Mechanical and chemical characterization of solder joints after accelerated ageing tests

Isothermal ageing is a widely used method in electronics to address the alleged long-term stability of soldered joints. Compared to common IEC 61215 module tests such as damp heat or thermal cycling it is relatively fast (< 90 h), does not require the lamination of a module and allows ribbon peel testing and metallographic analysis after the test. We study the mechanical and chemical solder joint properties after isothermal cell testing compared to thermal cycling and damp heat exposure of mini-modules. In this study solder joints of encapsulated Multi-Busbar (MBB) solar cells, which provide a multitude of single joints, are investigated after accelerated ageing tests. A special demountable one-cell-module setup enables chemical and mechanical characterization of solder joints after damp heat and thermal cycling tests. The observed fracture patterns are analyzed and designated to the degradation products characterized by Raman spectroscopy. Isothermal ageing and thermal cycling show non-chemical failure mechanisms caused by mechanical stress or intermetallic phase growth. The largest impact on contact adhesion and indicators for chemical degradation are found after damp heat testing. As opposed to the phenomenon of grid finger corrosion with 3BB cells reported in literature, where the EL measurement shows clear indication for a complete loss of electrical contacts and very low adhesion to the cell, the EL measurement of the DH aged MBB module shows that the electrical contact of the affected metallization is still intact compared to the initial EL image.