Medical equipment manufacturers are placing greater emphasis on higher resolution imaging, viewing, and displays in diagnostic equipment. As a result, EMI and RFI requirements are critical considerations when designing X-ray machines and ultrasound equipment. In addition to resolution requirements, diagnostic equipment is becoming increasingly portable, resulting in demands for smaller, lighter components that are still highly reliable. Because connectors are used extensively in medical diagnostic equipment, there are a number of design considerations manufacturers must implement to conform to these requirements. Materials and filtering of the connector play a key role in shielding to meet EMI/RFI requirements, while pin counts, pin spacing, and contact system design affect the size and life expectancy of the connector.

Materials

Fig. 1 - The unique design of the lightweight Chip-on-Flex (CoF) filter connector provides a significant performance improvement in thermal shock and vibration.
RF connectors are significant components in medical equipment applications, because their emissions can negatively affect self-care and patient monitoring systems, as well as diagnostic equipment, by distorting images and adding signal noise. For example, pacemakers must be resistant to EMI/RFI, since noise will significantly affect the operation of these critical devices. Additionally, patient monitoring devices must be resistant to EMI/RFI so that the data can be collected and transmitted without corruption or interference that can render it unusable.

Previously, a tesla value of 0.3 was acceptable for most medical RF connectors. The tesla is an International Standard Unit (SI), derived from the magnetic field, or more specifically, the magnetic flux density of a magnetic material. In today’s diagnostic equipment, however, the magnets used in MRI machines and other imaging and equipment have doubled in size, causing most coil manufacturers to request that products meet a 3.0 tesla value. To achieve the desired 0.1 tesla value and eliminate image distortion, designers have had to eliminate stainless steel and nickel plating from their connectors. Medical applications also require connectors to be manufactured with non-magnetic materials, such as brass, in lieu of steel, iron, or nickel. As such, connector manufacturers, such as ITT Interconnect Solutions, offer families of non-magnetic RF connectors that utilize a raw brass material using a special Boillat alloy that contains less than 0.01% iron and 0.05% nickel, thereby eliminating magnetic interference issues.

In addition, materials used in MRI and other imaging/diagnostic equipment must be able to withstand intense cleaning with potentially corrosive solvents, including ultrasonic alcohol solutions and bleach. If an improper material is used, the connector can oxidize and corrode. In addition to contact plating, base metals are also critical to maintaining connector reliability. ITT utilizes contacts made of beryllium copper with gold or nickel plating and their connectors with metal housings use either aluminum or zinc die-cast base materials. For molded parts, selection of the proper high-performance thermoplastic dielectric materials, such as PEEK (polyether ether ketone) and PPS (polyphenylene sulfide), is also essential to ensure durability and a long operating life.

Filtering

Filtered connectors play a critical role in managing and suppressing EMI, RFI, and surge spikes to ensure high performance of critical medical equipment. Some connector designs provide standard filtering capabilities, including individual isolated pin filtering of high-frequency noise, built-in ground plane barriers in the connector inserts, and filtering at the face of system boxes. A more effective connector signal barrier enhances these traditional filter attributes by offering the system designer complete flexibility in defining or changing individual circuit capacitance, ground, and electromagnetic pulse (EMP) performance during the design and development phase. Many connector designs also incorporate shield cans placed on the PC board to protect the circuitry from signal interference.

Connectors designed with proper shielding and EMI gaskets can typically satisfy virtually any EMI requirement. Electromagnetic interference can also be reduced through adjusting the capacitance value of the connector. Connectors with capacitance values ranging to 50,000 pF significantly reduce signal noise traveling through the device, thus directly enhancing the performance of the end system.

Filtering effects can be further enhanced through a spring probe pin/pad contact system. The contact system is comprised of the spring probe pin in the plug connector configuration and can be implemented across multiple sizes. Some manufacturers utilize an internal clip mechanism to ensure uninterrupted contact with the contact itself. This spring probe design reduces electrical resistance while addressing misalignment issues, making the contact system much more forgiving. Along with the high durability of mating cycles possible with this design, the contact system offers more reliable performance in harsh or critical environments.

Complex EMI/RFI electronic issues have driven connector manufacturers to develop higher-performance and more cost-effective EMI suppression filter solutions, including advanced technology such as a spring probe contact systems with Chip-on-Flex (CoF) filter solutions. CoF is a unique filtering technology utilizing a flex circuit that incorporates individual chip capacitors surface-mounted to a pad on the feed-thru contact. This new system approach replaces the traditional planar array block capacitor while providing a highly reliable filter solution. In addition, the superior thermal shock and vibration performance of the CoF solution identifies this technology combination as a cost-effective filter solution.

Pin Counts/Pin Spacing

Fig. 2 - A miniature zero insertion force high-reliability QLC connector that features a high pin count, a quick-disconnect latch, and EMI and RFI shielding for portable medical electronics applications, such as ultrasound machines and other portable imaging equipment.
In response to advancements in portable diagnostic equipment that continue to require increased functionality and higher performance in smaller and lighter packages, manufacturers have developed smaller connectors while maintaining or increasing the pin count. Engineers have been tasked with reducing a connector’s pin spacing to less than 1 mm, thereby considerably reducing the size of a connector. A tight pitch between contact spacing allows for high-density electrical signals in confined spaces.

Higher pin counts enable engineers to choose from a variety of grounding schemes to maintain signal integrity, while mixing power, signal, coax, or pneumatic contacts within a single insert arrangement to significantly increase versatility for a variety of portable medical equipment needs. These portable systems include ultrasound machines, MRI systems, CT scans, robotic surgery, and patient monitoring systems.

Contact System Design

Fig. 3 - The SMA Precision connector features the MIL-C-39012 Series SMA interface and envelope configuration, designed for use with a variety of subminiature coaxial cables. Base materials include stainless steel, brass, or low magnetic alloys.
Contact technology plays an integral role in the design of connectors for medical devices. Manufacturers must guarantee that the interconnect link for their systems can withstand severe shock, vibration, and thermal dynamics, thus maintaining signal integrity. Connectors must withstand these conditions for an extended lifespan.

Because highly reliable contact systems are paramount in medical equipment, some connector manufacturers design "hyperboloid" contact systems, which include multiple points of contact, positive wiping action, and a "closed entry" to guide the mating pin for thousands of cycles without degrading the signal integrity. Hyperboloid contact systems feature extremely low insertion force and are typically immune to shock and vibration, which is a significant design factor in portable medical equipment.

Circular "bayonet style" connectors, such as ITT's VBN and CIR Series connectors, are widely used for their positive locking mechanisms, quick disconnect coupling, and easy installation. Such designs may also feature a silicone grommet on the back of the connector, sealing the wires against humidity and water penetration, while an anti-vibration cable clamp keeps the wires in place.

Designers have also developed a type of quick-disconnect breakaway connector with a simple push/pull mating mechanism that reduces the time it takes to hook up medical gear. Rated at more than 5,000 cycles, this connector technology utilizes a unique spring probe pin/pad contact system, implementable across multiple sizes, to accommodate misalignment issues. Connector designs that employ a spring probe system allow the connector receptacle to house individual touch pad areas, providing a highly effective point of contact for electrical engagement. Individual touch pad contacts eliminate crevices where contaminants can accumulate (which can significantly reduce the possibility of dirt and dust particles jeopardizing the reliability of the portable medical devices in harsh field conditions). In addition, breakaway connectors can provide complete cable termination solutions (discrete wires, flex circuits, and composite cables) with a high-performance, EMI-shielding/ grounding electronics cable assembly. Such designs allow for shell-to-shell grounding at less than 10 milliohms, as well as EMI performance of greater than 85 dB at frequencies from 40 MHz to 10 GHz. Performance is further enhanced by termination processes that allow for 360-degree shield/connector coverage.

Other contact options include "pad/pogo" designs for ease of field maintenance and "twist pin" designs for reliable small form factor applications. Connector manufacturers are adapting these contact systems for other arenas — including military applications, radio and power supply cables, test equipment, and hospital bedside monitoring equipment — in order to provide reliable connections after a high number of mating cycles (constant connection and disconnection).

Conclusion

Achieving maximum performance and reliability in medical electronics equipment is largely dependent upon the individual components within the device. Connectors used in diagnostic equipment are required to provide reliable connectivity and signal integrity, with EMI and RFI suppression to ensure high performance and proper functionality of the critical medical equipment. The materials, filtering system, pin count, and contact system used to develop a connector play a critical role in providing the above requirements. Further, as more medical equipment is designed for portable operation, connector designs must be smaller and lighter with increased functionality, while maintaining the high performance, durability, and reliability found in the original stationary equipment.

This article was written by Keith Teichmann, Worldwide Director of Marketing & Product Management at ITT Interconnect Solutions, White Plains, NY. For more information, Click Here 


Medical Design Briefs Magazine

This article first appeared in the September, 2010 issue of Medical Design Briefs Magazine.

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