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and other contaminants are sufficiently soluble in either water or saponified water. It also depends on good access to the residues; consequently, a minimum component standoff that permits cleaning is required if it is possible for flux to get underneath the component body during assembly. It is almost as important that the board be thoroughly dried because water is an excellent medium for galvanic corrosion. Proper drying can be quite difficult even with substantial air flow since water has a much lower vapor pressure than CFCs do. If the component standoff is low, capillary action holds water in the small gap. If water cleaning is done in midprocess (e.g., before a reflow or wave-soldering step), plastic components may absorb moisture; in this case, the board must be baked out to prevent package cracking in subsequent high-temperature processes (see Secs. 57.2.3 and 57.4.2.2). The rework process should not be overlooked in planning a flux and cleaning strategy. Compared to the automated processes that proceed it, it is typical to use a more aggressive flux and more of it to do rework. Use of a halide-free flux or proper cleaning after rework is essential to prevent cleanliness-related reliability problems. Finally, the cleaning process itself can damage the PCA. Ultrasonic cleaning can damage components with internal wire bonds or die attach. It has also been observed to cause fatigue cracking of solder joints to LEDs and SOT-23s when the energy density was high because these components have terminations that are mechanically resonant near the generator frequencies. Solvent cleaning can attack the polymers used in solder masks, PCBs, conformal coatings, and components. D-limonene (terpene)-based solvents should be tested carefully for compatibility with exposed plastics and metals. 57.4.2.5 Electrical Test and Depanel. The electrical testing and depanel processes can impose large mechanical stresses on the PCB and its components. In-circuit electrical test utilizes a bed of nails or two beds in a clamshell arrangement to contact each electrical node on the board.The probes must contact the board with sufficient force to make good electrical contact. If the board is not properly fixtured or if the loading in a clamshell fixture is unbalanced, the resulting deflections can cause solder joint or component cracking. These cracks may cause electrical failures immediately or after some period of service. Depaneling, the process of separating individual images from a larger panel, is done by a variety of methods. The associated mechanical deflections or vibration can cause component cracking or solder joint fatigue. 57.4.2.6 Rework. Rework, whether repair of open or shorted solder joints or replacement of defective components, has a significant negative effect on component reliability. If there were not enough other incentives for low process defect rates, the effect on product reliability would be enough. Reworking the quality in does not bring the board to the quality level that would have been reached if the boards were built right the first time. Some of the ways rework processes can adversely affect reliability are described here. Thermal Shock During Rework. Thermal shock to components is a concern during rework as it is during reflow. The maximum heating or cooling rate is driven by the requirements for ceramic capacitors and should not exceed 4 C/s.31 Rework of large through-hole components, such as pin grid arrays (PGAs) and large connectors, poses special problems. If it is improperly done it can result in PTH failures. Because the damage during these large thermal cycles is cumulative, the number of rework operations at a given site should be monitored and limited to a safe number. The number of cycles that will cause a fatigue crack to initiate in the copper in the barrel and propagate to failure depends on the aspect ratio of the PTH, the type and thickness of plating in the hole, the substrate material, etc. Due to the large number of joints that must be melted at once and the large thermal mass of the components, rework of large TH components is often done with a solder pot. The thermal shock caused when the molten solder hits the board can cause PTH cracking due to z-axis expansion.A preheat step (to about 100 C for FR-4) helps to reduce the damage.The time the board is in contact with the solder fountain should also be minimized since dissolution of the copper plating inside the PTH occurs during this time. Thinning the plating in the PTH tends
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