barcode scanner asp.net mvc CONFORMAL COATING, ENCAPSULATION, AND UNDERFILL MATERIALS in Software

Creation QR Code ISO/IEC18004 in Software CONFORMAL COATING, ENCAPSULATION, AND UNDERFILL MATERIALS

40.8 CONFORMAL COATING, ENCAPSULATION, AND UNDERFILL MATERIALS
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When required, the final steps in the assembly of a printed circuit board are the applications of conformal coatings, encapsulants, and underfill materials. Conformal-coated circuit boards are shown on the bottom of the stereo photograph in Fig. 40.32a; the same circuit board,
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FIGURE 40.32 Stereo photographs showing (a) conformal-coated (bottom) and encapsulated (top) versions of a surface-mount circuit board and (b) underfilled flip-chip components. (Courtesy of Sandia National Laboratories.)
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encapsulated, is shown in at the top of the photograph. An example is shown in Fig. 40.32b of underfilled flip-chip devices. The role of the conformal coating is to protect the circuit board and components against large-scale contamination from dirt, water, liquid chemicals, corrosive gases in air pollution, mold, and fungus. Because the layers are very thin (<0.25 0.50 mm) and relatively soft, they do not offer significant protection against handling damage or mechanical loads in shock-andvibration environments. Protection against the latter conditions is obtained from encapsulants, also called potting materials, and underfill materials. Underfill materials perform a function similar to adhesives by securing the component to the circuit board. The difference is that the underfill material is applied after soldering and must fill the entire gap between the package and the circuit board. In the case DCA/FC, the underfill also improves the thermal mechanical fatigue reliability of the solder joints. The details of these materials are discussed in other chapters. However, it is important to understand their role in the overall circuit board assembly process. First of all, the functions of these three materials require that they have good adhesion to all surfaces. Therefore, the printed wiring assembly must be cleaned of any flux residues as well as those residues left behind from the cleaning procedure.
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Second, the application of conformal, encapsulant, and underfill materials must be tightly controlled in order for them to function properly. The nominal thickness and its uniformity are critical for conformal coatings. A coating that is too thin is a less effective barrier against harsh environments. Thickness and uniform coverage have become even more critical as many designers are relying on conformal coatings to mitigate the consequences of Sn whisker growth on 100 percent Sn coatings. On the other hand, a conformal coating that is too thick can interfere with rework procedures as well as prevent fit-up of the circuit board into racks and backplane cabinets. In general, visual inspection is used as a nondestructive method to control the application processes. In the case of encapsulant or potting materials, there must be complete filling of the mold, especially around the components, to prevent stress concentrations that can damage surfacemount solder joints. Potentially damaging stresses can also be generated when the density of the potting material varies excessively about the circuit board.The resulting variation of physical and mechanical properties of the foam encapsulant (thermal expansion coefficient, modulus, etc.) can lead to excessive board bending that damages solder interconnections, particularly those of large components (e.g., BGA devices). Therefore, it is necessary to control the encapsulant material itself as well as the casting process to prevent these defects. Unfortunately, most nondestructive evaluation techniques are not effective at finding encapsulation defects. Voids and density variations are not readily detectable by visual inspection or by x-ray analysis. In the case of x-ray, the inherently low density of encapsulant foams limits the contrast between filled and unfilled (void) regions. Scanning acoustic microscopy (SAM) has had some limited success when the encapsulant layer is relatively thin, which is not often the case. Witness samples taken during the casting process are used to confirm foam density. A number of studies have been performed that underscore the crucial requirement to control the underfill process used on flip-chip devices. Even small variations in underfill material properties (e.g., viscosity, percentage of filler, etc.) or the method for dispensing the underfill material between the die and substrate can significantly alter the long-term reliability of these interconnections. The accepted method of nondestructive testing used to control the underfill process is SAM. The thin layer allows this technique to detect voids in the underfill material, which when located near the solder interconnections can be responsible for a significant loss of thermal mechanical fatigue reliability. X-ray techniques can be used to monitor the density of the underfill material, specifically, the distribution of filler material within the layer under the die. Density variations can indicate a larger distribution of underfill mechanical and physical properties, which may affect long-term reliability performance of the solder joints. Quantitative image analysis can be coupled into SAM and x-ray analysis data to provide valuable process control tools for the factory floor.
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