Yield problems related to MEMS accelerometer - Page 1
Forum section: electronics / general

author: andyturk
author_rank: Frequent Contributor
timestamp: September 15, 2016, 02:04:50 pm
content: We're having some trouble with yield on a consumer product that incorporates one of ST's accelerometer chips. We're failing up to 40% of our assembled boards during testing because the accelerometer readings are out of spec. The test methodology is basically looking for offset along each of the X, Y, and Z axes... and our error bands are actually larger than what the OEM recommends. If we used their guidelines, we'd have an even lower yield.These failures a specific to the accel chip in that we can take a failing board, replace the accel by hand, and it'll be perfect. If we take an accel chip from a failing board and reflow it onto a previously working PCB, the failure moves with the bad part.We have two hypotheses: 1) some kind of SMT issue is causing stress on the accel package which causes these offsets. 2) Ham-handed depanelization at the CM. Our panels use mouse bites (not fully routed) and individual boards are snapped out using a jig. Perhaps the snapping process causes too much shock and damages some of the MEMS chips.Maybe 3) would be a bad reel of parts from ST, but I find that really hard to believe.Does anyone in the EEVBlog brain trust have experience with this kind of problem? How would you go about finding the root cause?EDIT: Our pre-production runs using the exact same parts and gerbers didn't have this problem. It only started happening during production, so is likely some kind of process issue.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 15, 2016, 03:06:12 pm
content: Take a panel and pull it through your entire process as you would do otherwise.Take another panel and test the circuits without depanneling.

author: sparkswillfly
author_rank: Contributor
timestamp: September 16, 2016, 05:12:57 am
content: And check your reflow profiles

author: wraper
author_rank: Unknown
timestamp: September 16, 2016, 06:41:15 am
content: Quote from: andyturk on September 15, 2016, 02:04:50 pmIs it possible to test them still in panel?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2016, 07:13:07 am
content: FYI, I've seen accel or gyros that warn to avoid copper directly underneath the chip. Easy to miss, as it's a regular QFN package. They want it soldered only around the periphery, nothing touching the pad.Doesn't sound like the same kind of thing, just one of the few bits of info I've seen on the thingys.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2016, 07:33:31 am
content: The way of breaking the panels could be a real problem - when breaking things apart, one can get really high amplitude vibrations, that can break things, sometimes even less sensitive parts than MEMs devices, like capacitors or inductors in an unfavorable position.

author: Milentije
author_rank: Contributor
timestamp: September 16, 2016, 08:31:16 am
content: Had a similar problem with IR/UV sensors. Reflow profile was the main problem. These were very delicate pieces..

author: andyturk
author_rank: Frequent Contributor
timestamp: September 16, 2016, 12:42:22 pm
content: I'm working on getting a complete panel for testing purposes. In the mean time, we'll re-check our layout against ST's guidelines. The reflow profile is another good suggestion...Thanks for all the good idea folks.

author: elecman14
author_rank: Unknown
timestamp: September 16, 2016, 03:42:50 pm
content: You can use a strain gauge mounted near the part to get a better understanding of the de-paneling process.I have seen vendors (not necessarily ST) have IC problems and not notify the customer / CM. It might be worth it to get a hold of them and see if any other customers are having problems with the part. You will want to get a picture of the reel label(s) before you give them a call. They will more than likely want to know date code /lot information. While you are at it you can probably send a part off to ST for failure analysis / testing. If they are tracking the PCBA that may be a good option.

author: tatus1969
author_rank: Super Contributor
timestamp: September 16, 2016, 03:55:27 pm
content: does your EMS offer vapor phase reflow? Could be worth a try if convection or IR causes too much mechanical stress. I can also imagine that mechanical tension from the cooling process could lead to offsets inhomogenic heat distribution between PCB and part?). Another possibility: bad orthogonality because of excess solder resist lifting the part.

author: Dago
author_rank: Frequent Contributor
timestamp: September 25, 2016, 06:16:09 am
content: Just stating the obvious but are you following the suggested (by the accelerometer manufacturer) reflow profiles and have read all appnotes regarding the mounting/soldering process?

author: janekm
author_rank: Unknown
timestamp: September 29, 2016, 08:19:13 am
content: One more thing to add to the list of things to check is wether the assembly house are using ultrasonic cleaning of the PCBs post reflow. I've seen this mentioned as a potential concern for MEMS devices.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: September 29, 2016, 10:07:04 pm
content: Whats the MSL rating and are they being stored appropriately?