Many boards will have some form of external connection, where an area of the track is used as one half of the connector. Traditionally this is carried out by plating a ‘gold finger’, with a significant thickness of relatively hard gold. For lower cost applications tin-lead is also a possibility.
For hand-held applications, such as calculators and phones, it is quite common for the key pads to be integrated on the board, using a conductive rubber key pad to bridge an interdigitated structure. As the connections to the top surface are made and broken frequently, the material must be something which does not deteriorate during exposure to the atmosphere, so ENIG is frequently used. Low-cost products, however, may use screen-printed pads of carbon or silver-loaded epoxy.
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