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Fatigue life modeling for PBA mounted components has become significantly more complex due to in-use simultaneous application of both vibration and thermal cycles. Vibration strains and thermal strains should be superposed for more representative modeling.
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15.6.2.4 Types of Edge Mounting. The problems, methods of analysis, and minimization of the effects of shock and vibration on PBAs are the same as in other engineering applications, and similar solutions can be used. PBAs are designed and manufactured in a wide range of shapes and sizes, with rectangular being the most common because of the shape of most electrical/electronic equipment, especially for plug-in PBAs. Though PBAs are a multidegree-of-freedom system, the fundamental mode is of primary importance because it has the large displacements that are the primary cause of fatigue failure in solder joints, component leads, wires, and connector contacts.6 Most vibrational fatigue damage occurs at the fundamental or natural frequency because displacement is highest and stress is maximized. Edge or boundary conditions are terms that are used to define the method of attachment of the PBA (or, more generically, a panel) to its mounting frame.The term free edge is used to define those edges that are not restrained and are free to move and/or rotate along the edge of the PBA out of their normal mounting plane. The terms supported edge or simple support used to define an edge that is restrained in out-of-plane movements but is allowed rotational movement around the PBA s edge. The terms fixed edge or clamped edge are used to define an edge that is restrained in both out-of-plane and rotational movements. Illustrations of the definitions for fixed edge, supported edge, and free edge, and their applications to plug-in mounted PBAs, are shown in Fig. 15.2.
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FIGURE 15.2 The method of mounting PBAs contributes to shock and vibration concerns, but can be reduced by good design practices.
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15.6.2.5 Board Deflection. The amount of strain on a PBA component is a function of the maximum deflection of the PBA when subjected to shock and vibration. Those components mounted closest to the center of the assembly are subjected to the greatest strain, as illustrated in Fig. 15.3. A set of empirical maximum deflection (d) calculations has been developed by Steinberg, and his latest equation7 has more parameters than previous equations, reflecting the sophistication and requirements for modern PBDs (units adjusted from inches to mm). d= k 0.00022 B ct L (15.3)
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FIGURE 15.3 The PB bends in a PBA during shock and vibration, with the most severe stresses applied to the components mounted centermost on the assembly.
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k = units conversion coefficient; for inches k = 1, for mm, k = 25.4 B = length of PB edge parallel to component located at center of board (worst case), mm L = length of component, mm t = thickness of PB, mm c = 1.0 for standard DIP = 1.26 for DIP with side brazed leads = 1.0 for PGA with four rows of pins (one row extending along the perimeter of each edge) = 2.25 for CLCC
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Analysis of the maximum deflection calculation formula reveals (as expected) that components with some compliance built into their component mounting and electrical terminations (such as the DIP and PGA) can be subjected to about twice the vibrational deflection as an SMT CLCC, provided component size, PB size, and PB thickness are equivalent. The latest equation for maximum deflection calculations is rated for 10 million stress reversals when subjected to harmonic (sinusoidal) vibration, and 20 million stress reversals when subjected to random vibration. It must be understood that this equation is a first approximation for predicting solder joint life. There are many factors that must be included for a more rigorous analysis and prediction. A more thorough discussion is found in Barker.6 15.6.2.6 Natural (Fundamental) Resonance of Printed Board Assemblies. The mechanical mounting of PBAs and their components is a key design consideration in the ability of the PBA to withstand shock and vibration. The overall size of the PBA is not a major factor, provided a suitable mechanical support structure is included as a part of the PBA s product definition requirements. There is a large matrix of ways to mount panels (PBAs) using various combinations of free edges, supported edges, fixed edges, and point supports and by calculating fundamental resonance. The following four examples compare the fundamental natural resonances of the same rectangularly shaped PBA using different edge-mounting techniques. Additional formulas for calculating other natural resonances are found in Barker,6 Steinberg,7 and others. In the following examples demonstrations of the sensitivity of PBAs to their methods of mounting the same PBA is used for direct comparative purposes (see Fig. 15.4). The following are the design requirements and material parameters that were used for the calculations:
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