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December 2005

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Subject:
From:
Guenter Grossmann <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Guenter Grossmann <[log in to unmask]>
Date:
Thu, 15 Dec 2005 16:38:12 +0100
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David

Lead free BGAs can be soldered with SnPb solder in a SnPb soldering process. Even if you stay below the liquidus of SnAgCu the liquid solder will solve the ball forming a lower melting alloy at the interface. The process of the solution of the ball into the solder ends if
- The alloy that formes due to the continuous solution of Cu and Ag into the SnPb has ist liquidus at the solder temperature
- The temperature is lowered below the liquidus of the SnAgCuPb alloy

This means that depending on temperature and time different fractions of the balls are are solver in the solder. we made some trials and used the temperature- time integral approach of James Vincent to characterise the influence of the production parameters on the solder structure. 
Originally I was worried that the interface where the original ball meets the solder will be the week point where degradation will take place. However, looking at the slides of Dave Hillman I see that the rupture takes place in the SnPb solder along the pad. This makes sense, since SnAgCu it mor creep resistant than SnPb and thus the strain occurring due to thermal cycling will be dominantly in the SnPb solder. I have no quantitative data in hand to say how much crack growth is accelerated due to this material combination.

A problem which is evident is the fact that you have no collapsing balls (or not the entire balls) to compensate for coplanarity problems of the balls. 

Best regards

Guenter





EMPA
Swiss Federal Laboratories  for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1 823 4054
mail:     [log in to unmask]

>>> [log in to unmask] 13.12.2005 15:30:24 >>>
Is it possible to get an acceptable solder joint if a Pb free BGA is put onto a board using SnPb solder paste and normal SnPb
profiles?
The lead free solder balls are SnAgCu 96.5/3/0.5.

Seems nigh impossible to get DDR2 packages with SnPb balls.

I'm designing a high end DSP into an industrial instrument. The DSP is not available lead free, nor does the application call
for lead free (thankfully).
There is no option to use a different memory type with the DSP. Nor is there any possibility of using a different DSP or FPGA
due to time/cost/sanity.
Don't want to explore going lead free on this one (for obvious reasons), and certainly do not welcome the expense of reballing
0.8mm DDR2 chips.


Best Regards

David Greig
______________________________
GigaDyne Ltd
Buchan House
Carnegie Campus
Dunfermline KY11 8PL
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk <http://www.gigadyne.co.uk/>
______________________________


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