Mark Finstad

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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Read more: 5 Options for Connector-Less Flex Jumpers

Nick Koop

Crosshatches help meet target impedance values and improve flexibility.

A crosshatch or mesh pattern is often seen in the copper plane of a flex circuit. It will look something like FIGURE 1.





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Read more: Why Do Some Flex Circuits Have Hatch Patterns in the Plane Layers?

Mark Finstad

The shorter the length, the stiffer the flex.

I have an application requiring two rigid PCBs mounted at 90° to each other. I would like to connect them with a flex zone and use rigid-flex construction. How close can rigid areas be to each other; i.e., what is the shortest flex length between rigid areas?

Most manufacturers can form 0.250" flex sections with no issues, and many can get down to less than half that. There are manufacturing and final-use implications of short flex sections between rigid areas on rigid-flex circuitry. The manufacturing issues affect cost, and the final-use issues could cause premature failure if the specified flex length is too small.

Manufacturing Issues

Alignment/misregistration. How a flex area is created in a rigid-flex circuit is important to understand to grasp the challenges. You probably know that copper-clad polyimide substrates are present in both flex and rigid areas of a rigid-flex, not just in the flex areas. The way the manufacturer makes an area “flexible” is by eliminating all rigid materials in those areas. This is done many ways, depending on the manufacturer, but virtually all include a punching operation to form “windows” in the prepreg adhesive and FR-4 layers. These rigid layers then are aligned to the flex layers, so the future flexible areas are prepreg-free. The smaller these areas, the more critical alignment is. For example, consider an application where the distance between rigid areas is 0.1". If the top prepreg layer is skewed to the right by 0.015" and the bottom prepreg is skewed to the left 0.015", the flex zone is now 0.07", not 0.1". This is a 30% reduction in flex area, which could have a significant impact in the final application. However, if the flex zone is an inch wide or more, a 0.03" reduction is inconsequential. Very short distances between rigid areas can have a significant impact on yields and therefore cost.

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Read more: What Is the Shortest Flex Length Between Rigid Areas?

Nick Koop

If the clearance is not at least 10 mils, yields may drop.

One of the often-overlooked aspects of a board design is the moat. Perhaps this conjures up images of Monty Python’s Holy Grail, but moat does not refer to the ring around a castle. Instead, this is the clearance between pads and a surrounding copper plane, sometimes also referred to as embedded clearance.

These clearances often are 0.004" to 0.005" wide. This may seem like plenty of room, but Pareto analysis tells us this can lower overall manufacturing yield. These clearances often lead to unexpected yield loss, depending on certain design and processing factors. Believe it or not, etching these moats or clearances is difficult, due to the closed-ended, circular nature of the clearances. They do not image or etch well and are prone to shorting.

One reason is that driving the energy into the resist can result in bleeding and create an imaging short. But etching is also more difficult, as the etchant flow is trapped in a dead-end donut. These can conspire to create unintended image/etch shorts.

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Read more: How Big is Your Moat?

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