barcode reading in c#.net PRINTED CIRCUITS HANDBOOK in Software

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Npoweractual = actual number of power cycles that occur before failure Npoweronly = number of power cycles to fail if only power cycles occur Nminiactual = actual number of minicycles that occur before failure Nminionly = number of minicycles to fail if only minicycles occur K = number of minicycles that occur per power cycle
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Equation 59.23 can be solved for the number of power cycles to fail, which yields a value of 1,311 power cycles or 107.8 years. It is important to employ a confidence factor (safety factor). Taking 2 as a standard safety factor, the final fatigue life is estimated at 53.9 years.
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59.3 MECHANICAL RELIABILITY
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As discussed in Sec. 58.3.11 of Chap. 58 the surface finishes used on packages have been shown to have a significant impact on the mechanical robustness of BGA packages. With increased I/O density, finer pitch, and new substrate technologies (e.g., flex substrates), BGA packages tend to be more fragile and susceptible to mechanically induced failures. These failures are mostly catastrophic in nature, but the loading conditions could vary from bending during production assembly to shock due to shipping or accidental drops. Both monotonic and cyclic conditions could result in catastrophic failure either in manufacturing or field operating conditions. This section focuses on the test methodologies used in quantifying and benchmarking different mechanically induced loading conditions on printed circuit assemblies (PCAs). In general, the mechanical loading conditions that an assembly undergoes could be classified by the strain and strain rate level, as illustrated in Fig. 59.8. As Fig. 59.8 shows, bend and shock tests each cater to different ranges of the strain/strain rate spectrum.
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Monotonic & Cyclic Bend Testing
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Shock Testing
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Strain Rate (mes-1)
FIGURE 59.8 Spectrum of strain/strain rate in which mechanical testing fits with the assembly process. The monotonic and cyclic bend testing focuses on high-strain, low-strain rates, whereas the shock testing focuses on low-strain, high-strain rates.
59.3.1 Monotonic Bend Testing Monotonic (bend-to-failure) testing is designed to simulate the assembly conditions to which a PCA is subjected in manufacturing. A variety of assembly steps impart loading conditions that are similar to a monotonic bend test, such as heatsink attachment, connector attachment, incircuit test (ICT), handling, and card-cage installation. These assembly steps could mechanically
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induce several different failure modes. Monotonic bend testing in a controlled condition serves two critical purposes:
A comparative assessment can be performed between different package types and material sets. The failure modes induced at different strain levels can be catalogued. Production operations can then be characterized with strain gauges and compared with the monotonic bend test data to determine whether excessive strain was applied on the PCA during any given assembly step.
To ensure consistency in results and to allow for ease of comparison, an industry standard test method has been developed and published jointly by IPC and JEDEC (IPC/JEDEC9702).35 This test method outlines in detail the requirements of the monotonic bend test method. The test setup is illustrated in Fig. 59.9. The typical failure modes observed at the solder joint interconnects are shown in Fig. 59.10.
Anvils Solder Joints PCB
Silicon Die
BGA Substrate
FIGURE 59.9 Schematic of four-point bend test setup. A four-point bend test setup is used to determine the strains-to-failure rate of the BGA solder joints.
Package Substrate
A B C D A B C D E F G
Legend Package Pad Lift/Crater Pkg Metal/IMC Interface Fracture Pkg IMC/Solder Interface Fracture Bulk Solder Fracture PWB IMC/Solder Interface Fracture PWB Metal/IMC Interface Fracture PWB pad Lift/Crater
Solder Ball
E F G
FIGURE 59.10
Different interconnect failure modes.
59.3.1.1 Strain Measurement in Production. A common procedure for performing strain measurements on live PCAs during production is outlined in IPC/JEDEC 9704.36 This provides a comprehensive guideline on ways to perform strain measurements and reporting the recorded data. It also includes sample guidelines on the amount of strain/strain rate considered acceptable as a function of board thickness. One assembly process that is particularly susceptible to
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