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FIGURE 51.8
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Plating void. (Courtesy of IPC.)
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ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
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FIGURE 51.9 Voids in finished coating. (Courtesy of IPC.)
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51.11.1.4.3 Hole Diameter. Hole size is the inner clearance diameter of the finished plated-through or unplated hole. A PTH is a hole in which electrical connection is made between internal or external conductive patterns, or both, by the deposition of metal on the wall of the hole. An unplated hole, termed an unsupported hole, is a hole containing no conductive material. Hole size measurement is performed to verify that the hole meets the intended design requirements. This size requirement is usually associated with the fit of component leads, mounting hardware, etc., plus adequate clearance for attachment metal. PTHs providing layer-to-layer interconnections, where no components are soldered into the hole, are called via holes.Via holes do not have a fit requirement, so only the plating integrity is critical. Drill blank plugs and optical magnifiers are used to verify hole diameter. The latter method is utilized when soft coatings over the copper are used.The optical method prevents deformation or removal of the soft coatings within the hole. When drill blank plug gauges are used, the inspector should use a soft touch to prevent damage to the hole. The drill blank gauges should be cleaned prior to use to prevent solderability degradation. Plating nodules are sometimes present in the hole and restrict the penetration of the gauges. Forcing the gauges into the holes causes the nodules to be dislodged, resulting in voids in the PTH side wall. 51.11.1.4.4 Lifted Lands. During visual examination there should be no evidence of the lifting of lands from the base material surface. Lifted lands can trap contaminates during the assembly process and are considered undesirable for assembly. 51.11.1.5 Marking (Legend/Silkscreen) Imperfections. The marking of part number, revision letter, part mounting information and orientation, etc., on a PCB is evaluated for location, size, readability, and accuracy. Marking imperfections are usually considered minor, but markings that are missing, incorrect, or partially obscured could have an effect on functionality. Marking that encroaches into the soldering area can also be a significant issue and should not reduce the solderable areas below what is stated in the procurement documentation. 51.11.1.6 Solder Resist Coating Imperfections. A solder resist is a coating material used to mask off or to protect selected areas of a pattern from the attachment of solder. A solder resist can also be used as electrical insulation between isolated conductors or to define the soldering area for ball grid array (BGA) patterns. Attributes inspected are registration, blisters, bubbles, adhesion, wrinkles, tenting, soda strawing, and delamination. Misregistration at land areas reduces or prevents adequate solder fillet formation or can expose adjacent conductors. The minimum land area to obtain adequate solder filleting should be reflected by the minimum acceptance criteria. Wrinkles, flaking, peeling, and delamination provide sites for moisture absorption and contaminants to settle on or into the PCB and can reduce the insulation between adjacent conductors, increasing the potential for electrochemical migration (ECM). When solder resist is used to define BGA patterns, the shape and position on or around the land is important to proper soldering of the BGA (see Fig. 51.10)
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FIGURE 51.10
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Soldermask defined BGA land. (Courtesy of IPC.)
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51.11.1.6.1 Tenting of Vias. Tenting of via holes is the process of covering holes with solder resist where soldering is not required or desired. This is done to prevent the potential for shorting during the assembly process and to allow for vacuum-based test fixtures to draw the assembly down onto test pins for functional test. The requirement here is that all vias that are required to be tented are covered by resist. 51.11.1.6.2 Soda Strawing of Solder Resist. A soda straw is a long tubular void along the edges of conductive patterns where the solder resist is not bonded to the base material surface or edge of the conductor (see Fig. 51.11). If this void is allowed to reach the point where the solder resist meets the outside environment, fluxes, fluids, and other contaminates may end up trapped in the soda straw, leaving the potential for future ECM or corrosion.
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FIGURE 51.11 Soda strawing between copper trace and soldermask. (courtesy of IPC.)
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51.11.1.7 Coverlayer Characteristics. Flexible and rigid/flexible PCBs are covered with either a coating similar to solder resist or a coverfilm adhered to by acrylic adhesive. Covercoat acceptability requirements are the same as those for solder resist. 51.11.1.7.1 Coverfilm Separations. When a film is adhered to the flexible PCB surface, separations between the film and PCB surface can occur. There should be no more than three small separations that do not reduce the spacing between conductors by 25 percent or below the minimum stated in the procurement documentation. Separations along the edge of the PCB are not allowed, as they can trap contaminates. 51.11.1.7.2 Adhesive Presence on Attachment Surfaces. The presence of adhesive from the coverfilm should not reduce the solderable surfaces below the minimum reflected on the procurement documentation.
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