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.

Brazed or welded fingers. This construction also yields unsupported fingers that extend beyond the circuit outline. The fingers can be thick copper (I recommend half hard or full hard copper), but more often they are a more durable material such as nickel. While this construction is more robust and tolerant to handle than sculptured construction, the fingers can still be damaged if mishandled. Cost implications vary from vendor to vendor, depending on the level of automation incorporated when the fingers are attached.

IDC contacts. Insulation displacement contacts use special pins with “claws” that pierce the polyimide insulation and contact the base copper (FIGURE 2). These contacts are reasonably durable and come in a range of configurations. The downsides are cost and limited minimum pitch between contacts, and only vendors with IDC equipment support such needs.

Supported fingers. This construction yields the least expensive and most robust final product. The fingers are only exposed on one side, so no pre-punching of the base dielectric or laser skiving operations is necessary (FIGURE 3). The base polyimide strengthens the fingers, making them less prone to damage during handling. If adhesiveless base laminate is used, the resulting construction can be hot-bar-soldered through the polyimide base to a matching set of fingers on a rigid PCB. Alternately, anisotropic adhesive can be used to make the connection the same way. Either of these connection methods provides a very low-profile and reliable connection. The downside is it is difficult to inspect the hot-bar-soldered or anisotropic adhesive connections because you would have to look through the polyimide to see the solder or adhesive joint. Some designers will specify the polyimide material between the fingers be removed. This isn’t free, but it adds only modest cost to the finished flex. The base polyimide remains on the back side of the fingers to help strengthen them, but the material between the fingers is removed to permit inspection of solder joints or anisotropic adhesive connections. If anisotropic adhesive is used, and the polyimide between the fingers removed, be sure the hot bar used to join the flex and rigid PCB has a non-stick finish! If not, there is a good chance the exposed adhesive between fingers could stick to the hot bar.