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Intermettalic layer analyssis

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Intermettalic layer analyssis | 28 September, 2007

We have the need to verify that proper intermetallic layer between a 100% Sn plated contact and the solder joint is being formed. Never having done this before: Is there any specifications, etc. that we can use as acceptability criteria?

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Intermettalic layer analyssis | 28 September, 2007

There is no specification. If there is: * An IMC => good * No IMC => bad

Intermetallic growths are diffusion limited, and therefore the thickness of the intermetallic is given by: z^2 = (Do)(t)exp[-Q/RT] Where Do (m^2 s-1) is the diffusion coefficient, T(�K) is the temperature, Q (J mol-1) is the activation energy and R (8.314 J mol-1 �K-1) is the gas constant. From previous work at CALCE, recommended values for the activation energy for 60Sn40Pb solder is 78 kJ and the diffusion coefficient was empirically calibrated as 1.5 x 10-5 m^2 s-1 . If you plug and chug, you will find that at 80C (353 K), ||Time||IMC Thickness micron ||10 sec||0.02 ||1000 sec (~15 min)|| 0.2 ||100000 sec (~1 day)|| 2 ||10000000 sec (~100 days)|| 20 You will find for a standard solder joint, there is drop off in strength after the intermetallic thickness exceeds approximately 8-10 microns. [C Hillman, DfR Solutions]

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