Small differences can have big consequences.

For as long as there have been printed circuit boards, the nominal thickness seems to have been set at 0.062″ – or in Latin, 1.5748mm, but call it 1.6mm for short. In practical terms, the standard dielectric materials available support this board thickness while providing anything from two to 20 layers. I imagine four layers is still the most common use case.

Larger boards will need more connections and require more stiffness. To manage connectivity and flatness requirements, standard PCB thickness targets ratchet up to 2.4mm and 3.2mm. On the low side we find 1.0mm and go down to 0.8mm. All these targets are related to using so-called gold fingers as a printed edge connector.

It’s about connectivity and solderability. This was handed down from the backplane and daughtercard configurations found in our tower computer systems. The motherboard has expansion sockets and the memory cards come with fingers to plug and play. As a result of this variety, many connector vendors that market to plated through-hole technology users will offer different pin lengths that fit the range of board thickness options.

Rigid-flex brings the best – and worst – of both worlds.

Combining all aspects of a flex circuit with a rigid board that makes full use of HDI techniques is one of the breakthroughs of our time. The stacking connectors for board-to-board or the typical flex circuits are bypassed. If you've ever tried to connect a flex circuit to a stacking connector, you know that's a bottleneck in the process – blindly positioning the flex connector over the mating connector can be fiddly to the point of destroying the connectors. Now what?

Rigid-flex projects remind me of digital/analog projects: the best of both worlds and the worst of both. Just for starters, if the team is taking this route, you know they are serious about holding things together with all possible integration. Both technologies are well understood on their own, though the rigid camp is larger and better understood.

Flex circuits on their own. Flexible printed circuits (FPCs) require more than a change of materials from their stiffer cousins. Additional tolerance must be designed into the data. Reason: The different types of material stacks used in the manufacturing process. For the most part, a flex will also have a rigid section where the connector is mounted. The stiffened area could also be extended to host the ESD protection, an LED or microphone; we're flexible.