Adhesion improvement for solder interconnection of wet chemically coated aluminum surfaces

Abstract

Reliable solder interconnection is essential for the performance and the long-term stability of photovoltaic (PV) modules. The rear electrode of standard passivated emitter and rear cells (PERC) is made of aluminum, which cannot be electrically contacted with the established solder interconnection parameters of PV module fabrication. The aim of this contribution is to address this recurring topic of contacting aluminum (Al) surfaces. Not altering the soldering parameters, cost efficient wet chemical coating processes are targeted to achieve strong and reliable solder joints on Al surfaces: a (single/double) zincate process (< 90 s) followed by electroless plating of nickel (Ni). We characterize the coatings and solder joints in terms of wettability, adhesion after soldering, fracture mechanism and microstructure. The fundamental understanding of the interaction of the involved materials (i.e. Al, Zn, Ni, Sn) is investigated on 200 µm thick monolithic Al foils. The Ni-coated foils show perfect wetting with liquid SnPb-based solder (contact angle < 30°), resulting in high peel forces of > 5 N/mm after soldering ribbons. We analyze the microstructure of these void-free solder joints, presenting cross section SEM images and EDX mappings. A detailed investigation of the fracture after the peel test shows a mixed fracture pattern and no dominant failure interface. Based on those results on Al foils, we apply the double zincate process with subsequent electroless Ni plating to the rear side of PERC-type solar cells. As the screen printed Al has not been optimized as substrate for soldering, peel tests show an adhesive failure between Al and silicon wafer. Neglecting this limitation, a successful Ni coating of different screen printed and fired Al rear side pastes could be demonstrated.