Effect of Cooling Rate on the Intermetallic Solder Joints

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Summary

Conclusions

• In dilatometry studies the transformation from monoclinic η' to the hexagonal η phase during heating can be observed as an expansion over a temperature range around the nominal 186°C transformation temperature.


• Cu₆Sn₅ with partial substitution of Cu by Ni retains its hexagonal crystal form over the range room temperature to 250°C.


• The partial substitution of Cu by Ni in the hexagonal Cu₆Sn₅ crystal reduces the lattice parameter in the a direction but does not significantly affect the lattice parameter in the
  c direction.


• At cooling rates as slow at 50 ° Cmin -1 metastable hexagonal η phase can be retained to 50°C.


• In isothermal interruptions to 100°Cmin -1 cooling at 160, 150 and 140°C transformation from the metastable hexagonal phase to the monoclinic phase has begun by 400 seconds. At 180°C the transformation did not begin within an hour. At 100 °C the metastable hexagonal η phase remained unchanged for up to 1000 seconds.


• On the basis of the results of rapid cooling and isothermal ageing experiments it is possible to start to map a temperature - time - transformation ("TTT") diagram for the η→η' transformation in Cu₆Sn₅.


• On the basis of a presumption that the different coefficients of thermal expansion of the hexagonal η phase and the monoclinic η' phase can be extrapolated into the temperature ranges where the phases are metastable it is hypothesized that whether the change from metastable η' to η is associated with an expansion or contraction depends on the temperature at which it occurs. That creates an opportunity for minimizing potential damage to the joint integrity by allowing the transformation to occur at a temperature at which the difference in coefficient of thermal expansion is close to zero.

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