Capacitors for Use in Hearing Aid Devices: Requirements and Technologies
Hearing aid technology has improved tremendously in the last two decades, and so is the technology used in the manufacturing of electronic components. Although the early designs of hearing aids were analog, most of today’s devices are digital. Unlike analog hearing aid devices, digital hearing aids use advanced digital technology and can implement various types of algorithms including noise reduction, feedback cancellation, and statistical data logging. Capacitors are fundamental components in both analog and digital hearing aids, and their characteristics greatly influence the overall performance of a product. It is therefore crucial for circuit designers to ensure that they select the right capacitors when designing a hearing aid.
Key considerations in selecting capacitors for hearing aids
Capacitors, much like other passive electronic components, come in different shapes, sizes, and designs. The performance characteristics of these components vary primarily depending on the technology they are based on. The choice of a specific capacitor technology is mainly determined by the requirements of an application. Whereas early electronic devices were based on through-hole printed circuit boards (PCBs), most of today’s electronic circuits use surface mount technology. Surface mount components are particularly suitable for applications with strict space constraints such as hearing aid devices.
The circuit of a typical hearing aid device consists of a multi-stage sound amplification system, a microphone, a speaker, and a battery. Unlike general purpose electronic systems, hearing aid devices demand electronic components with special performance characteristics. One of the key considerations for these applications is the size of a component. The high volumetric efficiency and impressive performance characteristics of tantalum capacitors make them a suitable choice for today’s hearing aid devices.
Although hearing aid devices come in different designs, all of them have one common characteristic: they are small in size and demand miniature electronic components. The in-the-canal hearing devices are inserted into the ear canal and are arguably the smallest in size. For these devices, the size of electronic components is a key design factor. Many capacitor technologies do not meet the strict space requirements of modern hearing aid devices. This requirement favors capacitor technologies that deliver high volumetric efficiencies.
One of the main uses of capacitors in hearing devices is in signal filtering. For this use, high capacitance values are required. Miniature surface mount tantalum capacitors are commonly used for this application because they offer large values of capacitance. Unlike most capacitor technologies, these components have higher volumetric efficiency and longer service life.
Conformally coated versus molded surface mount tantalum capacitors
The performance of a capacitor is significantly determined by the manufacturing technology used. For tantalum capacitors, the two most common types of surface mount components are conformally coated and molded tantalum capacitors. In terms of properties, both types have nearly identical electrical characteristics. Conformally coated components were commonly used in the early designs of hearing aid devices. For the new generation devices, most manufacturers are using molded capacitors because they offer better performance characteristics.
One of the main reasons why molded surface mount capacitors are preferred for use in modern hearing aids is that they offer impressive size uniformity and repeatability. In comparison, conformally coated components lack these characteristics. Considering the space constraints of today’s hearing aids, size fluctuations that are common with conformally coated components are undesirable.
Molded construction technology produces miniature tantalum capacitors with flat and smooth tops. This flat top design allows manual and automatic machines to pick the components with ease. For components that do not have smooth and flat tops, it is common for machines to miss them when picking them during assembly.
The irregular top surfaces of most conformally coated surface mount capacitors make them an unsuitable choice for most assembly line machines. This irregular surface also makes it difficult for manufacturers to mark components. Moreover, a flat top design allows equipment to pick components easily during inspection. The flat top design of molded tantalum capacitors makes them a suitable choice for use in modern hearing aids.
Different capacitor manufacturing technologies yield different solder reflow conditions. These solder reflow characteristics significantly determine the overall reliability of a component. For a molded tantalum capacitor, the leads of a component are specially designed to absorb thermomechanical stresses caused by solder reflow. Unlike molded tantalum components, conformally coated tantalum capacitors have poor solder reflow conditions. For these components, thermomechanical stresses can result in cracking and failure of a component. This characteristic makes conformally coated surface mount capacitors an unsuitable choice for hearing aids.
After solder reflow, electronic assemblies are usually cleaned using special solutions. For surface mount assemblies with cracks caused by thermomechanical stresses, the cleaning solutions can penetrate through the cracks and significantly affect the electrical performance of a device. This contamination is common for assemblies with conformally coated capacitors. In comparison, the leads of molded capacitors are specially designed to prevent such defections. The high tolerance of these components to contamination during cleaning makes them a better choice for hearing aid devices.
Apart from hearing aid devices, tantalum capacitors are also widely used in other medical appliances. This capacitor technology is commonly used in handheld medical devices, heart pacemakers, and other life and non-life support medical devices. These components have impressively high reliability, and this makes them a suitable option for critical applications. Moreover, these passive components have a long service life.
Capacitors are used in both analog and digital hearing aids for filtering and other applications. Due to space constraints, these devices demand miniature components with high capacitances and high reliability. The high volumetric efficiency, process compatibility, and impressive reliability of tantalum capacitors make them an unrivalled choice for hearing aids. The two most common types of surface mount tantalum capacitor technologies are conformally coated and molded tantalum capacitors. As compared to conformally coated surface mount capacitors, molded components have uniform and regular tops, high resistance to contamination, and impressive solder reflow properties. These characteristics make them an ideal choice for use in hearing aid devices and other medical appliances.