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Inferior Copper Deposits These may be caused by any of the following:
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Either low or excess additives Chloride out of range (i.e., too high or too low) Organic, metal, or sulfur (thiourea) contamination Excess DC rectifier ripple (greater than 10 percent) Low copper content with bath out of balance Roughness in drilling, voids in electroless steps, or other problems introduced in earlier processing
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Cracking and Ductility.
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Resistance to cracking is tested by the following:
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Solder reflow Wave soldering and cross-sectioning are acceptable alternatives. Elongation Two mil copper foil should exceed 10 percent elongation. Acid copper elongation should range between 10 and 25 percent. Frequent testing gives more meaningful results. Float solder test This test includes prebaking and flux, using a 5 to 20 sec. float in a solder pot (60:40) at 550 F, followed by cross-sectioning for evaluation. Copper foil bulge test This test measures tensile strength by puncturing copper at high pressure. DC ripple High values of rectifier ripple (8 to 12 percent) may cause inferior copper deposits and poor distribution of thickness.
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29.6.5.2 Visual Appearance. When plated, copper has a semibright appearance at all current densities. Unevenness, hazy or dull deposits, cracking, haloing around the PTH, and lowcurrent-density areas indicate organic contamination. Carbon treatment is required if these conditions persist. Burned, dull deposits at high-current densities indicate low additive content, contamination, solution imbalance, or low bath temperatures. If dull, coarse deposits result at low-current densities, the chloride ion is not in balance. When chloride is high or bath temperature too low, anodes may become heavily coated and polarized (the current drops). Decreased throwing power (surface-to-hole ratio), reduced bath conductivity, or poor-quality plating may also indicate contamination and are corrected as follows:
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Maintaining solution balance and chloride content at 60 to 80 ppm Circulating solution through filter continuously, passing through a carbon canister periodically, or by batch carbon treatment Analyzing organic additives by CVS or Hull cell Checking metal contaminations every three months Keeping the temperature between 70 and 85 F Checking anodes daily and replacing bags and filters (rinsed in hot water) every three to four weeks
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29.6.5.3 Problems. Table 29.5 lists problems that appear after copper plating. Two groups are listed, with the first group readily correlated to the copper-plating process. Thin, rough copper plating in the PTH may also be exhibited by outgassing and blowholes during wave soldering. Figures 29.16 through 29.25 illustrate some of these effects.
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ELECTROPLATING
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TABLE 29.5 Printed Wiring Board Copper-Plating Defects Defect in copper process Corner cracking Nodules Thickness distribution Dullness Uneven thickness in PTH Pitting Columnar deposits Step plating, whiskers Defect, overall manufacturing process Voids etching Inner layer smear Roughness Hole-wall pullaway Copper-copper peeling Soldering blowholes Cause Excess additive, organic contamination in solution Particulate matter in solution; also drilling, deburring residues See Sec. 29.5.2 Off-balance solution, organic contamination Organic sulfur (thiourea) contamination Additive malfunction, defective electroless, or preplate preparation Low additive, rectifier malfunction Excess or defective additive Cause Malfunction of electroless copper steps, also preplate cleaner Drilling or malfunction in smear removal Drilling or drilling residues Malfunction of smear removal or electroless copper steps Surface residues from electroless and/or image transfer Drilling roughness, voids, and thin plating
FIGURE 29.16 PTH gvertical cross section illustrating corner cracking.
FIGURE 29.17 PTH vertical cross section illustrating uneven, thick/thin copper plating.
FIGURE 29.18 PTH vertical cross section illustrating columnar copper deposit structure.
FIGURE 29.19 PTH vertical cross section illustrating copper voids.
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FIGURE 29.20 PTH vertical cross section illustrating rough, nodular copper plating due to drilling roughness. Nail-heading is also shown.
FIGURE 29.21 PTH vertical cross section illustrating hole roughness due to excessive etchback.
FIGURE 29.22 PTH vertical cross section illustrating copper hole wall pull-away.
FIGURE 29.23 Trace surface view and vertical cross section illustrating munchies (or mouse bites ) and pitting.
FIGURE 29.24 PTH vertical cross section illustrating wave-soldering blowholes and thin, rough copper plating.
FIGURE 29.25 PTH vertical cross section illustrating reverse etchback.
ELECTROPLATING
SOLDER (TIN-LEAD) ELECTROPLATING
Solder plate (60 percent tin, 40 percent lead) is widely used as a finish plate on printed circuit boards. This process features excellent etch resistance to alkaline ammonia, good solderability after storage, and good corrosion resistance. Tin-lead plating is used for several types of boards, including tin-lead/copper, tin-lead/tin-nickel/copper, solder mask over bare copper (SMOBC), and surface-mount (SM). Fusing is required on all tin-lead-plated surfaces. Thickness minimums are not specified. The preferred composition contains a minimum of 55 percent and a maximum of 70 percent tin. This alloy is near the tin-lead eutectic, which fuses at a temperature lower than the melting point of either tin or lead and, thus, makes it easy to reflow (fuse) and solder. (The composition of the eutectic is 60 percent tin, 37 percent lead, with a melting point of 361 F.) Fusing processes include infrared (IR), hot oil, vapor phase, and hot-air leveling for SMOBC. Plating solutions currently available include the high-concentration fluoboric acid peptone system as well as low-fluoboric, nonpeptone, and a nonfluoboric organic aryl sulfonic acid process. These processes are formulated to have high throwing power and give uniform alloy composition.1 The sulfonic acid process has the advantage of using ball-shaped tin-lead anodes. Table 29.6 gives details of operation and control of two high-throw tin-lead (solder) baths.
TABLE 29.6 Tin-Lead Fluoborate: Operation and Control High HBF4/peptone Operating conditions Lead Stannous tin (SN+2) Free fluoboric acid Boric acid Additive Temperature Cathode current density Agitation Anodes Type Composition Purity Bags Hooks Length Current density 1.07 1.88 oz./gal 1.61 2.68 oz./gal 47 67 oz./gal Hang bag in tank Use as needed by Hull cell and Ah usage. 60 80 F 15 18 A/ft2 Solution circulation Bar 60% tin-40% lead Federal Specification QQ-S-57130 Polypro Monel Rack length minus 2 in 10 20 A/ft2 Low HBF4/proprietary 1.4 3.0 oz./gal 3.0 4.5 oz./gal 15 25 oz./gal Same Use as needed by Hull cell and Ah usage. 70 85 F 10 30 A/ft2 Mechanical and solution circulation
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