Electro-plating of certain metals
Due to their very high corrosion resistance, good electrical conductivity, low contact resistance, as well as the good solderability of the gold, gold coatings find wide use in electronics and electrical engineering. Typical layer thicknesses at a few 100 nm (e.g. for a soldering aid) to a few μm is used as a corrosion protection.
Alkaline Cyanide Deposition of Gold
The electrolyte here is based on the highly toxic potassium dicyanoaurate(I) = K[Au(CN)2]. This solution contains approximately 68% gold and dissociates in aqueous solution in K+ and [Au(CN)2]- ions. The latter migrate to the anode and dissociate there to Au+ and (CN)- ions. The gold ions migrate back to the cathode, where they are neutralized and deposited on the cathode.
The anode used is either soluble gold or gold-copper electrodes, or insoluble platinum-plated titanium electrodes.
Neutral Cyanide Deposition of Gold
This electrolyte is also based on potassium dicyanoaurate but does not contain any free cyanide (no free (CN)- ions). Insoluble platinum-plated titanium electrodes are used as an anode.
Acidic Cyanide Deposition of Gold
Here too, potassium dicyanoaurate is the gold source in the electrolyte, which additionally contains cobalt or nickel, as well as citric acid. As a result, shiny gold layers can be obtained, which are comparatively hard because of their relatively large proportion of organic constituents and have a low ductility.
As anodes, either insoluble platinum-plated titanium or stainless steel is used.
Strongly Acidic Cyanide Deposition of Gold
For this purpose, trivalent potassium tetracyanoaurate(III) = K [Au(CN)4], which is also stable in strongly acidic solutions, forms the metal supply of the electrolyte. Furthermore, mineral acids such as sulphuric or phosphoric acid are added.
Cyanide-free Deposition of Gold Sulphites
Instead of the highly toxic cyano compounds, the electrolyte is based on Ammonium disulfitoaurate(I) = (NH4)3[Au(SO3)2] or sodium disulphitoaurate(I) = (Na)3[Au(SO3)2] (alkali metal sulphite). The [Au(SO3)2]3- ions of the solution decompose near the cathode into Au+ and (SO3)2- ions, the gold ions are reduced to gold on the cathode and deposited.
In addition to dispensing with the highly toxic cyanidic baths, gold layers deposited from sulphite electrolytes have the advantages of excellent macro-scattering ability (= high deposition rates also at current-degraded points of the electrode) and high ductility.
For this reason, our gold bath NB SEMIPLATE AU 100 is based on a sulphite electrolyte.
A high brilliance of the deposited gold requires a smooth surface with fine, defined crystalline structure. For this purpose, it is necessary to promote the formation of nuclei during the growth of the gold, while at the same time suppressing the growth of crystals.
This requirement is met, depending on the electrolyte, by the addition of elements such as arsenic, thallium, selenium and lead as well as ethylenediamine, which control the growth of the crystallites by means of a locally selective passivation or a chemical buffering directly at the location of gold deposition.
Nickel Plating with Nickel Sulphate
The main metal supplier is nickel sulphate as hexahydrate with the formula NiSO4·(H2O)6, or as heptahydrate (NiSO4·(H2O)7). Nickel chloride as hexahydrate = NiCl2·(H2O)6 serves to improve the anode solubility as well as conducting salt to increase in the electrical conductivity of the electrolyte. Boric acid (H3BO3) serves as a chemical buffer to maintain the pH value.
The nickel sulphate dissociates in aqueous solution into Ni2+ and (SO4)2- ions. The Ni2+ ions are reduced to nickel on the cathode, which is deposited there as a metallic coating. The sulphate ions migrate to the copper anode and form new copper sulphate there, which is dissolved in solution, by consuming the anode.
Deposition of Nickel with Chloride Electrolytes
Pure (i.e. nickel sulphate-free) chloride electrolytes consist of NiCl2·(H2O)6 as a metal supplier and conducting salt in one, and boric acid as a chemical buffer.
Compared to nickel sulphate electrolytes, nickel chloride baths allow for a deposition with lower electrical power because of their higher electrical conductivity. However, nickelchloride baths are more expensive and more corrosive than nickel sulphate baths.
Nickel Deposition with Nickelsulphamate
The main metal supplier of this electrolyte is nickelsulphamate 4-hydrate with the formula Ni(SO3NH2)2·(H2O)4, nickel chloride = NiCl2 to improve anode solubility and boric acid (H3BO3) as a chemical buffer for maintaining the pH value.
The nickelsulphamate dissociates in aqueous solution into Ni2+ and (SO3NH2)2- ions. The Ni2+ ions are reduced to nickel on the cathode, which is deposited there as a metallic coating. The sulphate ions migrate to the nickel anode and form new nickelsulphamate there by consuming the anode.
Nickelsulphamate has a very high solubility in water, so that very metal rich baths with high current densities and deposition rates can be prepared, which nevertheless achieve nickel layers with good mechanical properties. The use of a nickelsulphamate-based electrolyte is particularly recommended when thick and stress-free layers are required at the same time. The deposited nickel layer is very ductile and provides good protection against wear and corrosion.
For this reasons, our nickel bath NB SEMIPLATE AU 100 is based on a nickelsulphamate-based electrolyte.
Prerequisites for Shiny Nickel Filmes
Which surface properties lead to a bright (nickel) surface is not yet fully understood for nickel, even if a very smooth, fine-crystalline structure plays an important role.
A fine crystalline surface requires, on the one hand, a high nucleation density, on the other hand, that the growth of these nuclei to larger crystallites is suppressed.
Brightening Agent (primary Brightners)
Additives such as sulphonamides, sulphonimides and sulphonic acids cause a grain refinement of the growing nickel layer, which has a generally high ductility.
Brighteners and Levellers (secondary Brightners)
Brighteners and levellers as additives enable shiny layers, although less ductile.
Deposition of Tin with Tin(II)-sulphate
Here the electrolyte solution consists of a sulphuric acid tin(ll)-sulphate. The tin sulphate dissociates in aqueous solution into Sn2+ and (SO4)2- ions. The Sn2+ ions are reduced to tin on the cathode, which is deposited there as a metallic coating. The sulphate ions migrate to the tin anode and form new tin sulphate there, which is dissolved in solution, by consuming the anode.
Deposition of Tin with Tin(II)-methane Sulphate
Here the electrolyte consists of methane sulphonic acid (CH3SO3H) and its salt, tin(ll)-methane sulphonate. This salt dissociates in aqueous solution to Sn2 + and (CH3SO3)2- ions. The Sn2+ ions are reduced to tin on the cathode, which is deposited there as a metallic coating. The methane sulphate ions migrate to the tin anode and form new tin(ll)-methane sulphate there, which is dissolved in solution, by consuming the anode. Our tin electrolyte NB SEMIPLATE SN 100 is based on tin(ll)-methane sulphonate and methane sulfonic acid.
In electronics, electro-chemical copper-plating is used, among other things, for the construction of printed circuit boards as well as through-connections.
Alkaline Cyanidic Depositions of Copper
In this case, the metal carrier is copper(I)cyanide (CuCN), which is not soluble in water, but in aqueous solutions of NaCN or KCN, with soluble cyanide complexes being formed via
CuCN + 2 NaCN → Na2[Cu(CN)3].
The deposited copper layers show a very good adhesion strength.
Sulphuric (acidic) Deposition of Copper
As an alternative to the highly toxic copper(I)cyanide, the electrolyte for sulphuric based deposition consists of copper sulphate (CuSO4) dissolved in diluted sulphuric acid. The copper sulphate dissociates in Cu2+ and (SO4)2- ions in aqueous solution. The Cu2+ ions are reduced on the cathode to copper, which is deposited there as a metallic coating. The sulphate ions migrate to the copper anode and form new copper sulphate there, which is dissolved in solution, by consuming the anode.
The sulphuric acid not only serves to improve the conductivity of the electrolyte, but is the prerequisite for a coherent, uniform layer deposition.
Our nickel bath NB SEMIPLATE CU 100 is made of copper sulphate dissolved in diluted sulphuric acid.
Electro-plating Deposition for Silver
In (micro)electronics, silver layers are used because of their good electrical properties: Among all metals, silver has the highest electrical conductivity.
Cyanidic Depositions of Silver
Since silver cyanide (AgCN) is almost insoluble in water, potassium cyanide (KCN) is added to the electrolyte, increasing the concentration of free cyanide. Depending on the concentration of free cyanide, the equilibrium concentrations of the soluble cyanide complexes dicyanoarate = [Ag(CN)2]-, tricyanoarate = [Ag(CN)3]2- and tetracyanoarate = [Ag(CN)4]3- adjust.
Cyanidic-free Depositions of Silver
As an alternative to the highly toxic silver cyanide, a whole series of less or non-toxic complexing agents, for example iodide, sulphite, ethylenediamine or thiourea.