Sophia,

Are you asking if any SnPb intermetallic phases form at 80C? The answer is
no. If you are asking if any CuSn intermetallic phases form at 80C, the
answer is time dependent. 1 second -- No; 10 years -- Yes.

Intermetallic growth is diffusion limited, and therefore the thickness of
the intermetallic is given by

z^2 = (Do)(t)exp[-Q/RT]

Where Do (m2 s-1) is the diffusion coefficient, T(oK) is the temperature, Q
(J mol-1) is the activation energy and R (8.314 J mol-1 oK-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 m2 s-1 .

If you plug and chug, you will find that at 80C (353 K),

Time                                    Intermetallic Thickness
10 sec                          0.02 microns
1000 sec (~15 min)              0.2 microns
100000 sec (~1 day)             2 microns
10000000 sec (~100 days)        20 microns

You will find for a standard solder joint, there is drop off in strength
after the intermetallic thickness exceeds approximately 8-10 microns.

Best Regards
Craig


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Sophia Johansson
(EMW)
Sent: Thursday, January 18, 2001 3:23 AM
To: [log in to unmask]
Subject: [TN] Phase transistions in solder joints


Hi,

Does anyone know if there occurs any transitions that weaken the solder
joint if the working temperature is between 80-90 C. The solder is a SnPb
63/37. Maybe the question should be put like this; is there a build up of
weak intermetallic phases at about 80C?

Kind regards,

Sophia Johansson

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