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18 SMT Magazine • April 2014 found elsewhere [8] . Figure 4 depicts ECM caused by dendrite formation between two copper conduction lines at 2.5 V and 10 V potential difference and on a chip capacitor at 10 V. For- mation of dendrites can cause electric shorts, compromising the functionality of the device intermittently or permanently. Cathodic corrosion Some metals used for electronic systems are soluble in acidic and alkaline environments over a wide range of potentials and pH. For ex- ample, the Pourbaix diagram [7] for Al and Zn shows solubility in acidic and alkaline environ- ments above the equilibrium dissolution po- tential, which itself is a function of pH in the alkaline range. In a microgalvanic cell, oxygen reduction takes place at the cathode, which pro- duces OH - ions. Production of OH - ions at the cathode surface shifts the pH to alkaline values, which causes the metals like aluminium to dis- solve. Therefore, although in principle the met- als should be cathodically protected, due to the change in pH a new dynamic equilibrium is created at the cathode surface at which cer- tain metals could dissolve. Among the metallic materials used for electronic applications, alu- minium metallization lines on IC chips are sus- ceptible to this type of corrosion. A schematic of the anodic and cathode corrosion process is shown in Figure 5. Galvanic corrosion This type of corrosion is attributed to the use of metals/alloys with widely varied electro- chemical properties. On PCBAs, galvanic corro- sion manifest in many ways and are considered CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues fEATurE figure 4: Dendrite formation due to ECM: (a) Copper comb pattern at 2.5 v, (b) Copper comb pattern at 10 V, (c) SeM picture of dendrite between copper lines, and (d) ecM on chip capacitor at 10 V. c