Compression contact connectors provide many performance and reliability advantages for medical device design.
BY MARK PAGE & THOMAS MOWRY
Electrical and electronic connectors are a vital and often challenging aspect of medical device designs. In choosing the optimum interconnect solution, system designers must balance the issues of maintaining exceptional performance and high levels of reliability with their concerns for space, cost, and ease of use.
Many designers keep a catalog (mental or physical) of traditional two-part connectors, and consequently build their product around those familiar parts. But today’s design challenges demand new and innovative approaches, requiring exploration of alternate designs, such as compression contact connector technology.
First developed over 35 years ago for NASA applications, the compression connector concept provides medical device designers great flexibility, small size, and cost effectiveness, while providing unparalleled reliability and durability required in today’s design challenges.
Compression contact connectors can be used in a wide variety of applications. For example, compression contacts may be installed into a socket frame to connect ASICs, microprocessors, or LCD displays for medical imaging devices. Since compression contact connectors have low resistance and highly reliable connections, they are used in a variety of patient monitoring devices.
In addition, due to the versatility of compression contacts in effectively handling relatively high current to low voltage signals, they are ideal for handheld devices, such as glucose or oxygen level monitoring devices. Finally, because the compression contact can be insert-molded into robust and sealed insulators, they are well suited for external applications on portable X-ray, infusion pump, or patient monitoring equipment.
Two termination types
Compression contact connectors are composed of two main termination styles: pure and hybrid compression.
- Pure compression style (Fig. 1) features a series of single contacts held within an insulator, and each end of the contact uses cantilever beams or compression spring contacts. This style requires that both ends be compressed against a PCB or device to effectively provide continuity.
- Hybrid compression style (Fig. 2) involves a hybrid compression connector using a single insulator, but one contact end is typically attached to a PCB by through-hole or surface-mount (SMT) soldering techniques. The other side of the connector uses a compression spring or cantilever beam to make contact to a device or other PCB.
 FIGURE 1. An illustration of a pure compression style. |
null
 FIGURE 2. An illustration of a hybrid compression style. |
In addition to the termination styles, compression contact connectors can be used in right-angle (90°) to parallel (180°) applications, with the contacts pre-formed then inserted into an insulator; or, the compression contacts can be insert-molded, and later post-formed to achieve the form factor desired. (Consult the connector manufacturer for additional functional details and for application-specific technical and design help.)
The solderless advantage
Compression contact connectors offer a multitude of advantages, such as space savings, lower installed cost, high levels of performance, and resilience to temperature-related anomalies.
One of the connectors’ greatest advantages is that they use a solderless assembly process. In addition to cost and time savings, eliminating the solder process minimizes the risks of temperature damage or component distortion, and also eliminates the risk of solder joint failure.
Another temperature-related concern addressed with the use of compression spring contact connectors (CSC connectors) is that thermal mismatch of components can be tolerated. In the example of large ceramic ASIC packages, the integrated circuit may have a different thermal expansion than the PCB, which over time is known to result in cracked and failed solder joints. A compression connector between the ASIC and PCB provides the needed flexibility through years of thermal cycles.
The compression contact is designed to provide reliable normal force with low mass, and can flex to accommodate movement, vibration, and shock; therefore, it’s resilient to those situations. Since normal force is a function of compression and contact material, a wide range of normal force and deflection ranges can be accommodated.
Minimum board-to-board spacing is required, and high contact density is possible. Single-piece compression contact connectors typically require less space than is needed for pin and socket connectors, reducing signal path length and providing low inductance values with very high pin counts. For example, a LGA interposer can fit 2,400 contacts in a 2.0-inch (50-mm) square area. In addition to space savings, CSC connectors let designers reduce system part count. Having fewer BOM items improves reliability as well as reduces the hidden costs of inventory, soldering, and assembly labor.
Finally, compression contacts can be post-inserted or insert-molded. Insert-molded connectors provide an effective dirt- and fluid-resistant contact-to-housing seal. This feature lets you use the connectors for I/O applications in environments where the device needs to be cleaned with aerosol or liquid solvents, such as in portable patient monitoring devices. Additionally, a great benefit to the medical community is that because compression contact connectors do not require soldering, the device can be more readily disassembled and repaired in the field. In this case, the damaged device that is connected to the system via the pure compression style can be unmated and easily replaced without soldering and without rework to the mating components.
Mechanical mounting
Although there are many benefits that compression contact connector designs offer, there are also some considerations that the medical device designer should note. The primary consideration is that mechanical mounting is required. When a single-piece pure compression connector is used, a secondary mechanical feature may need to be incorporated to provide proper registration of the connector. Examples include those needed to insure proper connector position (i.e., with locating posts), connector retention to the PCB (i.e., screw lock), or significant compression force evenly across all contacts (i.e., top and bottom support hardware).
Another factor is that higher compression forces may be required. High pin-count connectors, such as ASIC or other interposer applications, require that the PCB and device be designed to accommodate and withstand the clamping hardware and resultant forces.
In addition, vendor assembly control can be an issue. Compression contact connectors typically mate two PCBs or devices, and due to the common practice of outsourcing the PCB assembly to two or more fabricators, the tolerance and layout (registration) of each PCB or device needs to be accounted for in the design process.
Putting them to work
Compression connectors can be better understood by looking at some successful applications:
- A handheld patient monitoring device (Fig. 3), such as for a multi-function, LCD module and battery connector.
 FIGURE 3. This handheld patient monitoring device incorporates multi-function compression contacts for both power and signal functions. |
The design challenge involves a single piece design that is capable of connecting an LCD display and two rechargeable batteries. Additional design criteria include the requirement to be compatible with automated assembly processes, and to have a high level of reliability, including shock and vibration.
The solution is to create a single-piece plastic housing that holds a battery compartment on one side, and a recess to hold the LCD module on the other. The customer’s assembly robot places the LCD module into the housing, then places the PCB over the locating posts integral to the connector, and heat-stakes the PCB in position, effectively attaching the LCD to the connector and PCB. This design eliminates the previously used battery wires and clips, as well as eliminates the soldering operation needed to attach the connector and LCD module to the PCB, thereby creating a cost effective solution.
- A portable medical imaging device, such as for a high-density, board-to-board-to-board interconnect.
The design challenge involves creating a high-density device that could couple three parallel PCBs with 592 I/O contacts in a 1.0-inch (25-mm) × 1.5-inch (38-mm) package (Fig. 4). The system needs to be highly reliable, as well as allow serviceability of the device (disassembly and reassembly).
 FIGURE 4. One of the dual 592 contact wafers, with alignment and attachment guides. |
The solution is to create a compression micro-contact connector with nearly 600 contacts that can be accurately positioned and retained between each of the three PCBs. The contacts are gold-plated and insert-molded for a tight tolerance registration and low resistance connection. To create the precision outline, a post-molded laser-cut operation is performed to obtain the desired mounting options and alignment holes. Two low-profile stainless steel plates with mechanical screw locks are used to support the top and bottom PCBs to effectively create the PCB wafer system.
This compression micro-contact connector results in a high performance yet cost-effective solution that fits into the handheld scanner. The custom connector design lets the technicians remove the handheld device, disassemble the head for cleaning, and reassemble the device in the field.
- A portable fluid pump with a modular control unit. The device consists of a central modular unit that serves as a power and control unit. Auxiliary electronic application devices (i.e., patient monitoring or data recorders) can be attached or mounted to the central device as required. This application features a very robust, liquid tight, I/O (input/output) connector to connect external auxiliary units to the main control module.
The design challenge involves requiring a connector that enables both power and signal circuits to run between the central module and the add-on auxiliary units. Additional criteria includes eliminating external cable assemblies, reducing system cost, allowing attachment of the auxiliary units by non-technical users, having a sealed system, and incorporating early mate and late break grounding features.
The solution is to design a custom male-to-female bracket that has a built-in compression contact connector incorporated (Fig. 5). To add a module, the nurse places the male bracket of the auxiliary unit into the female bracket on the center control module, which allows the auxiliary unit to rotate down into final position. As the bracket rotates, compression contacts press against mating contacts in the other bracket, make the electrical connection, and lock the two units together.
 FIGURE 5. The compression-on-compression interconnect method, with hinge/lock mounting bracket. |
The contacts in the bracket are well protected from damage and are sealed against spray-on disinfecting cleaners, allowing an auxiliary unit to quickly attach to, and be retained by, the main control unit without additional costly or cumbersome mechanical hardware. This design consequently makes all necessary electrical connections to run the system, both transportable and seamless to the user.
New options for better devices
Compression connector technology offers the medical device designer many valuable advantages. When coupled with the advances in computer automated design techniques, contact material selection, improved insert molding techniques, and rapid prototyping processes, compression contact connectors are giving medical device designers a multitude of new options to help create higher performance and user-friendly devices.
MARK PAGE (Mpage@teledyne.com) is director of sales and new business generation, and THOMAS MOWRY is a consulting engineer at Teledyne Interconnect Devices, San Diego, CA.
