Hi Ramon,
I wouldn't speculate without seeing a bad joint. :-)
Vladimir
-----Original Message-----
From: TechNet
To: [log in to unmask]
Sent: Wed May 09 10:40:29 2007
Subject: Re: [TN] SV: [TN] SV: [TN] Metalurgists, need assistance
Hi, Vladimir:
In this case, per Inge, only the large parts were affected. Possibly indicating that there was not enough temp. We know that the paste melts at 183 deg C. As it has been stated from others on the forum that the coolest part needs to be above Liquidus + 20 for 5 seconds. So something undesirable happens at in between temps that is overcome when the right temp and time is reached.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vladimir Igoshev
Sent: Tuesday, May 08, 2007 9:06 PM
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] SV: [TN] Metalurgists, need assistance
Hi Ramon,
They could exist but I'm not aware of it. However, I'd say that not much is going on with the solder balls in paste untill they melt down.
Regards,
Vladimir
-----Original Message-----
From: TechNet
To: [log in to unmask]
Sent: Tue May 08 15:02:21 2007
Subject: Re: [TN] SV: [TN] SV: [TN] Metalurgists, need assistance
Insufficient temp at reflow, ha. What happens to the 60/40 Sn/Pb balls at the start of and middle of the reflow prior to liquidity, Vladimir? Are there any experiment reports on the eutectic solder paste that shows pics about the paste to 185, 190, 195, 200 and 205 deg C for different seconds?
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Tuesday, May 08, 2007 2:45 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] SV: [TN] Metalurgists, need assistance
Vlad & TN gnus,
Hey, curious? I'm still concerned, but at a higher level, as said Newton. To begin with, I Xsectioned samples from failed part AND known good references. Both finished with 0.1 micron diamond milk. Etched for 10 second, 5 seconds, 1 second and no etching at all. Same result: voids in the failed parts, absolutely flat with references. I'll send photos to Mr Wallman, alias Steve.
Pic new xsections_1 : typical solder joint with void concentrations under the lead's foot. Not etched.
Pic new xsections_2 : details from under the lead's foot. Nickel plating seen clearly, and a very thin nickel IMC layer. All bright areas are predominated by Tin, very little Lead. Black spots are voids that contains a little of everything debris from polishing. Not etched.
Pic SEM_3 shows following: from bottom copper pad / solder joint with voids / lead with nickel finish. Not etched.
Pic SEM_1 shows upper part of solder joint, with lead copper uppermost.
Not etched.
Pic SEM_ 2 shows typical solder joint at higher magnification with predominating Tin and also little Lead and Copper. Not etched.
As I said earlier, the references show no voids at all like the above.
Preliminary, I have excluded Black pads and Skip plating. Furthermore, I don't think there are brittle copper IMCs either, even if there are relatively high Cu peaks everywhere. Think Cu peaks come from preparation. Nor is it likely with Ni IMCs, they belong to the interface parts. So, what is this? Someone mentioned a insufficient soldering process, and that is finally what I begin to think too. Against this speaks the very experienced and big company that makes these boards (one of the big elephants, not Ericsson, not American). On the other hand, even the best can fail.
The failure appears randomly around the world, much annoying for the customers, and the technicians scratch their heads.
Finally, I remind you all about the fact, that the fractures start close to the solder pads and spread to the middle of the solder joints. And only on the large FPGAs. No other components are hit.
Inge
-----Ursprungligt meddelande-----
Från: Vladimir Igoshev [mailto:[log in to unmask]]
Skickat: den 8 maj 2007 15:47
Till: TechNet E-Mail Forum; Hfjord
Ämne: RE: [TN] SV: [TN] Metalurgists, need assistance
So inge,
What the cross-section looked like? :-)
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Monday, May 07, 2007 4:57 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] Metalurgists, need assistance
Diet Coke! Man! Does not remove black pads on your teeth..
New samples are ordered, will try both oxides on paper and diamond on nylon.
If the customer pays, I may try even FIB or TOFSIMS. Both are superior, and even if the time tax is high, I must say that polishing, and repolishing and endlessly repeated polishing can be quite expensive as well.
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För David D. Hillman
Skickat: den 7 maj 2007 14:23
Till: [log in to unmask]
Ämne: Re: [TN] Metalurgists, need assistance
Hi George! Good comments! It does seem that the industry, at times, spends more time in describing a phenomena rather than resolving the root cause
so that we can avoid a reoccurrence. As for Inge's Monday dilemma, I would wait and see what the cross-section results reveal. If the observations are that the nickel plating grain boundaries look bad, then I would work
with my pwb fabricator on what went wrong in the plating bath. It the observations are that the nickel plating grain boundaries look good, I would chase either a poor solderability or a poor soldering process root
cause. As you noted, the plating chemistry suppliers have worked extremely hard on making the ENIG plating process more robust and "real" black pad
problems are less prevalent today. I have always viewed the solution to black pad as one of two choices: don't use ENIG or work very closely with the pwb fabricator. Here is an article reference which I think really does the best job to date, of describing the root cause of black pad (one of the contributors is our good fried Dick Coyle!).
Zeng, Steirman, Abbott, Murtuza, "The Root Cause of Black Pad Failure of
Solder Joints with Electroless Ni/Immersion Gold Plating", JOM, June 2006, pp. 75-79
The reality for us may be that no amount of plating bath control will completely eliminate the potential of having a black pad situation and that a risk assessment on a product basis is the final solution. Ouch, way too much thinking on a Monday before I have had a Diet Coke!
Dave
"Wenger, George M." <[log in to unmask]>
05/04/2007 07:56 PM
To
"TechNet E-Mail Forum" <[log in to unmask]>, <[log in to unmask]> cc
Subject
RE: [TN] Metalurgists, need assistance
David,
I hate the "B" word because it describes an optical appearance rather than a failure mechanism. I've heard the "B" word as well as terms like "weak Nickel" and "Black Line Nickel" used many times to describe ENIG failures. The reason I don't like these terms is because they are adjectives to describe what one thinks they see and don't describe the failure mechanism that causes the brittle interface failures. We use ENIG on an old telecommunications legacy product because it wouldn't be profitable to do any engineering evaluations to qualify another surface finish. However, since the root cause of the ENIG solder joint failure mechanism has never been understood or resolved to the point where one can turn it on or off, we avoid the use of ENIG surface finish on all new products. Even though the suppliers of ENIG chemistry have done a great job of monitoring their chemistry and educating board shops on the proper controls to reduce the occurrences of brittle solder joint failures they haven't been able to eliminate these type of failure.
Ingemar is one of the few people I know of on TN that not only has a great understanding of solder joints but appreciates the process control details one needs to follow in PCB fabrication as well as PCBA to avoid un-reliable solder joints. His current problem (i.e., not being able to understand the failure mechanism or predict when it is going to happen) is the major reason we avoid the use of ENIG surface finish.
Let's just say Ingemar does a cross section and confirms that there is "mud flat" nickel grain boundary attack, what does he do on Monday?
Your advice about having a beer because it is Friday and not worrying about this until Monday only will help this weekend but come Monday what does he do. I assume that he's already using a "good" board shop and a "Good" ENIG chemistry. I know what we did and it worked for us but as an engineer I'd really like to what the ENIG failure mechanism is and how to avoid it.
Regards,
George
George M. Wenger
Andrew Corporation Wireless Network Solutions Senior Principal FMA / Reliability Engineer 40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office] (732) 309-8964 [Cell]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, May 04, 2007 4:45 PM
To: [log in to unmask]
Subject: Re: [TN] Metalurgists, need assistance
Hi folks and thank you Steve for being "TechNet Picture Central"! Sorry
but just to warn everyone but I am going to use the "B" word shortly.
Inge
- take a close look at picture 4. As Werner pointed out the gold is non-wetting the nickel in number of locations. What really jumped out at me is that "mud flat" appearance on some of the nickel surface regions - a "mud flat" appearance is a classic indication of black pad! I recommend you complete a cross-section to confirm that you have nickel grain boundary attack.
But since it is Friday, go have a beer and worry about this on Monday!
Dave Hillman
Rockwell Collins
[log in to unmask]
Steve Gregory <[log in to unmask]> Sent by: TechNet <[log in to unmask]>
05/04/2007 01:45 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Steve Gregory <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Metalurgists, need assistance
Hi Inge!
Got your pictures posted. Took me a bit, I was swapping our wave solderpot from 63/37 to HMP. Done now. I'm getting quicker, I can do it in 20-minutes now. Took me about 45-minutes when I first did it...
Anyways, here's the links:
http://stevezeva.homestead.com/files/Pic1.jpg
http://stevezeva.homestead.com/files/Pic2.jpg
http://stevezeva.homestead.com/files/Pic3.jpg
http://stevezeva.homestead.com/files/Pic4.jpg
http://stevezeva.homestead.com/files/Pic5.jpg
http://stevezeva.homestead.com/files/Pic6.jpg
-Steve-
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Friday, May 04, 2007 12:22 PM
To: [log in to unmask]
Subject: [TN] Metalurgists, need assistance
Have an issue with (randomly) lifted FPGA leads.
Pic 1: good joint after pull test
Pic 2: ditto but high magnification
Pic 3: "rotten" joint after pull test
Pic 4: ditto but high magnification
Pic 5: "rotten" cross section
Pic 6: ditto but high magnification
60Sn on ENIG, obviously a brittle region. Suspect Kirkendall voids, but the pockets with zillions of Tin spheres (0.1 to 2 um in size) points at insufficient peak temperature to make 100% melt and good solidifying. If these little balls were Lead, I could understand, but they are Tin!
Never seen like. Any "quickhead" out there, need fast advice.
Note: Pic 2, you can see the underlying board pad nickel as dark areas.
An experienced analyst will hopefully recognize. No Phosphorous peaks!
My Friday headache.
Inge
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