ELECTRODEPOSITION OF CERTAIN METALS
Electroplating of gold
Areas of application
Gold coatings are widely used in electronics and electrical engineering due to their good electrical conductivity, very high corrosion resistance, low contact resistance and good solderability. Typical coating thicknesses range from a few 100 nm (e.g. as a soldering aid) to a few µm as corrosion protection.
Alkaline cyanide deposition of gold
Here the electrolyte is based on the highly toxic potassium dicyanoaurate(I)=K[Au(CN)2]. This compound contains approx. 68 % gold and dissociates in aqueous solution into K+ and [Au(CN)2] ions. The latter migrate to the anode and dissociate there to form Au+ and (CN) ions. The gold ions migrate back to the cathode, where they are neutralised and deposited on the cathode. Either soluble gold or gold-copper electrodes or insoluble platinised titanium electrodes are used as the anode.
Neutral cyanide deposition of gold
This electrolyte is also based on potassium dicyanoaurate, but contains no free cyanide (no free (CN) ions). Insoluble platinised titanium electrodes are used as the anode.
Acid cyanide deposition of gold
Here too, potassium dicyanoaurate is the source of gold in the electroyte, which also contains cobalt or nickel, as well as citric acid. This produces shiny gold layers that are comparatively hard due to their relatively high proportion of organic components and low ductility. Insoluble platinised titanium electrodes or stainless steel are used as anodes.
Strongly acidic cyanide deposition of gold
Trivalent potassium tetracyanoaurate(III)=K[Au(CN)4], which is also stable in strongly acidic solutions, forms the metal carrier of the electrolyte. Mineral acids such as sulphuric acid or phosphoric acid are also included.
Cyanide-free deposition of gold with gold sulphites
Instead of the highly toxic cyano compounds, the electrolyte is based on ammonium disulphitoaurate(l)=(NH4)3[Au(SO3)2] or sodium disulphitoaurate(l)=(Na)3[Au(SO3)2] (alkali gold sulphite). The [Au(SO3)2]3- ions in the solution decompose near the cathode into Au+ and (SO3)2- ions, the gold ions are reduced to gold at the cathode and deposited. In addition to dispensing with the highly toxic cyanide baths, gold layers deposited from sulphitic electrolytes have the advantages of excellent macroscatterability (= high deposition rate even at current-dependent areas of the electrode) and high ductility. For these reasons, our gold electrolyte NB SEMIPLATE AU 100 is based on a sulphitic electrolyte.
Luster formation
A high lustre of the deposited gold requires a smooth surface with a fine, defined crystalline structure. To achieve this, it is necessary to promote nucleation during gold growth while at the same time suppressing nucleation. Depending on the electrolyte, this requirement is met by adding elements such as arsenic, thallium, selenium and lead as well as ethylenediamine, which control the growth of the crystallites via locally selective passivation or chemical buffering of locally different pH values directly at the site of gold deposition .
Electroplating of nickel
Deposition of nickel with nickel sulphate
The main metal supplier here is nickel sulphate as hexahydrate with the formula NiSO4-(H2O)6, or as heptahydrate (NiSO4-(H2O)7). Nickel chloride as hexahydrate=NiCl2-(H2O)6 is used to improve anode solubility and as a conductive salt of to increase the electrical conductivity of the electrolyte. Boric acid=H3BO3 serves as a chemical buffer to maintain the pH value. The nickel sulphate dissociates into Ni2+ and (SO4)2- ions in aqueous solution. The Ni2+ ions are reduced at the cathode to nickel, which is deposited there as a metallic coating. The sulphate ions migrate to the copper anode and form new copper sulphate there, consuming the anode, which goes into solution.
Deposition of nickel with chloride electrolytes
Pure (i.e. nickel sulphate-free) chloride electrolytes consist of NiCl2-(H2O)6 as metal supplier and conducting salt in one, and boric acid as chemical buffer.
Compared
with nickel sulphate electrolytes, nickel chloride baths allow deposition with lower
electrical power due to their
higher electrical conductivity. However, nickel chloride baths are more expensive and
more corrosive than nickel sulphate baths.
Deposition of nickel with nickel sulphamate
The main metal supplier of this electrolyte is nickel sulphamate 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 to maintain the pH value.
The nickel sulphamate dissociates into Ni2+ and (SO3NH2) ions in aqueous solution. The Ni2+ ions
are reduced at the cathode to nickel, which is deposited there as a
metallic coating. The sulphamate ions migrate to the
copper anode and form new
copper sulphate there, consuming the anode. Nickel sulphamate has a very high water solubility,
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
nickel sulphamate-based electrolyte is particularly recommended,
if thick and stress-free layers are required at the same time.
The deposited nickel layer is very ductile and offers
good protection against wear and corrosion.
For these reasons, our nickel bath NB SEMIPLATE Ni 100 is based on an electrolyte based on nickel sulphamate.
Requirements for bright nickel coatings
The surface properties that lead to a shiny (nickel) surface are not yet fully understood for nickel either , even though the smoothest possible, fine-crystalline structure plays a major role. A fine crystalline surface requires a high nucleation density on the one hand, on the other hand, the growth of these nuclei into larger crystallites is suppressed.
Brighteners (primary brighteners)
Additives such as sulphonamides, sulphonimides and sulphonic acids cause a grain refinement of the growing nickel layer, which tends to have a high ductility.
Brighteners and levellers (secondary brighteners)
These additives have a levelling effect to produce high-gloss coatings, albeit with lower ductility.
Electroplating of tin
Deposition of tin with tin(II) sulphate
The electrolyte consists of a sulphuric acid tin(II) sulphate solution. The tin sulphate dissociates in aqueous solution into Sn2+ and (SO4)2- ions. The Sn2+ ions are reduced at the cathode to tin, which is deposited there as a metallic coating. The sulphate ions migrate to the tin anode and form new tin sulphate there, consuming the anode, which goes into solution.
Deposition of tin with tin(II) methanesulphonate
The electrolyte consists of methanesulfonic acid (CH3SO3H) and its salt, tin(II) methanesulfonate. This salt dissociates in aqueous solution into Sn2+ and (CH3SO3)- ions. The Sn2+ ions
are reduced at the cathode to tin, which is deposited there as a
metallic coating. The methanesulphonate ions migrate to the
tin anode and form new
tin(II) methanesulphonate, which goes into solution, consuming the anode.
Our tin electrolyte NB SEMIPLATE SN 100 is based on tin(II) methanesulphonate and methanesulphonic acid.
Electroplating of copper
Areas of application
In electronics, electrochemical copper plating is used, for example, for the construction of conductor paths in printed circuits and for through-hole plating.
Alkaline cyanide deposition of copper
The metal carrier here is copper (I) cyanide (CuCN), which is not soluble in water but is soluble in aqueous solutions of NaCN or KCN, whereby soluble cyanide complexes are formed via CuCN + 2 NaCN → Na2[Cu(CN)3]. Copper layers deposited from these show very good adhesion.
Sulphuric acid deposition of copper
As an alternative to highly toxic copper(I) cyanide, the
electrolyte for sulphuric acid deposition consists of copper sulphate (CuSO4) dissolved in diluted
sulphuric acid. The copper sulphate dissociates in aqueous solution into Cu2+ and (SO4)2- ions. The Cu2+ ions
are reduced to copper at the cathode, where they are deposited as a
metallic coating. The sulphate ions migrate to the
copper anode and form new
copper sulphate, which goes into solution, consuming the anode. The sulphuric acid not only
serves to improve the conductivity of the electrolyte, but is also the
prerequisite for a coherent, uniform
layer deposition.
Our copper bath NB SEMIPLATE CU 100 is based on copper sulphate dissolved in diluted sulphuric acid.
Galvanic deposition of silver
Areas of application
Silver layers are used in (micro)electronics due to their good electrical properties: silver has the highest electrical conductivity of all metals .
Cyanide deposition of silver
As silver cyanide (AgCN) is almost insoluble in water, potassium cyanide (KCN) is added to the electrolyte, which increases the concentration of free cyanide. Depending on the concentration of free cyanide , equilibrium concentrations of the soluble cyanide complexes dicyanoargentate=[Ag(CN)2]-, tricyanoargentate=[Ag(CN)3]2-, and tetracyanoargentate=[Ag(CN)4]3- are achieved.
Cyanide-free deposition of silver
As an alternative to the highly toxic silver cyanide, a whole range of less toxic or non-toxic complexing agents are used, such as iodide, sulphite, ethylenediamine or thiourea.