Ask the Experts
November 7, 2018
Cause of Cracking on SOT23 Components
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?
Expert Panel Responses
Sinceyou indicate that this is a recent issue, it seems to me that something haschanged in the process even if it seemed insignificant to whomever authorizedit. Of course there are a lot of unknowns here and not a lot of information togo on, but the three top thoughts that immediately come to mind are warpage ofthe board fabrication especially with 0.031" substrates through reflow thatcould severely stress solder connections and component bodies resulting indestruction of the component.
This could originate from changed reflow settingsor something downstream at the board fabrication supplier impacting the boardfabrication quality itself. Another thought is that the component and/or solderconnections are being stressed through improper depanelization or singulationwhich 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 notbeing broken apart by hand under any circumstances!
A final thought is that itcould be originating from the packing process - perhaps something is physicallycontacting the component in packing or upstream during shipping. I think thatthe bottom line here is to realize that since this is a recent issue, somethinghas changed in the process and needs to be corrected. Good luck!
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.
It appears that there is some impact damage on the sideof the component near the horizontal crack, although it is hard to tell fromthe single picture.
The crack does not appear to be stress or heatrelated, although again it is hard to tell. If it only occurs in a singlelocation and always on the same side of the part, it could be that as the CCAsslide down a ramp out of the reflow oven or out of the wash they are allowed tobang into the CCA in front of them, which would explain physical damage only toa SOT-23 near the trailing edge of the CCA, but not the others populatedfurther inside. Take a good look under at 20x-30x scope, and see if you canverify any impact damage at the sides of the part. If it is impact damage, andit is severe enough to cause that type of cracking, there should be some typeof telltale scratch or chip at the point of impact.
Richard D. Stadem
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.
Itseems that mechanical damage after placement is the most likely scenario. Isthe problem position near a board edge by any chance? I'm wondering if theproblem location is prone to damage from tooling used to rack or otherwise holdor support the boards? Also look at other points in the process wheremechanical damage might happen at the problem location.
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.
Asonly some of the parts are cracking, I would measure the thermal profile ofboth the good and the affected bad parts and see if there is significantdifference between the two. As we know, components can suffer from hightemperature ramp rates and moisture penetration. It may be possible tocorrect the issue by altering the thermal profile or by pre-baking thecomponents. This is only one possible cause ... There are others.
M.O.L.E. Line Product Manager
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.