During the design process, specific parts and materials are selected and configured to meet specified requirements and to achieve desired objectives. The primary requirements concern performance, reliability, and maintainability, with the objective of meeting these requirements on schedule and within budget. Formal engineering education aims at the design process from a performance point of view. When parts and materials selection becomes a consideration, design engineers are not so well educated. Cost, schedule, and performance risks increase as a result.
To ensure the uniform application of parts and materials by all design engineers, an Approved Parts List (APL) must be issued at the start of Full-Scale Development (FSD). In addition to providing design engineers a baseline from which to select parts and materials, the APL also serves to introduce discipline into the design process since the use of any nonstandard parts or materials requires engineering justification prior to approval.
Results in recent pioneering defense systems acquisition programs have proven that specified derating criteria (fixed upper limits on allowable stresses affecting operating life) support low-risk design engineering. These criteria, invoked in government contract specifications and corporate engineering design policy, assist design engineers in making proper parts and materials selection and application decisions. At the same time, engineering design policy must require proven design solutions such as standard circuits and mechanical designs: techniques for designing for production assembly, test, and inspection and other successful techniques for reducing design risk.
To assure that the contractor's derating criteria in his corporate engineering design policy are kept up-to-date with government and industry standards, the government should review and approve the contractor's derating criteria prior to contract award.
One of the most critical factors to consider when determining the proper application of electronic parts is thermal stress, since one of the most common causes of electronic part failures is thermal overstress. The use of conservative thermal stress derating criteria provides an effective means of reducing part failure rates. In order to determine part thermal stress levels, the design engineer cannot wait until thermal overstress failures occur during testing, since the cost and schedule impact of redesign may be unacceptable. Therefore, it is critical that thermal stress analyses are performed on the design as soon as is practical. Equally important is the feedback of the results of the analysis into the design to effect design changes, not simply the reporting of the result to meet a contract data requirement, as has been done so often in the past. The thermal analysis must be continuously updated as part of the overall design effort.
As working models are constructed, the thermal analysis results should be confirmed by thermal survey measurements and the measured temperatures compared to derating criteria, as well as fed back into the analytical models.