Effect of the Strengthening Mechanism on the Response of a Solder Alloy to Strain Rate and Ageing

The function of the strengthening mechanism in Sn-based Pbfree solder alloys is to inhibit the movement through the Sn crystals, which make up the bulk of the solder volume, of the dislocations that can otherwise move relatively freely along crystal slip planes. Available mechanisms include, particle strengthening, solid solution strengthening and grain refining. The widely used Sn-3.0Ag-0.5Cu alloy (SAC305) relies on particle strengthening by the fine eutectic Ag3 Sn intermetallic compound that is dispersed in the Sn phase in the interdendritic spaces. However, these fine particles with their high surface area:volume ratio are thermodynamically unstable and by the process known as Ostwald ripening gradually coarsen, even at ambient temperature, so that their effectiveness as obstacles to dislocation movement fades and the strength of the solder declines towards that of unalloyed Sn. The realisation that the particle strengthening effect of the Ag is only temporary has triggered a search for alternative strengthening mechanism and solid solution strengthening has been identified as a promising candidate. The solid solution strengthening effect is not degraded by ageing and is therefore more stable than that provided by particle strengthening. In the as-soldered condition the particle strengthening by Ag3 Sn is effective, delivering good performance in accelerated thermal cycle testing. However, the flow stress of the particle strengthened alloy is sensitive to strain rate. At high strain rates that increased resistance to strain means that the stress is transmitted largely undiminished to the solder substrate interface or to the underlying laminate where brittle fracture can easily propagate. It is for this reason that SAC305 is notoriously susceptible to failure in drop impact. While it is well recognised that the performance of SAC alloys in drop impact is improved by reducing the Ag content reliability in other stress conditions is compromised. While the flow stress of a solid solution strengthened Ag-free Pb-free alloy can be as high as that of SAC305 it might be that because of the different mechanism is different it might be less sensitivity to strain rate than a particle strengthened alloy. In the work reported in this paper BGA solder spheres of three alloys with different strengthening mechanisms, particle strengthening with Ag, solid solution strengthening with Bi and a combination of both were reflowed to a Cu substrate and the resulting ball tested in shear impact at displacement speeds of 10mm/s, 1000mm/s and 2000mm/s in the as reflowed condition and after ageing for 500h at $150 ^{\circ}\mathrm{C}$. In the interpretation of the results account is taken of the variation in fracture modes in the 20 repeats undertaken for each test condition (alloy, shear speed, as-reflowed and after ageing).