barcode scanner input asp.net FAILURE MECHANISMS OF PCBs AND THEIR INTERCONNECTS in Software

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57.2 FAILURE MECHANISMS OF PCBs AND THEIR INTERCONNECTS
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This section will discuss the most important failure mechanisms of PCBs and the interconnects between PCBs and the components mounted on them. The discussion of PCB failure mechanisms will be more detailed since interconnect failures have been described far more extensively elsewhere. Whatever the environmental stress or the material response, these failures ultimately manifest themselves in terms of the functionality of the assembly, first as a change in electrical resistance between two points and then as electrical shorts or opens.
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PCB Failure Mechanisms PCB failure mechanisms fall into three groups: thermally induced failures, of which platedthrough-holes are the most important example; mechanical failures; and chemical failure mechanisms, of which dendritic growth is the most important example.
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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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57.2.1.1 Thermally Driven Failure Mechanisms. PCBs are exposed to thermal stresses in a variety of situations. These may be either prolonged exposure to an elevated temperature or isolated or repeated temperature cycles.These temperature cycles can cause various PCB failures. The most important sources of thermal stress are:
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Thermal shocks and thermal cycles during PCB manufacturing. Thermal shocks are usually defined as temperature ramps faster than 30 C/s, but include any ramp fast enough that temperature differentials play an important role. Examples include solder mask cure and hot-air solder leveling. Thermal shocks and cycles during printed circuit assembly. Examples are glue cure, solder reflow, wave soldering, and rework using a soldering iron, hot air, or molten solder pot. Ambient thermal cycles in service. Examples are going from inside to outside temperatures or ground to upper atmospheric temperatures, and elevation in box temperature due to heat dissipation from functioning electronic components.
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The primary PCB failure mechanisms accelerated by these thermal stresses are plated-through-hole cracking and delamination of the laminate. Plated-Through-Hole Failures Due to Thermal Shocks or Cycling. Plated-through-holes (PTHs) are the most vulnerable features on PCBs to damage from thermal cycling and the most frequent cause of printed circuit board failures in service. PTHs include holes for through-hole (TH) components and vias that make electrical connections between layers. Figure 57.3 shows the common failure locations. Most organic resinmatrix substrate materials are highly anisotropic, with a much higher CTE above the glass transition temperature Tg in the through-thickness (z) direction than in the plane of the woven matrix cloth (the x-y plane of the board). Since above Tg the CTE climbs sharply, aggressive thermal cycles can result in large strains in the z direction and, consequently, on the PTHs (see Fig. 57.41). The PTH acts like a rivet, which resists this expansion, but the Cu barrel is stressed and may crack, causing electrical failure. Figure 57.4 also illustrates the increasing strain on the barrel associated with a high temperature excurFIGURE 57.3 Schematic diagram of a platedsion. Failure may occur in a single cycle or may take place by through-hole in a cross section of a four-layer printed initiation and growth of a fatigue crack over the course of a circuit board showing common failure locations under number of cycles. For high-aspect-ratio through-holes subject thermal stress. to repeated thermal shocks from room temperature to solder reflow temperatures (220 to 250 C) during board fabrication (e.g., hot-air solder leveling) and assembly (reflow, wave soldering, rework), it is not unheard of to encounter failures after 10 or fewer of these thermal cycles. On a physical level, the number of thermal cycles to failure is affected by the strain imposed on the Cu in each cycle and the fatigue resistance of the copper. These factors are in turn controlled by a number of environmental, material, and manufacturing parameters. Lowcycle metal fatigue, in which most of the strain is plastic strain, can be treated approximately with the Coffin-Manson relation: Nf where Nf e ef m = = = = 1 2 ef e
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number of cycles to failure strain strain ductility factor, which correlates closely with tensile ductility constant near 2.
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