Hello. We have been using ICS-41350 microphones for several years, with 2 microphones in each product. We have already assembled several thousand products using the same technology, taking into account all your recommendations for installing and soldering MEMS microphones.
But when assembling the last batch, in the amount of 6,000 pieces, we found that about 30% of the microphones behave strangely. After mechanical impact on our product (it is mobile, for example, it can be placed on a table or taken from a table), which naturally leads to a pulsed, but not very strong, sound effect, some microphones have a characteristic transient process and then there are strong noises, as if the internal converter becomes unstable (at the same time the microphone still continues to sense external sounds, but much weaker). The same repeated exposure can bring the microphone out of this state. Foreign objects were excluded from entering the microphone chamber, as the microphone was disassembled and the camera with a membrane was examined through an electron microscope.
Here are the images as an example, there are two channels, the microphones of both channels are located on the same board next to each other. In one channel, the microphone continues to behave normally, in the other, after a characteristic transient process, it becomes unstable.
Some microphones start behaving this way without any external influence.
Please clarify the situation and help me understand the reason for this behavior of the microphones, either this is a low-quality/fake batch, or there is damage during soldering (although everything has been checked and it seems unlikely).