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Printed circuit board real estate continues to be coveted. Boards are getting smaller and smaller as more and more capabilities are built on them. To keep up with this trend, complex packages and devices and miniature devices are cropping up. Many would argue that a 0402 chip component is small, but small becomes subjective when you consider 0201s and 01005s. The 01005 component s sizes typically range from 0.10 mm (0.004 in.) 0.304 mm (0.012 in.) to 0.20 mm (0.008 in.) 0.40 mm (0.016 in.), depending on the supplier. Note that the 0201 typical is 0.60 mm (0.0236 in.) 0.30 mm (0.0118 in.), thus it s quite a jump. With devices on this size order, the design considerations are large and the propensity for defects is higher. Inspection must catch the defects, but at these sizes, approaching the eye of a needle, it is not practical to conduct inspections visually, and thus such automated test and inspection become paramount. Automated test and inspection continues to spur the development of camera and algorithm technology to keep up with the throughput, coverage, and size requirements demanded by the industry. Miniaturization in itself is quite a driver for these technologies. Add this to the material changes that are going on with materials changes such as lead-free materials, and the need for automated test and inspection only increases. AOI systems including three-dimensional (3-D) and two-dimensional (2-D) solder paste and pre- and postreflow component inspection are typically lead-free-ready, as are x-ray test systems, with a few minor programming updates. The advancement of the miniaturization movement also drives technologies such as multichip modules (MCM), system-in-a-package (SIP), and so on. These devices or boards, depending on how one refers to them, have many interconnects among themselves in addition to the requirements that they pose when the entire MCM, SIP, etc., is adhered to a printed circuit board. The number of hidden joints increases, the types of hidden joints increase, and the complexity of the attachment to the SMT board in general increases. All of these are contributors to the ongoing need for increase defect containment and defect coverage that only automated inspection and testing bring.
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Visual inspection is the visual comparison by a human of some attribute of the printed circuit assembly with specified standards that describe the acceptable range for that attribute. The inspector normally picks up the printed circuit assembly or places it under a microscope and carefully observes particular attributes, such as the condition or bend radius of component leads or the wetting of solder joints to a lead. Visual inspection always involves human judgment in comparing the attribute to its conditions of conformance to standards.
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General Inspection Issues As Fig. 53.1 indicates, visual inspection can occur after each of the several printed circuit assembly process steps. Visual inspection often has different purposes, depending on where in the assembly process it occurs. These purposes fall into the following major categories:
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Quick detection of an assembly process step that is not operating within its normal range Detection of process defects as specified by the customer or industry or internal standards
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Solder Joint Inspection Issues Visual inspection after the reflow and wave-soldering process steps can also be just a quick scan for obvious defects to detect a process condition outside of control limits. In this case, the operator visually inspects for solder bridges, large solder balls or solder splashes, lifted leads, and a
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number of other improper conditions. Often, however, visual inspection after the soldering process steps is aimed at finding solder joints that do not meet specifications. Visual inspection of solder joints against specification can cover 100 percent of the visual solder joints on an assembly. Inspecting samples of solder joints in conjunction with a documented process control system is also done. Inspecting for this purpose can be a lengthy process, taking as long as a half-hour for assemblies with 4,000 solder joints being measured against military specifications. As a rule of thumb, this kind of visual inspection has a throughput of about 5 joints per sec. compared to automated inspection techniques, which have capabilities of over 100 joints per sec. Thus, inspection of solder joints against specifications normally requires visual inspectors dedicated to this function with no other responsibilities. Visual inspection may still provide a low-cost alternative to automated inspection in some low-cost regions, which is one major factor contributing to its continued usage. The visual inspector must be very familiar with the specifications of the attributes for each solder joint type. Each solder joint can have as many as eight different criteria for defects such as insufficient solder, misalignment, etc. Each assembly typically has more than six different solder joint types such as BGAs and QFPs, corresponding to different component packages with individual requirements for each. As an example, Fig. 53.7 shows the specification attributes for one component type (rectangular passive chips). Table 53.1 gives the corresponding conformance criteria for each attribute for this component type. More importantly, the visual inspector must be highly trained to make an accurate judgment of conditions on the borderline between good and bad. For instance, accurately determining whether a fillet height is onequarter of the way up a component side that is only 0.05 mm high to begin with takes a lot of practice. Visual inspectors typically do not use any tools to help make these judgments. Rulers or calipers are very difficult or impossible to use to measure solder joint dimensions or thicknesses. Using reticules in microscopes in conjunction with coordinate-measuring machines is possible, but is usually much too time consuming to be done on a regular basis.
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