Thanks for the additional insight Dave.
I knew this wasn't going to be simple, which is why I posted my question here :-) I have learnt couple of things that have helped inform my decision and would like to thank all the other TechNet respondents for their comments/info also.
I hope to be able to reciprocate one day.
Rgds,
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan
CF40 1XX
Tel: 01443 425275 (direct)
Fax 02380 484882
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From: [log in to unmask] [mailto:[log in to unmask]]
Sent: 27 September 2012 12:59
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 - 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]>
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SubjectRe: [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
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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]>
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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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