Choosing Semiconductor Components for Medical Products

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI  February 1998 Column

INTEGRATED CIRCUITS

The selection decision depends on which type of integrated circuit best meets the product's technical and economic requirements.

The drive to reduce costs by moving patients from hospital to ambulatory care as rapidly as possible has increased the demand for portable medical devices and challenged medical device designers to find semiconductor components that can meet these devices' technical and economic requirements. Portable medical devices must consume less energy and take up less space than nonportable devices. They also may require remote or wireless communication features. Three types of integrated circuits (ICs) are generally used in medical applications: standard ICs, semicustom ICs, and mixed-signal application-specific integrated circuits (ASICs). Ten technical and economic issues should be considered when choosing which component to use: power, size, design flexibility, reliability, functionality, design security, electromagnetic compatibility, cost to design, cost to manufacture, and time to market (Table I).

Table I. Ranking of technical and economic variables for each semiconductor manufacturing option according to the amount of benefit offered.

STANDARD COMPONENTS

Standard off-the-shelf analog and digital semiconductor components include microprocessors, analog-to-digital converters (A/Ds), digital-to-analog converters, and operational amplifiers in addition to memory devices such as RAM, ROM, and EEPROM. The chief benefit of using standard semiconductor components is that they already exist, decreasing time to market and design cost. However, design flexibility is low—limited to products that already exist—and the components may not be able to meet the more rigorous miniaturization and power consumption demands of portable medical equipment. Moreover, designs using standard components are easy to reverse engineer, decreasing design security.

SEMICUSTOM COMPONENTS

Semicustom integrated circuits rectify some of the limitations of standard ICs. They provide designers with more flexibility because digital functions can be designed into a standard gate array, programmable gate array, or digital ASIC. Limited analog functions also can be added. The product's overall power consumption and size can be decreased while the digital portion of the design is tailored to meet precise requirements.

The time to market of products using semicustom ICs is generally longer than those using standard components. Digital ASICs are almost always production qualified within one year from start of design, although designers using a field-programmable gate array can significantly decrease this time. The design cost will also be higher because the semicustom IC must be designed and fabricated for the specific application. However, new digital design software is available that allows designers to work at the circuit behavioral level using high-level hardware description languages. This software allows designers to simulate the behavior of their high-level designs, verifying circuit behavior and performance. Once this is done, designers can run synthesis software to compile the behavioral descriptions and produce a list of cells and their interconnections (a netlist) ready for ASIC layout. Other software can draw a useful schematic from the synthesized netlist. Because all of the device's digital functions are integrated into a semicustom IC, it is more difficult to reverse engineer, increasing design security.

Semicustom ICs solve many medical device performance, size, design flexibility, and design security problems. However, semicustom ICs are generally limited to digital functions, which may not be sufficient for a device's requirements.

CUSTOM COMPONENTS

Custom, mixed-signal ASICs combine analog and digital circuits on a single chip. Designers can opt for exactly what they need—specifying an 11-bit A/D, for example, instead of an off-the-shelf 12-bit A/D. Mixed-signal ASICs can reduce the number of components used in a circuit board from 20 or 30 to as few as 5 or even 1. The accompanying reduction in the number of long-wire connections and board interconnects makes devices using ASICs more electromagnetically compatible, i.e., less likely to generate and less susceptible to electromagnetic fields. Further, end-product reliability increases when the number of components making up the circuitry decreases, and manufacturing costs fall due to fewer manufacturing steps and components purchased and inventoried.

Because mixed-signal ASICs permit designers to specify exact pinout configurations, connections and packaging are simpler and more direct. In addition, if ASICs are obtained as flip chips, they—and thus the product—can be smaller. Custom ICs provide smaller size, greater energy efficiency, and better design flexibility than standard or semicustom ICs. Moreover, attempts to reverse engineer the product are almost impossible because most or all of the circuitry is included on one IC.

These benefits come at a price. The design cycle for products using custom mixed-signal ASICs is significantly longer. A minimum of one year from design start to receiving qualified production parts is the norm, but the time can be much longer depending on the ASICs' complexity. Engineering costs will be higher because—unlike all-digital ICs, which frequently can be designed in-house—mixed-signal ASICs must be created by experienced analog designers who are typically only available through outside sources. The complexity of an ASIC and, therefore, its development time and cost, becomes greater as a result of increased digital gate count and analog functions. Thus, design work must be initiated with enough lead time to compensate for these factors.

Identifying ASIC Designers. Mixed-signal ASIC development requires the manufacturer to match the needs of the medical device with the capabilities of potential partners and existing technology.

Questions to ask at this stage include:

The answers provided by potential partners will help the manufacturer identify qualified sources for designing custom ASICs.

Budgetary and Technical Information. Once the list of potential ASIC designers has been narrowed down to those who are technologically qualified, the manufacturer should meet with each potential vendor for an in-depth review of the requirements and end product and ask for a budgetary quotation.

All economic aspects—such as exploring cost reduction trade-offs between bare die, flip chips, and packaged parts—should be reviewed. Manufacturers should compare manufacturing costs using standard or semicustom ICs against those using ASICs. A higher cost for an ASIC may be offset by reductions in procurement, inventory, manufacturing, or assembly expenses.

Low annual volumes and the potential liability for medical product ASICs may preclude some vendors from responding to the request for quotation. Those quotations that are received should be accompanied by a discussion of pertinent technical issues and information on application-specific experience.

Questions to be asked at this stage include:

The manufacturer should be wary of budgetary quotations that do not address these technical issues and provide only a price. Unless the ASIC is extremely simple, some technical discussion should always be included with the quotation.

Manufacturing and Financial Considerations. The manufacturer should also study each vendor's financial strength and access to manufacturing facilities. The length of time that the vendor has been in business and the portion of the manufacturer's business devoted to ASICs and to the medical market are all-important. In addition, the device manufacturer should ask whether the vendor has its own fabrication facility (fab) and, if so, how many. If the vendor is simply a design house that can use any fab, the manufacturer should question how committed the fab will be to the manufacturer's business now and in the future. A vendor with limited financial or manufacturing capabilities will probably not be a successful partner.

The manufacturer's overriding objective should be to select a vendor who will be involved with current and future products and ASICs. Price can be most beneficially addressed in the final stages of evaluation, when only fully qualified candidates are being considered. Paying more for an ASIC may make sense if the main cells have already been designed and proven. A higher nonrecurring engineering expense may make sense if a new architecture saves space and decreases the amount of software code needed. In short, price is just one of many parameters to consider when selecting an ASIC vendor.

Management Objectives and Philosophies. Once a vendor has been selected, the manufacturer should arrange regular meetings throughout the design process. Because some product specifications may have changed during the quotation period, all requirements should be reviewed before the design starts. If some specifications are more flexible, that fact should be noted. Adequate resources and technical support should be provided for the project, even if the entire design is being completed by the ASIC partner. Some trade-offs probably will have to occur as the design takes shape; a slight change in a noncritical specification may result in significant yield improvement, decreased design time, or decreased risk.

The manufacturer should be an integral part of all design reviews and insist on periodic updates of milestones and development schedules. There should be advance agreement on the definition of a prototype or first article part; what comprises design validation, preproduction, and production; and what is required by both parties before releasing the final ASIC to production.

If the end assembly will be produced at a contract manufacturer, the manufacturer should meet with the contractor during the design process. Mechanical samples from the ASIC vendor should be taken to the contract manufacturer as early as possible.

The device manufacturer should know how much testing of the ASIC is required and how to test the end product in which the ASIC will be used. Furthermore, the manufacturer should be prepared to relax testing requirements where possible to keep piece part prices low.

CONCLUSION

The demand for portable medical products will continue to grow, increasing the requirements for lower-power operation, smaller size, better performance, greater reliability, and design flexibility. Medical device manufacturers should consider their options when choosing ICs for their products and take care to consider a variety of technical and economic issues when looking for a vendor.

Jim Gentile is manager of ASIC products at Mitel Semiconductor (San Diego).

Copyright ©1998 Medical Device & Diagnostic Industry