Intermetallic compounds in solar cell interconnections: Microstructure and growth kinetics

- Lead-based and Bi-based solders for PV module technology are reviewed.
- The formation of intermetallic compounds in solar cell interconnections is described.
- Changes in microstructure of the interconnection during thermal aging are discussed.
- Elemental composition of intermetallic compounds is investigated with EDX.
- The growth of intermetallic phases is modeled using outdoor temperature data.

The microstructure and intermetallic phase growth in solder joints of photovoltaic modules are investigated because of their significance for interconnection reliability. Interconnector ribbons with Sn60Pb40, Sn62Pb36Ag2, Sn43Bi57, Sn41Bi57Ag2 and Sn91Zn9 are soldered on the front busbars of industrial crystalline silicon solar cells. Cross sections are inspected using microscopy, SEM and EDX. The interconnections are isothermally aged, whereas the intermetallic layer thickness is determined successively. The microstructural changes in the bonds are characterized. Grain coarsening, volume increase of intermetallic compounds, Sn-penetration into the metallization and growth of grain boundaries between the phases are found. The composition of the intermetallic phases within Sn91Zn9-bonds is discussed. A diffusion model is used to simulate the intermetallic layer growth after 3000 h at 85 °C and thermal cycling from −40 °C to 85 °C for 600 cycles. A prognosis of the phase growth within the photovoltaic module after 25 years at the location Freiburg in Germany is made. It is found that the Ag3Sn-phase using Sn43Bi57 extends to 6.6 µm after 3000 h at 85 °C as compared to 2.6 µm for Sn60Pb40. After 25 years in Freiburg the Ag3Sn-layer within Sn43Bi57 joints is predicted to be 2.4 µm whereas within Sn60Pb40-bonds only 2.2 µm.