R21 Electrolytic methods in electronics

Many everyday objects have been subjected to some form of surface treatment, take, for example, automobile parts, kitchen utensils, cans for preserving food, building materials (for window frames or roofing) etc. Similar techniques are used in the production of electronic parts, such as printed circuit boards, electrical contacts and capacitors. For the most part electrolytic treatments are used, which can be illustrated by the following examples.

1. Electrolytic deposition of metals and alloys
The aim of electrodeposition is to modify a metal’s surface to obtain certain surface properties: hardness, wear- and corrosion-resistance, gloss etc. The adhesion between the deposited layer and the substrate must be perfect. The principle of electroplating is simple: electrolysis, i.e. a sustained redox reactions. The object to be treated is the cathode, the anode being a conductive inert material (Pt or Ti alloys) or a pure sample of the material to be deposited. Illustration R21 shows the plating of a printed circuit board with copper, the printed circuit boards being the negatively charged cathode and the copper the positively charged anode.

The electrolyte is the most critical component of the cell. It contains a suitable salt of the metal to be deposited, usually complex salts since these tend to have a higher stability and solubility than common salts such as chlorides and sulphates. In the example in illustration R21, the electrolyte is a mixture of copper(II) sulphate and sulphuric acid. The tricks of the platers’ trade lie in the formulation of the bath, small quantities of various (usually organic) additives being present, which are responsible for the deposited layer acquiring the required properties of :

• Gloss: the additives control the micro-roughness of the deposited layer. The mechanism of this effect is usually unclear.
  
  
• Wetting: hydrogen gas is usually formed during the electrodeposition. Gas bubbles can become trapped under the deposited layer, giving adhesion problems.
  This can be avoided by appropriate additives.
  
  
• Layer structure: certain additives modify the crystalline structure of the deposit, giving improved physical properties (ease of soldering, corrosion-resistance, hardness).

The thickness of the electrodeposited layer is in the range of 1-100 µm.

2. Electroless deposition of surface layers
As with electrodeposition, the aim here is to reduce metal ions to metal, which is then deposited as a continuous layer on the surface of the substrate. In electroless deposition the driving force for the reduction is provided by an additional electro-active component in the bath: a reducing agent. For this process to work well, the surface of the substrate must have a sufficiently catalytic character, that both the reduction of the metal ions and oxidation of the reducing agent can occur rapidly on the (catalytic) surface. To ensure that the surface of the metal substrate is sufficiently catalytically active, it is pretreated with an etching bath, the metal thereby achieving the highest possible surface area and traces of oil etc.being removed at the same time.

Phosphoric acid and formaldehyde are typically used as reducing agents in electroless deposition formulations.
Since the power of the reducing agent can be pH-dependent, the buffering of the bath is also important. Once again, the deposited layer is
1-100 µm thick.

3. Electrochemical conversion of surfaces
The presence of a passive layer on the surface of a metal can increase its corrosion resistance, isolate it electrically or improve its appearance. In this case oxides, phosphates or chromates are typically deposited on the surface.

• Anodising
  This process is used for aluminium, titanium, copper, steel, tantalum and niobium surfaces. During anodising, the surface of the metal is converted into its oxide, the metal being the anode of an electrolytic cell with a solution of sulphuric, phosphoric or oxalic acid, as an electrolyte.
  The half-reactions involved in the care of the anodizing of aluminium are:
  
  
  
  The overall electrolyte reaction is:
  
  
  The current required varies between 1 and
  100 A/m².
  Depending on the duration of the treatment, layerthicknesses between 0.5 µm (capacitors) and 100 µm (building materials) can be obtained.
  
  
• Phosphating
  Phosphating is mainly used to prepare surfaces for further coating with paint or organic coatings. The corrosion protection and adhesive properties of the metal are noticeably improved by this priming layer. The most important applications are the phosphating of steel and aluminium for use in the automobile and building industries.
  The chemical and electrochemical processes at work here have not been fully characterised and the technique is empirical rather than scientific.
  
  
• Chromating
  Chromate solutions, despite their toxicity, are much used for depositing protective and decorative layers especially on aluminium and zinc. The most common applications are in the food industry, e.g. chromate treatment of aluminium cans.
  In recent years the toxicity of chromates has triggered a movement away from chromating to chromophosphating or phosphating.