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CHEMISTRY OF ELECTROLESS PLATING
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Chemical reactions which take place in electroless plating have been treated well in the literature.4,21 However, they are discussed again here because some of the due comments are relevant to the chemistry of electroless plating.
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Electroless Plating Solution Chemical Reactions Main ingredients of most electroless plating bath formulations consist of NaOH HCHO CuSO4 EDTA pH adjustment Reducing agent (formaldehyde) Source of copper iron Cheleting agent
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31.6.1.1 Copper Deposition. The chemical reaction of copper deposition may be represented by Cu(EDTA)2 + 2HCHO + 4OH Cu0 + H 2 + 2H2O + 2CHOO + EDTA4 (31.1)
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Under the alkaline pH condition, the cupric ions would normally combine with OH to produce cupric hydroxide [Cu(OH)2], a useless precipitate. When a cheleting agent such as EDTA is added, it prevents cupric hydroxide formation by maintaining the Cu2+ in solution. Once the deposition of metallic copper starts through catalytic sites, the reaction in Eq. (31.1) continues because of the autocratic nature of the reaction. 31.6.1.2 Cannizzaro Reaction and Formaldehyde Concentration. While this main reaction, represented by Eq. (31.1), takes place, other undesirable side reactions proceed, also in
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competition with the reaction. One of the major difficulties is the lowering of formaldehyde concentration in the solution due to its disproportionate consumption in alkaline solution. This is known as the Cannizzaro reaction, which may be characterized by 2HCHO + OH CH 3OH + HCOO (31.2)
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This reaction continues independently. Fortunately, methanol (CH3OH) one of the byproducts of the Cannizzaro reaction, tends to shift the equilibrium of the reaction in Eq. (31.2) to the left and thus prevents the decrease of formaldehyde concentration by an unproductive reaction. By controlling the plating conditions properly, the wasteful consumption of formaldehyde by the Cannizzaro reaction can be retained within 10 percent of the consumption in the reaction in Eq. (31.1). 31.6.1.3 Fehlings-Type Reactions. In addition to the Cannizzaro reaction, the following side reaction also competes for formaldehyde: 2Cu2 + + HCHO + 5OH Cu2O + HCOO + 3H 2O (31.3)
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31.6.1.4 Other Formaldehyde-Related Reactions. Under conditions which would favor the Fehlings-type reaction [Eq. (31.3)], spontaneous decomposition of uncatalyzed plating solutions produces precipitation of finely divided copper with attended vigorous evolution of hydrogen gas. The finely divided copper produced is due to the disproportionate quantity of Cu2O under alkaline conditions: Cu2O + H 2 O Cu0 + Cu2 + + 2OH (31.4)
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Furthermore, formaldehyde may act as a reducing agent for the cuprous oxide to produce metallic copper: Cu2O + 2HCHO + 2OH 2Cu0 + H2 + 2HCOO + H2O (31.5)
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31.6.1.5 Filtering. The Cu0 nuclei thus produced according to Eqs. (31.4) and (31.5) are not relegated to deposition on substrate but are produced randomly throughout the solution and become the catalytic sites for further undesirable copper deposition. Continuous filtration with a filter of pore size 20 mm or smaller can improve extraneous copper due to these catalytic sites.25 The use of continuous filtering also improves the ductility of copper by eliminating codeposition of impurities. In most mass production facilities of PWBs by additive technology, two or three stages of filtering with filters of pore sizes down to 5 mm are common.
Use of Stabilizers Since the reaction represented in Eq. (31.3) indicates the greatest degree of instability of the plating bath, various measures are taken to counteract this reaction. Alkaline cyanide has been a popular stabilizer used for the CC-4 bath, since cyanide forms strong complexes with Cu+ (Cu+ + 2CN Cu(CN)2 ), but relatively unstable complexes of Cu2+. However, since cyanide also reacts with HCHO, it is difficult to control. 2,2 -dypiridil also chelates Cu+ and does not react with HCHO. Hence, it is a more favored stabilizer, particularly for those baths used in Japan. On the other hand, a dypiridil bath deposit begins to have less ductility after several plating cycles and weekly bath makeup may be necessary. Aeration of the plating bath is known to stabilize the solution, and vigorous aeration is commonly used to operate modern full-build electroless baths.
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