As their next-generation designs continue to shrink in size, medical equipment manufacturers are calling for resistors that deliver higher frequency, extended resistance values and tighter tolerance all in a small, surface-mount package. Thanks to the use of new materials, refined manufacturing processes as well as advanced packaging technologies, resistor manufacturers are making the grade and fine-tuning their resistor technologies to meet the demands for higher performance and smaller footprints.
While some of these new components focus on supporting higher frequency ranges, tighter tolerance or high resistance values, they all share one common characteristic smaller form factors. Resistor manufacturers such as AVX, RCD Components, SEI Electronics, and TT electronics IRC Advanced Film Division, are all making tremendous headway in the development of these higher precision resistors in smaller packaging.
SEI Electronics Inc., for example, recently introduced its new high-voltage chip resistor series, designated the HVC, that the company says outperforms its competitors in higher voltage withstanding and resistance range. The Raleigh, N.C.-based company is targeting two primary applications for these HVC series resistors. First, these devices can be used as overvoltage protection including lightening strike, surge and electrostatic discharge (ESD) protection of medical equipment such as defibrillators, medical imaging systems, neurostimulators, bladder control devices, and ECG and blood transfusion equipment.
Secondly, these components can be used in high-voltage power supplies in snubber or bleeder resistor applications for medical devices such as PET scanners, GAMMA cameras, radiation therapy systems and x-ray equipment.
The HVC resistors can replace current solutions such as axial-leaded film and wirewound resistors in medical applications with a much smaller and surface-mountable part.
SEI's director of sales Rob Hudspeth said the HVC resistors are capable of achieving exponentially higher withstanding voltage than competitive products. "Competing technologies typically claim voltage withstanding to 2.5 kilovolts. The way we manufacture this technology allows us to offer three to four times better voltage withstanding," he said.
These HVC chip resistors are capable of handling voltages up to 40,000 volts (potted) and up to 10,000 volts (in air). Other key specifications for the resistors include ohmic values to 2 teraohms, precision tolerances down to 0.1%, a temperature coefficient of resistance (TCR) to 10 ppm/°C, and a voltage coefficient of resistance (VCR) to 1 ppm/volt.
Available in limiting element voltages of 2,500V (1206), 5,000V (2010), and 10,00V (2512), the power ratings for the three chip sizes range from 0.3 watt to 1 watt.
SEI's HVC series of high-voltage chip resistors
SEI has improved the performance by using a proprietary fine film deposition process that enables the company to screen print longer and highly precise resistive traces on the substrate.
Debuting the new technology with the introduction of the HVC resistors, Hudspeth said the new process allows the traces to be deposited in such a way to prevent any whiskering effects associated with common screen printing techniques. Typically, resistor manufacturers use a common screen printing technique to print resistive material on a substrate such as ceramic.
"While whiskering effects associated with screen printing increase inductive noise our technology does the opposite. It reduces noise and allows us to achieve precision and very tight tolerances down to .1%. The advantages of our technology is that it provides a very high-voltage part with low noise and very low tolerances," Hudspeth said.
"If you plan to use a snubber or bleeder resistor on some of these high-voltage equipment such as ECG equipment or defibrillators you'll require a ground and the only way to do that is with higher resistance values. The higher the value, the better in this case," Hudspeth said.
The HVC resistors, in standard 1206, 2010 and 2512 case sizes, differ from competitive resistors in voltage ratings. (Custom package sizes are also available.) For example, the HVC resistor in a 2512 case size is rated at 2,500 volts and can handle peak pulses of 10,000 to 20,000 volts. In comparison, competitive products are typically rated at 800 to 1,000 volts with a pulse voltage of 2,500 V, said Kory Schroeder, SEI's engineering product manager.
Medical equipment manufacturers had to use larger and more expensive axial-leaded devices for high voltage (2,500 to 5,000-volt) applications, Schroeder said. They also used wirewound resistors for high-power applications but there are limitations with the technology in terms of inherent inductance and low frequency, he added.
"As engineers move toward miniaturization and surface-mount technologies, the HVC chip resistors offer another advantage," Schroeder said. "This is a chip resistor style that allows them to downsize and that certainly helps to make some of these high-voltage equipment portable where it might not have been previously possible," he said.
Prices for the HVC resistors range from $0.40 to $2.50 each, depending on values and quantity. Delivery is from stock to eight to ten weeks.
Earlier this year, SEI also expanded its line of precision thin-film nichrome chip resistors with the introduction of the RNC Series that provides a tighter tolerance down to 0.01% and a TCR to ±5 ppm/°C. The RNC resistors can be used in several applications including medical equipment. For more information, visit www.eeproductcenter.com/passives/showArticle.jhtml?articleID=17602893
A key challenge, particularly for battery-operated portable medical devices, which are becoming smaller, is power dissipation, which translates into the need for higher resistive values, said Jerry Seams, applications manager for TT electronics IRC Advanced Film Division (Corpus Christi, Tex.).
Seams said there is a trend to use high-precision resistors in voltage dividers that offer higher resistance values because they draw less power from a battery for both portable medical devices that are external to the body as well as for implantable devices.
The result of this need is IRC's new Chromaxx series of chromium silicon oxide resistors that are constructed with a higher range film. The thin-film resistors, which are available in both standard and custom values, offer resistance values up to 15 Mohms, which make them suitable for high value voltage dividers, chips and networks used in high input impedance medical and instrumentation applications. For more information on the Chromaxx series, visit www.eeproductcenter.com/passives/showArticle.jhtml?articleID=17602915.
Another challenge is meeting high-frequency demand for growing wireless data transmission for medical devices. Many of these portable and implantable devices can communicate via a wireless base station, which allows doctors to download the medical information, Seams said, and much of the wireless communications is done at very high frequency, about 2.4 GHz.
To meet these requirements, IRC has recently introduced a series of high-frequency chip resistor terminators, designated as the PFC-HF series. The company has been working over the past six to 12 months to characterize each of these products for their frequency operation. Currently, samples are available for the W0805HF and W0603HF parts. A 1206 device is under development.
The W0603HF has been tested to 6 GHz and offers a power rating of 100 mW (max.), while the W0805HF has been tested to 10 GHz and offers a power rating of 250 mW (max.). Both devices are rated at 50 ohms. A 75-ohm version is also under development.
Last month, IRC also released a new series of high-speed termination arrays. Designated as the TaNFilm CHC series, these high-density resistor networks have been tested to 40 GHz. A variety of package sizes, including custom configurations, are available for the 1.0-mm pitch and 0.65-mm pitch series. For examples, in a CD1065B package, the line terminator features frequencies up to 20 GHz and in a CC0910B package, the device offers up to 40 GHz.
The chipscale networks, packaged in ball grid arrays (BGAs), are available in a wide range of standard and custom resistance values from 22 ohms to 10 Kohms, with isolated or bussed schematics that incorporate from 10 to 40 resistors. The devices also feature absolute TCRs to ±100ppm/°C, and standard tolerances to ±1% with operating temperatures from -40°C to 85°C.
IRC's CHC series of high-frequency BGA resistor networks
The key benefit of this new CHC BGA series is the high frequency, Seams said. "They've been tested up to 40 GHz and offer a high density, packing up to 24 resistor elements, for example, into a 2010 size package," he said.
Available in production volume, typical pricing for the CHC Series devices is $0.80 each in quantities of 10,000. Delivery is from stock to 12 weeks.
Packaging also plays a big role in the miniaturization of resistors.
Medical equipment manufacturers eventually want to offer truly portable monitoring systems that offer real-time data acquisition, but this will require smaller communications devices, said Kevin Christian, technical marketing manager for AVX's Passive Micro Components Division (Myrtle Beach, S.C.).
The key driver will be miniaturization, Christian said, and one way to achieve it for passives is by using thin-film technology which can supercede the density of 0402 resistors, while achieving tighter tolerance. The wafer processing technology offers a space savings as well as improved signal integrity because the "resistive values are very precise, reproducible and verifiable," he said. "The way to do that is by laser trimming them to a very tight tolerance to create ultra-precision, small resistors."
Christian believes portable medical devices developed in the future will drive the need for BGA-style and high-density passive devices, particularly for telemetry applications that will add several components to the circuit board including communications chipsets.
AVX's PMC division uses a high-resistivity chrome-silicon (CrSi) material, which offers a much lower leakage current to extend battery life, in conjunction with BGA packaging for high-density terminations. The company also produces tantalum nitride resistors, as well as the CrSi resistors, on several substrates including glass, quartz and silicon in both BGA and wire bonded packages. Voltage ratings are available up to 1,000 volts. Most of the devices produced for medical applications are semi-custom in terms of layout, values and tolerances, Christian said.
Also utilizing thin-film technologies to produce smaller and higher precision resistors, RCD Components Inc. (Manchester, N.H.) offers a new family of ultra-precision thin-film chip resistors with tight absolute tolerance and TCR characteristics. Suitable as a replacement for bulk metal foil types, the new BLU series of resistors are available with resistance values from 4.7 ohms to 4.7 Mohms and maximum DC power ratings from 50 mW to 1W with tight tolerances to ±0.01%.
Available in standard case sizes of 0201, 0402, 0603, 0805, 1206, 1210, 2010 and 2510, the thin-film chip resistors can be used in a variety of medical, industrial, communications and portable electronics systems. For more information on the BLU resistors, visit www.eeproductcenter.com/passives/showArticle.jhtml?articleID=17700205.
In the future, the medical industry may also see a move toward more advanced and integrated packaging technologies such as those offered by Tessera Inc. The company focuses on developing advanced packaging products to allow electronic products to become smaller, faster and more reliable, said Craig Mitchell, vice president of marketing for Tessera (San Jose, Calif.).
While the company's advanced packaging technologies are heavily utilized by wireless product manufacturers, Mitchell said the technologies are beginning to be adopted by medical and military applications for their high reliability and small form factor benefits.
Typically, the company is asked to integrate semiconductor components in either chip-scale or multi-chip packaging but are now being asked to integrate passive components such as resistors, inductors and capacitors into these more advanced package solutions, Mitchell said.
"We are utilizing advanced packaging technology to address not just miniaturization but overall system design, passive component count and the size of each individual passive," Mitchell said.
While new designs are in the early stages of development, there are active discussions with medical electronics OEMs to utilize the advanced packaging technologies. "What's happening is that medical electronics manufacturers are now starting to realize that there is a vast amount of advanced packaging technologies that they have not leveraged, said Michael Perry, Tessera's vice president of medical products. "We're finding more interest from them to evaluate and then to integrate our type of packaging solution into their products."
Company contact list
Call (843) 946-0414
Call (919) 850-9500
RCD Components Inc.
Call (877) 723-2667
Call (408) 894-0700
TT electronics IRC Advanced Film Division
Call (361) 992-7900