Solidification processes of SnCu, SnAg and SnAgCu solder alloys and interface reactions to charactize solar cell interconnections processes

Lead-contained and lead-free solder joints have been well studied in electronics packaging applications over the past years. Within the solar industry, establishing the Si crystalline solar cell interconnection process as an environment friendly and RoHS-conform process, more substantial investigations are necessary to prove the process reliability and characterize interconnection formation in the Ag-thick film and Cu-ribbon interfaces. This paper focuses on the evaluation of material parameters of the joining processes for solar interconnections and quantifies the solder formation of the typical PV interfaces and the solder compositions. Therefore, varying solder alloys (content gradiation of SnCu, SnAg compositions) are investigated. We used DSC (differential scanning calorimeter) measurements to characterize the melting and solidification behaviour with and without the interfacial reactions. Influencing factors like peak-temperature and cooling rate within the context of PV manufacturing process will be discussed to convey a better understanding for the Ag-paste to solder to Cu-ribbon interconnection level for solar cell stringing. The results show that the conditions of solidification behaviour and microstructural properties are of significant importance for silicon-based solar cell interconnection processes.