PV welding strip is a key component in the solar panel, which is an important factor to improve the efficiency and durability of the solar panel (Figure 2). The high efficiency and durability of the solar panel can only be achieved by using high-quality PV welding strips properly installed in the solar panel. High quality PV welding strip can also improve the production efficiency of solar panels and reduce the scrap rate.

In order to ensure the high productivity of the series welding process, it is necessary to use high-quality, straight, soft and suitable welding strips. In the process of series welding and wiring, the precise placement of PV welding strip must also be ensured. High quality interconnection welding strip will certainly reduce the downtime and scrap rate of the series welding machine. Nowadays, the performance of butt welding strip of high-speed series welding machine is more demanding. The three main trends in the performance of PV weld strips are:
a) The new generation of fully automatic high-yield series welding machine requires more and more strict tolerances on flux thickness and strip straightness;
b) The thinner and thinner solar cells require lower yield strength of the welding strip (Rp0.2%);
c) The new solar panel design uses three interconnect strips per cell to replace two strips, reflecting the growing demand for smaller (narrower and thinner) strips. This in turn drives the capacity expansion of the precision tin plating production line for small interconnects.
Properties and requirements of PV welding strip
The conductor or base material in the PV tape is high conductivity and high purity copper (Figure 3). Copper used for PV welding strip is generally copper in the form of ETP, DIP or oxygen free copper (OFC: CD-110, CD-101, CD-102).
The copper wire is wound in a rolling mill to produce copper strip, and then tinned/flux plated on the tinning line. Some manufacturers use alternative strip slitting processes to produce strips, which are generally of low quality.
The size of PV busbar bare copper strip (the inlet material of tin plating production line) is 3-6mm wide and 0.2-0.5mm thick. For interconnection welding strip, the size of bare copper strip is 1-3mm wide and 0.08-0.2mm thick.
The tolerance of copper strip varies with the manufacturer. They mainly depend on the type of rolling mill used, the quality of input materials and the manufacturer's know-how. The following are typical tolerances for manufacturers with excellent rolling capacity: (a) Width tolerance: ± 8 μ m-±15 μ m ； (b) Thickness tolerance: ± 8 μ m-±13 μ m。
The mechanical properties of PV strips usually sought by solar panel manufacturers are:
● Tensile strength: ＜ 250MPa
● Elongation: ＞ 20%
● Curvature: ＜ 0.5% [5mm on the 1m long sample]
● Yield strength (Rp0.2%): a) Hard ＞ 120MPa; b) Soft ＜ 80MPa; Ultra soft ＜ 65MPa
The straightness (also known as curvature) of the PV strip is measured by the number of millimeters from the straight line on a one meter long strip sample. The maximum bending is determined by the series welding process, and its range is generally between<8mm/m and<5mm/m.
There are many different types of flux components used in PV strips. They depend on the series welding technology developed by solar panel manufacturers and local health and safety standards for solar panel manufacturing. Common flux components include lead-free flux (Sn 100), lead containing flux (SnPb 60/40), silver containing flux (SnAg 96.5/3.5; SnAgCu 96.5/3.0/0.5), lead containing silver flux (SnPbAg 62/36/2), low temperature flux (BiSn 57/43, BiSnAg 57.7/42/0.3). Flux coverage thickness range is 10 μ m-40 μ m. Tolerance: ± 10% - ± 30%. The most commonly used flux coating thickness is 20 μ m±4 μ m。
There are three methods for measuring the flux coating thickness. X-ray method is an off-line measurement method for single side thickness measurement. The manual micrometer is also an offline measurement method for measuring the total thickness of the two surfaces of the coating. Laser thickness measuring equipment is often used for online measurement, which can be used on the tinning production line to measure the total thickness of the two sides covered in the production process of PV welding strip.
The PV welding strip can also be used to inspect the quality of the coating layer with naked eyes or microscope. It should be free of defects, such as stains, fragments, dents, discoloration, bare copper visible through the flux coating, small pinholes and other mechanical defects.
Most of the above performance indicators and corresponding measurement methods are specified in the PV welding strip standard introduced in August 2011. At www.semi These standards, including (a) SEMI PV18-08111 and (b) SEMI PV19-08112, are available on the Web site.
PV finished products are installed on the spool/reel or disc. In Europe, the most commonly used spools for PV welding strip are DIN K125, K160, K200 and K250, and P4 and P10 are also used in Asia.
Key quality parameters of PV welding strip
All the above performance indexes of PV welding strip are important in itself. The type and purity of copper determine the conductivity of the material and the maximum softness of the strip. The composition of flux, the thickness of its covering layer and the covering composition affect the quality of solder joints, thus affecting the durability of solar panels.
The high extensibility of the PV strip is very important to prevent solder joint failure between the bus bar and the interconnection strip. This failure may occur due to the extension/tension caused by the temperature oscillation during the operation of the solar panel. During the service life of the solar panel, the continuous, sometimes particularly intense temperature oscillation makes the solder joints undergo tests during the service life of the solar panel (25 years on average).
Two parameters that are critical to most PV strip manufacturers are bending and yield strength. Many PV strip manufacturers find it difficult to obtain a high level of strip flexibility while ensuring its straightness. Getting enough flexibility and low bending may mean the difference between winning and losing the supply contract. Therefore, manufacturers must strive to improve their rolling, annealing, tin plating and material treatment technologies to meet the increasing requirements of product performance indicators.  
Key parameter: yield strength
The thermal expansion coefficient of copper is different from that of silicon. The interconnection bonding strip is welded on the silicon cell at about 200 ℃. Bending will occur when cooling after series welding. This may cause damage to the silicon crystal. The low yield strength interconnection strip reduces the stress in the silicon cell after series welding, thus reducing the scrap rate.
The use of thinner and thinner solar cells has promoted the requirement for lower yield strength (Rp0.2%) of the weld strip. Only a few years ago, 300 micron thick solar cells were widely used. They can withstand the stress of the weld strip with yield strength<120MPa. Now, 160-180 μ m thick batteries are widely used, and the yield strength of the matching welding strip is<70MPa -<80MPa. It seems that the average thickness of the battery will continue to decline, which will bring further pressure to the manufacturer of the welding strip to reduce the yield strength to less than 65MPa.
In order to reduce the yield strength of the PV strip, the manufacturer should seek improvements in the following areas:
● Select appropriate input copper material
● Select the correct annealing and rolling technology
● Ensure accurate handling of soft soldering strips passing through the transport system of the tinning production line
● Ensure good paying off and accurate winding of the winding line in the tinning production line.
Solar panel manufacturers who want to reduce the stress on the battery after series welding should check the paying off system on their series welding machine to avoid the weld band hardening and bending during the paying off process. Some solar panel manufacturers have adopted another solar panel design, each cell has three or even four smaller welding strips (instead of two), further reducing the stress on the cell after series welding.
Key parameter: bending
Low curvature is important to ensure the straight placement of the interconnect strips during the series welding process. With the increase of welding speed, the production of solar panels has been fully automated. When the curvature of the interconnection welding strip used in the process is too large, the opportunity for high yield full-automatic series welding is troubled by unnecessary downtime. Excessive curvature of the welding strip may even cause an increase in the rejection rate of defective solder joints or series welders. The current target curvature is usually ＜ 5mm/m. The trend towards more stringent bending requirements indicates that manufacturers may require bending<3mm/m in future solar panels.
In order to minimize the bending, the PV strip manufacturer has to seek improvement in the following aspects:
● The accuracy of the winding layer on the spool requires precise mechanical structure and precise process control;
● The consistent weld strip quality, especially the tolerance of the thickness of the covering layer shall be small;
● Select the spool with proper size.
The manufacturer is very clear about the limit of the possible minimum curvature on the edge of the spool, and the welding strip at the edge of the spool changes direction during placement. The minimum possible bending on the spool depends on the size of the weld strip and the drum diameter of the spool.
However, in order to improve the placement of the welding strip before welding, the solar panel manufacturer or the series welding machine supplier can check the possible improvement of the pay off system on the series welding machine. Increasing the spool size also helps to reduce the bending at the edges of the spool.
PV welding strip production: comparison between Plasma PREPLATE tin plating and traditional tin plating
Traditional copper wire tin plating is to pass the copper wire through the molten tin/flux pool, and then vertically wipe and cool the tinned copper wire in the cooling tower. Intermetallic bonding can only be achieved when the line surface is clean and properly activated. Acid cleaning or soaking is a traditional method for cleaning the copper wire surface before surface activation, which is realized by dipping flux. Flux dipping is a dirty and environmentally damaging process, which is also harmful to operators.
Before the copper strip enters the tin bath, the PlasmaPREPARE process anneals, cleans and activates its surface so that tin can adhere without using flux. Flux free tin plating accelerates the formation of the intermetallic layer, so the tin plating speed is much higher than that of the traditional process.
The Plasma PREPARE process can be adjusted to anneal the copper strip to any required softness. Complete recrystallization and small grain size with yield strength as low as 50MPa can be obtained. Annealing is carried out online together with tin plating to reduce the use of soft materials. Small stress and mechanical deformation reduce the possibility of improving the yield strength and bending of the winding line of the rolling production line and the paying off line of the tin plating production line. It is important to recognize the need for precision processing and precise winding in the case of ultra soft solder strips.
The precision transportation system for handling ultra soft solder strip will be an expensive investment required by each tinning production line. Therefore, a faster tin plating production line can reduce the capital input of the transport system per unit of output.
The traditional tin plating line requires pickling, rinsing and flux before tin plating. These wet processes not only have environmental problems; They are also harmful and annoying to operators. Flux contamination of the tin bath results in high cost flux waste. The use of wet process also produces many production parameters that must be precisely controlled. The production of expensive precision products with strict tolerance requirements (such as interconnection welding strips) requires strict control of production conditions to prevent excessive scrap rate. This is often difficult to achieve when wet processes are involved.
The differences between the two processes can be divided into differences related to production efficiency and differences in finished product quality. Despite the high capital investment, PlasmaPREPARE tin plating process provides many cost saving benefits that can be converted into a fairly long-term:
● Compared with the traditional process of 5m/min-60m/min, the production speed is up to 150m/min, which means less tin plating production lines, less machine land and less manpower;   
● Process stability, increase of normal production time and less bobbin replacement mean less materials are used and fewer operators are involved in PlasmaPREPLATE tin plating production line;   
● The plasma prepared dry surface replaces the pickling, washing, drying, flux dipping, waste removal and waste treatment used in the traditional process;   
● No flux production, less tin waste;
● Fast conversion between different products and performance specifications;
● The production cost is low according to the power, manpower, chemical cost used, their utilization, maintenance, etc;
● Online quality control in PlasmaPREPARE tin plating process makes the product quality consistent, with low scrap rate and less rework.
Since the PlasmaPREPARE tin plating process was first introduced into the production of PV solder strips in 2007, it is the stability of the PlasmaPREPARE process, consistency of product quality, low cost operation, low scrap rate and less tin waste that have won the trust of many PV solder strip manufacturers.