The Ins and Outs of Connectors in Medical Equipment
- Wednesday, 01 September 2010
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.
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.
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.