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MEMS microphones and contamination

Question asked by nyberg.jonas on Nov 25, 2014
Latest reply on Dec 11, 2014 by nyberg.jonas
Hello!

I have a very specific question about MEMS microphone and noise induced by contamination. Refering to http://www.st.com/st-web-ui/static/active/jp/resource/technical/document/application_note/DM00103202.pdf, it is stated in chapter 2.1.1 that microphones can exhibit a high SNR or high noise output if contaminated. The high SNR I can understand if for example the MEMS membrane get sticky with say soldering flux. But what is the mechanism for just high noise at the output?

The background to my question is that I'm developping an acoustic camera using 32 MEMS microphones. I got as far as ordering my first prototype PCB and everything worked fine at first but after while I started to see very high intermittent noise at the outputs of some of the mics. The noise comes and goes with time but has gotten worse over time. I confirmed that the phenomena is actually high random noise by recording all the mic signals in a quiet environment and compared the recordings to normally working mics. Then I ran an FFT on all the signals and for the high noise one's I could see an strong otherwise quite white noise shape but also very strong near DC components (around 3Hz).

After this I looked at the sound-inlets through a microscope and saw that some of the inlets to the noisy mics were completely clogged with soldering flux. Could this then alone be the reason for the intermittent noise?
My logic says that if the sound inlets and the mic is contaminated with flux I would only get a very insensitive mic but not higher "standby" noise levels?
BTW, I also checked the integrity of all the clock signals etc. and they looked fine.

I really appreciate if any one would know if this is my problem alone.
Thanks in advance,
Jonas Nyberg
  

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