Topic

Corrosion

From general experience of life, we probably have some idea about what corrosion is and have experienced the higher levels of corrosion that occur in the presence of moisture and the hostile gas species, such as chlorine, ammonia, sulphur dioxide, hydrogen sulphide and oxides of nitrogen, that are often present in the atmosphere. Whether our experience is of tarnished silver cutlery or rotting exhaust boxes, what we have seen is corrosion, defined as ‘the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment’.

The phenomenon of corrosion involves reactions which lead to the creation of ionic species, by either loss or gain of electrons. Take the case of the rusting of iron, where metallic iron is converted into various oxides or hydroxides when exposed to moist air. The equations for this reaction are:

Because the iron loses electrons, chemists refer to this as an oxidation process. In this case, the resulting product is a mix of hydroxides; where copper reacts with acid residues creating chlorides and sulphides, this would equally (and somewhat confusingly) also be referred to as an oxidation process.

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Factors affecting corrosion

Corrosion of bare conductors will happen at a rate that varies substantially, depending on the conditions. Table 1 indicates some of the factors that affect such corrosion. Note that we are considering unprotected materials, and that the addition of an effective surface coating can substantially affect the outcome for the better.

Table 1: Factors that affect corrosion (after Viswanadham, 1998)
  Factor
Conductors Nature of the material or alloy
Surface condition/roughness
Conductor configuration
Conductor-conductor spacing
Substrate Composition
Moisture absorptivity
Structure
Nature of any reinforcement
Environment Temperature
Humidity
Corrosive elements (type; concentration)

 

Another factor that affects the rate of progress of corrosion is the nature of the ‘corrosion product’. If the material produced by corrosion is insoluble and forms an impervious and tenacious layer, the corrosion reactions becomes self-limiting, as the corrosive medium can no longer diffuse through the corrosion product. A useful example of this is the oxidation of aluminium, which forms a thin protective layer of aluminium oxide; as ‘anodised’ aluminium, this is a good example of self-passivation.

If, on the other hand, the corrosion product is soluble or porous, corrosion will continue until the material is depleted, and no further reaction can occur. This is seen with the rusting of iron, where the oxide/hydroxide ‘rust’ has a different crystal structure from the iron, and creates only a porous, poorly adherent layer which does not protect against continued attack.

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Galvanic corrosion

The corrosion situation is more complex if more than one metal is present within the system. Which metal will undergo oxidation is determined by the ‘standard electrode potential’, which is measured against an arbitrary zero created by a ‘standard hydrogen electrode’. Table 2 gives standard electrode potentials for many of the metals used in electronic packaging – the more positive the electrode potential, the more ‘noble’ the metal is, and hence the less prone to oxidise and hence corrode.

When dissimilar materials are connected together, the less noble metal will corrode relative to the more noble one. Thus, given a corrosive environment, one would expect the tin-lead of the joints to be corroded before the copper of the tracks, and this is indeed usually the case.

Table 2: Standard oxidation electrode potentials for common metals used in electronic packaging (after Viswanadham, 1998)
Metal Metal ion
in equilibrium
Electrode potential
at 25°C (V)
Gold
Au3+
1.498
Palladium
Pd2+
0.987
Silver
Ag+
0.799
Copper
Cu2+
0.337
Cu+
0.521
Lead
Pb2+
–0.126
Tin
Sn2+
–0.136
Nickel
Ni2+
–0.250
Iron
Fe2+
–0.440
Fe3+
0.331
Chromium,
Cr3+
–0.744
Aluminium
Al3+
–1.662

Conditions for corrosion

Corrosion can occur with no external applied potential, and is merely dependent on the thermodynamic free energy of the reaction – if that free energy is negative (that is, heat is given out) then the reaction will take place spontaneously.

The rate of reaction is governed by a complex set of factors, but will always increase with temperature and depend on the concentration of the reacting species. For most metals of interest in packaging, corrosion rates below 30% relative humidity are too slow to be significant.

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Corrosion and semiconductors

More than just moisture is needed for corrosion to take place in a semiconductor package. Moisture needs to be combined with ionic contamination. The problem of corrosion in an integrated circuit can thus be split into two parts:

External sources of ionic contamination can be the result of soldering, cleaning or contact with human skin. Within the integrated circuit, contamination can potentially come from both the die and the moulding. However, in modern plastics the ionic content is very low, so corrosion is mainly the result of reactions between moisture and:

The integrity of the die passivation is a major factor in building in resistance to corrosion, and experience with plastic packages has shown a relationship between the presence of cracks in these layers, and resulting corrosion. These micro-cracks may be caused by improper handling before moulding, or be present initially. Sources of micro-cracking during wafer fab include brittleness caused by incorrect deposition conditions and topographic reasons, such as poor step coverage.

Key requirements for reliability are therefore:

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