Dave

We made quite a bit of testing on this mixing situation. 

First of all we determined the influence of the solder parameters on the structure of the mixed solder joints. Through the whole trials we produced joints where the SnPb solder alloyed flawless with the leadless balls. The main problem was the coplanarity of the balls which might be responsible for non wetting if the balls don't collapse.
After producing Pb free BGAs on a 1.6mm PCB with lead free solder that showed a clear line of distinction between the ball and the solder we cycled the specimens -20 /120°C with a temperature gradient of 2°C/min and a dwell time of 30 minutes at 120°C and 60 minutes at -20°C together with totally lead free produced BGAs and SnPb BGAs.
after 4000 cycles (nearly 2 years) we couldn't find a significant difference between the different solder techniques. As a matter of fact the PCBs below the pads degraded faster than the balls. 
It might be that in fast temperature cycling the result will be different.


With best regards

Günter Grossmann

>-----Ursprüngliche Nachricht-----
>Von: TechNet [mailto:[log in to unmask]] Im Auftrag von David D. Hillman
>Gesendet: Mittwoch, 26. September 2012 14:26
>An: [log in to unmask]
>Betreff: [SPAM] Re: [TN] Leaded Paste and Lead Free BGAs - Do they mix?
>Wichtigkeit: Niedrig
>
>Hi Bob - yes, not have collapse would be a very bad idea. Soldering a
>lead-free BGA in a tin/lead soldering process automatically creates a
>mixed metallurgy situation which can be a solder joint integrity issue
>depending on the product use environment.  The degree of solder joint
>microstructure mixing/uniformity has a direct impact on the solder joint
>integrity. The customer's suggestion of not having solderball collapse
>would result in the tin/lead solder paste and the lead-free solderball
>being very distinct zones in the final solder joint which would be a
>worst case scenario.
>
>Dave Hillman
>Rockwell Collins
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>
>Robert Kondner <[log in to unmask]> Sent by: TechNet
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>09/25/2012 07:26 PM
>Please respond to
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>Subject
>[TN] Leaded Paste and Lead Free BGAs - Do they mix?
>
>
>
>
>
>
>Hi,
>
>
>
>I have a customer using leaded paste with lead free BGAs. I think they
>want
>to have the balls NOT collapse for better optical inspection.
>
>
>
>Sounds like a really bad idea to me but I have no explanation where I
>can
>say as why.
>
>
>
>Can anyone make suggestions either way?
>
>
>
>Thanks,
>
>Bob K.
>
>
>
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