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April 2014 • SMT Magazine 15 cal cells, which are either electrolytic or galvan- ic in nature (Figure 2) [2] . A typical example of an electrolytic type corrosion cell formation on PCBA surface is closely spaced conduction lines with opposite bias which is connected by a wa- ter layer, while galvanic type cells are formed for example by the coupling between the gold and underlying electroless nickel and the cop- per layer on a ENIG contact. Numerous other examples of both types can also be found on a PCBA. However, a pure water layer on a clean PCBA surface has only limited conductivity to intro- duce any significant leak current or corrosion effects. But the PCBA surface is seldom clean due to the process related residues [3-5] (Figure 1), which dissolves into the water layer. Water lay- ers with dissolved ionic residues are more con- ducting, which cause first-level corrosion effect such as the increased levels of leak current caus- ing functionality problems. The leak current is generated by faradic reactions involving disso- lution (from the positive electrode) and build- ing up of metal ions in the solution layer, which over a long period of time—depending on the level of potential bias, thickness of water layer, and materials involved—will results in electro- chemical migration (ECM). Electrochemical migration results in the for- mation of metal dendrites between two adjacent electrodes connected by a water layer, resulting in an electric short. Electric shorts caused by these dendrites can be intermittent or perma- nent. Other forms of corrosion can also occur, depending on the materials and other param- eters involved. However, ECM is a common fail- ure mode on a PCBA surface if metals such as Sn, Pb, Ag, Cu, and Au are involved. The pres- ence of ionic residues on the PCBA surface also influences the formation of a water layer; all ionic residues are hygroscopic in nature, there- fore absorbing water at relatively low levels of humidity on a clean surface, while thickness of the water layer also changes with the amount of contamination. The following sections will provide a more detailed discussion on various aspects of electronic corrosion. Corrosion Failure Mechanisms Corrosion failure mechanisms of electronics can be divided into two major categories: (i) the electrochemical corrosion in aqueous environ- ment and (ii) the gaseous corrosion caused by the corrosive gases. Other factors such as wear and vibration can superimpose on corrosion, causing varieties of tribo-corrosion such as fret- ting. Gaseous corrosion at room temperature or slightly elevated temperatures can also be assisted by aqueous conditions, and therefore it is difficult to differentiate between the elec- CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues fEATurE figure 2: Two types of electrochemical cell formation possible on a PCbA surface. left: galvanic cell due to coupling between two different metals through water layer. right: Electrolytic cell formation between similar metal/alloy parts due to opposite potential bias.