In recent years, medical equipment has been developing towards the trend of smaller and smaller volume; Small implantable devices can make patients feel more comfortable and less disturbing to the body. In order to meet the needs of implantable medical devices for smaller hybrid components, people continue to improve the hybrid layout and packaging technology of microcontroller (MCU) - or application specific integrated circuit (ASIC) - and power supply system. This paper discusses the selection process of passive components in order to reduce the space of hybrid components and circuit boards in medical equipment.introductionPassive components are produced in large-scale manufacturing facilities, and the differences between different batches are reduced through excellent process control. Compared with commercial components, medical components require a higher level of reliability and performance in a smaller size. In terms of component manufacturing methods, various schemes can be adopted to reduce the space occupied by hybrid components and circuit boards, and improve reliability at the same time.
Capacitor selection standardEach capacitor technology has its own unique attributes, which should be regarded as part of the specific reference standard when selecting products for final application.For implantable devices, housing size 0201 (0.024in × 0.012in) multilayer ceramic capacitor (MLCC) to provide decoupling or tuning function for radio / telemetry system. The maximum MLCC size is 2225 (0.22 in × 0.25in), usually used as a resonant capacitor for in vitro medical equipment.
The shell size of solid tantalum capacitor for medical equipment is 0402 (0.045in × 0.026in × 0.024in), its low height helps to save space. In addition, there are 1210 t housing dimensions (0.138in × 0.11in × 0.063in) reliable and large capacity fixed tantalum capacitor.The existing size of silicon-based capacitor is 20MIL × 20MIL products with a maximum capacitance of 1000pf were launched. Silicon capacitors can be bonded by epoxy resin or low eutectic chips and are wired. In addition, there are 0402 surface mount "flip chip" silicon-based capacitors with shell size and maximum capacitance of 27pf. Silicon based capacitor technology supports reliable broadband operation (20GHz), and has the characteristics of high Q value, low DC resistance (DCR) and high self resonant frequency (SRF).Selection standard of magnetic components
Most magnetic components are custom designed to fit the limited space of specific medical device applications, in cooperation with engineers from medical device manufacturers and magnetic component companies. The magnetic components customized for implantable devices are generally composed of skeleton transformer, ring coil transformer, molded inductor and antenna with unique performance and shape. In addition, a variety of core materials and shapes can be used to optimize performance to meet the needs of different applications.After discussing the trade-offs of size, price and performance, you can target the most cost-effective and best performing components that match the space requirements. Once the design work is completed, strict manufacturing process, control and test procedures can be formulated to ensure that the products reach the highest quality level in terms of size and magnetic properties and achieve the best reliability. Small design usually needs 3D CAD simulation to achieve accurate component layout and prototype design.In the manufacturing process of customized magnetic components, a variety of special air core coil, skeleton and ring coil winding equipment will be used. This equipment has strictly controlled key electrical performance requirements. Inspection equipment such as optical measuring instruments will be used for the measurement of key dimensions. Custom designed test bench and fixture can monitor and test electrical parameters. These automatic test stands can be used for data analysis to ensure the manufacturability of the design.
The size and shape of magnetic components used in different medical devices vary greatly, depending on the specific application scenario. 0402 small size inductor (0.040in) × 0.020in) for telemetry / communication applications. These inductors can be wired, made of ceramic core, and the maximum inductance can reach 150nhHigh frequency wired RF spiral inductors are available in two sizes: (0.030in × 0.030in × 0.020in) and (0.050in × 0.050in × 0.020in）。 These inductors have excellent neutral performance in the RF band and are suitable for bias, tuning and lumped element filter applications.Selection standard of resistor
The housing size of standard thin film and thick film surface mount resistors is between 0402 and 2512. The selection criteria of resistors include pulse processing capacity, working voltage, working temperature and long-term stability. The size range of wired resistors is 0.015 inch × 0.015in × 0.010in (rated power 125MW) to 0.055in × 0.055in × 0.010in (the maximum resistance is 30m Ω; the working voltage is 100V).Thin film resistors support dense circuit arrangements while providing the advantages of highly reliable resistor films. The resistor tolerance is as low as 0.01% and the resistance temperature coefficient (TCR) is as low as 5ppm, which is conducive to the accurate adjustment of amplifier, transceiver circuit and power distribution. For medical applications, these chip resistors can cover any standard resistance value (10 Ω ~ 25m Ω).High reliability test
For medical equipment applications, avoiding catastrophic failure and drift failure of passive components is a top priority. In the final analysis, the product reliability prediction is based on the supplier's test data and the application operating temperature specified by the medical device manufacturer within a defined time range. The process control of passive component suppliers is an important factor to achieve high reliability. The reliability of passive components can be determined by raising the temperature under rated or higher voltage and carrying out the life test for a specified period of time, so as to ensure that the components can be used in important medical equipment applications. Passive component testing is based on customer requirements and U.S. military standard (MIL) specifications (if applicable).
The reliability prediction of passive components can be carried out using the on-line modeling program based on mil-217 manual or iec863. The following is an example of supplier reliability simulationThe fixed tantalum test standard is based on mil-prf-55365. Fixed tantalum capacitors undergo internal qualification and periodic maintenance tests at elevated temperature and voltage. For key applications, tantalum capacitors are designed, manufactured and tested according to the limits that can meet customer customized requirements.The following table shows the failure rate prediction based on Weibull test of fixed tantalum capacitors.
Qualified resistors for critical medical applications in accordance with mil-prf-55342. Resistor failures fall into two categories: fatal failures (e.g., resistor open or short circuit) and drift failures (resulting in poor circuit operation).The resistor performance obtained from the test can be compared with the limit value of mil-prf-55342, as shown in the table below.The internal reliability test of custom magnetic components is based on mil-prf-27 standard. The tests carried out include weldability, solvent resistance, terminal strength, impact and vibration, moisture resistance and thermal shock. These tests are detailed in mil-std-202 and other ASTM or JEDEC standards.
summaryWith the help of small medical equipment, the operation can be simpler and less invasive, so as to facilitate doctors' operation and reduce patients' pain. With the introduction of smaller and newer passive components, better manufacturing and testing technologies are also needed to improve the quality of components. Suppliers of new small-size passive components may need to increase equipment and automation investment to obtain the process capacity required by medical equipment manufacturers. Qualification test and reliability test in accordance with customer requirements and industry standards are the requirements of the development process.