Hi George,

Great analysis of the FA method,

Best regards,

> John Burke
> Engineering Fellow
> SanDisk Corporation
> 951 SanDisk Drive, Milpitas, CA 95035
> T: + 1 (408 597 1423)| M: + 1 (408 784 4377) 
> [log in to unmask]
> On Apr 9, 2016, at 8:34 AM, George Wenger <[log in to unmask]> wrote:
> 
> Hi Steve,
> I haven't been on TN for some time (my 3 year old grandson has been taking up much of my time).  Greg Munie had forwarded your TN post and Dave Hillman's reply and I sent an email repl to Greg that he suggested I post on TN so here it is:
> 
> From: "Greg Munie" <[log in to unmask]>
> To: [log in to unmask]
> Sent: Friday, April 8, 2016 12:39:07 PM
> Subject: FW: [TN] Recommendations for Failure Analysis / Root Cause of PCBA Failure...
> 
> 
> 
> Dis looks like a job for George . . 
> 
> From: [log in to unmask] [mailto:[log in to unmask]] 
> Sent: Friday, April 08, 2016 3:20 PM
> To: Greg Munie <[log in to unmask]>
> Subject: Re: [TN] Recommendations for Failure Analysis / Root Cause of PCBA Failure...
> 
> 
> 
> Dis would be a job for George if George still worked in a Lab like he had in Lucent or Andrew but doesn't have that now.  I believe that Dave Hillman is correct that with two PCBAs with shorts there are many labs that could most likely find the root cause for the failure.  I didn't see Steve's resopnse to David's question about if Steve believes the problem to be an internal short.  I know what I would do if I were doing the failure analysis and the first thing would be to ask Steve many more questions about the PCBA and when the failures are detected before I did any physical analysis of the two PCBAs because the analysis is destructive and if you don't find the root cuse you're out of luck
> 
> After reading Steve's email and looking at the two photos a couple of things come to mind.  The short could be an internal PCB short or it could be an external PCBA short.  From the photograph the surface finish on the PCB appears to be ENIG and it also appears that there isn't a cleaning process done after PCBA solder assembly process.  My first questions would be what solder paste is used for the assembly?, Is the PCBA a double side surface mount reflow assembly?, is there a wave soldering assembly performed and if there is what flux is used?  I'd also ask for PCB layer drawings and an electrical circuit diagram.  I would do a high resolution scan of the top and bottom sides of both of the failed PCBAs and inspect both failed assemblies in detail under a microscope. Before doing any cleaning of the PCBAs or cross sections of the area that is shorted I would systematically begin removing surface mount components that are soldered across the 12V power. I'd carefully examine the bottom of each removed component and the PCB surface from which it was removed for any debris, dendrites, or shorts and also measure the removed component to see if it is shorted as well as re-measure the via to see if the PCBA is still shorted.  An alternative to doing doing a component by compnent analysis is to do an agressive cleaning and drying process on the PCBA and then measure the via to see if the short is still present.  The clening process I would us is to do ultrasonic cleaning in a terpine cleaner like EC-7 followed b ultrasonic cleaning in IPA to remove any terpine residue and then do a long (4 to 8 hour) bake at 100C to drive off any moisture that might be traped under components or in component flexure cracks and then check the via to see if it is still shorted.  If it isn't then the problem is an external short rather than an internal PCB short.
> 
> 
> 
> Since the failures are occuring after 111 to 472 days field exposure I believe there is a strong possibility that the shorts are external and are occuring under or within components rather than internal to the PCB.
> 
> 
> 
> If the cleaning and component removal doesn't eleminate the short than I would do careful transmission xray imaging around the via that is shorted.  Actually I would do this x-ray imaging of the shorted via before doing the cleaning and componet removal just to have the images.  The x-rays may not be able to see if there is a short but I would still do them.  If the x-rays son't show anything and the via is still shorted I think I would begin a very slow and careful lapsectioning of the via and examine each of the PCB layers for shorts as the laping progresses through the PCB.
> 
> 
> 
> Sorry for the long winded answer but I hope it helps Steve find the root cause for his problem.  Feel free to share my response with Steve or Dave or any lab Steve might work with.  I haven't made any recommendations for an external lab because there are so many.  There are deffinately some labs out there that I wouldn't use because I'm not sure they really have the capabilityy and all they want is your money.  The one lab I've actually never used but I know their capability and would certainly reccomend is George Westby's Universal Instruments Lab in binghamton, NY.  Although the are really good the one drawback is they are expensive.  I think their minimum is $1000.
> 
> Regards,
> 
> 
> 
> George
> 
> 
> 
> George M. Wenger
> Failure Signature & Characterization Lab LLC
> 609 Cokesbury Road, High Bridge, NJ 08829
> (908) 638-8771 (Home) (732)-309-8964 (Cell)
> [log in to unmask]

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