Printed wiring assemblies: Reliability stress screening (NT ELEC 018)

Abstract

  New electronic equipment in the first hundreds of hours of operation (the initial period) has a failure intensity that is typically 5-20 times as high as the steady state failure intensity during the period of operation. As long as this is the case Reliability Stress Screening (RSS) is a method with which one can improve the quality of the product in the initial period. It must however be emphasised that RSS shall be flexible to its extent and should ideally be terminated as soon as the design, components, raw materials and processes are debugged. The early failures in an equipment are caused by one or more weak or damaged (flawed) components and sub-assemblies. In order to remove these flaws the equipment or its PWA’s is subjected to a Reliability Stress Screening (RSS) process that transforms the flaws into permanent faults. In this way the weak and flawed components and sub-assemblies can be located and repaired resulting in good and strong equipments. Since the primary purpose of the RSS process is to remove the flaws to a predetermined quality level it is a production process that 100% of the production must pass. The RSS process will as a secondary advantage give information about design failures, component failures and process failures. This information should be used for a reliability growth program. The difference between a screening process and a reliability growth program is shown in Fig.1. RSS will further give information that can be used to release an individual product or lot for delivery. Another secondary advantage of an RSS process is the possibility to reduce the extent of incoming inspection. Tests of complex IC’s are very costly and there is always a risk of damage during these tests and all handling. Further the test program coverage is not 100 % and functions really used in actual application may not be tested. During an RSS function test the really used functions of the IC are tested. When the product during the initial period has approximately the same failure intensity as during the period of operation the RSS process can be terminated. But as a safeguard against a bad lot of components or an assembly process that goes out of control, it is advisable to keep the RSS equipment ready for operation and to send samples of the production through the RSS process. In that way 100% RSS may be resumed at short notice should a bad lot of components arrive, or a process go out of control.