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EDN MOMENT Space Shuttle Columbia is delivered, March 25, 1979 Search Login Register Patrick Gormally -March 24, 2014 Share Tweet 0 Like 0 Over the years medical devices have continually been made

To fulfill the need for smaller hybrids within the implantable medical device, continuous improvements in the hybrid layout and packaging techniques for the microcontroller (MCU) or

1 EDN MOMENT Space Shuttle Columbia is delivered, March 25, 1979 Search Login Register Patrick Gormally -March 24, 2014 Share Tweet 0 Like 0 Over the years medical devices have continually been made smaller; smaller implantable devices offer improved patient comfort and less disruption to the body. To fulfill the need for smaller hybrids within the implantable medical device, continuous improvements in the hybrid layout and packaging techniques for the microcontroller (MCU) or application-specific integrated circuit (ASIC) and power systems are being implemented. This article discusses the passive component selection process to reduce hybrid and board space in medical devices. Introduction Passive components are produced in large-scale manufacturing facilities with good process controls to reduce lot-to-lot variability. Medical devices require higher levels of reliability and performance in smaller sizes than commercial components. Component manufacturing methods can facilitate a variety of options that reduce hybrid and board space and increase reliability. Capacitor selection criteria Each capacitor technology has unique properties that should be considered as part of the specifying criteria for the end application. MLCCs (multilayer ceramic capacitors) for implantable devices in the 0201 case size (0.024 in x in) are used for decoupling or for tuning in radio/telemetry systems. The largest MLCC is the 2225 size (0.22 in x 0.25 in), which is often used in medical devices outside of the body as a resonant capacitor. Solid tantalum capacitors for medical device applications are available in the 0402 case size (.045

in x.026 in x.024 in) with low profiles to save space. Reliable, high-capacitance solid tantalums are also available in the 1210 T case size (0.138 in x 0.11 in x 0.063 in).

Surface-mount flip-chip silicon capacitors are available to 27 pf in the 0402 case size.

Magnetics selection criteria Most magnetic components are custom designed to fit into limited space for specific medical device applications in collaboration between the device manufacturer and the

Custom magnetics for implantable devices typically consist of bobbin- style transformers, toroid transformers, molded inductors, and antennas that have unique properties for performance and shape.

Tradeoffs in size, price, and performance are discussed to zero-in on the most cost-effective and highest performance device that fits in the available space.

2 in x.026 in x.024 in) with low profiles to save space. Reliable, high-capacitance solid tantalums are also available in the 1210 T case size (0.138 in x 0.11 in x in). Silicon-based capacitors are available in the 20 mil square size and in capacitances up to 1000 pf. Silicon capacitors can be mounted by epoxy or eutectic die attach and are wire- bondable. Surface-mount flip-chip silicon capacitors are available to 27 pf in the 0402 case size. Silicon-based capacitor technology provides a reliable wideband operation (20 GHz), high Q, low DCR, and high SRF. Magnetics selection criteria Most magnetic components are custom designed to fit into limited space for specific medical device applications in collaboration between the device manufacturer and the magnetics company engineers. Custom magnetics for implantable devices typically consist of bobbin- style transformers, toroid transformers, molded inductors, and antennas that have unique properties for performance and shape. Additionally, various core materials and shapes are used to optimize performance and fit requirements for each application. Tradeoffs in size, price, and performance are discussed to zero-in on the most cost-effective and highest performance device that fits in the available space. Once the design is complete, exacting manufacturing processes, controls, and test procedures are developed to ensure the highest level of quality and reliability for both dimensional and magnetic performance. Smaller form factor designs often require 3D CAD simulation for accurate component layout and prototyping. In the manufacturing of custom magnetics, a wide variety of specialized air coil, bobbin, and toroidal winding equipment is used. This equipment produces tightly controlled critical electrical requirements. Inspection equipment such as optical gauging is used for measurement of critical dimensions. Custom-designed test stations and fixtures allow monitoring and testing of electrical parameters. The use of these automated test stations allows for data analysis for use in design for manufacturability. The size and shapes of magnetic components used in medical applications varies greatly depending on the application. Small 0402 size inductors (0.040 in x in) are used in telemetry/communications applications. These inductors can be wire-bondable and are manufactured with ceramic cores and have inductance values up to 150 nh.

High-frequency wire-bondable RF spiral inductors are available in two sizes: (0.030 in x 0.030 in x 0.020 in)

3 High-frequency wire-bondable RF spiral inductors are available in two sizes: (0.030 in x in x in) and (0.050 in x in x in). These inductors perform well in the RF band for biasing, tuning, and lumped-element filters. Resistor selection criteria Standard thin and thick film surface-mount resistors are available in case sizes ranging from 0402 to The criteria for resistor selection includes pulse handling, operating voltage, operating temperature, and long-term stability. Wire-bondable resistors range in size from in by in by in with a 125 mw power rating, to in by in by in with values up to 30 MΩ and operating voltage of 100 V. Sample Vishay wire-bondable resistors Thin film resistors provide the ability to densely populate circuits, while offering the benefit of a highly reliable resistor film. Tolerances as tight as 0.01 % and TCR as low as 5 ppm provide for fine tuning of amplifiers, Tx/Rx circuitry, and power distribution. For medical applications, these chip resistors are available in any standard resistance value (10 Ω to 25 MΩ). High-reliability testing Avoiding catastrophic and drift failures in passive components for medical devices is a primary concern. Ultimately, product reliability predictions are based upon the suppliers test data and medical device manufacturers' specified application operating temperatures over a defined period of time. Process control at the passive component supplier is an important factor in achieving high reliability. Establishing reliability of the passive component and qualifying components for use in critical medical device applications is done by performing life testing at elevated temperatures at the rated voltage, or higher, for a specified time. Passive component testing is based upon customer requirements and MIL specifications where applicable. Reliability prediction for passive components is available online using modeling programs that are based upon MIL Handbook 217 or IEC863. Below is an example of a supplier reliability modeling:

Solid tantalum test standards are based upon MIL-PRF-55365. Solid tantalum capacitors undergo internal qualifications and periodic maintenance testing at elevated temperature and voltage.

The table below shows failure rate predictions based on Weibull testing of solid tantalum capacitors. Resistors are qualified for critical medical applications in accordance with MIL-PRF-55342.

Resistor performance after testing can be compared to MIL-PRF-55342 limits, as shown in the table below. Internal reliability testing of custom magnetics is based on MIL-PRF-27.

4 Solid tantalum test standards are based upon MIL-PRF Solid tantalum capacitors undergo internal qualifications and periodic maintenance testing at elevated temperature and voltage. For critical applications, tantalum capacitors are designed, manufactured, and tested in accordance to limits that meet custom requirements. The table below shows failure rate predictions based on Weibull testing of solid tantalum capacitors. Resistors are qualified for critical medical applications in accordance with MIL-PRF Resistor failures fall into two categories: catastrophic failures, such as open or shorted resistors, and drift failures, which result in poor circuit operation. Resistor performance after testing can be compared to MIL-PRF limits, as shown in the table below. Internal reliability testing of custom magnetics is based on MIL-PRF-27. Some of the tests performed are solderability, resistance to solvents, terminal strength, shock and vibration, moisture resistance, and thermal shock. These tests are detailed in MIL-STD-202 and other ASTM or JEDEC standards. Summary Smaller medical devices allow for less-invasive, less-complicated procedures, making it easier for

5 the physician to perform and easier on patients. As newer passive devices with reduced sizes are introduced, better techniques in production and testing are necessary to improve the quality of the components. Suppliers of newer, smaller-form-factor passive components may require equipment investment and automation to achieve the process capability required by medical device manufacturers. Qualification testing and reliability testing in accordance with customer requirements and industry standards are requirements of the development process. Also see: Roundup: PCB protection/coatings Product how-to: Uses of copper barrier capacitors in MRI RoHS Recast (RoHS2) An update on RoHS Electronic features of implantable circuits Share Tweet 0 Like 0 Write a Comment To comment please Log In

Diagnosing the Future of Medical Applications

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