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FIGURE 57.19 Examples of alternative construction using Cu-Invar-Cu to achieve low-x-y CTE: (a) metal core (sides A and B may be multilayer boards) and (b) metal plane constructions. (After F. Gray.34)
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under components, dry film solder mask should be used. However, excessively thick solder mask, particularly dry film over closely spaced traces, can result in crevices. If the solder mask cannot flow enough to adhere closely to the board, the resulting crevices can entrap contaminants such as flux that can accelerate corrosion later. LPI solder mask provides excellent coverage, resolution, and alignment to other features, but it generally cannot be used to tent vias. IPC-SM-840 defines the performance and qualification requirements for solder mask. 57.5.1.3 Metal Finish. The metal finish on the SMT and TH pads can have an impact on PTH reliability and on the reliability of the solder joints made to these pads. Common metal finishes for solder-mask-over-bare-copper (SMOBC) boards include hot-air solder leveling (HASL or HAL), organic-coated copper (OCC), and electroless NiAu. Galvanically plated CuNiAu and CuNiSn made by another processing route are also available. These finishes provide a solderable finish for later printed circuit assembly. The pros and cons of the various finishes are discussed in turn. Of the common metal finishes, HASL is the only one which can directly reduce reliability of the board. In a typical HASL process, the board receives a severe thermal shock when it is dunked into a bath of molten eutectic Sn-Pb solder. The PTHs can survive only a certain number of solder shocks without failure; this process uses up one of these thermal cycles before the board leaves the fabricator.
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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
20 Coefficient of thermal expansion (10 6/k)
Unconstrained FR-4
Heating Cooling
0 60
20 60 Temperature ( C)
FIGURE 57.20 Example of the low overall CTE that can be achieved for a metal core construction similar to Fig. 57.19a. Two 0.055-in (1.4-mm)-thick multilayer FR-4 boards were bonded to a 0.085-in (2.2-mm)-thick copper-Invar-copper core. Data shown are for the third thermal cycle. (After F. Gray.`)
Organic-coated copper provides a consistent, flat, solderable metal finish. Exposed copper after printed circuit assembly has been a persistent reliability concern, because it is generally not permitted on HASL boards. While exposed Cu on HASL boards is associated with poor solderability, which may be due to contaminants that were not removed before the HASL process, there is little evidence that exposed Cu on a properly processed OCC board causes reliability problems. Surface insulation resistance (SIR) testing shows that OCC boards have comparable or better performance than HASL boards in high-temperature, high-humidity storage tests. CuNiAu boards fabricated either with the NiAu as the Cu etch resist or by the SMOBC process followed by electrolessly plating Ni and Au can confer improved PTH reliability. There are two mechanisms for the observed improvement: the enhanced rivet effect provided by the Ni and the elimination of Cu dissolution during solder shocks such as wave soldering or PGA rework. For high-aspect-ratio holes, electroless Ni confers an additional benefit because the plating thickness in the barrel is more consistent than for conventional electroplating. In the simple picture of PTH failure shown in Fig. 57.3, the comparatively low CTE metalplated PTH acts as a rivet that resists the z-axis expansion of the PCB. Because Ni has a higher elastic modulus than Cu, it strains less under the stress imposed by the expanding PCB. Consequently, adding Ni plating lowers the strain imposed on the Cu and lessens the amount of fatigue damage. In this model, the Ni protects the Cu, increasing PTH life. The ability of the Cu to withstand the forces imposed on it by thermal expansion of the PCB is also dependent on the thickness of the Cu in the PTH. Unfortunately, in the SMOBC process all subsequent steps after pattern or panel plating reduce the Cu thickness from the plated amount. Nickel plating is resistant to the etches and developers used in later processing steps, so it protects the underlying copper from thinning due to dissolution. The HASL process and rework of large through-hole connectors or pin grid arrays (PGAs) can have particularly negative effects. Cu dissolves rapidly into molten eutectic Sn-Pb solder. During the HASL process or component removal and replacement with a solder fountain, large amounts
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