scan barcode asp.net mobile ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES in Software

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ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES
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the completed PCBA. The wire should be spot-bonded along its route and must not be applied to lands, pads, or components. The interval of staking must be defined in the applicable engineering documentation, but the wire must be staked at all changes in direction of the wire. No jumper wires may be staked to components that are socketed or any moving part. Jumper wires must be staked or taut enough to prevent the wire from rising above the height of adjacent components. No more than two jumper wires may be stacked on a given route. When a jumper wire is attached to leads on the secondary side of the PCBA or to axial components on the primary side of the PCBA, it must form a full 180 to 360 loop around the component lead. When a jumper wire is soldered to other component package styles, the wire should be lap-soldered to the component lead. Jumper wires may be installed into a PTH with another component lead for Class 1 and 2 equipment; however, this is unacceptable for Class 3 equipment. Jumper wires may also be installed into via holes (see Fig. 52.40).
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FIGURE 52.40a Example of jumper wire connected to plated through-holes (IPC).
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FIGURE 52.40b Example of wire connected to a via hole with a lead installed.
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REFERENCES
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1. ANSI/J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies. 2. MIL-STD-2000, Standard Requirements for Soldered Electrical and Electronic Assemblies. 3. ANSI/J-STD-002, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires. 4. ANSI/J-STD-003, Solderability Tests for Printed Boards. 5. IPC-A-610, Acceptability of Electronic Assemblies.
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ASSSEMBLY INSPECTION
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Agilent Technologies Gilbert, Arizona
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Stig Oresjo
Agilent Technologies Loveland, Colorado
53.1 INTRODUCTION
This chapter covers why manufacturers inspect printed circuit assemblies, how they have implemented and enhanced visual inspection, what automated inspection systems they are using, and how they have implemented these automated systems. The scope of this chapter includes only inspection of printed circuit assemblies during the assembly process, as typically shown in Fig. 53.1. Thus, it includes inspection of solder paste after the paste printing process step, components after the component placement process step, and solder joints after the solder reflow process step. Not included, however, is incoming inspection of components and the bare printed circuit board (PCB). The focus of this chapter is on production use of inspection, not the collection of measurements during process development in a research and development (R&D) environment. 53.1.1 Visual Inspection Manufacturers of printed circuit assemblies have always visually inspected their boards at various points in the assembly process. There are a variety of reasons for visual inspection of assemblies, including eliminating obvious process defects quickly and meeting military specifications. With the advent and growth of surface-mount technology (SMT), visual inspection of printed circuit assemblies has also grown in importance and prevalence. SMT solder joints must carry a much bigger burden for mechanical or structural reliability than do platedthrough-hole (PTH) solder joints. The pin-in-through-hole solder joint carries much of the mechanical burden, helping to keep the component attached to the printed circuit board. However, with SMT solder joints, it is often only the solder that keeps the component attached to the board. In some cases, only visual inspection could judge the mechanical reliability of an SMT solder joint. As SMT geometries have continued to shrink and solder joints have become denser on printed circuit boards, visual inspection has become more difficult and thus its results have become less consistent and reliable. In addition, components such as pin grid arrays (PGAs)
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PRINTED CIRCUITS HANDBOOK
FIGURE 53.1 Generic manufacturing process for surface-mount technology printed circuit assemblies, including the possible locations within the process for inspection or testing of mechanical or structural attributes and electrical characteristics of an assembly.
or ball grid arrays (BGAs) completely hide their solder joints from view, which makes visual inspection not possible at all for those joints. As Fig. 53.2 indicates, achieving a high assembly process yield is more important as the number of solder joints per assembly increases.
FIGURE 53.2 Printed circuit board yield decreases dramatically with increases in the number of solder joints per assembly if the defect rate per solder joint remains constant. For instance, at a 40 defects per million opportunities (DPMO) defect rate, the yield drops from 96 percent at 1,000 joints per assembly to 45 percent at 20,000 joints.
53.1.2 Automated Inspection Inspection is an important source of process information without which high yields are difficult to obtain. Consequently, manufacturers have employed the following range of techniques either simply to enhance visual inspection or automate inspection fully:
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