Medical equipment and devices must be designed with rigorous standards to function in environments where patient safety and product reliability are critical. Today, there is great demand for innovative, feature-rich, and affordable electronic equipment to be used in diagnostic labs, hospitals, and clinics, as well as for consumer self-care. Engineers are constantly challenged to deploy differentiating leading-edge electronics that demonstrably improve patient care in a cost-effective manner for in-home patient care and portable monitoring and therapeutic devices.
Mechanical membrane switches and increasingly feature-rich capacitive user interfaces are used in a wide range of medical equipment and devices — defibrillators, EKG leads, electronic sensors, glucose meters, infusion pumps, patient monitors, and portable oxygen units, as well as disposable dental, medical, and surgical equipment. Therefore, it is important to closely compare and understand the respective technologies behind user interface and switch options.
Membrane Switch Technology
Membrane switches provide a lightweight and lower-cost option for integrating user interfaces and electronic components into medical devices. Adapted from the white goods industry, membrane switches not only provide an excellent value-to-performance ratio, but are also rugged, easy to clean, and intuitive to use.
Low-profile control panel assemblies are ideal for touchpad applications, combining both the switch contact system and custom electronics into one complete package, which eliminates the need for an additional motherboard or display board. Standard membranes are simple designs comprised of thin, micro-motion assemblies, with one or more layers of silver or carbon conductors printed on polyester substrate layers. Pressure-sensitive adhesives are used to bond the layers together. Some companies, such as Molex (Lisle, IL), offer advanced manufacturing capabilities, such as printed double-sided silver circuits that produce the equivalent of multi-layer PCBs or that eliminate printed “crossovers” for environmentally exposed applications. These advances are customized in a variety of configurations to fit smaller, more demanding, cost-competitive applications. Assemb lies can be designed as thin as 0.70 mm (0.028").
Available in many standard styles, membrane switches can be customized in a variety of configurations to fit specific applications. The flexible switch substrate allows for the addition of simple electronic components, eliminating the need for additional LED or display-board mounted electronics below the switch substrate. Custom integrated membrane switch assemblies can incorporate LEDs and other electronic components — such as photo diodes, resistors, and capacitors — connected to the membrane-switch substrate using conductive epoxy technology. Depending on the equipment and functionalities, more sophisticated membrane applications can integrate copper flex circuits and PCB assemblies, LCDs, plastic housings or metal backers, and lenses. Membrane options include non-tactile smooth surfaces (i.e., microwave ovens, treadmills), poly-domes, and silicone rubber keypads with carbon or gold contacts.
Tactile metal domes are frequently used to provide the “snap” or tactile feedback and audible sound when a switch is user-actuated. A more economical option than rubber, stainless steel snapdome arrays for membrane switches are produced through a high-speed stamping process, with auto-placement on the membrane to ensure reliability and a consistent user response. The mechanical contact system is extremely reliable in rugged environments and offers a superior tactile response that is consistent from switch-to-switch. Metal dome or embossed arrays offer easily integrated low-cost contact systems that still allow for custom contact configurations.
Rubber membrane keypad assemblies increase switch travel and tactile feedback for a more desirable user experience. Rubber provides a more discrete key appearance with multiple surface finishes for enhanced aesthetics. Cosmetically appealing 3D rubber keypads offer the reliability of a membrane or PCB-substrate switch, with the three-dimensional look and tactile feel of a silicone rubber keypad. A rubber keypad utilizes a variety of contact systems ranging from carbon pills to gold pills, and to actuating tactile metal domes in a membrane-switch assembly. Mounting methods used to apply the rubber actuator to the switch may vary. One simple method of mounting a rubber actuator is to use a double-sided, pressure-sensitive silicone rubber adhesive. Switch options can include patented Molex rocker switches, hard keycaps, and IMD (in-mold decorating). A recent innovative mounting method includes a simple pull-through protrusion molded into the rubber itself. The protrusion is then pulled through a printed circuit board substrate, where an interference fit secures the rubber component to the assembly.
Tactile and non-tactile membrane switches can be converted to bonded LED assemblies by adding light-emitting diodes. Membrane switches with embedded LEDs for backlighting and status indication may feature embossed windows for enhanced viewing angles and fully automated component bonding. LED/display flex assemblies allow for flexible mounting configurations and multiple circuit substrate options. Flexcircuit substrates offer a three-dimensional packaging alternative to the combination of printed circuit board, connectors, and wires. This allows for a single interconnect solution that is smaller, thinner, lighter, and highly reliable.
LED-display assemblies are ideal in a variety of medical equipment applications and can improve manufacturing assembly processes with reduced EMI (electromagnetic interference) and cosmetic options. Placing an LED assembly on the top surface of a user interface produces a wider viewing angle, increased visual brightness, decreased light bleed, improved assembly, and easier use of curved surfaces. Value can be found by creating a single LED assembly to replace light pipes or bulky and costly wire harnesses. Display assemblies may be designed with a large number of indicators, and combined with switches, backers, seven-segment LED-displays, LCDs, and other components to enhance or expand the performance and functionality of the displays.
Applying LEDs to flex circuits can be accomplished using high-speed SMT processes and bonding technology. These two innovative methods of bonding and encapsulating components provide customers with greater value and a lower cost interface. Silver printed circuits on polyester cannot handle soldering temperatures, so Molex began using anisotropic (Z-axis) conductive adhesives, subsequently developing a fully conductive epoxy to accommodate the high-speed SMT lines.
Membrane switches are mechanical, with moving parts that reduce product longevity and increase maintenance requirements. However, initial capital investment is generally lower. Membrane panels require only simple circuits without complex electronic components or design elements. The pressure needed to depress keys is consistent and firm enough to necessitate that depression be intentional. So, accidental activation is less likely than with capacitive switches. Membrane switches are extremely resistant to shock and easy to shield, offering good protection against static discharge and EMI/RFI (radio frequency interference). An internally vented sealed design offers resistance against dust and moisture. The compact size of membrane-switch technology can make it the best fit for smaller, more demanding, and cost-competitive applications.