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April 2014 • SMT Magazine 21 CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues ment to form a new film, which will be de- stroyed during the subsequent movements. In this way, a large number of oxide debris will be produced, which will affect the electrical prop- erties by increasing the contact resistance. This results in current spikes and damage to the electronic device. Due to fretting corrosion, the switches in an oscillating movement (10 cycles/min) have ex- perienced an increase in the connection resis- tivity from 1 milliohm to 1 ohm within 20 min. It is obvious that the use of gold, which does not produce an oxide layer, as the connection material could solve this problem except for the cost. Another way of solving the problem is to use a good design for switches and contacts to ensure mechanical stability. Corrosion Susceptible Materials and Components The materials used today in electronic de- vices include all classes of materials such as polymers, plastic, and elastomers, ceramics, and metals and alloys. Among them the most important for the corrosion reliability are met- als and alloys. A number of metals and alloys— from the noble gold to metals such as silver, tin, lead, aluminium, copper, nickel, iron, etc.—and combinations of these are used in the form of alloys (e.g., solder alloys or components termi- nals) or multi-layer coatings (e.g., PCB surface finishes such as ENIG). Polymers and plastics used in electronic de- vices are also susceptible to degradation in vari- ous climatic conditions, but to a lesser degree, and not life-threatening like corrosion prob- lems related to metallic parts. However, poly- mers and plastic parts on the device can act as a medium for the adsorption of humidity, gases, and ions, thus accelerating corrosion inside or outside of the assembly. A typical example is the transport of water molecules through the interface between the glass fibers and matrix in glass epoxy laminate and the resulting CAF for- mation between embedded copper conduction lines. Similarly, the surface morphology of the polymer or ceramic parts can also have an im- pact on the water layer formation due to the presence of porosity and roughness acting as fEATurE figure 6: galvanic corrosion of mobile phone key pad due to galvanic coupling: (a) Mobile phone key pad, (b) Schematic of the multi-layer coating, (c) corrosion of ni and cu under layer due to porosity in the gold layer, (d) SeM Be image showing porosity in the gold layer, and (e) a list of standard electrode potential of various metals.