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| May 24, 2013
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Circulation Over 51,000
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| July 25, 2012 |
| Cause of Cracking on SOT23 Components |
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We've recently encountered cracking with SOT-23 parts. The EMS contractor says the machines have been checked and there are no problems. The cracks occur only at one place on the board, and components from the same reel are placed at three other positions without issues. I suppose board support is the most likely cause. Could there be any other explanation? |
| B.A.
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Since
you indicate that this is a recent issue, it seems to me that something has
changed in the process even if it seemed insignificant to whomever authorized
it. Of course there are a lot of unknowns here and not a lot of information to
go on, but the three top thoughts that immediately come to mind are warpage of
the board fabrication especially with 0.031" substrates through reflow that
could severely stress solder connections and component bodies resulting in
destruction of the component.
This could originate from changed reflow settings
or something downstream at the board fabrication supplier impacting the board
fabrication quality itself. Another thought is that the component and/or solder
connections are being stressed through improper depanelization or singulation
which can definitely cause this issue to occur again with thinner 0.031" substrates.
Make certain that they are using a singulation system and that they're not
being broken apart by hand under any circumstances!
A final thought is that it
could be originating from the packing process - perhaps something is physically
contacting the component in packing or upstream during shipping. I think that
the bottom line here is to realize that since this is a recent issue, something
has changed in the process and needs to be corrected. Good luck!
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David Bonito
Sales & Marketing Manager
Technical Manufacturing Corp.
David has been active in all areas of the contract electronics manufacturing industry for over fifteen years. He is currently in charge of all Sales and Marketing related activities for Technical Manufacturing Corporation.
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It appears that there is some impact damage on the side
of the component near the horizontal crack, although it is hard to tell from
the single picture.
The crack does not appear to be stress or heat
related, although again it is hard to tell. If it only occurs in a single
location and always on the same side of the part, it could be that as the CCAs
slide down a ramp out of the reflow oven or out of the wash they are allowed to
bang into the CCA in front of them, which would explain physical damage only to
a SOT-23 near the trailing edge of the CCA, but not the others populated
further inside. Take a good look under at 20x-30x scope, and see if you can
verify any impact damage at the sides of the part. If it is impact damage, and
it is severe enough to cause that type of cracking, there should be some type
of telltale scratch or chip at the point of impact.
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Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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It
seems that mechanical damage after placement is the most likely scenario. Is
the problem position near a board edge by any chance? I'm wondering if the
problem location is prone to damage from tooling used to rack or otherwise hold
or support the boards? Also look at other points in the process where
mechanical damage might happen at the problem location.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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As
only some of the parts are cracking, I would measure the thermal profile of
both the good and the affected bad parts and see if there is significant
difference between the two. As we know, components can suffer from high
temperature ramp rates and moisture penetration. It may be possible to
correct the issue by altering the thermal profile or by pre-baking the
components. This is only one possible cause ... There are others.
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Mark Waterman
Engineer / Trainer
Electronic Controls Design, Inc. (ECD)
Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.
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Reader Comment
I would agree David Bonito in his assessment. A
SOT23 is a pretty rigid part and the least likely to be cracked due to machine
forces. More than likely your contractor has a new de-panelizer or even a new
employee not fully trained in proper PCB handling.
Joe Johnson
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