Piezoelectricity, piezoresistivity and dielectricity discovered in solder

The performance and reliability of soldered joints are important for electronic packaging. This work provides the first observation of piezoelectricity (generating electric field and capacitance), piezoresistivity (changing the resistivity) and dielectricity (giving polarization) in solder. These effects influence electrical conduction. The solder may encounter stress (particularly thermal stress) during use in a soldered joint. The solder studied is the tin–lead eutectic alloy. The stress is tensile in the direction of electrical measurement. Although the piezoelectric coupling coefficient d₃₃ of the solder (− (7.5 ± 0.8) × 10⁻⁶ pC/N) is very low compared to those of the well-known piezoelectric materials, the resulting electric field and capacitance are substantial and the changes are reversible upon unloading. The relative permittivity (2 kHz) (a dielectric material property) is very high ((3.973 ± 0.017) × 10⁶) compared to those of conventional dielectric materials, but is comparable to those of previously reported steels. The polarization is due to the movement of the free electrons; it opposes the applied electric field. The permittivity increases reversibly with increasing tensile stress and is attributed to an unidentified reversible microstructural change. The permittivity increase contributes to d₃₃ by only + (4.3 ± 0.6) × 10⁻⁷ pC/N, which is opposite in sign from the abovementioned overall negative d. The gage factor (fractional change in resistance per unit strain) associated with piezoresistivity is high (85 ± 6). The resistivity increases substantially and reversibly with increasing stress; it is attributed to the microstructural change and is disadvantageous for electrical conduction. However, the piezoelectricity and piezoresistivity enable the solder to sense its own condition (stress) without embedded or attached sensors.