Alternative Strengthening Mechanisms for Lead-Free Solders
Authors: Keith Sweatman, Takatoshi Nishimura, Wayne Ng, Michihiro Sato, Tetsuro Nishimura Company: Nihon Superior Co., Ltd., Nagase & Co., Ltd. Date Published: 9/17/2017
Abstract: The search for lead-free solders that can deliver sustained reliability in harsh environments such as the engine compartment of motor vehicles is prompting a reexamination of the characteristics of an alloy that determine its reliability. This re-examination is being undertaken in the context of evidence that has been accumulating of the temporary nature of the reliability boost that came as a bonus with the silver added to the tin-copper eutectic to achieve a further 10°C drop in melting point to 217°C. That reliability boost comes from the strengthening effect of a fine dispersion of particles of Ag3Sn intermetallic compound formed as part of the ternary eutectic that, during the last stage of solidification, freezes isothermally from the liquid remaining between the primary tin dendrites. While in the as-soldered condition these particles are sufficiently closely spaced to act as effective obstacles to the movement of dislocations through the ß-tin crystal, as they coarsen in response to the thermodynamic pressure to reduce their surface area:volume ratio they become less effective as dislocation obstacles and the strengthening effect fades. One approach to maintaining the resistance of the solder to deformation has been to add more elements that form intermetallics such as nickel and antimony to provide additional particle strengthening. However it is difficult to achieve the particle spacing that is effective as an obstacle to dislocation movement and in any case all such particles have the same tendency as Ag3Sn to coarsen and migrate in service conditions. A method of increasing solder joint reliability that is looking increasingly promising is solid solution strengthening which has the advantage that if the solubility of the strengthening element is sustained the effect is resistant to deterioration in service conditions. In this paper the authors will review these approaches to alloy development and report test results of an alloy that relies on solid solution strengthening to achieve a level of reliability comparable with that of alloys strengthened with silver.