Designing Connectors for Portable Medical Equipment

Engineers designing connectors for portable medical equipment must take several factors into consideration, including size and insertion force, shielding from interference, and preventing electrical shocks. Here’s a closer look at mechanical, electrical, and safety requirements, as well as customer concerns.

Mechanical considerations
Connectors must often be small but have high pin counts. Customers are also demanding more mating cycles (connect-disconnect) along with consistent and reliable connections. In many cased, customers also want zero-insertion-force (ZIF) designs.

To meet demands for smaller connectors with more pins, designers are working to cut pin spacing to less than 1 mm. This can shrink the size of the connector by more than 60% compared to older connectors with the same number of pins. As a result, some connectors boast 260 contacts, each with a 0.8-mm contact pitch, in PC-board mountings that measure just 74 × 19.4 × 18.5 mm. Having more pins lets engineers choose from a variety of grounding schemes to maintain signal integrity, and this makes these connectors well suited for portable ultrasound machines, patient monitors, and test equipment.

Engineers have devised several approaches to extending connector life, including new materials and entirely new designs. For example, modern connectors with rugged nickel-plated aluminum housings can have minimum rated lives of 20,000 mating cycles with no performance loss. Such connectors can also be mated and unmated in less than 2 sec and cost 25% less per mated line than high-density rack-and-panel versions.

Designers have also come up with a quick-disconnect breakaway connector that includes a simple push/pull mating mechanism rated at more than 5,000 cycles. And the coupling mechanism’s canted spring cuts the time it takes to hook up medical gear.

Breakaway features that remove the danger of tearing connectors off equipment or out of walls are another recent innovation. Breakaway connectors are often used in portable medical imaging and diagnostic equipment because they are rugged and reliable enough to withstand field use. Some breakaway connectors feature a spring probe pin and pad contacts for durability and to withstand harsh environments. The probe pin in the plug connector works across multiple sizes. An internal clip ensures individual pins and sockets remain electrically connected and accommodate misalignments. The spring probe lets the connector receptacle house individual touch-pad areas, providing reliable electrical contacts. Further, the spring probe and touch pads make connectors easy to clean in the field. The individual touch pads, for instance, contain no crevices that let contaminants accumulate.

ZIF connectors, besides being easy to engage and disengage, also rate high in terms of mating cycles, durability, and minimizing cross-talk. This lets them serve well in patient monitors and portable imaging equipment like ultrasound devices. ZIF connectors often use landed contacts, which eliminate engagement forces and reduce wear on the contacts to the short time they are pressed together and lightly wiped past each other during cam-and-lock operations. As a result, contacts in the plug and receptacle do not touch each other while connector halves are being engaged. Not only do these connectors have minimum rated lives of 10,000 mating cycles, they can be mated in less than 2 sec.

Electrical considerations
Once engineers determine a connector’s mechanical characteristics, electrical issues come into play, including contact resistance and shielding requirements.

Contact resistance impedes current flowing through the connector. One way to decrease this resistance is by choosing the right material. For example, gold plating over contacts made of high-conductivity copper alloys lowers resistance. If strength is a concern, consider using beryllium copper as the base material. Beryllium copper also has low stress relaxation which boosts the number of mating cycles the connectors will withstand.

Spring-probe-and pin-pad designs mentioned earlier also reduce electrical resistance, thanks to the internal clip that always provides a highly conductive path.

Shielding against EMI and RFI signals, another consideration, is critical for devices such as pacemakers and patient monitors. Signal noise can affect a pacemaker’s operation and corrupt data in monitors. In these applications, it is also vital that connectors use nonmagnetic materials because magnetic emissions degrade image clarity and increase signal noise. As a result, connector manufacturers rely on stainless steel, alloys, and brasses, as they offer non or low-magnetic fields, thus keeping EMI/RFI from interfering with equipment. Shielding effectiveness lets some connector manufacturers offer EMI performance greater than 85 dB at frequencies from 40 MHz to 10 GHz.

Another method of minimizing effects of EMI/RFI is to overmold the connector cable. This is often accomplished by attaching a stainless-steel shield over the shell (the shielding lies between the wires and connector jacket), and then premolding or overmolding the end of the cable to the connector. So when there is EMI/RFI, it is absorbed by the overmolded cable, thus minimizing insertion loss and any electrical variations. The overmold also adds tensile strength to the cable.

Some connectors use springs and shield-locking mechanisms to ensure pressure around the perimeter of the mated connector is uniform and creates an EMI/RFI shield. By eliminating EMI/RFI disruptions, signal noise can’t affect pacemakers, nor can it corrupt data or images traveling from or stored in patient monitors and diagnostic equipment. And shield cans placed on PC boards protect circuits from signal interference.

Filter connectors also play a critical role in managing and controlling EMI and RFI. Some connectors have standard filtering features, including individual isolated-pin filtering for high-frequency noise, built-in ground plane barriers in connector inserts, and shield cans on PC boards to protect circuits from signal interference.

But the filter-design approach is more effective. It lets engineers define and change individual circuit capacitance, ground, and electromagnetic-pulse (EMP) performance during development.

To ensure medical devices work, especially in critical applications, engineers must design interconnects that are reliable and maintain signal fidelity. This can be done by using breakaway connectors, EMI-shielding, and grounding-electronics cables. Such designs allow for shell-to-shell grounding at less than 10 mΩ, as well as EMI performance of greater than 85dB at frequencies from 40 MHz to 10 GHz. Performance is further enhanced by termination processes which allow for 360° shield/connector coverage.

Complex EMI/RFI electronic issues have driven connector manufacturers to develop higher-performance and more-cost-effective EMI-suppression methods, including spring-probe contacts a chip-on-flex (CoF) filter. CoF filters, using a flex circuit with chip capacitors, are surface-mounted to a pad on the feed-thru contact. This replaces traditional planar-array block capacitors and while provides reliable filtering. In addition, the filters perform well despite thermal shocks and vibrations.

Safety considerations
From a safety aspect, portable medical devices need finger protection and touchproof connectors. IP2X, a finger-protection standard, requires that a connector’s live or electrified parts cannot be touched by a human finger. Because portable medical equipment may need to be repaired in the field, touchproof connectors prevent health-care professionals and patients from getting shocked when they touch a connector. Touchproof construction often involves placing a plastic plunger over male pins, letting only the female contacts touch the male pin. Making pins on the active side of the connector touchproof using any means eliminates the risk of shocks.