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Subject:
From:
"Ingemar Hernefjord (KC/EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Ingemar Hernefjord (KC/EMW)
Date:
Wed, 6 Sep 2006 09:08:03 +0200
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text/plain (454 lines)
I'm inclined to discounter carbonate too now, because the percentage of Carbon and Oxygen does not correspond to Lead Carbonate. The C and O peaks are rather random, probably minor residues and oxides. Close analysis of the crystals themselves point at all Lead. Note this: the Lead crystals are exactly the same for ALL the balls, both in terms of appearance and in terms of amount. They look all the same! That tells against the flux residue > carbonate theory, because the flux residues can not be so exactly and evenly positioned. But the mechanical forces may be coincidental and even.

You mention the eutecticum used for soldering the balls on the ceramic body and its pads. Now, this is interesting, because the  crystals appear ONLY in/around the interface between the high Tin alloy and the high Lead alloy, see the three pictures I'll send to Steve. If you look at the surface of the  high Lead (the ball), you can see strange 'lamella'-like structures. This is not the normal superficial 'skin'. A crystalograph might tell what that means. I have a 5 cm thick compendium on crystalography from the school time, but it takes too long a time to update my shrinked brain. Hope there is someone out there who can interpret the odd solder ball exteriour.

Wonder how many of you have delivered boards with Lead whiskers on the CBGAs without knowing. We can't be the only one!

Inge


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leo Higgins
Sent: den 5 september 2006 20:36
To: [log in to unmask]
Subject: Re: [TN] PbCO3

The presence of the carbonate was considered a marker for incomplete removal of the rosin flux used in the classical C4 bump (95Pb-5Sn)FC assembly while at another company.  The presence of flux residue was carefully analyzed with solvent extraction and analysis techniques during process qual to eliminate this risk.  

The solder balls on the CBGA were 90Pb-10Sn, and were joined to the ceramic with eutectic Sn-Pb paste.  We did not encounter the carbonate issue on the BGA balls. 



Best regards,
Leo

          
Leo M. Higgins III, Ph.D.
Sr. Vice President, Engg and Technology
ASAT, Inc. 
13809 Research Blvd., Suite 635
Austin, Texas     78750

office phone   512-249-4758
mobile           512-423-2002
[log in to unmask]
www.asat.com



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of David D. Hillman
Sent: Tuesday, September 05, 2006 9:56 AM
To: [log in to unmask]
Subject: Re: [TN] PbCO3


Hi Brian! Absolutely - you and I are on the same page. I added my comments just so folks understood that we were most likely not dealing with a
Sn63Pb37 solderball thus Inge has a situation were Pb could be a major player in a reaction (as you detailed below).  What has me puzzled is that room temperature and standard humidity shouldn't be generating PbCO3. Inge must have a contaminate initiating the reaction which the cleaning process is not impacting. Have you ever observed the CBGA packaging (e.g ESD bag,
tray) being a PbCO3 contaminate source?

Dave



                                                                       
             Brian Ellis                                               
             <[log in to unmask]>                                         
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             09/05/2006 08:55          TechNet E-Mail Forum            
             AM                        <[log in to unmask]>,              
                                       [log in to unmask]    
                                                                        cc
                                                                       
                                                                   Subject
                                       Re: [TN] PbCO3                  
                                                                       
                                                                       
                                                                       
                                                                       
                                                                       
                                                                       




The point I was making was that if the solderball were pure lead, or nearly so, any oxidation would not be the preferential tin. If you look at an SnPb phase diagram, there could be a solid solution phase of practically pure alpha down to room temp (98.1% at 0°C), whereas at 10% Sn , it would be a mix of beta in alpha up to ~150°C. If you have lead oxide and hygroscopic contaminants, such as are found in many un- or
poorly- cleaned flux residues, then the "carbonate growth" cycle could be primed.

Brian

David D. Hillman wrote:
> Hi folks! The "typical" solderball alloy for a CBGA is 90Pb - 10Sn 
> which keeps the CBGA from collapsing onto itself during reflow. 
> Because the Sn percentage is lower the usual tin oxidation reactions 
> are not going to be as dominating. I believe the reason Inge is 
> observing only the reaction exclusively on the CBGAs is the solderball 
> alloy.  The real question is what is initiating the reaction!
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
>              Brian Ellis
>              <[log in to unmask]>
>              Sent by: TechNet
To
>              <[log in to unmask]>         [log in to unmask]
>
cc
>
>              09/04/2006 04:53
Subject
>              AM                        Re: [TN] PbCO3
>
>
>              Please respond to
>               TechNet E-Mail
>                    Forum
>              <[log in to unmask]>
>              ; Please respond
>                     to
>                 Brian Ellis
>              <[log in to unmask]>
>
>
>
>
>
>
> Just had a thought, Inge. Could your CBGA maker be conforming to RoHS, 
> by using high melting point balls with lots of lead, such as 99%?
>
> Brian
>
> Ingemar Hernefjord (KC/EMW) wrote:
>> Triple Hmmm!
>>
>> I used Gilette, which is little rough, but it works fine: dropped a 
>> board in LN2. When at -196 knocked the backside of the board, and the 
>> CBGA jumped off. Could then study the cleanliness. No flux residues 
>> or whatsoever.
>> Yepp, Vigon is indeed very active, but why does the phenomenon appear 
>> only on CBGAs? We have other BGAs on the board, and these have never 
>> shown such growths.
>>
>> I'm attempted to go on quite another track for a while. One guy 
>> proposed whisker growth. And, when you look at the SEM image, you 
>> find something looking like pressing toothpaste out of its tube: 
>> whiskers? The guy (who was it?) meant that CBGAs build up mechanical 
>> stress unlike other components, and hence create whiskers.
>>
>> Lead whiskers? Quadruple Hmmm. The litterature is very thin about 
>> Lead whiskers,  not to say nearly non-existent. A few cases are 
>> reported, but not regarding any BGAs, as far as I know. However, I 
>> will study the few samples I got (am not allowed to touch the sharp 
>> boards)and see if it can be Lead whiskers, or other type of Lead 
>> crystals. According to Sod's law, I may be the one to be haunted by Lead whiskers...
>>
>> Inge
>>
>>
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
>> Sent: den 4 september 2006 09:38
>> To: [log in to unmask]
>> Subject: Re: [TN] PbCO3
>>
>> Double Hmmmm!
>>
>> Applying Occam's Razor to this. Put everything you have done to one 
>> side for a moment and just imagine you were at the early stages of 
>> investigating this problem.
>> You would describe a situation where you are using a reputable resin 
>> based flux and then clean them in a reputable semi-aqueous cleaner.
>> Everywhere is OK, except under a large component.
>> My immediate thought would be that you are not cleaning under the 
>> component as well as you think or at least are not washing/rinsing 
>> out the cleaner.
>> Vigon is a very high pH material and therefore chemically active.
>> How do you know you have cleaned under the device? Are you lifting it 
>> off to do something like ROSE?  If there is a flush though problem 
>> then ROSE will not detect it.
>> My apologies for pointing out what is basic stuff like this to 
>> someone with your knowledge/experience, but sometimes it is easy to 
>> overlook something when it is too simple.
>>
>>
>> Regards
>>
>> Mike
>>
>>
>>
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
>> Sent: Sunday, September 03, 2006 10:19 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] PbCO3
>>
>> Hmmm....
>>
>> I have excluded residues that stem from reactions with the solder 
>> mask and other reactions from solvents and fluxes. The reason is, 
>> that such residues appear immediately after the soldering process, 
>> while our residues GROW slowly during weeks and months. And only on CBGAs.
>> Furthermore, we have investigated the space between the CBGAs and the 
>> board, and it's absolutely clean, because we use a very effective and 
>> powerful Vigon 200 washing machine.
>> So, it is still a mystery.
>>
>> Inge
>>
>>
>>
>> ----- Original Message -----
>> From: "Mike Fenner" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Thursday, August 31, 2006 4:20 PM
>> Subject: Re: [TN] PbCO3
>>
>>
>>> I can puzzle out how you could get carbonate but not how you would 
>>> get lead  carbonate. Lead is unreactive compared to other metals 
>>> present and they  would react in favour of the lead, or even if the 
>>> lead reacted would  displace it in the corrosion cell reaction.
>>> If you are getting significant carbonate formed so quickly then this 
>>> would  imply a fairly high acidity on the work, which is hard to 
>>> relate to that  found in electronic grade fluxes.
>>>
>>> You have nothing to lose by trying an activated rosin base flux to 
>>> reduce this stuff -whatever it is - and floating it away as a 
>>> slag/dross in the rosin, but I doubt it would be effective unless 
>>> you
>> re-melted the solder.
>>> If
>>> you were to do that it would be similar to some purification 
>>> processes for solders [and other metals,]
>>>
>>>
>>> Regards
>>>
>>> Mike Fenner
>>> Indium Corporation
>>>
>>> T: + 44 1908 580 400
>>> M: + 44 7810 526 317
>>> F: + 44 1908 580 411
>>> E: [log in to unmask]
>>> W: www.indium.com
>>> Pb-free: www.Pb-Free.com
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar 
>>> Hernefjord
>>> (KC/EMW)
>>> Sent: Thursday, August 31, 2006 9:21 AM
>>> To: [log in to unmask]
>>> Subject: [TN] PbCO3
>>>
>>>
>>> We got lots of such growth on CBGA balls recently. After
>>> soldering/cleaning: nada. After one week in stock: signs of growth.
>>> After one month: unacceptable crystals. Now, being insoluble in 
>>> water and many solvents, some specialists advise you to mix an RA 
>>> with ethanol, heat this mixture and dip the part until the crystals vanish.
>>> My question to TN is whether there is alternative methods? Some 
>>> accept
>>> PbCO3 growth as being harmless, but this is not our intention, 
>>> because loose particles are semiconducting and can create leakage 
>>> current
>> paths.
>>> Brian's book "Cleaning and Contamination of Electronics and
>> Assemblies"
>>> mention there are five groups of white residues, but, oddly enough, 
>>> lead carbonate is not included. The chemistry world of soldering 
>>> residues is so complicated, that I have to ask you for help.
>>>
>>> Thanks in advance
>>> Ingemar Hernefjord
>>> Ericsson Microwave Systems
>>>
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