copper deposition
Although various reducing agents can be used to deposit copper coatings, only formaldehyde copper plating solutions are of practical interest. Formaldehyde autocatalyzes the reduction of copper ions in alkaline solutions (pH = 11–14) at room temperature; here, the copper ions need to bind to form complexes. Suitable Cu2+ ligands for electroless copper plating solutions are polyols (polyhydric alcohols, hydroxyacid anions) and compounds with tertiary amine groups and hydroxyl groups (hydroxylamine, EDTA, etc.). In practice, tartrate, EDTA and tetraoxypropylethylethylenediamine (Quadrol) are often used.
During the copper plating process, accompanied by the main reduction reaction,
The Cannizzaro reaction consumes formaldehyde and a total of 3 to 6 moles of CH2O are used to deposit 1 mole of copper. During copper plating, a large amount of alkali is used, including the Cannizzaro reaction. The consumption of OH– can be determined according to the following equation (moles of species):
Various formulations of copper plating solutions have been developed that are completely stable and suitable for long-term development (for example, Solution B in Table 28.2). Three types of electroless copper plating solutions are distinguished in the literature: (a) low deposition rate solutions (0.5 to 1.0 μm/h), suitable for depositing copper underlayers; (b) solutions with a deposition rate of 4 to 5 μm/h ( ie exhibit higher autocatalytic effect); (c) Copper coatings deposited from solutions of high ductility and strength (for example, solution C in Table 28.2). All these solutions essentially have the same composition: they differ mainly in their additives. In addition, highly ductile coated printed circuit boards are produced by additive processes at higher temperatures (>40°C) and relatively low copper deposition rates.
nickel plated
Electroless nickel plating using hypophosphite as a reducing agent is a popular process. 12,13 The autocatalytic reduction of nickel ions by hypophosphite occurs in both acidic and basic solutions. In stable solutions with high coating quality, deposition rates can be as high as 20 to 25 μm/h. However, this requires relatively high temperatures, around 90°C. Because hydrogen ions are formed in the reduction reaction,
A high buffering capacity of the solution is necessary to ensure a steady state process. For this reason, acetates, citrates, propionates, glycolates, lactates or glycines are added to the solution; these substances, together with buffers, can form complexes with nickel ions . It is necessary to combine Ni2+ ions into complexes in alkaline solutions (here, in addition to citrate and glycine, ammonia and pyrophosphate can be added); moreover, this combination is desirable in acidic solutions, Because free nickel ions precipitate with the reaction product (i.e. phosphate) and hinder further use of the solution.
Nickel plating solutions have fewer stabilizers added than copper plating solutions; nevertheless, they are added to ensure long-term solution stability.
Phosphorus is always present in the coating when reduction is performed by hypophosphite. Its amount (in the range of 2 to 15 mass percent) depends on pH, buffer capacity, ligands, and other parameters of the electroless plating solution.
Borohydride and its derivatives can also be used as reducing agents for electroless nickel plating solutions. While the reduction of nickel ions by borohydride requires temperatures of 60 to 90°C, dimethylaminoborane (DMAB) is capable of using small amounts of boron (0.5 to 1.0 mass percent) in the temperature range of 30 to 40°C . Neutral and alkaline solutions can be used, similar in composition to hypophosphite solutions.
Cobalt, Iron and Tin Plating
The deposition of cobalt is similar to that of nickel - the same reducing agents (hypophosphite, borohydride and their derivatives) are used and the reduction relationships are similar. 14 The reduction of cobalt is more difficult, however, the deposition rate of cobalt is lower than that of nickel; the north and south tides remind you: it is difficult to deposit cobalt from acid solution. The obtained Co-P and Co-B coatings are of particular interest due to their magnetic properties .
Electroless iron plating is more difficult, and only one sufficiently effective iron plating solution is known in which Fe ions form complexes with tartrate and NaBH4 is used as a reducing agent. The Fe-B coating (about 6% B) obtained a rate of about 2 μm/h in alkaline solution (pH 12) and sediment at a temperature of 40 °C .
It is rather difficult to realize an autocatalytic tin deposition process. A sufficiently efficient tin deposition method is based on the disproportionation of tin(II) in alkaline media. 15 In 1 to 5M NaOH solution at 80 to 90°C, a deposition rate of several microns per hour can be obtained
Precious Metal Deposition
Electroless silver plating is the oldest electroless metal plating process; however, it currently lags behind nickel or copper plating.
Unstable disposable ammonia silver plating solutions (with glucose, tartrate, formaldehyde, etc. as reducing agents) are usually used. The coating thickness of this solution is not very large (<1 μm). This unstable solution is more suitable for aerosol spray.
More efficient electroless silver plating solutions were developed using cyanide Ag(I) complexes and aminoborane or hydrazine as reducing agents: deposition rates of 3 to 4 μm/h at temperatures of 40 to 50 °C, at These solutions are very stable in the presence of stabilizers. Sufficiently stable electroless silver plating solutions can be obtained using metal ions such as Co(II) compounds as reducing agents.
Gold coatings can be deposited using various reducing agents: however, the solutions are generally unstable. Solutions with sufficient stability have been developed using stable gold cyanide complexes using borohydride or DMAB as reducing agents. 16 At temperatures between 70 and 80°C, the folding deposition rate reached 5 Am/h and a gold coating of sufficient purity was obtained.
Thin gold coatings can be deposited on plastics by aerosol spraying: using amine gold complexes and hydrazine as a reducing agent, thicker coatings can be obtained (deposition rate up to 0.4 μm/min).
Palladium coatings are readily deposited in alkaline solutions using hypophosphite as a reducing agent, where Pd2+ ions form complexes with ammonia, EDTA or ethylenediamine. Palladium plating is performed at 40 to 50°C and the deposition rate of the Pd-P (4 to 8 P) coating is in the range of 2 to 5 μm/h.
Coatings of platinum, ruthenium and rhodium can be deposited using borohydride or hydrazine as reducing agents. Treatment rates in stable solutions were low (0.5 to 2 μm/h).
deposition of metal alloys
Coatings of about 60 different mass compositions can be deposited containing two or more metals.
Metals such as copper, iron, zinc, tin, rhenium, tungsten, molybdenum, manganese, thallium, and platinum group metals can be introduced into nickel and cobalt coatings, as well as nickel, cobalt, tin, zinc, cadmium, antimony, bismuth, lead, and Gold into copper coating. In the electroless deposition of metal alloys, the thermodynamic relationship to the deposition of alloys by electroplating techniques is valid; it is evident that it is difficult to introduce hard-to-reduce metals such as chromium and manganese into the coating. Furthermore, in the case of chemical reduction, another factor—the catalytic properties of the metal—became evident.
The incorporation of significant amounts of additional metal into nickel, copper, etc. coatings is only possible if the metal is catalytic or at least inert to the oxidation of the reducing agent. The alloy may contain as much as 100% metal catalyst, as much as 50% catalytically inert metal, and as little as 10 to 20% metal inhibitor. The deposition rate decreases when less catalytically active metals are introduced.
