Abstract
c-Si solar cell interconnection damages from thermal cycles emanate from cumulative damage contributions from the various load steps in a typical thermal cycle. In general, a typical thermal cycle involves five thermal load steps, namely: 1st cold dwell, ramp-up, hot dwell, ramp-down, and 2nd cold dwell. To predict the contributions of each of these load steps to creep damage in soldered interconnections, each of the respective load steps needs to be profiled to accurately fit a function capable of predicting the damage contributions from a given number of thermal cycles. In this study, a field thermal cycle profile generated from in situ thermal cyclings at a test site in Kumasi, a hot humid region of sub-Saharan Africa, is used to predict damage in solar cell interconnections from accumulated creep energy density using finite element analysis (FEA). The damage was assessed for two different solder formulations, namely: Pb60Sn40 and Sn3.8Ag0.7Cu (lead-free). The results from the FEA simulations show that the cooling (ramp-down) load steps accounted for the highest accumulated creep energy density (ACED) damage in solder interconnections. The ramp-up load steps followed this closely. The cumulative contributions of the two load steps accounted for 78% and 88% of the total damage per cycle in the Pb60Sn40 and Sn3.8Ag0.7Cu solder interconnections, respectively. Furthermore, a study of the damage profiles from each of the five load steps revealed that each of the damage functions from the various load steps is a step function involving the first two thermal cycles, on one hand, and the remaining 10 thermal cycles on the other hand. The damage from the first two thermal cycles can be predicted from a logarithmic function, whereas the damage from the remaining 10 thermal cycles is predicted using six-order polynomial functions. The ACED results computed from the damage functions are in close agreement with the results from the FEA simulation. The functions generated provide useful relations for the prediction of the life (number of cycles to failure) of solder interconnections in solar cells. The systematic approach used in this study can be repeated for other test sites to generate damage functions for the prediction of the life of c-Si PV cells with SnPb and lead-free solder interconnections.
Highlights
The reliability of soldered interconnections of solar PV modules and other electronic devices is usually assessed using accelerated thermal cycling tests
Syed’s model is based on creep strain energy density that relates to the deformation stored internally throughout the volume of the solder joint during thermal loading
This model offers a more robust damage indicator in the solder joint since creep strain energy density captures the entire deformation in the joint
Summary
The reliability of soldered interconnections of solar PV modules and other electronic devices is usually assessed using accelerated thermal cycling tests. Xuejun et al [7], in their study on the effects of dwell time and ramp rates on lead-free solder joints in flip-chip ball grid array (FCBGA) packages, reported a decrease in fatigue life with an increase in dwell time from 15 to 30 min, and up until 90 min. Manock et al [9] compared 10, 30, and 60 min dwells with 10-min ramps between (0 ◦C–100 ◦C) temperature boundaries Their results show a decrease in a characteristic lifetime in cycles to failure with an increase in dwell time. Several studies have reported conflicting outcomes on the effects of ramp rate on fatigue life of solder joints, but a significant majority [5,10,11,12,13] agree that the number of cycles to failure decreases with an increase in ramp rate. The reported differences in cycles to failure are explained by the resulting large change in creep strain energy density per cycle at faster ramp rates
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