Figure 4.57 Micrographs of Viromet solder alloy: (a) as-sintered and (b) polished and etched
This is extremely advantageous from a reliability perspective and shows the potential of
microwave sintering to improve the properties of lead-free solders, which generally have a
lower ductility than traditional eutectic tin–lead solder.
On-going studies on solder alloys and composites using microwave sintering further
confirm the advantages of microwave processing. Sn-3.5Ag solder powder was procured from
Qualitek Singapore Pte Ltd and used for characterization studies. This solder powder was
compacted at room temperature into a billet of 35 mm diameter and 40 mm height, followed
by microwave sintering for 6 minutes and 45 seconds, reaching a temperature of 221 C,
corresponding to the melting point of the solder alloy. Hot extrusion was performed after
microwave sintering at a temperature of 221 C. An unsintered sample was compacted and
extruded at the same temperature for comparison purposes.
The effect of microwave sintering on the tensile properties of Sn-3.5Ag solder alloy is
shown in Table 4.28. For unreinforced Sn-3.5Ag solder alloy, the microwave-sintered sample
clearly exhibited improved tensile properties over the unsintered sample and the reference
values provided by the manufacturer.
Microwave-sintered Sn-3.5Ag solder alloy displayed an improvement of 68% over the
reference value and 38.5 % over the unsintered sample for 0.2 % yield strength.
Similarly, the UTS value is the highest in the case of the microwave-sintered sample. This
is a clear indication of the ability of microwave heating to effectively sinter and improve the
strength of Sn-3.5Ag solder alloy in just a few minutes.
The failure strain did not exhibit significant differences between the unsintered and
microwave-sintered samples but was significantly superior to the reference value of 24%
provided by the manufacturer.
In another study, the addition of different weight percentages of indium to Sn-3.5Ag
solder alloy was investigated. Carefully weighed mixtures of indium and solder powder were
blended and compacted using the same methodology as described in A.8 of Appendix A.
The compacted billet was microwave sintered for 1 minute 25 seconds, reaching a
temperature of 160 C, and extruded at room temperature. The hardness and tensile
properties are shown in Table 4.29.
From Table 4.29, it can be observed that the addition of 1 to 2 weight percent of indium
increased the microhardness and strength of Sn-3.5Ag solder alloy and marginally reduced
the failure strain. Preliminary studies using differential scanning calorimetry (DSC) have
shown that the addition of indium lowers the melting point of Sn-3.5Ag solder alloy by
approximately 2 C per unit weight percent of indium added. Further tests are currently in