EoW November 2013

Technical article

There are three types of solder coat thickness measurement technique: • X-Ray: off-line measurement used for one-side thickness measurement • Manual micrometer: off-line measure- ment used for measuring the total thickness of two sides of the coat • Laser: in-line measurement that can be deployed on the tinning line and is used for measuring the total thickness of two sides of the coat during PV ribbon production PV ribbon is also inspected visually or with a microscope to examine coating quality, which should be without defects such as stains, debris, burrs, dents, discoloration, bare copper visible through solder coating, small pinholes and other kinds of mechanical defects. Most of the above specifications and corresponding measurement techniques are defined in the standards for PV ribbon that were introduced in August 2011. They are available at www.semi.org and include: • SEMI PV18-0811: Guide for Specifying a Photovoltaic Connector Ribbon • SEMI PV19-0811: Guide for Testing Photovoltaic Connector Ribbon Characteristics Finished PV ribbon products are packed on spools/reels or discs/pancakes. The most common spools used for PV ribbon in Europe are DIN K125, K160, K200 and K250 and in Asia also P4 and P10. All of the above PV ribbon specifications are important in their own way. Type of copper and its purity determines material conductivity and the maximum level of softness achievable for the ribbon. Solder composition, its coat thickness and coating composition influence the quality of solder joint and panel durability. High elongation of PV ribbon is important to prevent failure of solder joints between the bus bar and interconnect ribbon, which may occur due to stretching/ tension due to temperature oscillations during the panel operation. Continuous daily, sometimes extreme temperature fluctuations during the lifetime of the solar Critical quality parameters for PV ribbon

• PV bus bar: width [3mm-6mm] x thickness [0.2mm-0.5mm] • Interconnect ribbon: width [1mm-3mm] x thickness [0.08mm-0.2mm] Copper ribbon tolerances vary among producers. They depend mainly on the type of rolling mills deployed, the quality of input material and the know-how of the manufacturer. Typical tolerances for producers with good rolling capabilities are: • Width tolerance: ±8 micron-±15 micron • Thickness tolerance: ±8 micron-±13 micron The mechanical properties of PV ribbon that are commonly sought by panel manufacturers are: • Tensile strength: <250 MPa • Elongation: >20% • Camber: <0.5% [5mm on 1m long sample] PV ribbon straightness, also known as camber, is measured in terms of millimetres off a straight line on one metre long ribbon sample. Maximum level of camber is determined by the stringing process and typically ranges between <8mm/metre and <5mm/ metre. There are different types of solder compositions used in PV ribbon. They depend on the stringing/soldering tech- nique deployed by the panel manufacturer and the local health and safety standards related to panel manufacturing. Common solder compositions are as follows: • Lead-free solder: Sn 100 • Lead containing solder: SnPb 60/40 • Silver containing solder: SnAg 96.5/3.5; SnAgCu 96.5/3.0/0.5 • Lead and silver containing solder: SnPbAg 62/36/2 • Low temperature solder: BiSn 57/43; BiSnAg 57.7/42/0.3 Solder coat thickness ranges from 10 micron to 40 micron, with tolerances between ±10% and ±30%. The most common solder coat thickness is 20 micron ±4 micron. • Yield strength (Rp0.2%) • Hard/semi hard >120 MPa • Soft <80 MPa • Super soft <65 MPa

soft and properly soldered ribbon has to be used. Accurate laying of the PV ribbon also has to be ensured during the stringing, tabbing process. Good quality interconnect ribbon will inevitably reduce stringer downtime and its scrap rate. Today’s high-speed stringers require ever more demanding ribbon specifications. The three key trends in PV ribbon specifications include: • Ever tighter tolerances of solder thickness and ribbon straightness are driven by new generation fully automated, high-output stringers • Lower ribbon yield strengths (Rp0.2%) are required for increasingly thin solar cells • New panel designs utilise three interconnect ribbons per cell instead of two, reflected in a growing demand for smaller (narrower and thicker) ribbons. This in turn drives capacity expansion of precision tinning lines for small interconnect ribbons ▲ ▲ Figure 2 : Solar panel with interconnect ribbons soldered onto cells and bus bar around the panel perimeter The conductor or base material in the PV ribbon is high-conductivity, high-purity copper. Copper used in PV ribbons is typically ETP, DIP form copper or oxygen-free copper (OFC: CD-110, CD-101, CD-102). Copper wire is rolled in a rolling mill to produce copper ribbons, which are subsequently tinned/soldered in a tinning line to produce PV ribbon. Some producers use an alternative process of copper strip slitting to produce copper ribbons, which are generally of lower quality. The size range of bare copper ribbons (inlet material for tinning line) is as follows: PV ribbon specifications and requirements

▼ ▼ Figure 3 : Cross-section of a typical hot-dip tinned copper ribbon

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November 2013