2.1 Background

Temperature cycling, as an acceptance test of production assemblies, is widely used as a test screen for the detection of workmanship and parts defects. It usually is used in conjunction with vibration and is particularly applicable to electronic equipment. As the design process matures, design problems should diminish significantly and approach zero. If extensive temperature cycling is employed during hardware development, as it should be, then "design" failures during the production program should be minimal, and "workmanship" and "parts" problems should predominate. The number of parts problems is influenced by the extent of the screening accomplished at the parts level. However, significant part problems are frequently detected by temperature cycling at higher levels of assembly, even when the individual parts have been subjected to high reliability screening at incoming receiving inspection.

As a part of their long-life assurance study (Reference 1) for the National Aeronautics and Space Administration, the Martin Marietta Corporation, Denver Division conducted a survey of 26 manufacturers and government agencies to review and analyze current temperature cycling practices. Out of this came some clear guidelines for cost-effective temperature cycling as a means of stimulating latent defects for corrective action prior to delivery. Typical examples of such defects which can be screened out by temperature cycling at the acceptance test level are listed in Table 1.

Martin Marietta Aerospace Packaging problems, such as bridging of conformal coating Shorts and opens in transformers and coils Defective potentiometers Intermittent solder and weld joints Shorted power transistor Defective capacitors Cracked dual  inline integrated circuits Collins Radio Co. Poor solder joints, welds, seals Nearly shorted wire turns and cabling due to damage or improper assembly Fractures, cracks, nicks, etc., in materials due to unsatisfactory processing Out-of-tolerance parts and materials NASA-MSC (Apollo) Resistor core cracked due to absence of elastomeric buffer coating Hairline crack in transistor emitter strap ground connection Damaged mica insulation washer causing transistor short Improper staking of tuning coil slug causing erratic output Cold solder joints Open within multi-layer boards due to mishandling in processing Diode internally open at low temperature Drift problems Decca Radar Limited Defective transistor Intermittent shorts in coils Lugs shorted to ground Drift and erratic operation problems Supplier B Problems with small gage wire (less than No. 40) in motors, transformers, and other electromechanical devices Failure of plastic encapsulated parts Radiation Incorporated Drift problems Integrated circuit problems

Table 1. Typical Examples of Defects Screened Out By Temperature Cycling