Adding solderable fingers to connect rigid boards.

When designing a flex jumper between two rigid PCBs, where no room exists for connectors, are solderable pins or tabs that extend out of the edge of the flex circuit a possibility? If so, what are the cost and reliability implications?

Quite a few options are available, each with its pros and cons and cost implications. This month, we look at the possibilities.

Sculptured fingers. This construction yields unsupported copper fingers that extend beyond the circuit outline. The fingers are typically ~0.010" thick. These can be made by starting with very heavy copper (usually half hard) and etching down all areas other than the fingers, or starting with thinner copper and plating up only the finger areas to meet the desired overall finger thickness. The fingers can then be formed to best fit the desired applications (FIGURE 1). The cons to this construction are cost and handling issues. Adding sculptured fingers to a flex circuit will have a modest cost impact, but the biggest downside is these parts are fragile. The fingers can be easily bent out of shape during shipping or handling on the production floor and are difficult to realign once damage occurs.

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