【Advantages of Using P-Flex®】Increase the frequency response of MEMS microphones with flexible substrates

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Advantages of Using P-Flex® for MEMS Microphones

MEMS microphones are used in various electronic devices including smartphones because of their small size and suitability for mass production. Most of them are surface-mount type and are mounted on PCBs.

Types of MEMS Microphones: Top Port Type and Bottom Port Type

MEMS microphones have a small hole in the case to capture sound. If the hole is on the top cover side, it is called a top-port type, and if it is on the PCB side, it is called a bottom-port type. The figure here is a cross-sectional view of a MEMS microphone from Cirrus Logic’s application report. (Left: Top port type, Right: Bottom port type)

Two advantages of using P-Flex®, which is thin and has a high degree of form-flexibility

To maximize sound performance, the thickness of the case, gasket (*), and protective mesh layer in the sound capturing area should be reduced. The thickness of the PCB also has an important factor since the bottom-port type sound is drawn in through the PCB. It is recommended that the sound input section should ideally be 0.5 mm thick, with a maximum thickness of 4 mm (Microchip application note).

By mounting the MEMS microphone on P-Flex®, a flexible substrate made of 25µm (polyimide and thinner substrates), it is possible to accommodate the recommended thickness of the input portion of a bottom port microphone type.

P-Flex® can also be used to separate the microphone from the main PCB to prevent vibration coupling for both top-port and bottom-port microphones.
It is made possible to separate the MEMS microphones from noise sources such as buzzers, motors, and mechanical switches mounted on the main PCB.

(*)Terminology: A gasket is something that is pinched and compressed to seal the gap and at the same time prevent leakage of fluid or entry of foreign matter from the outside

Difference in frequency response with substrate thickness

Here is a figure taken from an application report by ST Micro, simulating the thickness of the sound input of a MEMS microphone at two different thicknesses (cylindrical tube of 1.5 mm length = standard PCB, cylindrical tube of about 0.3 mm length = FPC). The simulation results for the two substrate thicknesses indicate that the use of flexible substrates is better to obtain a flat frequency response over a wide range, up to the audio band and near ultrasonics.
Another advantage of flexible substrates is that they are not limited to two dimensions and can be softly positioned in the appropriate location on the enclosure. This feature also contributes to optimal mechanical design of the final product.
In addition to these features of flexible substrates, P-Flex® has the advantage of being environmentally friendly due to the Pure Additive manufacturing process.