Medical Electronics: Putting it all Together

28 February 2005 (Last Updated February 28th, 2005 18:30)

Packaging and interconnection design play a vital part in medical electronic equipment. Devices must achieve the functional and performance targets that the market demands.

Medical electronic equipment manufacturing is a global industry with shipments valued at an estimated $60bn in 2004, according to the market research firm Bishop & Associates, Inc. This increasingly competitive market environment challenges equipment designers to meet a complex set of objectives. These include marketing, manufacturing and regulatory requirements.

From the marketing perspective, the designer must address the need for safety, high reliability, cost-effectiveness and an optimal mix of features, functions and performance. Lower cost is a primary manufacturing objective, generally involving smaller equipment size, fewer components, supply chain efficiencies and, increasingly, outsourcing. Related key issues include shorter time-to-market and regulatory compliance.

"The designer must address the need for safety, high reliability, cost-effectiveness and an optimal mix of features, functions and performance."

Packaging and interconnection design plays a crucial role in meeting overall design performance objectives. Interconnection design covers five zones of application, as shown below.

Zone 1: interconnection inside component packages. Zone 1 includes interconnections within a component, such as a semiconductor or LCD display panel. Advances in Zone 1 connectivity have had the greatest impact on miniaturising electronic equipment design. These advances include the flip chip, stacked chips, multi-chip modules (MCM) and system on chip (SOC).

Reducing the number of individual chip packages and associated passive components significantly reduces the required board space. With this comes a bonus: the intimate proximity of one die to another significantly reduces both parasitic losses and power consumption while increasing system performance.

Portable devices such as multi-function patient monitors, home diagnostic equipment, portable ultrasonic imaging systems, in-canal hearing aids and advanced electronic implants would not be practical without these innovations.

Zone 2: component package to circuit board. Zone 2 interconnections provide connection between components, such as integrated circuit (IC) packages and a circuit board or flex circuit. While component reliability and surface mount technology have reduced the use of IC sockets, there is a rapidly growing need to provide connectivity for various types of memory devices.

Zone 3: board to board, wire to board. Zone 3 is comprised of board-to-board connections. In large systems – an MRI scanner, for example – Zone 3 is likely to be a backplane assembly mounted in a card cage.

Individual circuit packs provide input/output, CPU, switching and power supply functions. Wire-to-board connections are also found. An alternative to the backplane approach is direct board-to-board stacking. This method of interconnection is common to smaller systems, but may be found in large systems as well. Often stacked boards, called mezzanine boards, will be deployed to enable future system upgrades. When a system upgrade is required, only the mezzanine card is changed, the overall system is essentially unchanged.

It is in Zone 3 where the impact of Zone 1 innovation is most evident. The functions of one or more boards may be consolidated into a single MCM or SOC package. Multiple MCM or SOC packages may be mounted on a single board, potentially replacing a card cage full of conventional circuit packs.

Zone 1 efficiencies enabled the design of a single-circuit board ultrasound scanner with functionality that previously required an estimated 20 boards. The complete elimination of the Zone 3 complexity combined with improved display technology make such devices possible.

Zone 4: chassis to chassis. Zone 4 includes connections between two or more sub-chassis within a single piece of equipment such as a CT or MRI scanner. It also includes ‘rack and panel’ connections that, for example, would provide ‘blind mating’ between individual functional modules and a multi-functional patient monitor.

The picture shows a multi-function patient monitor utilising a cathode ray tube (CRT) display and multiple function modules. The CRT and modules add considerable packaging and interconnection complexity as well as weight to these devices. The CRT requires heavy shielding, a high-voltage power supply and high-voltage wiring. The modules require an additional layer of connections.

Because monitors of this type tend to be almost permanently mounted on the wall, equipment utilisation is often less than optimal. In contrast, a truly portable, multi-function patient monitor would offer better utilisation and increased flexibility.

Again, Zone 1 efficiencies combined with advanced TFT-LCD display technology enabled a new generation of medical equipment. Zone 3 and 4 packaging complexities are eliminated. The heavy CRT, shield and high-voltage power supply, along with the attendant EMI/RFI headaches, are eliminated as well.

The advantages of this new generation of monitors are safety, cost effectiveness, improved utilisation and reliability.

Zone 5: equipment to equipment, equipment to patient. Zone 5 typically addresses system-level connections between pieces of equipment, such as a camera and an imaging workstation. These connections are similar to those employed to connect personal computers to networks and peripheral equipment. More interesting are the connections between diagnostic or therapeutic medical devices and the patient.

These include ‘patient cables’ for diagnostic ECG and ECG monitoring, blood pressure, respiration, blood oxygen, and body temperature monitoring cables, neural stimulator cables, defibrillator paddles, electro-surgical probes and return electrodes, ultrasound transducer cables and other patient interface cable devices.

What makes this category of interconnection particularly interesting is the rich diversity of application environments and circumstances of use. In many instances, standard off-the-shelf connector products do not adequately address the application need. As a result, manufacturers of connectors have collaborated with medical equipment designers to develop either custom design solutions or modifications to existing connector designs in order to meet the specific needs of an application.

The evolution of ECG patient monitoring cables is a case in point. Device manufacturers wanted an exclusive input-output interface that would make it impossible to plug a monitoring cable into the wrong type of equipment, thereby creating a potentially hazardous condition. The connector industry modified a MIL-C-5015-type circular connector to provide a unique insert and keying orientation exclusively for ECG cable and monitor applications.

"The use of unique insert arrangements, keying and colour coding has made medical equipment both safer and more user friendly."

The common method of lead wire attachment to disposable electrodes had been the common clothes snap. When it was ascertained that the pressure of mating the snap could displace the saline gel within the electrode or, in some cases, cause injury to infants and elderly patients, connector makers responded with the squeeze-clip design.

As multi-function monitors and various other types of equipment evolved, the need for better keying and identification methods at the equipment-to-patient cable interface was recognised.

Connector manufacturers responded by evolving their standard product lines to meet the specific needs of the medical electronics industry. The use of unique insert arrangements, keying and colour coding has made medical equipment both safer and more user friendly.

Eliminating the possibility of cross-mating patient cables is not the only interconnection challenge. Some categories of medical electronic equipment, most notably ultrasound and MRI imaging equipment, require the interface connectors to support a complex mix of signal types. These include power, radio frequency and digital signals.

To address this need, some connector manufacturers have developed customisable modules that may be configured to meet the exact signal requirements of the application. A connector manufactured by ODU enables the equipment designer to custom configure the connector interface for each type of surface coil based on the specific signal mix required. The connector receptacle integrates well with the panel, while the plug is insert-moulded to the cable assembly. The result is a very elegant solution to a complex interconnect problem.

The medical industry represents a significant opportunity for connector manufacturers. Connector shipments to the medical electronics sector were $650m in 2003. By 2008, the value of shipments to the sector will exceed $1bn, according to market research firm, Bishop & Associates, Inc.