Interconnect strategies for medical devices

Connectors play a critical role in electrical systems, but the interconnect requirements are often considered at the end of the product design phase. The impact of an interconnect is often appreciated only when poorly manufactured or incorrectly specified connectors fail, eroding system performance and even bringing the system to a halt. For designers, advances in connector design, materials, and manufacturing ensure availability of an interconnect solution matched to each application.

Quality and reliability of the interconnect directly influences system performance and reliability as a whole. The selection of a suitable connector depends on a design’s performance requirements, configuration limitations, operating conditions, and working environment. For example, the connector requirements for a healthcare application are dramatically different than those for a deep mining application, yet each requires maximum reliability from the connector.

Without proper connector consideration, the entire application can be left exposed to performance and reliability breakdowns. Today’s high-precision connectors are carefully designed and manufactured using a variety of high-conductivity alloys; application-specific platings; and high-temperature, high-strength housing materials. A main component of connectors and discrete interconnect systems is the pin receptacle, which is made by press-fitting stamped and formed multi-finger precision contact clips into a machined shell. (See Figure 1) This style of contact is a reliable and consistent way to connect critical components.

Manufacturing and materials

The receptacle uses precision-machined, specialized alloys designed to match application requirements. For precision connector lines, high-speed Swiss turning and CNC machines produce parts across a wide range of sizes while holding diameter tolerances of ±0.0005" or tighter. Internal stamped finger contacts provide connections for mating leads ranging in diameter from 0.008" to 0.102", as well as square and rectangular pins.

Using advanced machining capabilities, interconnect manufacturers such as Mill-Max can offer receptacles with termination styles including press-fit; solder mount; compliant press-fit; swage mount; and wire-termination options including solder cup, crimp, forked, and bifurcated. These manufacturing capabilities also can create specialized receptacles suitable for pressing into plated through-holes in printed circuit boards. Polygon press-fit features such as a square, hexagon, pentagon, or octagon are machined on the body or tail of the receptacle, providing stress relief when pressed into the plated through-hole on a PCB. Press-fit features are typically held to a tolerance of ±0.0005" to help maintain consistency during the press-fit operation, which is especially important if the application calls for solderless press-fit.

While precision manufacturing allows for a wide range of shapes and sizes, the materials used for interconnect manufacturing have performance and manufacturability characteristics that have been optimized for application requirements. A variety of primarily copper-based alloys are used in the manufacturing of machined electrical interconnects from highly ductile brass to high-strength beryllium alloys.

Brass is most commonly used as it exhibits excellent machinability, is suitable for a wide variety of applications, and is cost-effective. Phosphor bronze is a more ductile material, useful when additional strength and bending resilience are required. For higher current applications, tellurium copper’s high conductivity (93% IACS at 68°F) characteristic provides a low-resistance electrical path, producing less temperature rise.

Depending on the desired size and force characteristics, the internal contacts are three-, four-, or six-finger designs, stamped from either beryllium copper alloy C17200 (HT) or beryllium nickel alloy 360. Beryllium copper has emerged as the standard for the majority of applications due to its excellent strength, spring characteristics, durability, and conductivity. Beryllium nickel exhibits similar properties and is particularly suitable for use in temperature environments hotter than 150°C.

Critical healthcare

State-of-the-art connector technology provides options designed to meet specific performance and configuration requirements in diverse applications. More specifically, the combination of a precision machined outer shell and stamped internal finger contact provides the flexibility, quality, and reliability required for mission-critical applications.

In the healthcare industry, high-reliability receptacles are used on detector boards for CT scan equipment; in I/O connectors for portable blood analyzers and implantable devices for data acquisition and transmission; for socketing transducers that monitor and regulate vital signs, blood sugar, and other bodily functions; in the signaling circuits for medicine delivery pumps; as the power jack for medical and dental drills and saws; and in cables used on a variety of medical equipment and devices.

Pins and receptacles are often the building blocks for these interconnect systems. A typical healthcare interconnect application must terminate fine gauge wires to male and female components to make up cable assemblies. Accurately machined pins and receptacles, featuring wire termination options such as solder cups or crimp barrels, meet this need. The receptacle is fitted with a high reliability beryllium copper spring contact able to make a secure electrical and mechanical contact to the mating pin. All components are gold plated for protection, durability, and reliability. (See Figure 2)

This receptacle design of the machined shell fitted with the beryllium copper contact provides a gas-tight connection at the interface of the mating lead and the internal spring contact, which secures the interface against potential oxide-forming environmental conditions and helps reduce the effects of fretting corrosion that can occur in these applications.

As with any mission-critical application, healthcare systems present exacting requirements for interconnect design and performance, putting a premium on versatility and dependability.

Rugged connectors

Equipment designers and their customers expect high-quality interconnect solutions able to maintain robust connections regardless of environmental conditions and usage. Underlying high-reliability interconnect systems, advanced manufacturing methods build in strength and performance. For example, after stamping and forming, connector contacts are heat-treated to yield contacts with excellent stress relaxation properties, defined as the resultant loss in spring force with time at constant strain and elevated temperature.

An interconnect’s stress relaxation characteristic is key for both high cycle life, repeated insertions and extractions, and stationary mating scenarios where a connection faces extended exposure to harsh environmental conditions or shock and vibration. Typically, high quality interconnects are able to withstand 15G-force vibrations and 50G-force shocks with no electrical discontinuity greater than 1µs.

Although beryllium copper is the default material for standard internal spring contacts for receptacles, applications exposed to high temperatures can take advantage of more temperature- and force-resistant materials. Beryllium nickel exhibits little or no stress relaxation when exposed to 225°C for 1,000 hours. (See Figure 3) Along with high-temperature performance, beryllium nickel contacts inherently withstand higher forces.

Interconnect flexibility

In more advanced interconnect systems, beryllium nickel contacts fit into the same receptacle shell as their beryllium copper counterparts and can be specified as a design evolves from the workbench to the field. With more sophisticated interconnect solutions, engineers can test their designs, modify strategies, and even switch to different connector materials and sizes without redesigning the overall interconnect design.

For applications requiring optimal environmental protection or for PCB press-fit applications, gold plated shells and contacts are well-suited because both the outer shell and internal contact are individually plated and the design engineer has the flexibility to choose tin, tin/lead, gold, or silver plating.

Tin or tin/lead-plated shells with gold-plated internal contacts are a cost-effective option for solder mount receptacles accepting gold-plated mating leads. Despite the different combinations of metals, the gas-tight press-fit between the contact and shell eliminates the chance of oxidizing interactions. This gas-tight connection ensures that no corrosion arises at the contact-shell junction due to environmental conditions such as high humidity or exposure to gases.

In addition, interconnect platforms such as Mill-Max’s allow designers to change pin sizes due to the product’s built-in wide mating lead acceptance range. While some interconnect systems have a lead acceptance range of 0.004", enhanced interconnect contacts have a much greater range, generally 0.010", with larger contacts enjoying a range up to 0.020". The wide acceptance range of receptacles translates into greater tolerance on the mating lead size and position. If a piece of equipment is initially designed to receive a cable of a particular size and that cable is revised to use larger or smaller pins, the wider acceptance range available in a versatile receptacle design may support changes in specifications.

A flexible interconnect platform allows engineers to choose higher- or lower-force contacts for most mating lead sizes. In the Mill-Max system, for example, 32 of the 39 contacts have at least one alternative force option. Lower forces are desirable for applications such as high-pin-count interconnects; delicate, soft, or flexible leads or wires; socketing leaded glass-sealed (hermetic) devices; and to ease field replacement and repair in tight spaces.

Conversely, higher forces are desirable for ruggedized applications facing high shock and vibration, fretting corrosion, high-current connections, and long-term static connections. Higher-force connectors can help overcome oxides caused by environmental conditions, which is especially advantageous in circuits with low currents.

Conclusion

Interconnect quality and reliability play a fundamental role in determining overall system quality and reliability. For designers, highly developed interconnect systems provide a broad array of solutions, precision-machined from alloys designed to meet varying needs for ductility, manufacturability, strength, and temperature resistance. Drawing on these precision parts, engineers can create an interconnect solution optimized for the unique requirements of each application.

Mill-Max Manufacturing Corp.

www.mill-max.com