Hi,
All replies are very interesting and I do appreciate all help I am getting here.
I realised the pictures quality is not great and that is hard to judge from them.
Here a bit more I have done and my plan to go forward.
1. There is nickel over copper measured as 89% in the P+ area.
2. I did ask to measure the BGA solder ball heights across 'A' and 'T' path at the outer rows. Unfortunately I can't tell if the below variations are within limit for my package size (PBGA pro ASIC APA600 from Microsemi).
A1 - 401um
A8 - 383um
A16- 404um
T1 - 381um
T8 - 376um
T16 - 384um
3. I asked BGA supplier to send me the report on package warpage as per Jedec.
I did ask the PCB manufacturer to send me a report on PCB bow and twist as per TM-650.
4. The initial cross-section with separation was performed on the module with only BGA chip populated onto the PCB. I have second cross-section done from the same subcontractor, same PCB, BGA but on fully assembled module. We found no separations at all.
Not sure if there is any correlation between not fully populated and populated assembly in terms of temperature gradient.
Surprisingly in the second report there was no solder resist registered pads (our requirement) and there was no separation.
The time frame between both report is 30 days. Can ask Steve to upload this report as well.
5. I am planning for another cross-section but this time would like to use different lab. Can someone suggest a Lab within US that I can send our board for cross-section? Any suggestion please send directly to my email.
6. I think George mentioned a very important point in terms of using elevated temperature which may 'assist' brittle effect being more exposed.
Does re-balling the BGA with leaded alloy and leaded assembly help to address problem with brittle effect?
We want to keep PCBs with ENIG finish for our class 3 flight instrumentation equipment.
Of course this is only my assumption but because brittle effect is strictly related to the assembly itself, using full leaded process and lower the temperature may mitigate this issue.
Andy
Sent using BlackBerry®
-----Original Message-----
From: Steve Gregory <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Wed, 27 Nov 2013 16:16:42
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory
<[log in to unmask]>
Subject: Re: [TN] Separation between the BGA sodler ball and pad on the PCB
Hi George!
Thanks! Got it posted, here 'tis:
http://stevezeva.homestead.com/Brittle_ENIG_Failures.pdf
This is why I love the Technet, what may be old to you may be gold to others...I'd venture there are those who have never seen this paper, myself included.
Once again thanks!!
Steve
-----Original Message-----
From: Wenger, George M. [Contractor] [mailto:[log in to unmask]]
Sent: Wednesday, November 27, 2013 1:39 PM
To: TechNet E-Mail Forum; Steve Gregory
Subject: RE: [TN] Separation between the BGA sodler ball and pad on the PCB
Hi Steve,
Attached is a really old PDF file of ENIG brittle interface fractures that show how you can get a lot of useful information from a well polished and micro-etched cross section. Although the failures in this file are really old tin-lead failures before switching to Pb-Free, if you have room to post the file it might be helpful to Andy and others.
Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Wednesday, November 27, 2013 12:46 PM
To: [log in to unmask]
Subject: Re: [TN] Separation between the BGA sodler ball and pad on the PCB
Hi Andy,
Got all of your pictures and attachments. All the cross-section pictures are here:
http://stevezeva.homestead.com/A1.jpg
http://stevezeva.homestead.com/A16_1.jpg
http://stevezeva.homestead.com/A16_2.jpg
http://stevezeva.homestead.com/T16_1.jpg
http://stevezeva.homestead.com/T16_2.jpg
Your dye & pry is here:
http://stevezeva.homestead.com/256BGA-mod_for_TechNet.pdf
Lastly, here is the reflow profile:
http://stevezeva.homestead.com/Andy_s_Reflow_Profile.jpg
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of andi1978
Sent: Wednesday, November 27, 2013 6:56 AM
To: [log in to unmask]
Subject: Re: [TN] Separation between the BGA sodler ball and pad on the PCB
George,
First apologize for not timely replying to this topic, I am living within Europe time zone.
Here more facts about this condition and as well to clarify some questions already posted.
I did re-sent pictures again and in addition will send 2 pages of our dye & pry report document we did in March this year (from the same subcontractor).
As George W. already indicated we can eliminate lab issue and poor sample preparation doing the dye & pry test. Fortunately I am in possession of such a report from March 2013 before our BGA cross-section from September 2013. The short message is that there are evidences of dye ingression at the PCB pad level and only existing in the corners of the BGA package. To clarify George's question on completed inter-metallic fusion. What I can see from pictures I sent to Steve, the solder paste perfectly melted with the BGA solder balls and the sample board was fully cooled. The cross-section was done after the initial reflow (single thermal processing).
The whole board is double sided assembly but because of the need for cross-section, only the BGA component was populated (no other parts were assembled).
There was a request to send the reflow profile to Steve, have done that now. That's all I know and far I can go myself.
I kind of excluded PCB warpage due to small size of the sample board (65mm/70mm/1.5mm) and that this PCB was held in the thermal panel secured with 4 clamps during reflow (single pass).
Regards,
Andy
27 November 2013 3:02 "Wenger, George M. [Contractor]" <[log in to unmask]> wrote:
Andy,
Once Steve has your photomicrographs of the cross section posted it will be much easier to provide meaningful advice. Without a photograph and additional information one can only guess. You indicated separation occurred after the reflow process was completed because there is evidence of completed inter-metallic fusion. I assume when you say "after the reflow process you most likely mean after the solder melted and then cooled below liquidus. Is this a single sided reflow assembly or a double sided reflow assembly? Where the separations discovered after the first reflow or the second reflow?
Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Andrzej Zielinski
Sent: Tuesday, November 26, 2013 10:33 AM
To: [log in to unmask]
Subject: [TN] Separation between the BGA sodler ball and pad on the PCB Dear Colleagues ..
I am facing an issue with the BGA part where in three locations we found separation between the BGA solder balls and pads on the PCB (ENIG finish).
This condition was discovered during the cross-section that we have requested from our local lab.
I can share few pictures I have taken from our report (can't attach the whole document as this report is confidential to our business).
Basically this BGA is the plastic ACTEL package 896 pins, SAC305 finish, soldered with leaded process using 62Sn/36Pb/2Ag solder paste. Temperature was slightly elevated to accommodate SAC305 solder finish from the BGA package.
The separation condition we have observed was only found in location A1, A16 & T16 which are the corners of the BGA package. As well all 3 separations occurred after the reflow process was completed because there is an evidence of completed inter-metallic fusion.
In location A16 & T16 you can see the copper is protruding of the pad but I have been told this is not an issue and cause of the separation.
I was considering package or PCB warpage as the cause of this separation or something related to cooling the board after reflow.
I asked our lab to measure the height of the solder balls across row A & T and they all deem to be ok.
I have asked to check for the phosphorus content as the pad level to see if this is related to black-pad symptom and the phosphorus was measured as 10.3%.
So far the board assembly house is adamant their reflow profile is correct.
PCB manufacturer is confident with their PCB and pointing to the assembly house.
There is a thought about poor sample preparation where the separation issue would be caused by the lab (during cross-section).
We are attempting to perform another cross-section on different board but here I am not really sure if this is the right way to go (extra cost).
Because I have never seen such a defect, I am not sure what should be the next step to take.
Any advice guys ?
Andy
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