Generating SCDs for Piece Parts

The typical procedure for using a non-standard part begins with the generation of a source or specification control drawing. This document not only specifies all the electrical and mechanical parameters of the part, but also defines the qualification and quality conformance requirements. When generated for chip components, these drawings may be called detailed device specifications. Qualification is the initial testing that qualifies the part to meet the requirements of the drawing, and usually includes environmental testing as well as electrical testing. Quality Conformance Inspection testing (also known as QCI) qualifies the particular lot of parts, and is usually done at periodic intervals. QCI normally has a smaller subset of tests than qualification testing, so is less expensive. QCI is also known as group A, B, C, D and E testing. Depending on the type of parts, the “group” testing may be just group A and B, or A through E. In the QCI process, one or more units are destroyed, adding additional expense. There are various strategies for reducing QCI cost, which require the informed direction and approval of the customer.

The qualification process for chip components with a hybrid microcircuit is called element evaluation, and is performed in accordance with MIL-PRF-38534. This is also used for quality conformance inspection.

Magnetic components for space power supplies are almost invariably non-standard. The usual governing specification for magnetic components is MIL-STD-981, which builds on MIL-PRF-27 and related specifications. Like many other detailed parts specifications, MIL-STD-981 allows qualification (not QCI) by similarity, provided the “similar” part meets certain tests of “closeness” with the previously qualified parts. This permits cost savings in many instances.

Many of the parts will be covered by specification control drawings or source control drawings. The former type is used when the requirements are not highly critical and source qualification is not deemed mandatory. The latter restricts procurement of the part from approved sources. Source control drawings usually list requirements which must be satisfied to qualify a part.

In practice, source control drawings are frequently preferred over specification control drawings when the component engineer is aware of indefinable parameters that are important or that only one acceptable source exists. Another reason for source control drawings may be that the component engineer knows the recommended source has unique test equipment or capabilities not available elsewhere.