Implementing a sound parts control program requires more than policy directive such as DoD Directive 4120.19 or plans to use "MIL-STD" parts. Although MIL-STD parts may provide some level of device control, blind dependence on them to ensure system reliability will prove disappointing. Until adequate control programs are set up with vendors to ensure adequate source controls, the assumption that procurement of parts to military standards automatically ensures good parts will remain faulty. Also, cost savings are feasible by establishing controls for all parts suppliers at the earlier possible time; such controls are not available through current standard parts programs (e.g., Defense Electronics Supply Center).
A key element of parts control is an established corporate policy which ensures that certain steps are taken to control part quality, (both electrical and mechanical), independent of government contract requirements. When this policy is well defined and imposed on subcontractors, the most effective parts control measures have been initiated.
Any attempt to delay adequate parts control planning until rate production will add significant risk to the project. The parts that will ultimately be used in production designs should be qualified and assessed during FSD. During FSD the critical parameters that need to be measured, and the identification of which parts will require rescreening at incoming inspection, must be identified. This will help ensure that FSD is successful and set the stage for significant production cost savings. Savings in purchased parts quantities of up to 12 percent have been noted. Reduced rework and repair results in additional cost savings, which is the principal benefit of electronic parts rescreening.
Although the exact rescreen to perform will vary with part type, 100 percent environmental stress rescreening on electronic and semiconductor piece parts has been found to be economical. Digital Integrated Circuit (IC) devices benefit most from performance testing at temperature, while diodes and transistors can sometime be effectively tested at room temperature. Particle Induced Noise Detection (PIND) testing on cavity devices (MIL-STD 883, Method 2020) and solderability tests on leaded devices are strongly encouraged. Destructive Physical Analysis (DPA) is also recommended to identify process characteristic changes in the manufacture of electronic piece parts that could affect part performance and reliability.
An effective parts rescreening program requires that feedback techniques be established which notify suppliers of defective parts and require corrective action on the part of the vendor. In addition, visibility of incoming and assembly yield rates must be continuously maintained to identify poor suppliers and to develop more effective incoming and vendor screens. Early detection of parts problems is the key to a low risk transition to rate production. Ignoring these essential measurement and feedback techniques will lead to unexpected surprises and significant cost and schedule overruns in rate production. Developing an adequate testing and screening program at the vendor's facility should be pursued; however, parts rescreening at incoming inspection currently provides the best approach to solving the technical risk associated with piece part control.