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january 2014 docid025706 rev 1 1/18 AN4428 application note best practices in the manufacturing process of mems microphones introduction this application note serves as a reference concerning best practices in the manufacturing process of mems microphones.these products have undergone thorough quality and reliability testing as st ma nufacturing proce sses have been carefully studied and developed to achieve highly reliable devices. section 1: quality assurance details the various tests performed on the microphones. this document also provides recommendations to properly manage the microphones during processes performed by the customer such as verification of the device upon reception, proper handling of the device and production line considerations. www.st.com
contents AN4428 2/18 docid025706 rev 1 contents 1 quality assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 reliability tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.1 htol: high-temperature operating life . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.2 hts: high-temperature storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.3 pc (jl3): preconditioning (solder simulation) . . . . . . . . . . . . . . . . . . . . . 4 1.1.4 tc: temperature cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.5 electrostatic discharge (hum an body model, machine model, charged device model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.6 lu (ci): latch-up (overvoltage and curren t injection) . . . . . . . . . . . . . . . . 4 1.1.7 thb: temperature humidity bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.8 lts: low-temperature storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.9 repeated free-fall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.10 ms: mechanical shocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.11 vvf: vibration variable frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.12 mtc: moisture and temperature cycling . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.13 air compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2 final test information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.3 pick-and-place settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4 pre-shipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2 customer manufacturing considerations . . . . . . . . . . . . . . . . . . . . . . . 13 2.1 customer handling recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.1 contaminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.2 electrical overstress (eos) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.3 mechanical overstress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.1.4 cavity detachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 appendix a bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
docid025706 rev 1 3/18 AN4428 list of figures 18 list of figures figure 1. air compression test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 2. final test checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 3. pick-and-place machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 4. picking area for 4 x 5 microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 5. 4 x 5 pick-and-place process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 6. picking area for 3 x 4 microphones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 7. 3 x 4 new design nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 8. picker mechanical details for hlga 3 x 4 micropho nes . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 9. actual pick-and-place nozzle us ed in st manu facturing facilities . . . . . . . . . . . . . . . . . . . 12 figure 10. back-end process flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 11. example of eos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 12. spike suppressor configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 13. mems sensor crack example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 14. cavity detachment example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 15. cavity ceiling detachment example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
quality assurance AN4428 4/18 docid025706 rev 1 1 quality assurance 1.1 reliability tests this section lists all of the reliabilit y tests performed on the microphones. 1.1.1 htol: high-temperature operating life the device is stressed in a dynamic configur ation, approaching the operative max absolute ratings in terms of junction temperature, load current, and internal power dissipation. this test is intended to simulate the worst-case application st ress conditions . the test is performed to investigate typica l ic failure modes like oxide faults and metal degradation and to check overall ic parametric stability. 1.1.2 hts: high-temperature storage the device is stored in an unbiased conditi on at the maximum temperature allowed by the package materials, sometimes higher than the ma ximum operative temperature. this test is performed to investigate the failure mechanisms activated by high temperature; typically wire-bond solder joint aging, data retention faults, and metal stress-voiding. 1.1.3 pc (jl3): preconditioning (solder simulation) the device is submitted to a typical temperature profile used for surface mounting, after controlled moisture absorp tion. this test is done to investigate in general the effect of customer manufacturing soldering enhanced by package water absorption. as a standalone test it is used to investigate the level of mois ture sensitivity. as preconditioning before other reliability tests it is used to verify that the surface mounting stress does not have an impact on the subsequent re liability performance. 1.1.4 tc: temperature cycling the device is submitted to cycled temper ature excursions, between a hot and a cold chamber in air atmosphere. this is done to investigate failure modes related to the thermo- mechanical stress induced by the different the rmal expansion of the materials interacting in the die-package system. typical failure modes are linked to metal displacement, dielectric cracking and molding wire-bond failure. 1.1.5 electrostatic discharge (human body model, mach ine model, charged device model) the device is submitted to a high-voltage peak on all pins, simulating esd stress according to different simulation models. this test is needed to classify the device according to its susceptibility to damage or de gradation by exposure to electrostatic discharge. 1.1.6 lu (ci): latch-up (overvoltage and current injection) this test consists of forcing a current into an input pin or requiring a current from an output pin. under these conditions, removing such a current, no change of the magnitude of the supply current must be observed. this is done to verify the presence of bulk parasitic effect- inducing latch-up.
docid025706 rev 1 5/18 AN4428 quality assurance 18 1.1.7 thb: temperature humidity bias the device is biased in static configuration minimizing its internal power dissipation, and stored at controlled conditions (ambient temperature and relative humidity). this test is aimed to investigate the failure mechanisms activated in the die-package environment caused by electrical field and wet conditions. it is mainly oriented to highlight typical failure mechanisms of ic in these conditio ns like electro-chemical corrosion. 1.1.8 lts: low-temperature storage the device is stored in an unbiased condition at the minimum temperature allowed by the package materials, sometimes lo wer than the min. op. temp. this is useful for investigating the failure mechanisms activated by extrem ely cold conditions for prolonged time. 1.1.9 repeated free-fall the device is subjected to repeated mechanical drops. this test is performed in order to check the robustness of the mems microphone when subjected to repetitive mechanical stress. 1.1.10 ms: mech anical shocks the device is subjected to 10000 g / 0.1 ms, 5 shocks for each axis. it is intended to determine the compatibilit y of the componen t(s) to withstand moderate ly severe shocks as a result of suddenly applied forces or abrupt change in motion produced by handling, transportation or field operation. 1.1.11 vvf: vibration variable frequency the device is subjected to a vi bration with peak acceleration 20 g , 20 hz to 2000 hz where three perpendicular directions have been appli ed. the vibration variable frequency test is performed to determine the effect of vibratio n, within a specified frequency range, on the internal structural elements. 1.1.12 mtc: moisture and temperature cycling the device is subjected to cycled temperature and moisture exposure. the test serves to investigate device robustness against the combined stress of moisture and temperature cycles.
quality assurance AN4428 6/18 docid025706 rev 1 1.1.13 air compression this test consists of applying high-pressure ai r into the sound inlet of the microphone. this application of compressed air is repeated five times for 1 sec from different distances. the test is performed using an air gun and its purp ose is to check the robustness of the mems membrane. the device passes the test if there is no variation of the sensitivity. figure 1. air compression test
docid025706 rev 1 7/18 AN4428 quality assurance 18 the results of the st mems microphone in te rms of reliability are indicated in the table below. this table contains the data of the mp34dt01 used as an example. table 1. reliability test results of the mp34dt01 test name condition/method period or frequency preconditioning (jedec level 3) moisture sensitivity 3 1 week at < = 30c/60%rh peak body temperature = 260c reflow profile = j-std-020c final htol vdd(max) = 3.6 v; tamb = 125c jesd22a108 168 h 500 h 1000 h thb vdd(nom) = 1.8 v t = 85 c / rh = 85% jesd22a108 168 h 500 h hts ta = 120c jesd22a103 500 h 1000 h tc ta cycling: -40c/+125c jesd22a104 100 cy lts ta = - 4 0 c jesd22a119 500 h 1000 h esd hbm voltage +/-2000 v j edec / jesd22-a114e - esd mm voltage +/-200 v jedec/jesd-a115-a - esd cdm voltage +/-500 v ansi / esd stm 5.3.1 esda - latch-up and overvoltage current injection +/-200 ma overvoltage 1.5 x vmax eia/jesd78 - repeated free-fall drops from height = 1 m iec 60068-2-32 300 drops mtc ta cycling: +30c/+65c rh=+90%/+93% iec 60068-2-30 144 h ms 10000g/0.1ms 5 shocks for each axis mil std 883mil - vvf vibration with acceleration peak 20g, variable frequency from 20hz to 2000hz; applied in three perpendicular directions for 16 minutes on each direction, 48 min total jesd22-b103b 48 min
quality assurance AN4428 8/18 docid025706 rev 1 1.2 final test information after production the micr ophone must be sc reened to avoid shipping failing parts to the final customer. both electrical and acoustic parameters are tested. first of all the microphones are tested in terms of electrical behavior. the asic, on each pin, is rigorously analyzed to find any occurrence of open circuit or shorts. ad ditionally the device is analyzed to check for any evidence of current leakage. finally, re ference voltages and current consumption are checked. once the entire electrical parameters have been verified, the microphones are screened in terms of acoustic performance. it is sufficient to test the sensitivity and the frequency response in order to determine whether the microphone works properly or not. the sensitivity can be used to determine if the membrane is damaged in which case the thd, as well as the noise floor, is out of sp ecification. finally the sensitivity is checked across the audio band. each microphone is tested according to the previously mentioned criteria to avoid shipping failing parts. figure 2. final test checklist
docid025706 rev 1 9/18 AN4428 quality assurance 18 1.3 pick-and-place settings the process of assembly of the printed circuit boards is fully automated owing to dedicated machines. basically these machines are used fo r high-speed, high-pre cision placement of electronic components, like capacitors, resistors, or integrated circuits onto the pcb. these systems normally use pneumatic suction cups, and at the end, a nozzle accurately picks the device from a predetermined area. figure 3. pick-and-place machine in the case of microphones, this process must be rigorously controlled since the position of the nozzle, the force involved in the pick a nd place, and the mechanical parameters can damage the structure of the microphone. hence, a reference/safe pick area is defined for each microphone depending on the dimension of the package. the reference pickup area for the generic 4x5 top-port mems microphone is shown in figure 4 . figure 4. picking area for 4 x 5 microphones basically the safe area has been set in order to not pick up the component by the top of the microphone sound port, thus preventing damage to the mems membrane or an incorrect pickup and placement. additionally this safe area has been dimensioned, considering the tolerances of all the mechanical parameters involved in the process, i.e. position of the sound inlet, microphone package dimensions and pocket dimensions. suction cup nozzle device am17604v1
quality assurance AN4428 10/18 docid025706 rev 1 figure 5. 4 x 5 pick-and-place process the nozzle for picking up a 4 x 5 microphone is commonly a steel picker with a cylinder shape respecting the safe area indicated in figure 4: picking area for 4 x 5 microphones . the vacuum port at the bottom side of the picker is a hole with a diameter of 1 mm. the safe pickup area for the 3 x 4 microphone is almost the same but with different dimensions due to the different package outline. as shown in the previous design, the following area has been dimensioned, also cons idering the tolerances of all the mechanical parameters involved in the process; i.e. po sition of the sound inlet, microphone package dimensions and pocket dimensions. the 3 x 4 safe area is indicated in the following figure. figure 6. picking area for 3 x 4 microphones to optimize the microphone yield, a dedicated picker has been designed to fully fit the allowable safe area; hence, the currently used nozzle for the 3 x 4 microphones has 4 vacuum holes for each corner (refer to figure 7 ). am17605 1 steel nozzle device tape & reel
docid025706 rev 1 11/18 AN4428 quality assurance 18 figure 7. 3 x 4 new design nozzle this design ensures that the holes for the vacuum and the air blow are always away from the porthole of the device (4 vacuum ports at the corner of the device). additionally, the new design has also a recess, in the form of a cross, allowing the porthole to be left always at atmospheric pressure. under th ese conditions, the membrane will not suffer any sudden air disturbances during the picki ng or placing of the device in the tape and reel. figure 8 shows the detailed mechanical dimensions of the new picker, the positioning of the vacuum holes and the width of the recess. figure 8. picker mechanical details for hlga 3 x 4 microphones the safe areas of the 4 x 5 and 3 x 4 microphones are identical and differ only in the dimensions. the safe area of the 4 x 5 microp hones includes the safe area of the 3 x 4 microphones; hence, despite this difference, t he 4 vacuum port nozzle can be used for both microphone packages. the following figure shows the usage of the 4-hole picker in the st production line (finishing).
quality assurance AN4428 12/18 docid025706 rev 1 figure 9. actual pick-and-place nozzle used in st manufacturing facilities finally, other parameters must be rigorously c ontrolled for the proper handling of the device: the force of the vacuum and the pressure of the nozzle on top of the microphone package. the pressure of the nozzle depends on the velo city and the height during the picking of the device but also depends on the force set for the placement of the nozzle on the safe area. to correctly control the forces involved during the pick-and-place process, the following indications must be respected: ? do not allow more than 7 psi (a) vacuum force ? usually the picker is set to a minimum di stance from the component (100-200 m) ? recommended placement force on the safe area is below 500 g 1.4 pre-shipment the flowchart below represents the back-end process from testing to shipment. basically the devices are tested according to electrical and acoustic parameters (ate), they are placed in a tray, then they are placed in a reel using the automatic pick-and-place machine and finally the microphones are packaged and shipped to the customer. figure 10. back-end process flow a. 1 kpa = 0.145 psi (lb/in2) = 0.0102 kgf/cm2 = 0.0098 atm maximum allowed pressure on microphone package am17606v1
docid025706 rev 1 13/18 AN4428 customer manufacturing considerations 18 2 customer manufacturing considerations 2.1 customer handlin g recommendations the purpose of this section is to summarize, for typical failures th at can occur in the customer?s production line, an explanation of th e issue first and then to propose a solution or even give a recommendation to avoid these failures, usually caused by mishandling of the device. 2.1.1 contaminations when the component provides a pdm output th at when listened to or measured shows poor snr performance or very noisy output, the issu e is typically due to contamination of the mems membrane. contamination during pcb sawing, during pcb cleaning or washing can occur in a customer?s production line. further contamination can happen during device soldering if a vapor phase soldering process is used for the reflow or some soldering paste accidentally drops inside the sound inlet. to avoid these types of contaminations, the se aling of the sound inlet with foam/tape before sawing to prevent dust from entering the microphone is recommended. as a matter of fact, using a vapor phase soldering process must be avoided to prevent membrane damage and contamination. 2.1.2 electrical overstress (eos) when the component does not show any signal at the output or the output is tied to gnd (or to vcc), most probably the asic has been submitted to an electrical overstress. electrical overstress is an unexpected electr ical phenomenon, like a high-voltage discharge, that can cause irreversible damage to the asic such as burned areas or voids in the silicon. figure 11. example of eos to avoid this issue, it is highly recommended to verify the supply voltage vs. microphone operative and max absolute voltage specifications, or also to verify the voltage ripple when doing hot plug to testing jigs. a diode (spike suppressor) can be included in the testing equipment using the following configuration.
customer manufacturing considerations AN4428 14/18 docid025706 rev 1 figure 12. spike suppressor configuration a spike can also be caused by an electrostatic discharge. esd can be a problem when there is a wrong or poor connection of the operators or of the equipment to ground. additionally it must be carefully checked that esd does not exceed the declared values of the components. improving esd protection in the production line is highly recommended in order to avoid this kind of issue. 2.1.3 mechanical overstress if the component sensitivity is excessively far from the specification value, most probably the membrane has been damaged. such damage is typically caused by a mechanical overstress. the asic in the component is working properly, but performing a visual inspection reveals that the mems microphone membrane is damaged. the membrane can be affected by cracks or voids due to heavy mechanical ov erstress. such stresses can be caused by multiple reasons. when the microphone is moun ted on a module, it is important to control the module assembly on the final equipment (t ypically a laptop). during the assembly the screwdriver can hit the microphone damaging the membrane. in general, high shock events, above 10,000 g, can happen at other instances during handling due to equipment design or configuration. mechanical shocks can also be caused by the pick-and-place equipment, for example if the nozzle does not pick in the prope r area or if the machine picks or places the device with excessive force. finally, also the us age of air to clean pcb can be dangerous if the pressure is too high. figure 13. mems sensor crack example to avoid the issues listed above, the customer must pay attention in setting the production line parameters such as the path of the scre wdrivers on the modules (if any) and carefully set the equipment to avoid any kind of dangerous shocks. a further recommendation is related to the pick-and-place equipment. st strongly recommends using the nozzle with 4 vacuum holes specifically designed to mi nimize the issue related to mechanical overstresses in production line. for the same reason, the forces involved in the pick-and- am17607v1 mems crack
docid025706 rev 1 15/18 AN4428 customer manufacturing considerations 18 place process must be controlled. high pressure (few bars) for long distances (few mm) can make the part reach velocities around 3 to 5 meter / sec. in case of impact the resulting shock can be much higher than 10, 000 g. duri ng the picking process the picker is set to a minimum distance to the component (100-200 m). conversely, regarding the placing process, the recommended force must be set below 500 g. as a last recommendation, if using an air gun to clean the pcb, st reco mmends limiting the pressure of the air flow. 2.1.4 cavity detachment the malfunctioning of the device in terms of any audio parameter, sensitivity, snr, frequency response, thd, can be related to the detachment of the cavity. the detachment of the cavity is a phenome non where one a portion of the package is detached. this can be caused by free-fall of components over a rigid surface from more than 1 meter. another possible reason is the cutting of the pcb if there is the need to separate different sections of the board. cavity detachment is typically caused by mechanical overstresses but can also be caused by thermal stresses. as a matter of fact, if the reflow profile differs from the standard one, the glue on the package can be weakened, losing its fixing capability. as a last reas on the detachment can be caused by a force exceeding 2.5 g for tape removal (when tape is applied to cover the microphone inlet for top- port devices). figure 14. cavity detachment example figure 15. cavity ceiling detachment example to avoid detachment of the cavity, a good practi ce is to avoid any mechanical overstress as listed in section 2.1.3 . additionally, following the st recommended reflow profile and reducing the force to 1 kg for tape removal are further advised.
bibliography AN4428 16/18 docid025706 rev 1 appendix a bibliography 1. ipc/jedec j-std-020d.1. mo isture/reflow sensitivity classi fication for non-hermetic solid state surface mount devices 2. stmicroelectronics - an4211 guidelines for soldering mems microphones 3. stmicroelectronics internal document: good practice using mems microphone in production line v1.31 4. stmicroelectronics mp34dt01 reliability report
docid025706 rev 1 17/18 AN4428 revision history 18 3 revision history table 2. document revision history date revision changes 09-jan-2014 1 initial release.
AN4428 18/18 docid025706 rev 1 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a particul ar purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. st products are not designed or authorized for use in: (a) safety critical applications such as life supporting, active implanted devices or systems with product functional safety requirements; (b) aeronautic applications; (c) automotive applications or environments, and/or (d) aerospace applications or environments. where st products are not designed for such use, the purchaser shall use products at purchaser?s sole risk, even if st has been informed in writing of such usage, unless a product is expressly designated by st as being intended for ? automotive, automotive safe ty or medical? industry domains according to st product design specifications. products formally escc, qml or jan qualified are deemed suitable for use in aerospace by the corresponding governmental agency. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2014 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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