NAVMAT_P-9492[1]

1 NAVMAT P-9492 NAVY MANUFACTURING SCREENING PROGRAM DECREASE CORPORATE COSTS INCREASE FLEET READINESS MAY 1979 Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 2 PREFACE Continuing advances in electronics state-of-the-art plus increased emphasis on reliability and early development testing have increased the potential for providing a basically sound and inherently reliable design. As this potential has increased, so has the complexity and density of contemporary equipment packaging. This complexity amplifies the ever-present problems of detecting and correcting latent manufacturing defects. Equipment malfunction, after many hours of field operation, has often been attributable to something as simple as a wire which was improperly soldered. The occurrence of such a failure when equipment is installed on ship, shore, or in aircraft incurs high maintenance costs and results in low operational readiness rates. The ability to detect simple anomalies through even the most intense visual inspection and bench checkout has become a thing of the past because of the complexity of current equipment. Effective manufacturing screens for the purpose of stimulating latent defects, whether or not such screens resemble expected mission environments, have become an absolute necessity. The manned space program of the 1960's evolved what continues to be the most cost-effective manufacturing screens: temperature cycling and random vibration. The Naval Material Command is striving to replace current and ineffective temperature cycling and low-level sinusoidal vibration with more stringent temperature cycling and random vibration in manufacturing screens such as burn-in and acceptance testing. This report outlines, primarily for Navy contractors, an adapted and effective manufacturing screening program consisting of temperature cycling and random vibration. With the recognition that test facility cost has been a major obstacle to the use of random vibration, a technical report, which describes in detail a proven means to generate random vibration at low cost, is included as an appendix. Together, temperature cycling and random vibration provide a most effective means of decreasing corporate costs and increasing fleet readiness. W.J. WILLOUGHBY, Jr. Deputy Chief of Naval Material Reliability, Maintainability & Quality Assurance Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 3 TABLE OF CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 1.0 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 TEMPERATURE CYCLES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Findings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.1 Scope of Survey.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.2 Number of Cycles and Equipment Complexity. . . . . . . . . . . . . 5 2.2.3 Duration of Temperature Cycles. . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.4 Equipment Operating Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.5 Temperature Range and Rate of Change. . . . . . . . . . . . . . . . . 7 2.2.6 Design Compatibility with Temperature Cycling. . . . . . . . . . . . 7 2.2.7 Types of Defects Simulated. . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.8 Repairs and Failure-Free Cycles. . . . . . . . . . . . . . . . . . . . . . . 8 2.2.9 Board Level Temperature Cycling. . . . .. . . . . . . . . . . . . . . . . . 8 3.0 RANDOM VIBRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2 Findings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2.1 Verification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2.2 Application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.3 Synthetic Random Vibration. . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.0 RECOMMENDATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.1 Temperature Cycling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2 Random Vibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.0 REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 4 1.0 INTRODUCTION The reliability of a well-designed product is usually degraded to some extent in manufacturing it. A low but finite number of defects in both parts and workmanship is generally considered "normal" in manufacturing processes involving people and machines. To sustain the level of reliability inherent in the design, however, these defects must be discovered and corrected before the product leaves the factory. Otherwise, they will show up as product failures in service use with possibly serious military consequences and always with undesirable cost impact. Further, the discovery and correction of defects in the factory contributes significantly to the manufacturer's production costs, as do field returns for correction of defects under contract requirements and warranties. Both the Navy and its suppliers, therefore, have a vital interest in the most efficient and effective means for the earliest elimination of defects. Most Navy programs acquiring electronic devices and systems traditionally have depended on the final acceptance test to catch manufacturing defects. They have relied on this screen as a sufficient incentive to the manufacturer for the inclusion of additional pre-acceptance test screens of many different forms in the production operation. Some contracts have called out specific pre- acceptance tests (e.g., burn-in) for the primary or ancillary purpose of defect detection. For a variety of reasons, both technical and contractual, the vast majority of electronic devices and systems delivered to the Navy continue to contain manufacturing defects in parts and workmanship which could have and should have been discovered and eliminated in the factory. This publication provides guidance concerning the use of temperature cycling and random vibration as manufacturing screens for defects in both parts and workmanship. The requirements for such screens are called out in Navy instructions and reflected in contract requirements. Section 2.0 on temperature cycling is derived from a Martin Marietta report for the National Aeronautics and Space Administration on industry experience in assuring long-life hardware. Section 3.0 on random vibration has been prepared by the Grumman Aerospace Corporation under the direction of the Naval Electronic Systems Command. It summarizes the experience of Grumman and others supporting the NASA manned space program. Grumman recently has devised a technique to simulate random vibration at low cost without a sacrifice in effectiveness as a manufacturing defect screen. This technique is included as an appendix to this publication. Section 4.0 contains the minimum recommended thermal cycling and random vibration manufacturing screens to be used in the production of Navy equipment. Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 5 2.0 TEMPERATURE CYCLING 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 o 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 Packing problems, such as bridging of conformal coating Shorts and opens in transformers and coils Defective potentiometers Intermittent solder and weld joints Shortened 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 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 temperatures 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 and other electromagnetic devices Failure of plastic encapsulated parts Radiation Incorporated Drift problems Integrated circuit problems Table 1. Typical Examples of Defects Screened Out by Temperature Cycling Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 6 2.2 FINDINGS 2.2.1 SCOPE OF SURVEY The utility of temperature cycling as a workmanship screen is recognized by many aerospace companies. The approach to this cycling, however, varies widely from company to company. Table 2 shows the degree of variation in the temperature cycling approaches employed by the 26 aerospace companies and agencies surveyed. The table shows a lack of standardization in employing more than one temperature cycle. It also shows a temperature range between –65 °F and +131 °F (-54 °C to +55°C) to be most commonly used. Supplier/Agency No. of Cycles Recommended Temperature Employed (ºF) Temperature Range (ºF) Lockheed Missiles and Space Co. 8 to 10 -20 to 160 180 General Electric Co. 6 to 10 -65 to 131 196 Aerospace Corporation 6 to 8 Variable - Decca Radar Ltd. 20 5 to 131 126 Radiation Incorporation 10 to 25 -65 to 131 196 TRW Systems 6 Variable - Martin Marietta Aerospace 6 to10 Variable - Boeing Co. 3 t o12 -65 to 131 196 Hughes Variable Variable - Motorola 22 -65 to 160 225 Collins Radio Co. 9 to 25 -65 to 160 225 Honeywell, Incorporated (Denver) 12 -13 to 131 144 Hewlett Packard Co. 16 32 to 131 99 Grumman Aircraft Engineering Co. 4 to 6 Variable - Bendix Corporation 6 Variable - Delco (AC) Electronics 5 -20 to 120 140 Raytheon – Equipment Div. 5 32 to 160 128 RCA 3 Variable - Westinghouse 3 or 4 Variable - Sandia Corporation 3 or 5 -65 to 160 225 Texas Instruments 2 to 10 -67 to 131 198 Barnes Engineering Co. Variable Variable - Goddard Space Flight Center 1 Variable - JPL 1 Variable - Supplier A 5 -65 o 131 196 Supplier B 1 -65 to 165 230 Table 2. Summary of Temperature Cycles from Survey Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 2.2.2 NUMBER OF CYCLES AND EQUIPMENT COMPLEXITY Test and failure rate data provide some interesting insights into the most effective number of temperature cycles to use for workmanship screening. Figure 1 provides a comparative illustration of the number of failures as a function of the number of temperature cycles. Six to ten thermal cycles are required to eliminate most latent workmanship defects. Figure 1. Temperature for Defect Elimination Investigation of the equipment to which this data applied revealed a useful correlation between equipment complexity and the effective number of temperature cycles required. Six cycles appear adequate for black boxes of about 2000 parts, while 10 cycles are recommended for equipment containing 4000 or more parts, as shown in Figure 2. Hughes Aircraft Company has developed mathematical models to predict how many cycles are required to achieve a specified reliability. The number depends on the previous amount of screening the quality of parts used, and the exact thermal conditions and profile for the parts being screened. A significant finding is that many more than 10 cycles are sometimes indicated by these models. Similarly, when unscreened parts are used and temperature cycling of assemblies is employed as the main production screen, more than 10 cycles may be required. Programs of 16 to 25 cycles are not unknown. 2.2.3 DURATION OF TEMPERATURE CYCLES Much of the data in this report is derived from programs using AGREE testing per MIL- STD-781. The AGREE cycle combines temperature ramps, temperature soaks, and low level (29) vibration. The consensus is that the temperature soaks and the low level vibration play a very minor role and, therefore, the AGREE technique is essentially equivalent to a temperature cycling test, with the screening strength of the test dependent on the temperature range, the temperature rate of change, and the number of cycles. The AGREE cycle is shown in Figure 3. 7 Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 8 Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 9 Accordingly, the dwell times at high and low temperatures need only be long enough for internal temperatures to stabilize, a time which is dependent on the equipment design and which will vary from one program to another. The AGREE cycle in Figure 3 is NOT to be used. 2.2.4 EQUIPMENT OPERATING TIME The equipment should be closely monitored during the operating portions of the cycle. It is desirable to turn off the equipment during chamber cool-down, otherwise self- generated heat will prevent the internal parts from reaching the desired low temperature. 2.2.5 TEMPERATURE RANGE AND RATE OF CHANGE Temperature ranges of –65 °F to +131 °F are the temperatures most commonly used. Most parts will withstand temperature cycling with power off through a temperature range of –65 °F to +230 °F. Heat rise with power on under test cooling conditions should be calculated to limit the chamber temperature to a maximum safe value. The maximum safe range of component temperature and the fastest time rate of change of hardware temperatures will provide the best screening. The rate of temperature change of the individual electronic parts depends on the chambers used, the size and mass of the hardware, and whether the equipment covers are taken off. In general, the rate of change of internal parts should fall within 1 °F per minute and 40 °F per minute, with the higher rates providing the best screening. 2.2.6 TEMPERATURE RANGE AND RATE OF CHANGE Temperature cycling with good parts and packaging techniques is not degrading even with several hundred cycles. However, the packaging design must be compatible with the temperature cycling program or the acceptance test yield will be reduced (to zero in some special cases). This compatibility is established by temperature cycling the pre-production hardware. Some typical troublesome problems are: 1) Electronic components assembled on printed circuit boards impose loads on the solder joint, and temperature cycling may produce solder joint cracking. Heavy coats of conformal coating on even a stress relief bend can negate the beneficial effect of the bends. 2) Transistors mounted on plastic spacers and coated with conformal coating will produce cracked solder joints in a few temperature cycles if the leads are not stress relieved. This problem arises because the coefficient of thermal expansion for plastics is about 8 to 30 times greater than Kovar transistor leads, or Dumet diode leads. 3) Large multi-pin modules soldered into the printed circuit board may result in solder joint cracking, particularly if the conformal coating bridges between the module and the board. 4) Cordwood modules potted with a rigid, solid polyurethane or epoxy may produce cracked joints and even crush weak parts such as glass diodes on the very first application of a temperature cycle. 5) Filters, motors, and transformers containing fine wire (#40 or #50) may constitute a problem. To avoid the problem, wire sizes larger than #40 should be used. 6) Single or double sided printed circuit boards without plated-through holes are undesirable. 7) Breakage of glass diodes can be expected if great attention is not given to the encapsulating material and the process. Implementing temperature cycling is most compatible with printed circuit board construction and least compatible with large, complex, potted cordwood modules where failure means scrapping the entire module. Downloaded from http://www.everyspec.com on 2011-11-14T7:13:33. 10 2.2.7 TYPES OF DEFECTS SIMULATED An approximation of the types of failures detected in mature hardware by temperature cycling is: Design Marginalities 5% Workmanship Errors 33% Faulty Parts 62% These figures are based on the experience of eight manufacturers as illustrated in Table 3. 2.2.8 REPAIRS AND FAILURE-FREE CYCLES When multiple temperature cycling is used as an acceptance test, it is standard practice to allow repairs without requiring a repeat of the entire test. Some programs have required no failure free cycles, some have required the two final cycles to be failure free, and one program (involving very simple hardware) required 20 consecutive failure free cycles. It is recommended that one final failure free cycle be required, together with criteria for extending the number of temperature cycles as a function of the difficulty and magnitude of the repair. 2.2.9 BOARD LEVEL TEMPERATURE CYCLING The concept of augmenting the black box temperature cycling with additional cycling at the print