Keith Sweatman, Tetsuro Nishimura
Nihon Superior Co. Ltd.
Osaka, Japan
Stuart D. McDonald, Kazuhiro Nogita
University of Queensland
Brisbane, Australia
Transcript
While it has long been known that the copper-tin intermetallic that plays a critical role in the reliability of solder joints made with tin-containing alloys on copper substrates exists in two different crystal forms over the temperature range to which electronics circuitry is exposed during assembly and service, it has only recently been recognized that the change from one form to the other has implications for solder joint reliability.
Under equilibrium conditions the change from the hexagonal to monoclinic form occurs in the cooling solder joint at 186 degrees C. However, cooling rates after common commercial soldering processes are typically faster than the rate that would permit complete transformation under such equilibrium conditions. In this paper the authors report a study of the effect of cooling rates on copper-tin crystals.
Cooling rates from 200 degrees C ranged from 10 degrees C per minute to 100 degrees C per minute and the effect of isothermal aging at intermediate temperatures was also studied. The findings have important implications for the manufacture of solder joints and their in-service performance.
Summary
While it has long been known that the Cu6Sn5 intermetallic that plays a critical role in the reliability of solder joints made with tin-containing alloys on copper substrates exists in two different crystal forms over the temperature range to which electronics circuitry is exposed during assembly and service, it has only recently been recognized that the change from one form to the other has implications for solder joint reliability.
Under equilibrium conditions the change from the hexagonal to monoclinic form occurs in the cooling solder joint at 186°C. However, cooling rates after common commercial soldering processes are typically faster than the rate that would permit complete transformation under such equilibrium conditions.
In this paper the authors report a study of the effect of cooling rates on Cu6Sn5 crystals. Cooling rates from 200°C ranged from 10°C/minute to 100°C/minute and the effect of isothermal ageing at intermediate temperatures was also studied.
The extent of the phase transformation after each regime was determined using synchrotron X-ray diffraction. The findings have important implications for the manufacture of solder joints and their in-service performance.
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.
