1 Overview

According to the introduction, MEMS microphone includes MEMS sensor chip and application specific integrated circuit (ASIC) chip. MEMS capacitive sensor chip converts sound pressure into capacitance change. ASIC chip monitors the capacitance change and transmits its analog electrical signal to ADC (analog digital converter). It has small volume and strong heat resistance. It can be maintained for 8 ~ 10 minutes in reflow soldering. At the same time, it can withstand the impact of the vaporized flux and solvent in reflow soldering. After welding, the detection of its performance is very important. After reflow soldering, testing the performance of the mechanism before assembly will largely avoid the machine dismantling and scrapping caused by the detection of defects at the end of the finished product. In this stage, the test object is PCBA (print circuit board assembly), and the test process is called SA (sub assembly) test. At this time, the PCBA has been welded with MEMS microphone, ADC integrated circuit chip, etc., which has the function of receiving sound signal and converting it into analog signal and then transmitting it to the central processing unit for analysis. If poor performance is found, PCBA analysis and maintenance can be carried out.

Testing principle and important parameters of MEMS capacitive sensor chip

After passing the SA test, after the MEMS microphone protection net, the mechanism upper shell and the main body of the smart speaker (including the speaker and the main control board with power amplifier function) are assembled, it is necessary to comprehensively evaluate the performance of the MEMS microphone, which is directly related to the user’s experience. In order to avoid the bad performance caused by the mechanism assembly and affect the use, FA (final) is introduced Assembly) test, focusing on the performance evaluation of MEMS microphone in the whole machine performance.

2 SA test

2.1 test principle and important parameters

The monomer of PCBA is MEMS microphone, the sound transmission hole is at the bottom of the part, and the signal transmission is analog differential signal transmission. This kind of microphone usually consists of a very thin silicon sensitive film and a plate (or backplane) with points to form a capacitance structure. The capacitance change between the sensitive film and the plate can detect the deviation of the sensitive film caused by sound vibration. The distance between the two plates is changed when the sound wave collides with the moving plate, which changes the capacitance of the microphone, causes the imbalance of the charge on the plate, and produces the electric signal.

Through the data comparison between PCBA to be tested and the selected standard prototype, the corresponding response curve is obtained to determine the good and bad products. The main performance parameters are raw fr (frequency response) and fi (fault injection) fr. Frequency response is an important parameter to reflect the dynamic response of MEMS microphone to frequency. In the process of acoustoelectric conversion, the ratio of the open circuit output voltage of each frequency acoustic signal to the open circuit output voltage of microphone with specified frequency under the action of constant voltage and specified incident angle acoustic wave is called the frequency response of microphone, and the unit is dB.

The test concept of bad injection fr: put the microphone sound hole and its remaining body in two independent cavities respectively, and set a loudspeaker in the cavity where the non sound hole is located to make sound. Record the frequency response of the microphone during the test. If the frequency response is suppressed, it indicates that the microphone body and PCB are welded well, and there is no or only very weak sound from the path of the microphone non sound hole MEMS sensors.

2.2 test environment

The hardware required for the test includes: independent sound card (Creative sound card), dual channel power supply (model: bk9173), power amplifier (model: GX3), passive speaker (used for making sound during bad injection test), passive speaker (model: LS50), dedicated speaker CPU circuit board (used to receive the electrical signal from UUT, CPU circuit board and UUT (unit under) Test (to be tested) are connected by FPC (flexible printed circuit board), scanner (to read the barcode of the tested object), PC (to communicate with CPU by network cable), USB interface is used to control the power controller, RS232 is used to control the test box, sound card and power amplifier are used to control the passive speaker and LS50 sound). The dotted line part indicates the test box, which includes two independent sound insulation boxes. At the same time, there are reserved holes in the middle part of the two boxes, so that the sound from LS50 in the lower box can be transmitted to the microphone transmission hole on the UUT. The UUT is placed on the silicone carrier in the upper box. The microphone transmission hole corresponds to the reserved hole in the box one by one, and other parts remain isolated from the lower box.

2.3 test process

Using LabVIEW to write the test control program, and through the network cable to generate the curve of the signal sent back by the CPU, the performance of the microphone can be judged directly by observing the curve. The test program drives LS50 to send out 10 Hz ~ 10 kHz sound source through sound card power amplifier. At this time, the microphone is in the signal receiving state. The power supply provides microphone bias DC voltage of 3.3 V and LED power supply voltage of 5.0 v V (LED is used to indicate whether the working voltage of the whole microphone array is normal). After receiving the sound signal, the microphone converts it into analog electrical signal and transmits it to the analog-to-digital conversion chip. After ADC processing, the digital signal is transmitted to the speaker dedicated CPU circuit board for more complex internal calculation. After completion, the data is transmitted through the network to the test software, and the software analyzes the data and standard The test data of the quasi prototype are compared to generate the corresponding curve and complete the raw FR test.

The fr deviation between each microphone in the microphone array and the corresponding position in the standard prototype shall not exceed ± 1dB. Similarly, after the raw FR test, the program drives the passive speaker of the upper voice cavity to make sound, and the microphone receives the sound signal. In order to simplify the test, only the frequency response of the microphone at 100 Hz is analyzed. If the test result exceeds the upper limit, it indicates that the microphone has defects in SMT welding, and the typical bad phenomenon is that the grounding ring is not sealed. Because the mass production stage can not pass the X-ray full inspection of all the microphone welds on PCBA, so the elimination of electrical test is very important.

2.4 selection of standard prototype

As mentioned above, our test is a relative test, that is, the test results of the object to be tested are compared with the test results of the standard prototype, and the deviation should be guaranteed within a certain range. Take the FR test as an example, if the fr deviation of the microphone at a certain position in the microphone array exceeds the microphone at the corresponding position of the standard prototype, it indicates that there is a problem with the microphone of the object to be tested, which may be a welding problem, or a welding problem Maybe it’s the performance of the microphone itself. The welding problems can be confirmed by X-ray detection, and the body performance problems can be checked by cross validation method. After confirming the problem point, carry out targeted repair. Because it is a relative test, the selection of the standard prototype is very important. Next, we will introduce the selection process of the prototype.

First of all, the microphone manufacturer needs to prepare a batch of microphones that have passed the strict monomer test. Under the condition of 94 dB SPL and 1 kHz, the monomer sensitivity is – 38 dbv / PA (± 1 dB). Next, the SMT process is used to solder the monomer to PCB to form microphone array. X-ray full inspection of SMT solder joints, and finally in our test environment test, get the prototype of the original curve. The fr of each microphone in the PCBA array must be 0 dB (± 0.5 dB) at 1 kHz. The test data of the prototype is stored in the fixed position of the test program, so that the program can be called and compared when testing the normal products to be tested.

3 FA test

After the SA test, the PCBA with good performance is obtained. After a series of assembly, the complete machine is formed, that is, the finished machine seen by the end user. The performance of microphone array in the finished machine can be evaluated through the SPL (sound pressure level) test of FA, and the defective products can be intercepted in the assembly factory.

3.1 test principle and important parameters

Similarly, the FA test is also a relative test. A part of the prototype selected in SA is assembled into the whole machine. The intelligent speaker and microphone array mentioned in this paper are assembled on the top of the whole machine, which is convenient to pick up sound from multiple angles. Sound check software is used to build the software test environment, and FR and phase are used to evaluate the performance. FR is used to determine whether the performance of the microphone body is damaged during the assembly process, and whether the microphone sound transmission hole is blocked during the assembly process; phase is used to determine the position matching between the microphone sound transmission hole and the reserved hole of the mechanism, and whether the reserved hole is too large or too small and the position deviation can be reflected in the test curve.

3.2 test environment and process

The SPL test box with only one cavity is used. The passive sound source LS50 is placed on the top of the test box. The whole machine to be tested is placed directly below the sound source. The whole machine communicates with PC through network cable. The sound check test system in PC controls RME sound card through USB interface, and transmits the set sound source file to power amplifier GX3. GX3 amplifies the signal and transmits it to LS50. LS50 sends out sound wave of fixed frequency band as required for microphone array to pick up. The microphone array picks up the sound wave, which is processed by the CPU inside the whole machine, and then transmits the signal to the sound check software through the network cable to generate the curve, so as to determine the performance of the whole machine.

4 Conclusion

With the development of consumer intelligent electronic products in the direction of more portable and more stable, the requirements of human-computer interaction are higher and higher, and the application of MEMS microphone is more and more widely, which has great attraction to medical treatment and automobile. Aiming at the test of MEMS microphone array used in smart speaker, this paper describes the factory end production test scheme. The test principle and process mentioned in this paper is a good reference for the product test of MEMS microphone application in different fields.

Editor in charge: PJ

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