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electrical test contact and in-circuit testing. OSP deposition on selective gold can prevent the intended contact functionality of the gold pad. Skip plating can be a concern. Very thin solder mask residues will prevent OSP coating. Because the OSP coating is invisible, it cannot be inspected, so skip plating will lead to non-wetting at assembly. OSP is the coating most sensitive to handling, storage, and shelf-life. Copper under OSP oxidizes with time and heat, so weak fluxes present soldering issues. The Pb-free temperature increase has narrowed the OSP process window to the point where PTH solderability may not meet IPC standards.[1 3]
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Silver was chosen as a PCB surface finish from the earliest days of PCB fabrication due to some inherent advantages. Silver is the best electrical conductor and exhibits the lowest contact resistance of any metal. As a precious metal, it is more resistant to oxidation than other more base metals. On early PCBs, electroplated silver was discovered to form metal dendrites between conductors. During the 1990s, new immersion silver was introduced as an HASL alternative, and users developed data showing that dendrite formation was not an issue. With the Pb-free transition, immersion silver gained widespread use. OEMs with specific needs specified silver to overcome issues with other finishes. Immersion silver is a galvanic displacement process with a typical thickness range of 0.1 to 0.4 mm. The fabrication process is easy, but the coating can suffer from tarnish when left exposed after assembly in field use. Heavy tarnish can be a predictor for corrosion and functional loss.
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Chemical Coating The PCB copper is prepared using separate cleaning and microetch steps. Over time, these steps can be taken for granted, but improper use can cause many defects. The predip bath removes excess acidity from the PCB and prevents chlorides from entering the silver bath, where silver chloride would quickly precipitate. The silver bath is quite simple chemically, relying on the galvanic displacement of less noble copper with two atoms of more noble silver. Proper rinsing and drying is essential.
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Fabrication Process Table 32.7 shows the fabrication process for immersion silver surface finish, including the chemicals used and dwell times.
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TABLE 32.7 Process Flow for Immersion Silver Process Cleaner Microetch Predip Silver Postdip (optional) Chemicals Aqueous solvents, detergents, and emulsifiers. Acid can help undercut residues and remove thick copper oxides. 1 2 mm of Cu is oxidized to Cu++ and dissolved. Normally, the ingredients of the silver bath without the silver metal itself. Silver, acid, grain refiners, chelation Ag+ + Cu(0) Ag(0) + Cu++ Can be an antitarnish or an extra cleaner for demanding ionic cleanliness applications. Time (min) 1 4 0.5 1.0 0.5 0.5 3.0
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Most immersion silver in large production is applied in conveyorized equipment. The cleaner and microetch may use flood or spray application, but the silver solution is applied using flood. Because the silver process is very mild to the PCB materials, rinsing is very important to ensure that contamination from all prior fabrication steps is removed. Often, silver is applied in part form after the PCB has been routed, depanelized, and electrically tested. This is to preserve the silver surface, which can be sensitive to handling, contamination, and fingerprints. In smaller-scale production and prototype manufacturing, silver can be applied in vertical process tanks.
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Advantages and Disadvantages Table 32.8 compares the advantages and disadvantages of using immersion silver as a surface finish.
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TABLE 32.8 Advantages and Disadvantages of Immersion Silver as a Surface Finish Silver advantages Flat; fine-pitch assembly Inexpensive finish Simple process Widely available Cu/Sn solderjoint No Pb Reworkable High productivity at fabrication Easy thickness measurement Excellent surface contact functionality No degradation with multiple reflow soldering Silver disadvantages Handling-sensitive Solderjoint microvoid history Exposed silver on final assembly can corrode Tarnish Silver had a history of dendrite formation Attack at the solder mask interface
Failure Modes The silver surface readily shows tarnish from exposure to sulfur and chloride environments. Excessively tarnished parts have limited solderability. As a galvanic process, aggressive silver baths can attack copper circuitry. Attack can occur at the interface with solder mask or at the electroless copper layer on sidewalls. The attack is more severe at high thicknesses over 0.50 mm and if the PCB is reworked with multiple passes through immersion silver. Copper attack can result in electrical opens. Aggressive silver baths, when deposited on rough copper, can also attack on pad surfaces, causing caves in the copper. Studies show that caves can result in layers of very small voids at the copper-solder interface after soldering. These structures, termed microvoids, can dramatically reduce solderjoint strength and produce electrical opens. Each of the preceding failure modes is worsened when the silver bath plates nonuniformly. The chemicals need to be formulated and operated in such a way to provide uniform galvanic exchange metal deposition.
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