MEMS microphones dominate new cellphone designs

As designers continue to integrate new features into sleek new cellphone designs, there continues to be a drive for integrated components and for size reductions of each individual component.

As designers continue to integrate new features into sleek new cellphone designs, there continues to be a drive for integrated components and for size reductions of each individual component.

In clamshell or similar types of designs where each half contributes to the overall thickness of the phone, component height is the limiting factor that determines the thickness of the phone.

Thin phone designs have proven to be very popular with consumers, as demonstrated by the popularity of recent phone models launched in the market.

One stand-alone component that is subject to these design pressures is the microphone.

In some cellphone designs the microphone is the limiting component that determines the thickness of the design, and requires special modification of the phone case to accommodate the microphone height.

Traditional electret condenser microphones (ECMs) have reached the 4 x 1.5mm size (without acoustic boot) but are approaching the limits of the technology and are unlikely to shrink much further.

The future technology in microphones is MEMS (micro-electro-mechanical systems).

MEMS is a technology that enables the manufacturing of small mechanical components on the surface of a silicon wafer.

In the case of a microphone, a back plate and diaphragm are built on the surface of a wafer along with the necessary electrical connections.

The diaphragm is then "released" through chemical etching so it can vibrate freely with incoming sound.

The changing capacitance of the charged capacitor formed by the back plate and diaphragm transduces sound into an electrical signal.

MEMS microphones take advantage of semiconductor processing and have inherent advantages over ECMs.

For example, the MEMS microphone produced by Knowles Acoustics (SiSonic) uses a charge pump to produce a constant charge on the diaphragm, resulting in better isolation from power supply noise.

In addition, the MEMS can recover from moisture condensation as the charge on the diaphragm is renewed by the charge pump once the condensation dries.

A MEMS microphone is not susceptible to charge decay over the life of the phone or to sensitivity shifts due to supply voltage changes.

It is inherently less susceptible to vibration because of the smaller diaphragm mass.

MEMS microphones are also perfect candidates for chip-scale packaging since integration with amplifiers, A/D convertors, and other IC devices is a relatively straightforward packaging task.

MEMS microphones have two main characteristics that make them particularly suitable for thin phone designs: small size and the ability to withstand reflow temperatures.

One of the most notable differences between a MEMS microphone and an ECM is sise.

The back plate and diaphragm in a MEMS microphone are approximately 10x smaller than those in the smallest ECM.

This inherent small size allows packaged MEMS microphones to start at approximately the same sise as the smallest ECMs, with the potential to shrink much further as MEMS microphone technology matures.

A smaller microphone consumes less PCB space and requires smaller height allowances, making it ideal for a thin phone designs.

The other main characteristic that distinguishes MEMS microphones is their ability to withstand standard solder reflow temperatures.

The ability of microphones to withstand reflow temperatures without impacting performance is so unique that many engineers simply refer to MEMS microphones as "reflow mics" to distinguish them from traditional ECMs.

There are obvious manufacturing benefits from using auto pick-and-place tools and solder reflow to install the microphone, but reflow mics also enable a new approach to acoustic path design that make them attractive for thin phone designs.

A typical cellphone design incorporates the primary microphone somewhere near the bottom of the keypad, ideally with the acoustic hole in the front case of the phone near the speaker's mouth.

Since the acoustic path length from the outside world to the microphone must be kept as short as possible to maintain optimum performance, the microphone must be on the front of the PCB near the acoustic hole in the case.

However, in a thin phone design virtually all components need to reside on the back side of the PCB, opposite the keypads.

This creates a height issue that is a barrier to creating a thin design.

With a MEMS microphone, the mic can be reflowed onto the back side of the phone PCB with other components, while utilising an acoustic path that goes directly through the PCB to the acoustic hole in the front case of the phone.

To achieve this, the microphone package is designed with an acoustic port hole in the bottom along with the signal solder pads.

The mic port hole aligns with a corresponding hole in the PCB, and a solder pad around the port hole provides an acoustic seal between the microphone and the main phone PCB.

A simple gasket on the front side of the PCB provides a seal to the phone case, and a hole in the case completes the acoustic path to the outside world.

In many designs, the gasket between the PCB and the phone case can be implemented by a simple extension of the rubber seal between the keypads and case.

A short, elegant acoustic path like this allows speech to travel through the phone case, gasket, and PCB and directly into the microphone, while allowing the microphone to reside on the back side of the PCB with the other components.

This approach to acoustic path design puts virtually zero height requirements on the front side of the PCB and allows for tighter height tolerances on backside components for thinner overall phone designs.

MEMS technology is radically changing the microphones in cellphone designs.

Because MEMS-based microphones are inherently smaller than traditional microphones and can be reflowed with other components, they provide the flexibility needed by engineers who must design smaller, thinner phones today.

The ease of integrating additional features through new companion ICs will also make MEMS microphones the logical choice for tomorrow.

MEMS microphones and their many advantages will inevitably win a place in the majority of new cellphone designs.

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