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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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Interconnect failures due to thermal fatigue of solder joints can be reduced by closely matching the x-y plane thermal expansion properties of the substrate to at-risk components. Large leadless ceramic components that are used because of their hermeticity pose a particular risk. Possible approaches include altering the laminate reinforcement material, adding constraining metal cores or planes, and switching to a ceramic substrate. The first two approaches are discussed here. A more extensive discussion of these options can be found in Ref. 33. A lower x-y plane thermal expansion coefficient laminate can be obtained by replacing the continuous-filament E-glass used in most FR-4 PCBs with an alternative material. The CTE decreases as the fraction of silica dioxide (SiO2) decreases and the level of quartz (as well as the cost) increases in the progression E-glass, S-glass, D-glass, and finally quartz, which has a CTE about one-tenth of E-glass. Aramid (Kevlar) actually has a negative CTE, but it is available in only a few glass styles. Some of the disadvantages of aramid fibers are higher z-axis expansion and higher moisture absorption relative to glass fibers that can result in decreased susceptibility to PTH failures and corrosion-related insulation resistance failures, respectively. Aramid fiber is also used to make nonwoven paper fabric which has a lower modulus, but also a much smoother surface because there is no weave pattern. This form has better dimensional stability and reduced microcracking during thermal cycling. Low-thermal-expansion metal cores or planes can also lower the overall substrate CTE because they constrain the expansion of the polymer material they are laminated to (see Fig. 57.19). Copper-Invar-copper (CIC) is the most widely used material for constraining metal cores (also termed polymer-on-metal or POM construction), followed by coppermolybdenum-copper (CMC). The PCB and core are bonded with a rigid adhesive, usually in a balanced construction to minimize warping. Other special processing is also required. The CTE of the assembly can be estimated using a simple model for composite structures most often written as CTE(overall) = Ea t Et
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where E, , and t are the elastic modulus, CTE, and thickness, respectively, of the various layers. A more sophisticated model can be found in Ref. 34. An example of the low overall CTE that can be obtained using a CIC core is shown in Fig. 57.20. Unfortunately, the constrained x-y axis expansion results in increased z-axis expansion that can reduce PTH reliability to dangerously low levels, especially in an environment in which the full mil-spec thermal cycle of 55 to +125 C is imposed. Consequently, use of polyimide is recommended with CIC cores. Because of its high Tg and low CTE below Tg, polyimide imposes much lower strains on the PTH for a given thermal cycle than other dielectrics. Constructions utilizing constraining low-CTE metal planes usually use CIC layers in place of ground and power planes in a standard multilayer board. The same PTH reliability concerns that hold for CIC core boards apply to these PCBs as well. PTH reliability can be improved by using polyimide resin and by using CuNiAu or CuNiSn metallization in the PTHs. These substrates are easier to manufacture than metal core boards because standard PCB fabrication techniques can be used for the most part. Resin material can affect fiber/resin delamination, one of the prerequisites for conductive anodic filament growth. Measling occurs at about 260 C for FR-4, but may occur at lower temperatures for boards with more hygroscopic resins. 57.5.1.2 Solder Mask. The three major types of solder mask liquid screen-printed, dry film, and liquid photoimageable (LPI) come with different benefits and concerns from a reliability perspective. The solder mask material should be selected for its compatibility with the heat and solvent characteristics of the assembly process, its capability to provide good conformity over surface features on the PCB, and its ability to tent vias if required. Since many of these characteristics are product-specific, only a few general guidelines can be provided here. Where tenting of vias is required to keep solder, moisture, or flux from wicking up
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