Hi Guenter -  very good data! And yes, if you use a temperature range of 
-55C to +125C, 15 minute dwells and ramp rates of 10C/minute then you will 
get different results. As this discussion highlights,  printed wiring 
assembly test conditions  play a key role in the integrity of the solder 
joints under investigation. One of the toughest tasks we face as 
technologists is assessing and comparing different data set results. The 
more published data we can review, the better we'll be able to understand 
lead-free soldering processes.

Dave



"Grossmann, Guenter" <[log in to unmask]> 
09/27/2012 08:46 AM

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AW: [SPAM]  Re: [TN] Leaded Paste and Lead Free BGAs - Do they mix?






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
>[log in to unmask]
>
>
>
>
>Robert Kondner <[log in to unmask]> Sent by: TechNet
><[log in to unmask]>
>09/25/2012 07:26 PM
>Please respond to
><[log in to unmask]>
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>To
><[log in to unmask]>
>cc
>
>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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