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15.6.1.5 Extracting Printed Board Assemblies. A number of unique principles have been developed and applied to solve the various problems of extracting PBAs from their plug-in enclosures. The result has been a proliferation of proprietary and a few industry-standard-like extraction systems. The most common industry-standard extractor is injection-molded plastic hardware that is free to partially rotate when attached to the PBA with a pressed-in pin. Many of these PBA extraction tools use a minimum of PB space, thereby maximizing available PB area for components and conductor routing. They also protect both the PBA and the associated mating connector(s) from damage during the extraction process. The following should be considered when selecting among the many different types of PBA extraction tools:
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The area of the PBA available for attachment The extractor s effect on the PBA-to-PBA mounting pitch The need for special provisions in the PBD, such as mounting holes, mounting clearance holes, and notches The size of the extractor, especially if the extractor is to be stored in the equipment with which it is used The need for an extraction device that is permanently attached to the PBA, usually by riveting The need for specially designed considerations, such as load-bearing flanges, in the PBA mounting chassis or cage hardware The suitability of the extractor to be used with a variety of PB sizes, shapes, and thicknesses The cost of using the extractor, both in piece price and added design costs The degree of access required inside the equipment to engage and use the extraction tool
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Shock and Vibration Shock, vibration, flexing, and bowing can be functional performance and reliability concerns for PBAs more so for larger PBAs. For many PBAs, the worst-case exposure to shock and vibration occurs in nonfunctional or operational usage, during shipping and other forms of transportation from one location to another, or possibly in functional use when the functionally operating product containing PBAs is inadvertently dropped on the floor. Other PBAs are designed to withstand specified levels of shock and vibration in transportation and in use. The design requirements for shock and vibration vary, depending on each family of general requirements. For example, there are nonoperating shock and vibration withstand requirements for vehicular, train, ship, and air transportation for domestic and international shipment for Level 1, 2, or 3 products, which include various procedures and requirements for packaging. The shock and vibration functional design requirements are many and varied, and are very dependent on the application. Some sources of shock and vibration are very obvious, while others are very subtle. The levels and duration of shock and vibration vary significantly in each application: An electronic sensor mounted on a vehicle s axle is different from the radio mounted on the dashboard. There are the differences among ground-based, rackmounted, industrial control equipment, aircraft, aerospace, and munitions applications. Some vibrations are subtle, low-level continuous, and are frequently caused by electric or gas motordriven rotating machinery and equipment. Continuous low-level vibrations can induce mechanical fatigue in some electrical/electronic equipment. 15.6.2.1 Shock. Mechanical shock can be defined as a pulse, step, or transient vibration, wherein the excitation is nonperiodic.5 Shock is a suddenly applied force or increment of force, by a sudden change in the direction or magnitude of a velocity vector. With few exceptions, shocks are not easily transmitted to electronic equipment by relatively light mounting
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frames and structures. Most shocks to electronic equipment in the consumer, commercial, and industrial markets are due to dropping during handling or transportation, the exceptions being electronic sensors or equipment mounted to heavier mounting frames, such as vehicle axles and punch-press-like equipment, or military equipment subjected to air dropping or to explosive forces, such as munitions. Most shock impact forces result in a transient type of dampened vibration which is influenced by the natural frequencies of the mounting frame. Generally, shock either results in instantaneous failure or functions as a stress concentrator by reducing the effective strength of the connection or lead for subsequent failure due to additional shock(s) and vibration. 15.6.2.2 Vibration. Vibration is a term that describes oscillation in a mechanical system, and is defined by the frequency (or frequencies) of oscillation and amplitude. PBAs that are subjected to extended periods of vibration will often suffer from fatigue failure, which can occur in the form of broken wires or component leads, fractured solder joints, cracked conductive patterns, or broken contacts on electrical connectors. The frequency(s) of vibration, resonances, and amplitude(s) all influence the rate to failure. Flexing and bowing in PBAs is the result of induced shock and vibration into the PBA. Different PBA mounting methods have differing susceptibilities to shock and vibration. In general, most small PBA functional modules are manufactured as components and are frequently encapsulated with a polymeric potting material into a solid mass, and they therefore have minimal shock and vibration requirements within the module. The plug-in PBA is restrained on one edge by the edge-board connector(s), and to some extent along the two sides of the PBA by mechanical guides. This leaves only one edge of the PBA free to flex from shock and vibration or to bow from residual manufacturing or assembly stress in the PBA. However, a handle along the free edge of the PBA or a restraining bar can be located across the center of the free edge of the PBA and mechanically attached for support at the ends of the restraining bar to the PBA mounting hardware (card cage). Generally, the mating edge-board connector has a molded-plastic body that provides mechanical support to the mated edge-board connector and has sufficient compliance in the electrical contacts to maintain good electrical connections within the connector s performance specifications. Mechanically mounted PBAs can be more of a shock and vibration concern for three main reasons: 1. The PBA can be very large, sometimes as large as 600 mm2. More frequently, the maximum width is 430 mm (the width of a standard electronic chassis) and less than 600 mm long. Although most are less than 300 mm2, this still creates a large area and can be a problem if unsupported. 2. The PBA is misused as a mechanical support structure for high-mass components, such as magnetic components (iron-cored transformers and inductors), power supplies, and large (in physical size) function modules. 3. The PBA is not included in the mechanical design definition. 15.6.2.3
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Major Shock and Vibration Concerns. These include the following:
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Flexing between PBAs may cause shorts between adjacent PBAs or to the enclosure. The fundamental mode is the primary mode of concern because it has the large displacements that cause fatigue damage to solder joints, component leads, and connector contacts. Continuous flexing of a PBA will fracture component leads and, more important, surfacemounted component solder joints, due to mechanical fatigue failure. (Mechanically induced flexing or vibration in assembled PBs is used under controlled conditions to induce failures in solder joints for quality and reliability studies.) Movement of a PBA within its mechanical guides will be amplified due to shock and vibration resonance or harmonic resonance.
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