring the system assessment with available historical predecessor data, test or field data, and process grade scoring profiles.

Once a system or assembly structure is designed, the failure rate value that is calculated by any model is an inherent or "seed" failure rate because it represents only the physical attributes of the components that comprise the system or assembly, subject to the environmental conditions and operating profile characteristics associated with its application. The failure rate that the system or assembly will actually experience in the field may be potentially better or worse than the inherent failure rate, depending on the processes used by the respective manufacturers to ensure that:

• Designs are reliable and robust.
• Parts of acceptable quality are selected and controlled.
• Manufacturing practices do not degrade reliability performance.
• Management processes encourage good requirements definition and design practices.
• Maintenance activities do not induce failures.
• The number of "cannot duplicate" incidents is minimized.
• Wearout and infant mortality issues are understood and addressed.
• Reliability growth is emphasized throughout the design and development phases.

PRISM quantifies how each of the above factors contributes to system reliability by grading the processes for each system failure cause. By answering a series of questions for each of nine specific process grade types, you establish a scoring profile whose total value is translated into a quantitative pi-factor multiplier. This score then accounts for the process-related variability by impacting the predicted failure rate positively or negatively. When you use Relex PRISM, you can even use process grades with other reliability models, including MIL-HDBK-217 and Telcordia (formerly Bellcore).

A Process Grade file is the unique set of answers for the nine different process grade types. The default Process Grade file supplies factors that represent "average" conditions. The set of answers in any Process Grade file may be common to a particular manufacturer, to a particular assembly, or to the entire system. For example, default answers for design process questions are shown in the following figure.

Process Grades File

After selecting the process grade type to be defined (Design), you answer questions by selecting from the choice lists associated with the Response fields. Asterisks following question numbers indicate that answers are required to calculate and use the process grade pi factors in an assembly or system reliability prediction. To simplify the scoring process, you can select the Only Show Required Questions check box so that only questions that require answers display. However, you can tailor the scoring even further by answering any relevant optional questions.

As you answer questions, the following totals are calculated and displayed:

• The number of questions answered from all of the questions presented.
• The number of questions answered from those that are required.
• The weight, which is the sum of the weights for all of the answered questions.
• The score, which is the sum of the calculated scores for all of the answered questions. (The calculated score of any question is equal to its weight times the answer value.)
• The total weight, which is the sum for all weights for all questions presented.
• The grade, which is a percentage, based on all of the questions. (The grade is calculated by dividing the total score by the total weight and then converting this value to a percentage.)

Infant Mortality is the only process grade that is defined differently. It is computed with a function of temperature screenings and vibration screenings that are performed prior to the delivery of the product to the customer. The Infant Mortality process grade addresses the:

• Type of temperature and vibration screening tests.
• Specific conditions (stresses) of the tests.
• Detection efficiency associated with the tests.
• Estimated percent of potential infant mortality failures resulting from temperature cycling and vibration for each screening test type option.
• Instantaneous failure time baseline (in months).

An example of the temperature screening information required by the Infant Mortality process grade is shown below.

Infant Mortality Process Grade

As you provide the appropriate responses for the nine process grade types, you can either save the Process Grade file under the existing name or under a new name. Once a Process Grade file is saved, it can be assigned to either one assembly or to any number of assemblies within a system (or even to any number of different systems). Because the Process Grade file contains known information about the processes associated with the assemblies and systems to which it is assigned, the process-related variability can be accurately reflected in the assembly or system reliability. The following figure shows project settings that control how process grades are calculated and applied:

Process Grade Options

• Process Grade Calculation Type. Used to select the type of process grade calculation (Logistics or Inherent), which are fully described in the subsequent paragraphs.
• Trickle Down Process Grades. Used to indicate whether the Process Grade file defined at the system level is to trickle down to all of the assemblies. Although the Trickle Down Process Grades check box is cleared by default, you can select it to trickle down the Progress Grade file assignment.
• Apply Default Process Grades. Used to indicate whether the default Process Grade file is to be automatically applied to assemblies when no Process Grade file is selected. Although the Apply Default Process Grades check box is cleared by default, you can select it to apply the default Process Grade file to all assemblies. For PRISM assemblies, this check box is ignored. As the note in the above figure points out, the default Process Grade file is always applied to PRISM assemblies.

The Inherent calculation model takes into account all processes that relate directly to the design and build of the assembly or system before external conditions, such as "Induced" or "Cannot Duplicate Failures," have an impact on the actual reliability achieved in the field. The Inherent model takes the form:

The Logistics calculation model adds factors that result from operation, maintenance, and repair of the assembly or system in the field specifically identified as "Induced" and "Cannot Duplicate Failures," and takes the form:

By accounting for all of the primary factors that influence the inability of a system to perform its intended function, PRISM adopts a broader scope to predicting reliability.

If you would like additional information about PRISM process grades or Relex PRISM, please email info@relexsoftware.com.