Designers of medical devices and systems need connectors that will help them address increasing complexity and smaller form factors, while at the same time ensuring high levels of reliability and performance under various usage models. Some connectors are inaccessible within the system making reliability critical. Other connectors are regularly used by surgeons, physicians, nurses, or technicians, so ease of use, and a high number of mating cycles are also important.
Depending on the application, connectors for medical devices and systems must comply with standards such as IEC 60601, ISO 80369-1, and ISO 13485, and may require severe environmental testing beyond typical industry standards and specifications.
Along with a usable model and specific standards, designers need to consider technical tradeoffs between non-return-to-zero (NRZ), also called pulse amplitude modulation 2-level (PAM2), and pulse amplitude modulation 4-level (PAM4) connector technologies to arrive at the optimal cost and performance for a specific use case.
Designers have a broad range of connector types to consider when identifying the best solution. To assist in the process, this article begins by briefly reviewing five important factors to keep in mind when specifying connectors for medical devices. It then presents examples of connector options from Samtec and closes with an overview of application considerations when integrating connectors in high-speed systems.
NRZ versus PAM4
NRZ transmits 1 bit of information per signal interval. PAM4 is a multilevel signal modulation format with a throughput of 2 bits per interval. In the NRZ eye, the top represents “1” and the bottom represents “0”, while the PAM4 signal consists of three stacked eyes formed using four voltage levels; 00, 01, 10, and 11 (Figure 1). The height of the eyes is an important consideration. The greater eye height of the NRZ signal results in better signal quality. NRZ is simpler to implement, has lower reflections, a better signal-to-noise ratio (SNR), and is lower in cost compared with PAM4. However, PAM4 is inherently faster and used in high-speed links such as multi-gigabit communications.
Figure 1: NRZ has a single eye (left) and transmits 1 bit of information per signal interval. PAM4 is a multilevel signal modulation format with three eyes (right) and has a throughput of 2 bits per interval. (Image source: Samtec)
Mechanical considerations when selecting connectors include contact pitch, mating type, termination style, and size (Figure 2). Pitch measures the center-to-center spacing of the contacts. It can be more than one number; the pitch between contacts in each row and the pitch between rows can be the same or different. Connectors on printed circuit boards (pc boards) can use horizontal, vertical, or right-angle mating. Retention force is another consideration that measures how easily the connector can be removed.
Figure 2: A small selection of the variety of available contact pitches, terminations, and sizes. (Image source: Samtec)
Common termination styles include through-hole, surface-mount, paste-in-hole, and press fit. Through-hole contacts pass through a hole in the pc board and provide strong connections between the pc board layers. Surface-mount connectors mount on the surface of the pc board and don’t require holes to be drilled. They can have smaller pitch spacings compared with through-hole connectors. Though-hole terminations are being supplanted by surface mount terminations in a growing number of applications.
Paste-in-hole connectors are mounted in holes that do not completely penetrate the pc board. To be used for surface-mount or paste-in-hole designs, the connector body material must be able to withstand solder reflow temperatures, and they need to have horizontal and vertical clearance around the leads to accommodate the required quantity of solder paste.
Press fit terminations are solderless and lower in cost but require special tooling for installation. They are pressed into a hole on the pc board and held in place by compressive forces. Less common termination styles include land grid arrays, ball grid arrays, wire wrapping, crimping, and screw terminations.
Ease of use
Contact resistance, mating cycles, and mating/unmating force contribute to connector ease of use in applications where connectors need to be regularly mated and unmated. The lower the contact resistance, the less power that is lost through the connector. A low mating/unmating force can contribute to ease of use, as long as the contact resistance remains low enough to meet electrical requirements. Connectors have limited mating/unmating cycle specifications, ranging from tens of cycles to many thousands of cycles. The cycle life of the connector must be matched to the needs of the application.
When connector contacts are mated, the contact is displaced, and the metal is flexed. The flexing is important and determines the force needed to mate and unmate the connector, and the contact resistance. Flexing also causes stresses in the contacts that results in both the mating/unmating force decreasing and the contact resistance increasing over time. Replacing the brass base metal commonly used in connector contacts with more expensive phosphor bronze will increase the cycle life. Phosphor bronze is more elastic than brass and less susceptible to stresses that limit the cycle life of bronze contacts.
IEC 60601, ISO 80369-1, and ISO 13485
There are numerous application-specific industry standards for various medical systems and devices. Three of the more general standards that need to be considered in all designs are:
- ISO 80369-1: This focuses on the design methodology to reduce the risk of misconnections between medical devices, or between accessories for different applications.
- IEC 60601 focuses on the general requirements for basic safety and essential performance including electromagnetic interference (EMI) and electromagnetic compatibility (EMC).
- ISO 13485 focuses on the quality systems needed for tracking the components and processes used in the manufacturing process. It is related to ISO-9001.
Testing beyond industry standards
Severe Environment Testing (SET) is a suite of tests developed by Samtec that extend beyond typical industry standards and specifications and includes:
- 250 mating cycles with 100% humidity
- Intense shock and vibration based on low-level contact resistance (LLCR) and event detection
- LLCR testing using 40 times the standard gravitational force (g) peak, 11 milliseconds (ms), half sine and 12 g RMS, 5 – 2000 Hertz (Hz), 1 hour/axis
- Event detection according to EIA-364-87, EIA-364-27 and EIA-364-28 using the same test procedure as the LLCR testing
- 500 temperature cycles
- Non-operating-class temperature testing where the connector is LLCR tested, exposed to -55 to 105°C for 100 cycles, then tested for LLCR again; exposed to -65 to 125°C for 100 cycles, and tested for LLCR again; the connector must maintain a change of ≤5 milliohms (mΩ) in LLCR to pass
- Dielectric withstanding voltage at an altitude of 70,000 feet
- Electrostatic discharge (ESD) testing is not usually performed on connectors but is included in SET
Connectors that handle 10,000 mating cycles
Designers that need up to 10,000 mating cycles can turn to Samtec’s TFM and SFM series, members of the company’s Tiger Eye interconnect system. These connectors are designed for micro, rugged, high-reliability, high-cycle applications, and are available in three pitches; 0.80, 1.27, and 2.00 millimeters (mm). These connectors have heat-treated, beryllium copper (BeCu) multi-finger contacts optimized for high-cycle applications and are designed for rugged environments (Figure 3). For example, the model TFM-105-01-S-D-A is a 10-position header with 1.27 mm pitch contacts.
Figure 3: Tiger Eye interconnects (left) are available in a variety of formats and sizes and provide a rugged contact system rated to 10,000+ mating cycles. The TFM-105-01-S-D-A (right) is a 10-position header with 1.27 mm pitch contacts. (Image source: Samtec)
The smooth contact mating surface does not stress the plating, providing lower contact resistance, longer plating life and longer cycle life. Solder can easily penetrate the micro slot on the tail providing greater solder joint strength. These connectors are polarized to guarantee proper mating, and optional friction locks improve connection security.
High-density, high-speed connectors
Applications that need high speed and high density can use Samtec’s SEARAY 1.27 mm open-pin field-press-fit arrays. These connectors have up to 500 contacts optimized for signal integrity and are available in vertical or right-angle mounting options (Figure 4). This system features up to 10 rows and 50 contacts per row to enable grounding and routing flexibility; a choice of 7 mm, 8 mm, 8.5 mm, and 9.5 mm stack heights; and can handle signals up to 28 gigabits per second (Gbits/s). For example, part number SEAFP-40-05.0-S-06 is a vertical mount design with 240 contacts and through-hole terminations.
Figure 4: SEARAY 1.27 mm high-density open-pin field-press-fit arrays are available in vertical and right-angle (shown above) options. (Image source: Samtec)
Connectors for PAM4 or NRZ
Applications that need higher contact densities and more than 28 Gbits/s speed can use the 56 Gbit/s SEARAY series. Their 0.8 mm pitch delivers twice the contact density of connectors with 1.27 mm pitches, are available with 7 mm and 10 mm stack heights, and can handle PAM4 or NRZ communications. Configurations are available with up to 12 rows of 60 contacts for a total of 720. These open-pin-field arrays provide maximum grounding and routing flexibility including differential signal pairs, single-ended signal transmission and power delivery (Figure 5). Part number SEAF8-20-05.0-S-04-2-K features 80 gold-plated contacts and surface-mount terminations. These connectors are SET qualified.
Figure 5: SEARAY high-density open-pin-field arrays provide maximum grounding and routing flexibility including differential signal pairs, single-ended signal transmission and power delivery. (Image: Samtec)
High-speed connector application considerations
When using high-speed connectors in medical applications, there are numerous factors that designers need to consider related to signal integrity and EMI, a few of these considerations include:
- Shorter is better. Shorter connectors deliver better signal quality. The shorter the connector, the shorter the time available for reflections and crosstalk to occur.
- The signal-to-ground ratio is important. In most instances, a ratio of 1:1 is optimal, but for connectors with large pin counts, a ratio of less than 1:1 may be needed for reliable high-speed, single-ended operation.
- Ground shielding of contact pairs is recommended for differential connectors carrying signals of 2.5 Gbits/s or faster.
- Misalignment can be a significant problem on pc boards with multiple connectors. Closely follow the manufacturer’s recommended termination connection specifications, and keep alignment pin hole diameter tolerances to ±0.002 inches (0.05 mm).
- EMI is not just a pc board problem. Board-to-board connectors can contribute to EMI concerns and need to be considered from the beginning as part of the overall design.
Selecting connectors for medical systems is an important and complex activity. Connectors need to be optimized to meet the mechanical durability, reliability, and ease of use requirements, in addition to meeting the electrical specifications and supporting communications protocols such as NRZ and PAM4. Adhering to relevant industry standards is important, but testing beyond the industry norms, such as with the Samtec devices mentioned here, is often needed to ensure the high levels of performance expected from connectors in medical devices and systems.
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