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Selecting passives for medical devices

06 May 2014  | Patrick Gormally

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In the past years, medical devices have become even smaller. The smaller implantable devices offer better patient comfort and less disruption to the body. To fulfil 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.

Multi-layer ceramic capacitors (MLCCs) for implantable devices in the 0201 case size (0.024 in x 0.012 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).

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 (20GHz), 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, moulded inductors, and antennas that have unique properties for performance and shape. Additionally, various core materials and shapes are used to optimise performance and fit requirements for each application.

Trade-offs 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 specialised 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 0.020 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) and (0.050 in x 0.050 in x 0.020 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 2512. The criteria for resistor selection includes pulse handling, operating voltage, operating temperature, and long-term stability. Wire-bondable resistors range in size from 0.015 in by 0.015 in by 0.010 in with a 125 mW power rating, to 0.055 in by 0.055 in by 0.010 in with values up to 30 MΩ and operating voltage of 100 V.

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