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FIGURE 32.2
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(a) HASL equipment (b) surface scanning electron micrograph (SEM).
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32.4 ELECTROLESS NICKEL IMMERSION GOLD (ENIG)
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Chemically, gold is the ideal element for the external coating of PCBs. Gold does not form an oxide, so it is virtually unaffected by temperature and storage conditions that might reduce the shelf-life of other finishes. In addition, gold dissolves nearly instantaneously into solder, promoting superior wettability. Gold is limited, however, in other ways. Gold embrittles solderjoints when present in excess of about 3 percent by weight, so gold coatings of 0.3 mm maximum thicknesses should be used in soldering. Gold dissolves very quickly into copper as well. To prevent the mixing of gold with copper, and the eventual solderability problems caused by oxidized copper at the PCB surface, a layer of nickel is deposited to separate the metals. ENIG does not rely on electrolytic plating with outside power; in practice, nickel is deposited as a phosphorous-reduced electroless nickel. 32.4.1 Chemistry ENIG deposition is dependent on the electromotive series of elements. Gold readily deposits directly on copper as an immersion bath, but as stated previously, a barrier of nickel is needed. In common practice, 3 to 5 mm of nickel is used in ENIG, with a thin gold coating of 0.05 to 0.15 mm (typically) to prevent nickel oxidation. Nickel does not deposit directly on copper without prior catalysis. Palladium or ruthenium metal forms an immersion deposit on copper as the catalysis or activation step of ENIG. Catalysis baths employ the immersion (galvanic displacement) chemical mechanism. As with all finishes, extra steps are required before the deposition of catalyst, Ni, and Au. All commercial ENIG processes rely on a cleaner and microetch prior to catalysis in order to remove trace residues from the solder mask and tin strip processes before final finish. Microetch is normally based on peroxide, persulfate, or monopersulfate systems. 32.4.2 Fabrication Process Table 32.3 shows the process flow for ENIG surface finish.
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TABLE 32.3 Process Flow for ENIG Time (min.)
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Process Cleaner Microetch
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Chemicals Aqueous solvents, detergents, and emulsifiers. Acid can help undercut residues and remove thick copper oxides. In acidic environments, copper is oxidized by persulfate (S2O8++), peroxide (H2O2), or proprietary monopersulfate acid mixes. Sulfuric acid is common to all types of microetch. 1 2 mm of Cu is oxidized to Cu++ and dissolved. Immersion deposit (galvanic displacement) Pd++ + Cu(0) Pd(0) + Cu++ (or) Ru+++ + Cu(0) Ru(0) + Cu++ Nickel is deposited through chemical (sodium hypophosphite) reduction. Rate control, by pH and temperature, determines phosphorous content, usually 8 11% by weight. Normally, automated controllers are needed to maintain this dynamic bath. Nickel can be measured optically or with titration. Ammonia maintains pH.
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Catalyst
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Electroless Nickel
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10 20
Immersion Gold
Immersion deposit (galvanic displacement) Au+ + Ni(0) Au(0) + Ni++ Gold is added as KAu(CN)2. Do not use gold salts containing Co or Ni.
8 15
PRINTED CIRCUIT BOARD SURFACE FINISHES
Due to the long dwell times, high operating temperatures, and complicated chemical reactions, the ENIG process is always conducted in vertical tanks. To achieve the throughput required in production, many ENIG systems make use of two or three electroless nickel (EN) tanks, which allows for heating new baths or nickel stripping. Using two active EN tanks greatly improves throughput, but the total process ordinarily exceeds 1 hour in cycle time. In addition to the normal plastic materials used in tank construction, stainless steel can be used for EN tanks. This is possible only with the use of anodic passivation, which employs an electrical bias applied to the steel to prevent reduction of nickel ions on the steel. By using steel tanks, fabricators avoid the frequent stripping of plastic tanks required due to the slow plate-out of nickel on rough surfaces and localized hot spots.
Advantages and Disadvantages Table 32.4 shows comparison of advantages and disadvantages of ENIG surface finishes.
TABLE 32.4 Advantages and Disadvantages of ENIG ENIG advantages Flat; fine-pitch assembly Surface contacts Widely available No copper dissolution No Pb Strong PTH rivet Long shelf-life ENIG disadvantages Expensive Brittle Ni/Sn solderjoint Not reworkable Solder mask attack Black-pad, black-line nickel Signal loss at RF frequencies Very complicated process
Failure Modes ENIG failure modes at fabrication are mainly due to the activity of the chemical process steps. Underactive catalysis yields skip plating in the subsequent nickel bath. Nickel skips allow copper to migrate through gold and prevent solder wetting. Overactive catalysis can lead to extra metal between circuit features, known as extraneous plating. This can cause short circuits. Overactive nickel baths deposit low phosphorous content, leading to the interconnection failure mode known as black-pad (described in Sec. 12.2). Overactive gold chemistry attacks nickel along nodule boundaries and produces black-pad. Such gold conditions may be detected with frequent x-ray fluorescence (XRF) thickness measurement. Underactive nickel and gold baths pose skip-plate problems. Peeling of gold films can result from too much rinse time after nickel or from poor rinse quality. Tape testing of ENIG is mandatory. Very rough nickel deposits are possible when the electroless nickel bath is not closely maintained. Nickel baths can destabilize, plate onto the tank walls, and produce inferior coatings. Lastly, soldered ENIG surfaces result in a nickel-tin solderjoint. This structure has been demonstrated to be less tolerant of physical shock than the copper-tin solderjoint. The brittle intermetallic is unavoidable with ENIG, so mobile devices usually do not employ ENIG solderjoints.
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