SOLDERING INDUCED DAMAGE TO THIN SI SOLAR CELLS AND DETECTION OF CRACKED CELLS IN MODULES

Abstract

Andrew M. Gabor, Mike Ralli, Shaun Montminy, Luis Alegria, Chris Bordonaro, Joe Woods, Larry Felton Evergreen Solar, Inc. 138 Bartlett St., Marlborough, MA 01752, 508-597-2317, [e-mail] Max Davis, Brian Atchley, Tyler Williams GreenMountain Engineering 500 Third St, Suite 265, San Francisco, CA 94107 ABSTRACT: The need to reduce PV manufacturing costs combined with the present shortage of polysilicon feedstock are driving a steady reduction in wafer and cell thicknesses. Processes, materials, and handling equipment must adapt to maintain acceptable mechanical yields and module reliability. The soldering of wires to the cells is one of the steps that becomes more challenging for thinner cells. Cells can break during the process or later crack in the modules due to damage incurred during the process. In order to maintain good yields and module reliability as we shift our String Ribbon wafer thickness below 200 microns, Evergreen Solar has developed tools to aid in process, equipment, and materials optimization and has developed improved methods of crack detection at the module level. In this paper we describe a cell breakage strength tester that we constructed as a quick feedback and quality control tool for improving and monitoring the soldering process. We also describe an electroluminescence crack detection system which we developed to give quick and nondestructive feedback for imaging the cracked cells in a module. Finite element modeling was used to explain why cells tend to crack more when loading the glass side of the modules as compared to the back side. Keywords: Module Manufacturing, Reliability, Soldering 1 INTRODUCTION The need to reduce PV manufacturing costs combined with the present shortage of polysilicon feedstock are driving a steady reduction in wafer and cell thicknesses. Processes, materials, and handling equipment must adapt to maintain acceptable mechanical yields and module reliability. The soldering of wires to the cells is one of the steps that becomes more challenging for thinner cells. Cells can break during the process or later crack in the modules due to damage incurred during the process. In order to maintain good yields and module reliability as we shift our String Ribbon wafer thickness below 200 microns, Evergreen Solar is studying the mechanisms involved in crack formation and is developing tools to aid in process and materials optimization and is developing improved methods of crack detection at the module level. 2 DAMAGE FROM SOLDERING The industry conventionally interconnects cells in the modules by soldering flat solder-coated Cu wires (ribbons) from the front side of one cell to the back side of the adjacent cell. The soldering operation can occur sequentially whereby the front-side busbars are soldered first in a tabbing operation, and the back solderpads or busbars are soldered next in a stringing operation. The soldering can be done by hand or with automated equipment. Lately, the trend is toward soldering the front and rear contacts simultaneously in automated combined tabber-stringers. A variety of heat sources can be used such as hot air, IR lamps, soldering irons, lasers, and induction coils. The soldering materials, process and equipment, as well as the upstream cell processing and materials can all potentially influence the yields of the cells in the soldering process. The work below explores these issues. 2.1 Potential sources of damage During the soldering operation, the cell and the wires heat up and expand and then later contract when the heat is removed Below the melting point of the solder, the differential contraction between the Cu and the Si, as shown by the CTE values in Table I, combined with thermal gradients, cause stress to build up in the system. In our model, this stress can cause the formation of microcracks in the Si and/or the propagation of existing microcracks. A possible solution to minimize the stress is to use wire with a lower CTE value than Cu. The literature [1] mentions Cu-clad Invar wire as a material with excellent fatigue properties that may work well in this case as the Cu can provide the required conductivity while the Ni-Fe Invar core can restrain the contraction of the wire. Solder composition is also an important variable. Due to the low yield strength of solder, it may accommodate some stress depending on its composition and the degree of brittle intermetallic formation. The effect of using a Pb containing solder can be seen in that the temperature differential over which the wire contraction can cause damage is lower due to the lower melting point. However, Evergreen Solar has never used Pb containing solder in its interconnect wire and chose not to explore this possible solution due to the environmental concerns surrounding Pb. For this work we used our standard Sn