TECHNET Archives

April 2016

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Robert Kondner <[log in to unmask]>
Reply To:
Date:
Mon, 11 Apr 2016 10:13:21 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (120 lines)
Hi,

 Did anyone ever report the drill to copper spacing used to design and fab the boards?

 This distance is larger than what many PCB designers realize and not all PCB fab houses seem to catch it. Removal of unused pads on internal layers is not sufficient, it is drill wall to other internal copper that is the issue. If you have 6 mil copper clearance and 5 mil rings on vias then you have 11 mil drill to copper. I recall 12mil as a minimum from some shops.  So if normal copper clearance rules are used to control internal routing  you can easily blow the drill to copper spacing.

And it gets much worse. If internal pads are removed early in the routing process then that 11 mil drill to copper clearance drops to 6 mils. So you can easily get shorts from via barrels to internal copper with the drill to copper rule violations.

Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of George Wenger
Sent: Sunday, April 10, 2016 9:56 PM
To: [log in to unmask]
Subject: Re: [TN] Recommendations for Failure Analysis / Root Cause of PCBA Failure...

Hi Steve, 

In my original email I said " 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"; well here are some of the questions. 

You said that you've been building these assembles for some time and have produced around 2000 assemblies and that the failures seem to be escalating and that the last 12 occurred after 111 to 472 field exposure hours. Have there been any failed assemblies at the time your customer gets them from you and test them? If the failures seem to be escalating do you think that the whole population may be at risk or have some of the earliest built assemblies not failed? If that is the case did your board vendor make any changes? You indicated there is a solder ball in the shorted via in your photo. In the copy of your photo that I've attached are the red arrows I put on the photo pointing to other solder balls? How thick are the PCBs? How many internal layers do the PCBs have? What is the PCB substrate laminate? 

After reading the additional information you provided I'm beginning to feel that CAF may be the mode of failure. The best CAF paper I've seen is the attached Zou-Hunt which I've attached. Have you cross sectioned a via (the same one that is shorted on a failure) on a bare board from your board supplier that you haven't assembled to see the structure and quality of the bare HASL board? If you see poorly drilled or poorly plated vias or via walls not completely covered with HASL that might increase the possibility that the failures could be CAF. M view of CAF is simple; if you have a potenital (power supply voltage), and an ionic source (plating chemistry, HASL flux, WSF assembly), and a pathway (woven fiberglass boards) and moisture (avionic environment) you can have CAF problems. 

There are many more questions and things going through my mine that might help confirm the problem is CAF. However, let me play devils advocate and cut to the chase; let's say you decide or the failure analysis confirms CAF as the root cause for the failure; WHAT DO YOU DO? Do you have the PCB fabricator change the PCB substrate material, do you change the PCB surface finish, do you change our solder assembly process and materials, and if you do how do you know you fixed the problem? 

Sorry for being so negative but I see the failure solution being much harder than the failure analysis. 

Regards,
George 

----- Original Message -----

From: "Steve Gregory" <[log in to unmask]>
To: "TechNet E-Mail Forum" <[log in to unmask]>, "George Wenger" <[log in to unmask]>
Sent: Saturday, April 9, 2016 8:11:14 PM
Subject: Re: [TN] Recommendations for Failure Analysis / Root Cause of PCBA Failure... 

Hi George, 

Those little young-un's can be a handful huh? I'm sure you're doing your job as all grandparents do and spoiling him to death (GRIN). Glad you're enjoying him! 

So I just want to give a little more information about the board. The was returned to us conformal coated, so what you may have mistaken as flux residues is actually conformal coating. The board finish is actually Sn/Pb HASL. We assemble the SMT with a Sn62/Pb36/Ag2 water soluble paste and is cleaned in a DI inline cleaner after reflow. The through-hole is stuffed and wave soldered, again using a water soluble flux. It is again cleaned in the inline cleaner. The board is inspected at the end of the reflow oven, and after wave solder, as well as going through AOI. Any touch-up that is done requires that the board go through the inline cleaner again, so I'm confident that the board is clean when we ship it to our customer. Unfortunately we don't electrically test the assembly, the test set-up is a custom deal that our customer doesn't want to part with, so they do the electrical test and conformal coat the assemblies. 

The red arrows were placed by our customer, and the probe marks that you see in the solder joints were done by what I assume were their test technicians with a DMM to isolate the fault. What isn't readily apparent in the photos is a slight darkening around the via, and you can see that the via looks like it blew a solder ball out that you see on the annular pad If you look carefully in the photos you can see tiny solderballs down inside the via, and if you you look at the via pad surface and compare the other via pad surfaces in the photo, you can see that the arrowed via did over-heat. 

The other assembly came to us with several IC's on the board with lifted pins which I assume were done trying to track down where the short was. This board also had a blown tantalum capacitor (which I didn't post a photo of), but the capacitor was installed with the correct polarity as you could still see the polarity stripe on the cap and see that it was installed correctly. So that was evidence in itself that voltage came from the wrong direction to that cap... 

So I've been reading everything I can find about CAF, and trying to understand the problem better. I think that probably that's what is happening given that it's taking time for the failures to occur. 

Steve 

On Fri, Apr 8, 2016 at 5:34 PM, 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] 






This email and any attachments are only for use by the intended recipient(s) and may contain legally privileged, confidential, proprietary or otherwise private information. Any unauthorized use, reproduction, dissemination, distribution or other disclosure of the contents of this e-mail or its attachments is strictly prohibited. If you have received this email in error, please notify the sender immediately and delete the original. 


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

ATOM RSS1 RSS2