Hi Earl

Sorry I didn't reply earlier but instead of thinking about technical problems I was engaged in surfing and swimming in the Mediterranean for two weeks. However, maybe some thoughts how I see the IMC issue might be still be a positive contribution to the discussion. I won't be using 1),2),... since my paragraphs do not correspond to yours.

a) The occurrence of IMC is give by the alloying process of some metals. In soldering these are Sn, Ni, Ag, Cu, Au.
b) The thickness of IMC's is determined by diffusion processes once an initial layer has been formed. Thus the thickness is time and temperature dependent. This means the IMC is also growing at room temperature but faster at solder temperature.
c) IMC's have another mechanical characteristic than solder. Solder creeps. It shows a continuous plastic deformation if a constant load is applied. Also at low load levels. Failure is do to damage accumulation caused by strain. IMC's are brittle with a high young's modulus. Failure is due to overstress. This overstress can be localised at the tip of a notch or crack.
d) There is no minimal thickness of IMC's to indicate a reliable solder joint. As soon an IMC is present we have the indication that solder and substrate have alloyed. That's all we need. Actually if the IMC's where not a part of the alloying process there would be no need for any. 
e) Whether a solder joint with a thick IMC is reliable depends on the load applied. As long as the stress applied in service or in a tests is far below the yield strength of the IMC the solder joint will fail because of the strain induced during deformation. If temperature cycles with high temperature gradients occur stresses higher than the yield strength of the IMC might be induced and the IMC will fail. 
I have the feeling that the problem occurs mainly with highly accelerated tests. Not only because of a catastrophic failure of the IMC but also because  IMC needles reaching into the solder will be very likely to snap of. This will reduce the load bearing area in the solder joint thus enabling faster deformation rates. If a tests is accelerated with short dwell times the faster deformation will cause larger deformation resulting in an earlier failure.

Some new insight for you? I hope so.

Have a great day.






Guenter Grossmann

Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :      xx41 1823 4054
mail:     [log in to unmask]

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