Figure 23: Log-log plot of isothermal creep data for Sn-Ag-Cu and Castin™ bulk solders.

Isothermal creep data from three independent sources are plotted as tensile strain rate (/second) versus tensile stress (MPa) in Figure 23. The data covers two orders of magnitude on the stress axis and eight orders of magnitude on the strain rate axis. Test temperatures are in the range -55°C to 150°C.

The original data, given in Table B.1, was digitized by Kil-won Moon of NIST. The raw
data was obtained from the following figures in their respective publications:

Table 10: Source of data, alloy composition and test temperatures.

These investigations of SAC properties are fairly recent, when compared to the Sn-Pb or Sn-Ag literature, since it is only recently that SAC alloys have received increased attention from industry.

Specimens

Table 12: Melting Conditions and Specimen Treatments (NA = not available).

Test specimens were dog-bone- or cylinder-shaped tensile specimens with specimen and/or gauge length and cross-sectional dimensions given in Table 11. Melting and cooling conditions as well as thermal treatment of specimens, when available, are summarized in Table 12. The reader is referred to the original publications for additional details on the specimen preparation.

The alloys have nominal compositions with elemental contents in the following ranges:

• 95.5 to 96.5% wt. for Sn.
• 2.5 to 3.9 % wt. for Ag.
• 0.5 to 0.8% wt. for Cu.
• Chemical analysis of the 95.5Sn-3.8Ag-0.7Cu (nominal) alloy studied by Schubert et al. gave a composition of 95.4% wt. Sn, 3.91% wt. Ag and 0.71% wt. Cu (from Table 1 in Schubert et al., 2001).

Comparing the specimen information from the three sources, note that:

• The specimens by Schubert et al. have a significantly smaller cross-section: 3 mm x 3 mm versus 11.28 mm and 13 mm in diameter for the specimens by Kariya et al. and Neu et al., respectively.

• The above dimensions compare to typical sizes of 0.1 to 0.2 mm (4 to 8 mils) for flip-chip solder joints, or 0.5 mm (20 mils) for Ball Grid Array (BGA) solder joints.

• The Kariya et al. specimens were rapidly cooled by water-quenching while the Neu et al. specimens were air-cooled although the cooling rate was not reported. The cooling conditions for the Schubert et al. specimens were not reported.

Microstructures

Table 13: Microstructure of bulk "SAC" specimens.

The microstructure of the as-cast SAC solders consists of the Sn matrix with precipitate-free b-Sn regions and eutectic (or near-eutectic) regions having Ag3Sn or Cu6Sn5 intermetallic phases dispersed in the Sn matrix. Specific microstructural features for the alloys and specimens under study are given in Table 13.

Test Procedures

Table 14: Test procedures and conditions.

The test conditions and procedures are summarized in Table 14. The isothermal tensile tests are of three types:

• Conventional "creep" tests (with constant load rates) where the linear portion of the strain-versus-time
  response gives the "minimum" or steady strain rate (see Figure 24a).
• Conventional "strength" tests (stress / strain tests ran at constant strain rates) where the maximum stress gives the Ultimate Tensile Strength (UTS) of the specimen at a given strain rate (Figure 24b). Once the UTS has been reached, the typical response of solder is further elongation of the specimen at constant stress or with a slow drop in load.
• Strain rate jump tests which are similar to the above stress / strain tests except that the strain rate was
  increased by a factor of ten after each increment of 1.5% strain.

Figure 23: Output of (a) creep test; (b) "strength" test.

Noteworthy observations about these tests are:

• In general, the first two types of tests give similar strain-rate-vesus-stress equations since the strain rate is constant during both the strength test and the steady-state phase of the creep test. However, one of the limitations of the creep tests is that they are not run at constant stress since it is easier to control loads rather than stress. The reported stress is the initial stress defined as the applied load divided by the initial cross-section of the specimen.
• As pointed out by Neu et al., 2001, the strain rate jump tests allow for the acquisition of tensile strengths at different strain rates from a single specimen. However, Neu et al. mentionned that the strength results may be affected by the history of prior deformations of a given specimen when jumping from one strain rate to the next.

The reviewed literature focuses on steady-state creep. To our knowledge, the other deformation modes, i.e. the initial, instantaneous plastic flow and primary creep have not been reported on for SAC-type alloys. Based on studies by Darveaux et al. (1992, 1995), these deformation modes are not negligible for neareutectic Sn-Pb and Sn-Ag alloys and are worthwhile investigating further for SAC solders.