Originally Published MEM Fall 2008

MINIATURIZATION

(click to enlarge) A micro-to-nano internal transition cable addresses the need for miniaturization and high reliability.

Medical instrumentation is benefiting from developments in the silicon chip industry, adding functionality, capabilities, and processing power. The changes are largely due to the latest advances in complementary metal-oxide semiconductor (CMOS) chips, which have reduced in size, requiring lower voltages and lower current levels. These integrated circuits (ICs) have enabled manufacturers to include more functions in tiny spaces. Consequently, designers have been able to streamline their products—particularly the front panels—to handle more services and higher levels of data acquisition and signal processing.

Good system design continues to require high-reliability connector cable systems—often with increased wire counts—to be routed to the main instrument panel. Inside the instrument, signals, data processing, and probe cues must be routed to a wide range of small, densely packed function-specific modules. Microconnectors at 0.050-in. (1.3-mm) spacing and nanoconnectors at 0.025-in. (0.635-mm) spacing can significantly reduce the size and weight of support interconnections in the new system technology.

Connectors: To the Box and Inside the Box

One of the key tasks facing product engineers is to design the two different yet integrated interconnection systems to accommodate both large external instrument panel requirements and their miniaturized internal systems. External cables may connect a system probe that provides data acquisition or a tool such as a laser, optical camera chip, or rotating implement to an instrument front panel. Internally, the design must be a compact interconnection system that extends the cable routing from the front panel to a multitude of miniature modules and printed circuit boards (PCBs).

(click to enlarge) Cable and connectors used externally must be rugged enough to handle harsh conditions and multiple mating cycles.

The complete wiring system carries signal, power, and acquired data lines into the front panel. It then distributes them throughout the instrument as required. Essentially, this demands an interconnection system that is rugged and serviceable on the outside and compact on the inside. As instrumentation increases in portability, these twin requirements must be accomplished while ensuring high reliability and versatility.

The micro-to-nano internal transition cable (see the photo) addresses this very issue. Signal power and data acquisition information are carried on cables that terminate to 0.050-in.-pitch microconnectors on the instrument panel. However, to meet the many demands of miniaturization inside the box, the terminating connectors on the internal modules are mainly nanoconnectors on a 0.025-in.-pitch spacing.

Transition cables are helpful for minimizing the number of components, cables, and connectors. The size issue is perhaps the greatest reason: cables that must be rugged to cope with regular handling are too bulky for internal use. Other issues include versatility, manufacturability, and upgradability. Miniature modules can be built or purchased and then plugged into an instrument with a minimum of technical expertise. If a module fails or an improved version becomes available, it can be simply removed and the replacement inserted.

In some instances, different modules are preferred for functional or geographical reasons. A transition cable also enables designers to separate out signals and route them to different modules. For example, the cable to the front panel might bundle video, storage, and data-processing elements, which can be split and rerouted to the correct modules using the transition cable. Because the transition cable can be screened and can include filtering close to the source, it can significantly reduce electromagnetic interference (EMI) and crosstalk.

(click to enlarge) Active-chip technology within the cable itself enables the cable to process information at the source.

To the Box. Connector and cable systems routed from outside services to the instrument front panel face conflicting demands (see the photo). Of course, the cable and connectors used externally must be sufficiently rugged to survive the rigors of everyday use in busy hospitals or surgeries where they will be routinely dropped, run over by gurneys, mated and unmated with little thought, and generally subjected to abuse. Despite such conditions, these cables must still plug smoothly into the mating panel connector without problems. Cable assemblies must also remain comfortable for the practitioner to handle throughout the equipment's life as well as retain their grip.

Such connectors must also mate many thousands of times with no degradation in performance or feel. Also, the cable must be flexible—especially when used in conjunction with a handheld instrument—so that the cable falls away from the clinician's hand without impeding his or her actions. The whole connector cable assembly must also be shielded from interference.

Finally, the cable and connector must be able to handle the massive amounts of data that are now routinely transferred back from the probe or service end of the cable. Such an increase in functionality is now possible with the implementation of active-chip technology (see the photo at left) out at the end of the cable itself.

Originally, probes used in medical instruments were fairly simple, merely collecting data that were then transferred back to the instrument for analysis. Increasingly, medical probes employ active-chip technology whereby ICs mounted inside the probe or sensor process the information at source, then transfer the results back for further analysis and storage. Immediate processing is necessary because the signals are so small that they would not survive the journey along a cable. ­Active-chip technology can also monitor usage, identify the probe that is being used, and act as an antipiracy measure. Examples of active-chip technology in use today include blood oximetry, laser treatment of wrinkles, catheters, dental cameras, and digital stethoscopes.

To ensure that the external cable and connector assembly can meet such requirements, it must be subject to exhaustive testing. Tests can include the following:

• Mechanical (flexibility, S-bend radius, gurney test, pull-off test, etc.).
• Electrical (EMI, crosstalk, impedance matching).
• Chemical (resistance to steam sterilization and cleansing processes).

Some products must also adhere to industry standards such as IEC 60601-1. Oftentimes, the quality of a good cable system is tested at the cable-to-connector interface, because this transition point is critical to maintaining a good jacket seal or EMI protection for the rest of the system.

Connector design support is often needed to ensure that a product meets or exceeds all of the challenges of getting a good signal into the equipment or driving a unique instrument out at the end of a cable. Requirements can include needing simple calculations of current and voltage or determining the number of pins needed. They can also include detailed analysis of the environment that the equipment must function in and the testing that will be required to implement the design. Professional companies also offer solid or 3-D modeling of products to help in the design process.

Experienced companies offer a wide variety of options based on the highest reliability and performance levels. These companies employ engineers trained to analyze the various product demands in order to recommend an appropriate interconnect system for the purpose. Such recommendations address not only external connection requirements, but also appropriate systems for distributing signals inside the box.

(click to enlarge) A micro-to-nano internal transition cable addresses the need for miniaturization and high reliability.

Inside the Box. The environment changes dramatically inside the instrument (see the photo). Temperature conditions may become much hotter (because of little or no airflow), and semiconductors often dissipate a great deal of heat. However, internal connector systems are unlikely to experience much handling or mating and demating by personnel other than trained operators. Similarly, these connectors are unlikely to be subject to the same degrees of shock and vibration.

Yet the unique challenges inside the box can be as difficult to address as those outside: electrical noise is more concentrated and sensitive inside the box, and as the amount of chip processing escalates, issues of signal speed, crosstalk, and EMI coupling can hinder the design process. Additional challenges include routing signals in and around boards and modules in multiple directions and elevations while trying to keep wire lengths as short as possible. (A key point to remember is that the best impedance match will be achieved when the shortest interconnection path is created.)

Additional functions can add weight to the instruments, which is unattractive to the user, especially if the instrument is portable or designed for home use by a patient. With advances in chip technology, however, voltages and current levels are decreasing. Whereas at one time it was common to see ±12 or ±5 V, nowadays equipment is more likely to run at 3 V or lower. Currents, too, have reduced from 10–20 mA to hundreds of microamperes. Coupled with the ever-present drive for miniaturization and increased signal count, these factors pave the way for use of nanominiature 0.025-in.-pitch connectors with smaller, more flexible wiring between systems. These new connector systems pass the highest quality and reliability requirements, such as the military standard MIL-DTL-32139, which exceeds tests demanded by the medical industry. Table I shows a breakdown of the specifications as laid down in this standard and the equivalent micro or 0.050-in.-pitch standard, MIL-DTL-83513. Yet their small size—occupying just one quarter of the volume and weighing 80% less than micro D connectors—means that they leave more room and weight allowance for other electronics inside the box. Nanoconnectors are available in high-density surface-mount formats to fit onto the smallest of circuit boards used in medical instrument modules. Solid-model designs are available online to enable designers to begin the interface process with connector suppliers.

The Designer's Dilemma

Electronic miniaturization pushes designers toward new disciplines that can stretch personal knowledge and experience. Requirements for highly flexible cable systems with simultaneously high EMI performance can go beyond previous design standards. Planning for sterilization, a good cable surface texture, and the ability to survive being run over by a gurney wheel pushes the limits of materials knowledge. Preparing a cable for continuous flexing pushes ingenuity as wire counts exceed current cable standards. New levels of high-performance cable and connector engineering are continuing to be developed to resolve these new design demands.

Conclusion

System designers can now benefit from the combination knowledge of multiple design requirements by tapping the cooperation of companies that have focused on the challenges of the transition from micro to nanointerconnection. Connector companies are addressing the very particular needs of the medical industry to deliver the best-engineered interconnection systems, moving from 0.050 to 0.025-in. dimensions within new, miniaturized, highly dense, portable systems.

The companies can provide online design resources and offer valuable personal expertise. As the evolution of medical instrumentation continues apace with more functions in smaller boxes, increased reliability, and better product performance, the connector industry will continue to develop its products to match the demands of industry.

Robert Stanton is director of technology and market development for Omnetics Connector Corp. (Minneapolis) and Bill Lee is business integration manager for Lemo USA (Rohnert Park, CA).