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1978 Edition, January 19, 1978

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Description / Abstract:

This handbook has been prepared specifically to guide engineers in the thermal design of electronic equipment with improved reliability. The primary purposes are: to permit engineers and designers, who are not heat transfer experts, to design electronic equipment with adequate thermal performance with a minimum of effort; to assist heat transfer experts, who are not electronic experts; to aid engineers in better understanding the thermal sections of Department of Defense specifications and standards for equipment; and to assist Navy personnel in evaluating thermal design during the various stages of equipment procurement and development.

This handbook recommends and presents electronic parts stress analysis methods which lead to the selection of maximum safe temperatures for parts so that the ensuing thermal design is consistent with the required equipment reliability. These maximum parts temperature must be properly selected since they are the goals of the thermal design, a fact which is often overlooked. Many thermal designs are inadequate because improper maximum parts temperatures were selected as design goals. Consequently, the necessary parts stress analysis procedures have been emphasized. Specific step by step thermal design procedures are given in chapter 4.

Proper operation at the desired performance and reliability levels can only be achieved if the electronic, thermal, and mechanical designs are all well executed and carefully integrated. Such a result can be accomplished best by the equipment designers, who must control all pertinent factors. It must be emphasized that the thermal design is fully as important as the circuit design.

Poor communications, lack of funds, and obsolete, incomplete, and unreliable data have hampered the development of proper thermal design. Methods of predicting the thermal performance of electronic equipment are becoming commonly known, but most organizations that have produced successful designs have achieved their goals by techniques peculiar to a specific equipment design.

This handbook supersedes certain Navy thermal design manuals previously published, such as NAVSHIPS 900, 192 (Design Manual of Natural Methods of Cooling Electronic Equipment), NAVSHIPS 900, 194 (Design Manual of Methods of Forced Air Cooling Electronic Equipment), and NAVSHIPS 900, 195 (Design Manual of Methods of Liquid Cooling Electronic Equipment).

This handbook covers the thermal design of shipboard, ground based, airborne (avionic), and space electronics. Simplified methods of design calculation, including nomographs and curves, are included so that engineers without heat transfer background can design acceptable equipment. Alternatively, the proper mathematical expressions are included along with comments on computer analysis methods so that experts in heat transfer can utilize these sophisticated techniques. When specific recommendations are given, care must be taken that the adoption of such recommendations does not conflict with the requirements of the contracting activity.

Throughout this handbook, equations intended specifically for use in design are labeled "D.E." (Design Equation).

We wish to express our appreciation for the productive cooperation we have received during this program from government agencies and industry. Source materials are listed in the References together with reference numbers at pertinent locations in the discussion. This handbook is not to be construed as an endorsement of any commercial products or techniques mentioned herein.

High temperature is a particularly insidious enemy of most electronic parts because it causes slow progressive deterioration rather than catastrophic failure. The mean time to failure of each part is a statistical function of its stress level and the entire complex of thermal history and chemical structure. Some authorities consider that inadequate cooling is presently the primary cause of poor reliability in military electronic equipment.

It is the failure of individual parts that leads to equipment failure. Electronic parts are prone to premature failure due to overstress (i.e., thermal, electrical or mechanical stress). Electrical and thermal stresses are closely interrelated and a reduction in electrical power dissipation correspondingly tends to ease the thermal stress. Examination of MIL-HDBK-217 (Reliability Prediction of Electronic Equipment) shows that failure rates of typical parts vary significantly with temperature. The following table presents a few extreme examples to indicate the effects: Table I.  Failure Rates

This information shows that the certain components (capacitors, resistors, coils, and transformers) are actually more temperature sensitive than transistors. Decreases in failure rates as great as those shown above are not always attainable, but very significant reductions can be and have been achieved by reduction of thermal stress (temperature). Cooling systems must be designed to control parts temperatures to the desired levels under all anticipated thermal environments.

Significant improvements in reliability and availability have been achieved by modifying the cooling systems in various existing shipboard and airborne electronic equipments. In several instances the reliability gains were as great as 500 percent in MTBF improvement. Even so the improved cooling systems were not of optimum thermal design; rather they could be better described as salvage jobs. Further, these equipments were used equipments composed of parts with a previous history of severe thermal stress. If optimum thermal designs had been applied with new parts, it was estimated that the reliability gains would have been increased by a factor of three. (Reference 1)

It has been proven that the additional cost of designing and implementing adequate thermal performance into equipment is very worthwhile. In several validated cost analyses, the average costs of thermally improving equipments were paid for by maintenance cost savings alone during the initial six months of equipment operation. The important and priceless gains in availability were excluded in the analysis and in this sense were free. Had these thermal improvements been incorporated into the equipments during their R&D phase, the costs could have been amortized, on the average, during the first three to four months of operation. Thus, on a life cycle cost basis the Navy can achieve significant economies through additional investments in adequate thermal design. (Reference 2)

Adequate thermal design results in the minimization of parts temperature excursions when power dissipation or environmental temperatures vary. Temperature cycling in excess of ± 15°C has been found to significantly reduce parts life and reliability. Failure rate increases as large as 8.1 have been observed when the temperature cycling exceeded ± 20°C. (Reference 3)