Connecting Solar Cells

Connecting Solar Cells

Tabbing and Bus Ribbon for

Solar Assembly

The interconnection of solar cells is a technology that has been around for hundreds of years, but is a relatively new application of the soldering process. By combining the metallurgical knowledge of solder joints (which has been developed through other applications) and new materials designed specifically for solar manufacturing, solar cells and cell strings can be effectively connected with high throughput, conductivity, and reliability.

BY JIM HISERT

Stringing of solar cells is used across the solar industry, and is a process that newcomers to the solar industry should be familiar with. However, it is a process that even experts still need to optimize. The top layer of a solar cell is a Transparent Conductive Oxide (TCO) to which solder will not adhere. Therefore, a metallization paste is used to bond to the TCO and provide a solderable surface for strips of solder-coated copper called tabbing or stringing ribbon. These ribbons are commonly applied as parallel strips that weave from the top of one cell to the bottom of the next to connect the positive and negative sides of the cells in series. Once connected, the tabbing ribbon channels electrical current to larger solder-coated copper strips, known as bus ribbon. Bus ribbon serves as an input/output for the entire solar array to the module junction box. Figure 1 illustrates the makeup of the four major types of solar cells and where these interconnect ribbons are used.

Tabbing Ribbon

PV interconnect products have a lot of nicknames: tabbing ribbon, interconnect wire, bus bar, stringing ribbon, flat wire, PV ribbon, as well as others. The dimensions are not yet standardized and even more varied than the names we use to describe them. There are three basic specification groups for tabbing ribbon: copper core specifications, finished material dimensions and tolerances, and solder coating specifications.

Copper core specifications include base copper purity, tensile strength, and elongation percentage. In addition, the copper is sometimes specified with a certain softness value, which helps to reduce stresses that are imparted on the wafer, as in the case of c-Si cells.

The second group of specifications deals with the physical dimensions of the tabbing ribbon. Although the copper is coated with solder, which increases the width and thickness of the finished ribbon, finished material dimensions are still based on the copper core. In addition to cross-sectional dimensions, camber of the finished ribbon is important when the material is used in stringing equipment.

Solder coating should be specified as well. Solder thickness is important when applying tabbing ribbon, bus ribbon, or (most likely) both types of PV interconnect materials. In almost all tabbing/stringing applications, the solder coating on the interconnect ribbon provides 100% of the solder used to form a metallurgical bond on top of solar cells. With this in mind, the solder coating should be more than just a shiny finish on the tabbing ribbon but what is the proper thickness for soldering? Solder manufacturers have been making precision solder-coated ribbon for quite a long time (and not just for tabbing/stringing). This experience has proven useful for determining how to control solder thickness and what thicknesses work in various applications.

Since RoHS and WEEE initiatives do not apply to this industry, many manufacturers use SnPb solder for interconnects, with Sn60 and Sn62 being the most popular. However, Sn/Ag is occasionally used and some manufacturers are exploring the use of SAC alloys (Sn/Ag/Cu), specifically SAC305.¹⁾ The alloy 96.5Sn/3.5Ag may not be the most well suited Pb-free alloy for tabbing however, not everyone agrees with this, as evidenced by the amount of Sn/Ag that is requested compared to Sn/Ag/Cu. Our industry uses Sn/Ag because it is now incumbent, but let��s examine why you should consider SAC305 (96.5Sn/3Ag/0.5Cu ) instead.

Melting Point

The melting point of SAC305 is 217 0C, compared to Sn/Ag at 221 0C. Why subject your solar assembly to more heat than necessary? Sn/Ag might be worthwhile if there were mechanical advantages, but are there any?

Reliability Data

Before the Pb-Free era, information was collected on certain Pb-free alternatives using Sn/Pb as a reference. There is simply more data available for Sn/Ag/Cu, which you can find from numerous and credible testing databases.

Strength and Raw Materials Cost

In addition to slightly higher tensile and yield strength, using an alloy with a small amount of Cu improves wetting?which in turn improves solder joint strength. The electronics industry has learned the benefits of Sn/Ag/Cu. Perhaps it is time for the solar industry to catch up.

Bus Ribbon

Bus ribbon is a very specialized interconnect that is used for photovoltaic modules. Much like tabbing ribbon, bus ribbon is made up of a copper ribbon, or flat wire, that is coated in solder. The solder protects the surface of the copper from oxidation and provides a layer of solder to form the solder joint. These solder-coated copper strips are used to collect the electricity from the strings. Both bus and tabbing ribbon are specified in the same way.

The main difference between these two interconnect materials is that bus ribbon is a bit wider and sometimes thicker than tabbing ribbon. Imagine tabbing ribbon as a road that travels across the solar cell. The bus ribbons serve as the highways to connect and tie them together. Bus ribbon is larger in cross-section because it has more electrical power to carry. To give you an idea of the size, bus ribbon is generally 5 mm-6 mm wide, although some applications require bus ribbon to be more than twice as wide.

Tabbing Flux

Solder fluxes are used with tabbing ribbon to form a solder connection with the metallization paste. Flux dissolves the oxides present on the surface of the tabbing ribbon as well as the silver metallization bonding strips on the top and bottom of the solar cell. Fluxes are typically liquid and consist of a chemical activator package, rosin or synthetic resin, and a solvent system. Historically, the solar industry has used fluxes formulated with alcohol solvents, but newer formulations with low VOC solvents are available. These newer low VOC fluxes are safer to use and have less environmental impact.

In both electronics assembly and the manufacture of solar cells, long-term reliability is of paramount importance. Therefore, care must be taken to insure that the flux selected for soldering will be non-corrosive and that the activator/resin system be designed to volatize or decompose during the peak temperature of soldering. This insures that no corrosive byproducts remain and the flux residue can safely remain on the substrate. The formulation technology and reliability testing for these no-clean fluxes were developed for electronics assembly and microelectronics applications by flux manufacturers serving these industries. These industries, circuitry line width and spacing are significantly less than what is used in solar cells, and even minute amounts of corrosive residues negatively impact the SIR (Surface Insulation Resistance) performance. Therefore, it is prudent for the solar module assembler to select a tabbing ribbon flux supplier that has experience in the electronics assembly and microelectronics industries.²⁾

New Trend

Tabbing is a process that can make or break a module assembly process. Although this is a relatively new application with new materials, solar cell stringing and busing use the same soldering principals that have been around for hundreds of years. This is good news for the solar industry because they can use this knowledge to effectively connect solar cells and cell strings with high throughput, conductivity, and reliability.³⁾

Jim Hisert is Applications Engineer at Indium Corporation (http://www.indium.com/).

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REFERENCES

1) Photovoltaic Stringing in Solar Cell Module Assembly, Karl Pfluke, SMT Magazine, May 2009

2) Fluxes for Soldering Tabbing Ribbon, Solar Materials Science Blog, Jim Slattery

3) Solar Material Science Blog

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