ACCEPTABILITY AND QUALITY OF FABRICATED BOARDS
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FIGURE 51.4 Base material defects: (a) blistering, (b) fiber exposure, (c) measling, (d) crazing, (e) crazing, (f) measling. (Courtesy of Sandia Laboratories and IPC.)
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188.8.131.52.2 Measling/Crazing, Delamination, Blistering, and Weave Texture/Exposure. The IPC formed a special committee in 1971 to consider base material imperfections and to define them better with illustrations and photographs. The conditions are defined and discussed here:
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Measling This internal condition occurs in the woven-fiber-reinforced laminated base material in which the bundles are separated at the weave intersection. Measling manifests itself in the form of discrete white spots or crosses below the surface of the base material. A report compiled by the IPC, Measles in Printed Wiring Boards, was released in November 1973. The report stated that measles may be objectionable cosmetically, but their effect on functional characteristics of finished products are [sic], at worst, minimal and in most cases insignificant. The IPC Acceptability Subcommittee readdressed the subjects of measles and crazing in 1994 and verified the 1973 findings. IPC standards allow measles in PCBs with the exception of high-voltage applications. As hole spacing has continued to decrease, new attention has focused on measles. There is a concern that measling could contribute to the formation of CAF on designs with close hole spacing, and the requirements for measles will likely adjust to accommodate these designs. Crazing An internal condition occurring in the woven fiber-based base material in which the glass fibers within the yarn are separated, crazing can occur at the weave intersections or along the length of the yarn. This condition manifests itself in the form of connected white spots or crosses below the surface of the base material, and is usually related to mechanical and/or thermally induced stress. Limited crazing is an acceptable condition as long as it does not reduce the space between conductive patterns below the minimum conductor spacing, the distance of crazing does not span more than 50 percent of the distance between adjacent conductive patterns, and no propagation results from thermal testing that replicates the manufacturing process. If crazing is present at the edge of the PCB, it should not reduce the minimum distance between PCB edge and conductive pattern. Delamination This is a separation between plies within the base material, between a material and the conductive foil, or between any other planar separations within a PCB.
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Blistering and delamination are considered to be major imperfections. Whenever a separation of any part of the PCB occurs, a reduction in insulation properties and adhesion occurs. The separation area could house entrapped moisture, processing solutions, or contaminants, and could contribute to electrochemical migration or produce other detrimental effects in certain environments. The delamination or blister area may also propagate to the point of complete PCB separation, normally manifesting during assembly operations. Last but not least is the effect on solderability in PTHs. Entrapped moisture, when subjected to soldering temperatures, has been known to create steam that blows holes through the plated side walls, exposing the resin and glass of the PTHs and creating large voids in the solder fillet. Blistering A localized swelling and separation between any of the layers of a laminated base material or between a base material and a conductive foil or protective coating, blistering is a form of delamination. Weave texture In this surface condition of base material, a weave pattern of glass cloth is apparent although the unbroken fibers of the woven cloth are completely covered with resin. Weave texture is an acceptable condition and should not be confused with weave exposure. Weave exposure In this surface condition of base material, the unbroken fibers of woven glass cloth are not completely covered by resin. Weave exposure is considered a major imperfection. The exposed glass fiber bundles allow wicking of moisture and entrapment of processing chemical residues into the base material. 184.108.40.206.3 Haloing. A mechanically induced fracturing or delamination on or below the surface of the base material, haloing is usually exhibited by a light area around holes, other machined areas, or both. Haloing should not penetrate more than 50 percent of the distance to the nearest conductor and should not reduce electrical spacing below the given requirement. 220.127.116.11 Conductor Pattern Integrity. Several methods can be used to determine conductor pattern integrity. Comparison equipment, overlays, and AOI machines are routinely used to compare the pattern to the digital image of the pattern. The most common conductor pattern inspection tool is the aided human eye using low (<3 ) magnification. The loss of adhesion and imperfections that reduce the conductor volume or spacing make up this category of imperfections. Voids in conductors and lands can be detrimental to function, depending on the degree of imperfection. Voids or pinholes reduce the effective conductor width, reduce current-carrying capacity, and can affect other design electrical characteristics, such as inductance and impedance. Voids in lands are also detrimental to solderability. Pinholes or voids in the conductor or land can undermine the finish coating. The presence of conductive elements in areas in which they were not intended can reduce spacing to a point where the insulation between conductors can be jeopardized. The degree that these imperfections affect the PCB depends on the intended design of the circuit. The imperfections in this group are defined as follows. (see Fig. 51.5).
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FIGURE 51.5 Dents, pits, pinholes and voids. (Courtesy of Sandia Laboratories.)
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Dent This is a smooth depression in the conductive foil that does not significantly decrease foil thickness.
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