New-Tech Europe | Oct 2016 | Special Edition For Electronica 2016

increase shear resistance of the connector to the PCB (FIGURE 3: Rugged features on insulators which increase the ruggedness of the connector) PLATING: Plating is another critical design factor that determines the success of the interconnect system. The best plating finish is whatever material meets your requirements at the lowest cost. Gold is generally specified for high reliability or low voltage and current applications. Even in very hostile environments, it will remain free of oxides which could cause an increase in contact resistance. Gold is also used for contacts that will see a large number of cycles because excellent electrical connection can be made with minimum contact pressure.

Gold is a significant cost adder to most interconnect systems. Of course gold is susceptible to price fluctuations. The impact of the price of gold on a connector has been alleviated with the adaptation of using less gold in non-critical applications. Originally, Bellcore specifications required 50 µ inches of gold plating over nickel, for applications including leads, cables, and grounding points. In a bid to remain competitive but still employ the advantages of gold, many companies lowered their minimum gold plating thicknesses to 30 µ or 10 µ inches, or even flash gold. Tin is a low-cost alternative to gold plating. It has good conductivity and solderability, but forms an oxide coating which must be penetrated for electrical connection. It is also susceptible to a form of corrosion

known as fretting. Tin is used in connector systems where fewer cycles are expected, or where the contact can be designed to apply high normal forces which will cause sufficient sliding (i.e., wiping action) during lead insertion, to break the tin oxide surface film. The wear caused by this wiping action is one factor that limits tin plated contacts to fewer cycles than gold. It also requires an increased plating thickness, often 150 µ inches. While economics are an important consideration, the requirements of a particular product are more likely to dictate which plating is used. Selective gold/tin plating is a popular plating option because it provides designers with the best of both worlds. The critical contact area has the reliability of gold, and the tail has the lower cost and solderability of tin.

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