Colin

The CTE is the difference of the coefficient of thermal expansion between varying materials. This is a constant for metallic materials. Polymers show a change in the CTE if you pass the glass transition temperature.
This means you can not have a CTE rate. If heat or cool the variation of the thermal expansion (positive or negative) of 2 joined materials will cause mechanical stress. If 2 stiff materials are joined the stress will cause elastic deformation possibly together with a certain amount of plastic, time independent, deformation. There are some materials that show a time dependent plastic deformation behaviour, very much like a wet chewing gum, where an applied load leads to continuous plastic deformation. This deformation is called creep.
The more stress you apply the faster the material deforms. Or, looking at it the other way, the faster you deform the more stress occurs.
Solder is such a material and lead free solders are more resistant against creep than tin- lead. If you heat or cool your PCB too fast in the process high stress is transferred across the solder joint say from the component to the pad which might cause components to crack of pads to delaminate from the PCB since the solder doesn't have enough time to compensate the differences in expansion by creep deformation. This effect is more pronounced with lead free solder than with tin- lead.

Your case:
- Forget the gold, it will dissolve in the solder joint during the soldering process
- Yes, there is a difference in the CTE of Ni and Cu. However, I never heard of a separation of the two layers
- I assume you are talking about a separation of solder joints in the solder- Ni interface. Yes, this is a potential problem. This interface consists of intermetallic phases which are brittle. This means that the formation of a crack depends on the force applied (coming from your cooling rate), and the geometry of the interface. In brittle fracture a sharp edged notch causes high local stress inducing the rupture. In ENING there are additional interface effects affecting the rupture behaviour. 
- As a summary: Fracture in the solder/pad interface has is cause in the formation of brittle phases, high stress due to rapid temperature change and interface effects. Which one is the main culprit for your problems is difficult to say.

Best regards

Guenter





>-----Ursprüngliche Nachricht-----
>Von: TechNet [mailto:[log in to unmask]] Im Auftrag von colin mcvean
>Gesendet: Freitag, 14. August 2009 05:04
>An: [log in to unmask]
>Betreff: [TN] Reflow profile
>
>Folks,
>
>From my previous question regarding pattern crack we are seeing on a
>wire-bonded flex-rigid PCB, I have now found more evidence that the
>presented FPCB is ok, but after reflow for SMT assembly, this is where
>the cracking is first found (rather than after wire-bond process). The
>reflow profile being used peaks at 245oC, but rapidly cools from peak to
>135oC in about 30s. This seems to me to be too rapid a cool rate, and
>with the pad metallics being 0.2uM gold over 10uM Nickel over 30uM
>copper pad. Are CTE rates on cooling the same as CTE rates for heating?
>I think I may have CTE mismatch on the coatings, which when cooled too
>rapidly, are causing the crack. Any metallurgists out there willing to
>comment?
>
>Thanks
>
>Colin
>
>
>
>
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