Electronics > Manufacturing & Assembly

Yield problems related to MEMS accelerometer

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andyturk:
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.

Ice-Tea:
Take a panel and pull it through your entire process as you would do otherwise.

Take another panel and test the circuits without depanneling.

sparkswillfly:
And check your reflow profiles

wraper:

--- Quote from: andyturk on September 15, 2016, 02:04:50 pm ---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.

--- End quote ---
Is it possible to test them still in panel?

T3sl4co1l:
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

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