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September 2012

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Subject:
From:
Vladimir Igoshev <[log in to unmask]>
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Date:
Thu, 27 Sep 2012 12:24:07 +0000
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Dave,

I'm always "confused" when people talk about grain coarsening in solders. 

I guess what you meant was  phase coarsening, which is a different thing all together.
Regards,

Vladimir

SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca

-----Original Message-----
From: "David D. Hillman" <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Thu, 27 Sep 2012 06:58:43 
To: <[log in to unmask]>
Reply-To: TechNet E-Mail Forum <[log in to unmask]>,
        <[log in to unmask]>
Subject: Re: [TN] Using Tin/Lead balled BGA in a Lead-Free process

Hi Peter - very good discussion! A couple items to consider: (1) with the 
leadfree soldering process temperatures being higher than tin/lead 
soldering process temperatures, achieving a more uniform, equalized solder 
joint microstructure is easier to accomplish; (2) understanding where the 
lead phase ends up after solidification is only part of the equation. 
Solder joint microstructure also rearranges itself in response to 
stress/strain conditions so we also need to understand where the lead 
might also move to. In tin/lead solder joints, grain coarsening is quite 
evident as the solder joint degrades. Similar reactions occur in leadfree 
solder joint microstructure but because of the different solder joint 
compositions, we have slightly different evolution. Finally, the industry 
is still testing and trying to characterize what role the other element 
additions such as Ag or Bi play - we know that in other metal alloys 
systems such as the aluminum alloys, having precipitation particles 
greater changes the alloy properties. Lots of work to yet be accomplished. 
Ah, the good ole days where we just worried about two elements!

Dave



Peter Barton <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
09/27/2012 04:48 AM
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Peter Barton    <[log in to unmask]>


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Subject
Re: [TN] Using Tin/Lead balled BGA in a Lead-Free process






Thank you  Dave and the others who have responded with comments and 
attached papers etc., a good number of which I already have copies of. 
Those that are new to me will be added to my ever growing library.
 
My initial concern was prompted by the fact that as the BGA balls are 
Sn/Pb and the paste is SAC305  the proportion of Lead in the resultant 
joint would be considerably higher than in the reverse scenario, which is 
much more frequently referenced. I have seen papers that show when a small 
proportion of Lead is present it is possible it can precipitate out into 
Lead rich regions during the cooling phase of the soldering process and If 
these regions are concentrated at joint interfaces it will  result in 
areas of weakness. 
 
I am no metallurgist but I was wondering if the significantly increased 
percentage of Lead in the joint would just make this possibility much more 
likely?
 
It would be great if we could run some test samples with various reflow 
profiles and have a lab analyse them, then conduct some accelerated life 
testing but that luxury (or the significant funds required) is not 
available to us at this time - I'm sure others have the same difficulty 
:-) 
 
In the meantime, given the uncertainty, and available evidence for other 
strategies we will go with reballing for now.
 
Thanks again,
 
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan
CF40 1XX
 
Tel: 01443 425275 (direct)
Fax 02380 484882
[log in to unmask]
 
 
 
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: 26 September 2012 16:36
To: TechNet E-Mail Forum; Peter Barton
Cc: [log in to unmask]
Subject: Re: [TN] Using Tin/Lead balled BGA in a Lead-Free process
 

Hi Peter - I agree with Vlad, we still have a mixed metallurgy situation 
for the solder joint so I would treat it as having the same solder joint 
integrity concerns that would need to be reviewed and assessed. 

Dave 


Peter Barton <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]> 
09/26/2012 09:45 AM 
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Subject[TN] Using Tin/Lead balled BGA in a Lead-Free process

 






Hi Technetters,

Interesting that this question has arisen for me when there has been a 
question posed here on the reverse scenario in the last couple of days. 
Firstly, I agree with other posters that full reflow and alloy mixing must 
be achieved when using Lead free BGA's in a Tin Lead process. There is 
quite a lot of data out there supporting this.

My problem is finding research data on using a Tin/Lead BGA on an 
otherwise fully Lead free assembly. This appears to be much more limited. 
So far I have only found one paper that refers to this particular scenario 
and some of it's conclusions are carefully qualified. 

Assuming that all the other the packaging materials for the Tin/Lead BGA 
part in question are capable of surviving a suitable reflow process to 
successfully solder the lead free parts whilst minimising thermal input, 
can anyone out there comment on potential reliability issues. For 
information the BGA balls are 63/37 Sn/Pb and the solder paste alloy is 
SAC305.

Many thanks in advance,

Peter Barton






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