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53.3 REASONS FOR INSPECTION
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Manufacturers inspect printed circuit assemblies during production for a variety of reasons. Most of these reasons fall into the following categories:
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Improvement of process defect coverage Improvement in ability to meet customer specifications Detection of process defects as quickly as possible after they occur Ability to decrease process defect rates through statistical process control (SPC)
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53.3.1 Process Defect Coverage The goal of higher defect coverage is to prevent any defective printed circuit assemblies from reaching assembly of the board and making it into the final product, whether final assembly is at the same site or at a separate customer site. Figure 53.3 shows a typical spectrum of SMT process defects. Some of these defects, such as misaligned solder joints and solder balls, can be discovered only by inspection, not by electrical, functional, in-circuit, or boundary-scan test. For instance, marginal solder joints are those that pass electrical tests just after assembly completion, but eventually fail because of inferior mechanical strength. Insufficient solder and components partially misaligned with printed circuit pads are the two most common causes of low mechanical strength.
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FIGURE 53.3 Typical process defect spectrum for surface-mount assemblies. Process defects marked E can be detected by electrical tests, those marked M can be detected by mechanical or structural inspection or testing, and those marked by E and M can be detected by either method.
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Voids in solder joints, solder balls away from solder joints, and cold or dull solder are other examples of defects that can be found only by some kind of inspection.
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ASSEMBLY INSPECTION
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Customer Specifications Many printed circuit assembly workmanship specifications including MIL-STD-2000A, IPC/EIA J-STD-001C, and, more recently, IPC-A-610D require inspection of solder joints to ensure conformance to these specifications. For instance, MIL-STD-2000A prescribes, for all defense and aerospace government contractors, the attributes of solder joints to be inspected and describes the conditions that result in nonconformance. Most commercial customers of contract manufacturers also have very specific solder joint specifications that require some type of inspection of printed circuit assemblies.Although process control is theoretically preferred over 100 percent inspection, practically speaking, a majority of printed circuit production lines include some kind of inspection to ensure conformance to specifications. As the inspection speeds of the automated inspection systems have improved in recent years, this has become even more prevalent. It is likely that this trend will continue in light of the continued usage of finer pitch devices and the dawn of material challenges, such as lead-free implementation.
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Quick Defect Detection and Correction Quick detection of process shifts or defects can lower rework costs. 53.3.3.1 Preventing Defects. If a particular process step has drifted out of its control limits or specification limits, discovering this fact as soon as possible will prevent more defects from occurring. If the problem is found quickly enough and corrected immediately, perhaps no actual defects will occur. 53.3.3.2 Lowering Rework Costs. Defects found earlier in the process are often easier to repair. For instance, if a defect in solder paste deposition is found before a component has been placed in the paste, it is fairly inexpensive to wipe the solder paste off and start over. If this same defect is found after the solder is reflowed, the solder joint itself will have to be touched up, a more difficult and expensive rework step.The same is true for repairing component placement defects before the solder is reflowed, particularly for missing or misaligned components. Additionally, when organic solderability preservative (OSP) boards are being utilized for leadfree soldering, for example, they cannot be reworked as easily. Using process control and defect prevention can prevent a large number of non-reworkable boards in this case. 53.3.3.3 Making Defect Diagnosis Easier. Defects found earlier in the assembly process are often more easily diagnosed, shortening the overall repair time. An example is the inspection of solder joints. Inspection of solder joints detects defects specific to the solder joints and quickly determines each defect s exact location and characteristic. Waiting until a later electrical test stage could make diagnosis more difficult because at that point there could be several causes of the defect, such as a defective component and defects in other connections, in addition to the actual defective solder joint.
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53.3.4 Statistical Process Control SPC requires reliable data that can be analyzed either in real time or historically. Visual inspection collects defect data, such as the number of solder joint defects per assembly right after the solder reflow process (either reflow or wave soldering). Some manual and automated inspection techniques also take quantitative measurements of key assembly parameters, such as solder paste volume or solder joint fillet height. To the extent that these data are repeatable, manufacturers use defect data or measurements to characterize the amount of process variation from assembly to assembly or from solder joint to solder joint. When the amount of variation starts to drift outside its normal range or outside its control limits, manufacturers can assess the assembly process and monitor or choose to take action until the process is adjusted to eliminate this drift. Historical analysis of the defect or measurement
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