barcode scanner code in asp.net BARE BOARD TEST OBJECTIVES AND DEFINITIONS in Software

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BARE BOARD TEST OBJECTIVES AND DEFINITIONS
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employed in an isolation test, they might achieve a similar result. But isolation testing of bare boards commonly occurs with 250 volts or less applied between networks, and hi-pot testing is often performed at values from 500 volts to several kilovolts. Hi-pot testing attempts to verify the strength of the insulating material between networks by subjecting it to so high a voltage that a catastrophic or avalanche-type voltage breakdown will occur if the insulator is subpar. In contrast, isolation testing on bare boards attempts to detect the small current flowing through contamination (or, for that matter, a hard short) before voltage breakdown occurs. Of course, if the insulator is very weak, then avalanche failures can occur at almost any voltage. Hi-pot testers tend to be benchtop devices with a pair of test leads and no switch matrix, and hi-pot test requirements usually specify that the voltage be applied for a sustained period of time. Hi-pot testing is valuable for inspection of very thin insulating core material, before circuits are etched. This can serve to detect z-axis faults or contamination in the material before value is added in subsequent processes. It can be impractical to perform hi-pot tests between all conductors of finished fine-pitch boards, as the atmospheric environment (air) between conductors will break down before high voltage is reached. The slow speed at very-high-voltage hi-pot testing and the costs of suitable fixtures and electronics present problems. For final product inspection, it is arguable that a very-high-impedance isolation test provides the superior solution.
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BARE BOARD TEST METHODS
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37.1 INTRODUCTION
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Although the main bare board testing technology is electrical, it is important to consider that nonelectrical methods are also important in the acceptance or rejection of bare printed wiring boards (PWBs). This chapter therefore includes detailed descriptions of both electrical and nonelectrical testing methods.
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37.2 NONELECTRICAL TESTING METHODS
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There are two nonelectrical acceptance/rejection methods, both based on inspection processes:
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Visual inspection Automatic optical inspection
37.2.1 Visual Inspection Visual inspection is a very manual approach in that it makes use of people, good lighting, some type of training defining what is acceptable and what is not, and good operator judgment. Usually a comparison to a known good product or the artwork is made. If the operator has seen the board often, he or she becomes more skilled at finding faults and looking for faults in likely locations. As product complexity has increased, we find that many modern products are not suited to this method. Many innerlayer defects are completely undetectable, and even the external layer complexity is visually overwhelming. Visual inspection often remains appropriate for detecting cosmetic defects, such as poor solder masking or physical damage. Such defects generally fall outside the realm of electrical testing as they are not detected by electrical means.
37.2.2 Automatic Optical Inspection There are computer-based visual inspection methods, referred to as automatic optical inspection (AOI). AOI equipment compares the board or its innerlayers to expected data and/or design rules that have been programmed into the controlling computer.These can be generally
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accepted parameters or design-rule-based parameters, or windows of acceptable dimensions for each specific feature on the board. As with manual visual inspection, faults found with this method can imply that there may be an impact on the board s functionality, but the board s functionality and interconnect are not directly tested. Distinctions between the aesthetics of the board s features and its fitness for use are difficult to differentiate, and may result in false failures. Rather than being used in final testing, AOI is used for inspection of innerlayers prior to lamination, with the goal of increasing final yield by weeding out the majority of defective layers prior to the addition of further value. As such, AOI can achieve significant financial benefit. AOI can detect some defect types not readily detected by electrical means, particularly mouse bites (brief narrowing of the conductor cross section). AOI may also be used to inspect the outside layers after lamination, but is not generally accepted as a quality assurance substitute for final electrical testing.
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