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sizes and pitches are required, which in turn necessitate closer tolerances on part placement. This is also the case of DCA, where the solder ball size and pitches tend to be as small as 0.10 mm and 0.25 mm, respectively. The CCGA is a variant of the BGA in which the solder balls have been replaced with solder columns. The columns allow the ceramic package to be assembled to an organic laminate circuit board of considerably larger thermal expansion coefficient by absorbing the higher strains created by the larger thermal expansion mismatch between the two materials. The columns are created from one of the high-melting-temperature, Pb-based alloys (e.g., 95Pb5Sn or 90Pb-10Sn) that will not melt during a eutectic Sn-Pb reflow process. The columns may also have a Cu spiral wound around them to enhance durability as those columns are susceptible to damage during handling and part placement. The accelerated development of surface mount components has generated packages and I/O configurations that have not yet been standardize, resulting in odd-form devices. Examples of odd-form components include surface mount switches and connectors as well as a variety of inductors, (Fig. 40.13) LEDs, and transformers. Typically, so-called surface-mount connectors may actually be mixed technology with through holes providing the mechanical strength needed for cable insertion and removal activities and surface-mount leads establishing the electrical connection. (The through-hole interconnections can be made by paste-inhole techniques or be soldered manually.)
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FIGURE 40.13 Odd-form components as exemplified by surface-mount inductors. (Courtesy of Sandia National Laboratories.)
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A number of assembly-related issues must be addressed with odd-form components. First, it is necessary that correct pad dimensions be designed on the circuit board. Also, the stencil must have the correct aperture size to print an adequate quantity of solder paste. The pickand-place machine may require custom tooling in order to handle these components. Lastly, odd-form parts are typically larger and heavier. Therefore, it is possible that they will not readily self-align while the solder is molten during the reflow process.
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The conversion to Pb-free solders has significantly affected surface-mount components. In the case of leadless passive devices and peripherally leaded packages, the traditional electroplated Sn-Pb finish has been replaced with 100 percent Sn coatings. The Sn coatings have raised concerns regarding the development of Sn whiskers that can potentially short nearby conductors in service. The Sn-Pb solder balls of BGA, CSP, and DCA technologies, having a melting temperature of 183 C, are being replaced with Sn-Ag-Cu alloys with a melting temperature of 217 C. In the case of DCA/FC and CCGA applications, the high-Pb alloys used for the balls and columns, respectively, will not melt during a Sn-Ag-Cu solder process used to create the second-level interconnections. 40.3.3.2 Dispensing. In surface-mount technology, three classes of materials must be dispensed on to the circuit board: adhesives, fluxes, and solder pastes. Each of these three categories, which utilize very similar equipment options for the actual dispensing action, is discussed in the following subsections. 40.3.3.2.1 Adhesives. Adhesives are used to secure surface-mount devices to the circuit board. An adhesive may be required when, for example, exposing surface-mount components to a wave-soldering process used to assemble a mixed technology circuit board. Also, larger components on double-sided circuit boards may be adhesively bonded in place to prevent them from falling off when the board is turned upside down for the second reflow step. Under these circumstances, the package weight exceeds the surface tension force of the molten solder that keeps smaller components on the board. The adhesive must be able to withstand the temperature conditions of the wave or reflow soldering process as well as the chemical activity of the flux. Adhesives may also be required to anchor larger surface-mount devices to the circuit board. This additional strength is needed for service specifications that include mechanical shock and vibration environments. (Adhesives are typically not used for through-hole components because clinching the leads provides a sufficient anchor to keep the component on to the board prior to and during the soldering process. After soldering, through-hole solder joints are sufficiently robust to withstand heavy shock and vibration environments. Nevertheless, in very severe environments, adhesives may be used to anchor through-hole components to the substrate.) It is important to control the quantity of dispensed adhesive since there must be sufficient material to perform the attachment function. On the other hand, too much adhesive can cause run-out onto the solder pad or component I/O, resulting in poor solderability. Some adhesives are prone to run-out or bleeding caused by their separation into individual liquid components that together comprise the adhesive material. When bleeding by a component liquid occurs beyond of the adhesive deposit, that bleeding liquid can contaminate nearby solderable surfaces. In the case of very dense circuit boards, excessive migration of the whole adhesive migration, or the bleeding of one of its components, can contaminate the pads of other components, affecting their placement and solderability. Although the function of the adhesive is to anchor components in place for the soldering process, the adhesive deposit remains as a part of the assembly after soldering, and as such must not interfere with next-assembly steps or negatively impact the long-term reliability of the circuit board. For example, some epoxies readily absorb moisture or other organic compounds.Those absorbed materials may outgas during subsequent temperature excursions that could contaminate critical surfaces (e.g., sensors) when the product is in service. Therefore, it is important to select only adhesive materials that are qualified for a particular application. Adhesives materials that are used in electronics assembly processes are typically based upon epoxies or silicones. Adhesives can also be described by the following four functional/materials categories: thermosetting adhesives, thermoplastic adhesives, elastomer adhesives, and toughened alloy adhesives. Each group differs in its compositions, the type of curing cycle, and its pre- and post-cure material properties. The curing cycle, which typically requires an elevated temperature/time profile, must not degrade the components present on the board or the laminate itself. It is understood that the added step of a curing cycle slows the overall assembly process. Thermosetting adhesives cure by heat or a catalytic reaction that cross-links the polymer chains. Once cured, these materials remain very strong and will not readily soften at elevated
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