The ever-shrinking connector

Higher electronic densities and diminishing printed-circuit-board (PCB) space have driven connector manufacturers to continually decrease centerline spacing between contacts. In many cases, spacing is now less than a millimeter. Spacing between contact centerlines for some earlier connectors was 0.156 in., or about 4 mm.

Next-generation connectors have 0.8-mm contact centerlines. But getting there brings up engineering and manufacturing challenges such as molding thin-walled parts, selecting plating materials, crimp orientation, user friendliness, and ensuring good electrical contact despite the smaller size.

In the past, board-mounted connectors typically contained pin, or post, contacts. The latest generation, however, has receptacle contacts, not pins, within the PCB-mounted connectors. The smaller dual-beam receptacle contacts are too fragile for handling and manual assembly associated with cable-side connectors. The cable half of the interconnection now contains pin contacts, which are crimped to discrete wire or ribbon cables. Receptacle connectors are preassembled at the manufacturer's plant and later handled with automated-placement equipment.

Contact-tail plating for surface-mount connectors has also changed. Earlier versions of contact mating sections were gold plated over a nickel substrate, and the contact-tail sections that attach to the PCB had tin/lead plating. Tin/lead plating is compatible with the solder processing of PCBs. But plating contact tails isn't feasible from a production point of view. Instead, the surface-mount contact tail section has a thin gold flash layer over nickel to make it compatibile with soldering.

Earlier versions of cable-side connectors attached to discrete wires. As contact centerlines decreased, conductor diameters became smaller and began to include ribbon and flat flexible cables. Discrete wire options now let designers route conductors in several different directions.

The small cavities of the new plastic housings, constrained by tight centerline spacing, influence crimp design. To fit the new geometry, crimps for discrete wires are placed in housings with different orientations. Small cavity dimensions also affect wire insulation. Insulation thickness must now be specified so that individual conductors, plus short lengths of insulation, fit inside the cavity diameter after an applicator crimps the wire to the contacts. In addition, the cavities leave no room for an insulation crimp. Pins are secured to the housing rather than left freestanding, adding reinforcement and preventing bent pins. Also, the pins have a flat, rectangular cross section like a blade, improving rigidity.

Another design issue when minimizing connector size is the plastic connector housings, especially the thin walls (about 0.01 in. or 0.25 mm) between cavities for the dual-beam contacts. Polymer mold-flow analysis of the geometry ensures thin walls fill completely.

Surface-mount connectors are pick-and-place ready with no need for caps that typically must be removed after soldering. The center section between dual receptacle rows on vertical SMT receptacles provides enough surface area for vacuum pickup. As for right-angle SMT receptacles, the pickup surface is integral to the plastic organizer. Other housing features make connectors even more user friendly. For example, a small ridge around the top perimeter of the connector eases disconnecting. The ridge can also be used to mount optional strain relief. A notch on the side of the connector helps users correctly orientate connectors for mating. In addition, two towers flanking contacts on board-mounted connector have slightly different shapes to help avoid mismating.