Connectors are getting smaller and denser. The question is, how small can connectors go? To answer this question, it is important to remember that the applications that use connectors demand a number of performance and reliability requirements, particularly in medical devices.
Application-specific connectors — and reduced-size connectors in particular — have come on quickly during the last five years. This is now fueling the rapid change in a combination of technological advances, starting with new solid model design and performance prediction software that help new advances in circuit chip technology. These changes have aided in driving increased performance and improved capabilities. These evolving circuit chips demand much lower voltages and current flow but run much faster and store and process significantly more data, while yielding a longer battery life. New software too has enabled designers the ability to model much smaller connector options that can be manufactured from a simple CAD rendering to a reality. New shell sizes and shapes can quickly be altered and cut out with CNC machines within minutes. These changes provide a faster prototype cycle at a reasonable price to designers that need to squeeze every last millimeter of space out of their interconnection systems. Oftentimes, designers select a standard connector to begin their circuit testing, but eventually find they need a custom design variation.
Determining Size Requirements
Connector size requirements are often based on a few key application factors and performance constraints, including electrical current load, signal integrity, environmental conditions, and circuit mobility. Many additional factors include awareness and compliance to biological protection, minimal open gaps and creep in connected mated pairs that can hoard contamination as well as a specific material selection to allow for sterilization.
Electrical signal noise in operating rooms must also be shielded to allow many electronic systems to run simultaneously without interference to each other. Even the limpness and feel of a cable that complements the connector is considered a characteristic that can assist in assuring patients that they are being served in a quality environment.
Electrical Current Load
Each connector contact must offer a low resistance interface with its mate and carry enough electrical current to satisfy the circuit it is serving. Fortunately, advances in chip technology have reduced the current flow rate, which enables the shrinking of interconnecting elements as long as they stay within a safe range for good performance (with some variations for power and signal surges). Current limits are often set by the diameter of the wire in the cabling itself, because wire length times resistance will set the performance and thermal capabilities of the interconnection system.
Using a low-resistance wire option that handles enough electrical current to run the equipment is often critical. So many machines are often running simultaneously, that high resistance can slow down the signal processing units or delay the triggering of emergency alarms. When connectors are designed with matched impedance, it also provides significant clarity in displays in the room.
As connectors and circuit modules squeeze into tighter spaces, circuits must still function independent of adjacent circuits. Designs must include protection against signal cross talk, as well as protection from electromagnetic emissions or reception of other signal noise in and about the system. Some modern shielding tricks are often built into the connector.
New shield materials often combine braided wire and aluminum foil that are wrapped around the length of the cable, inside the jacket material. Also, small ceramic filter discs can be included inside the connector housing that selects which frequency signal can pass through the cable and which are bled off to ground.
Ruggedized connector designs for performance in extreme environments are often controlled by specifications that ensure continuous signal flow during high shock and vibration, as well as performance during extreme temperature cycles. Blood perfusion monitors, for example, sit near the patient during operations to assure the surgeon that there is no internal bleeding that can't be seen from the operating table.
A miniature cable that is ruggedized and overmolded with medical-grade silicone has a connector at the end that mates to a disposable catheter. The catheter rests inside the brain and is small and delicate. In this case, the main cable protects the small catheter from being accidently moved or pulled on during its use. Eventually the catheter is removed and disposed of, while the connector and cable assembly goes off to the rigors of sterilization and cleaning.
In some portable applications, such as neonatal monitors, electrocardiogram systems, and other skin-sensitive detectors, connectors often face some additional challenges. Ingress protection (IP) ratings, such as waterproof sealing and resistance can be critical in portable cable and equipment. Omnetics Connector Corporation's IP68 Nano-Circular series is an example of how this problem is solved in today's market. These connectors are housed in one of the smallest form factors available and remain sealed.
Selecting the proper materials and processing can also produce connectors that exceed specifications in IEC 60601 and prevent any electrical leakage or shorting to the patient being treated. Elastomeric seal rings are often built into connectors to ensure that neither moisture nor dust penetrate through the connector and enter the circuitry. Smaller and smaller seals will be needed as connectors continue to miniaturize.
Connecting an active chip directly into portable equipment is an example of the demands on today's drive toward miniaturization. Applications such as robotic hands and medical probe tips, are early examples. Percutaneous connectors are now being used in neurological pain management devices, and miniature connectors are being used on cochlear devices. Even smaller connectors are being applied to neurological monitor and control devices for remote function control circuits.
Leading the charge for smaller connectors seems to be coming from brain mapping and neurosensing devices being applied in Parkinson's research, as medical designers are faced with trying to squeeze more functionality into portable electronics. It is here where size reduction begins. Standard designs are reviewed for potential form, fit, and function and then tailored using solid model designs to meet both the size and the reliability requirements. Pin-and-socket sizes can be reduced to minimum sizes that meet these criteria and still squeeze into the space allowed.
Circuit design can also help by combining signal systems for fewer lead counts. Some connectors are being used within a medical instrument during the assembly of the device. These connectors are mated once or twice in their lifetime and the design may be focused on longer-term reliability for shock and vibration of the unit.
Recently, the evolution of combining two or three connectors into one has taken center stage. Instrumentation designers are finding they can reduce the number of cables to the machine by asking for connectors to have both a power supply section and a separate signal processing function within the same connector. This saves space, weight, and in many cases cost. Instrument designers are adding multiple contact types into a single connector with the option of using: 1, 3, 5, or 10 A contacts in one shell. This option has as allowed OEMs to run the power and signal lines in conjunction with one another, saving design time and reducing the number of backup cables needed in stock.
The Challenge and the Market
Today's miniature connectors provide great performance, carrying charging power and signal routing in one element. This method of using a single flat-strip or lead-frame interconnection has led the way for higherspeed digital signal processing. Careful use of shape and spacing has allowed another level of high-density interconnection. As circuit speed increases and size decreases, the use of unique low-dielectric-strength insulator materials also paves the way for smaller connectors. Board-to-board connectors that use pins or press connections consume very little space and cause only minute aberrations in signal transfer.
How small connectors will go is still to be seen. Larger connectors are currently being replaced by smaller and lighter micro-circular connectors that use a 0.050 in. pitch, and those very same 0.050 in. pitch solutions are being replaced by nano variations at 0.025 in. pitch, handling multiple high-speed signals as well.