String welding of solar cell

String welding of solar cell

The previous article introduced the technology of single welding of solar cells, and this article will introduce the technology of string welding.

1. Purpose

Weld the welded monolithic interconnection strips flatly to the back electrode (ie, negative electrode) of the next cell to ensure a good electrical and mechanical connection. The interconnecting strips on the front and back surfaces are bright and there is no surfacing residue.

2. Bill of Materials

(1) There are 72 single-piece welded cells in total.

(2) Tin-coated solder tape (specification 1.5mm × 0.15mm, length 160mm, 12 in total).

(3) Flux.

(4) Analytical grade ethanol.

(5) Special template for tandem welding (see Figure 1).

String welding of solar cell
Figure 1 – Special template for tandem welding

3. Equipment and tools

(1) Constant temperature welding station.

(2) Special heating plate for connecting the template in series.

(3) Operating table.

4. Proper craftsmanship

(1) Wear work clothes, shoes, gloves and finger cots to prevent bare hands from touching the battery cells. The sweat and grease on the hands corrode the cells and affect the cross-linking of the cells and EVA.

(2) Before working, the work surface must be cleaned, the process hygiene should be done well, the environment should be kept clean, and contamination should be prevented.

(3) The temperature of the electric soldering iron is controlled between 360~370℃.

(4) Take the cells that have been single-soldered, 11 cells with an interconnection strip length of 230mm and 1 cell piece with an interconnection strip length of 160mm as a group, put them into the turnover box, and cover the upper cover of the turnover box. Then turn it over so that the back of the battery is facing up, and remove the lower cover.

(5) First, put 11 pieces of batteries with interconnecting strips of 230 mm in a row, pay attention to the negative pole of the battery piece facing down, and the lead wire of the latter piece is on the positive surface of the former piece, and finally take a cell with a length of 160mm interconnecting strip and put it into the template. Note that the two edges of each cell need to be in contact with the stopper, and there is no slope, as shown in Figure 2.

String welding of solar cell
Figure 2 – Serial connection process requirements

(6) Gently press the cell with the left hand, and place the edge of the cell parallel to the busbar against the positioning edge of the string welding template. The edge of the vertical busbar line is aligned with the positioning line of the string welding template, and the interconnection bar of the latter piece is welded to the back electrode of the former piece. The welding time of a single interconnecting strip should be controlled at about 2s, the speed should be uniform, and the angle between the electric soldering iron and the horizontal plane should be maintained at about 45°, and the head of the electric soldering iron should be in full contact with the surface of the interconnecting strip.

(7) Weld the last cell with a 160mm long interconnection bar, and the distance between the head of the interconnection bar and the edge of the back electrode should not exceed 5mm, as shown in Figure 2.

(8) Make one corner of the battery sheet close to the template. After aligning, press the interconnecting bar and the battery sheet with the fingers of the left hand to avoid relative displacement, but it is necessary to pay attention not to put too much pressure on the fingers.

(9) Visual inspection and self-inspection, rework if the quality is unqualified, and put the qualified welding template into the transfer template.

(10) The battery strings placed in the transfer template should be turned over twice. If there are small tin beads or foreign objects, use absorbent cotton and anhydrous ethanol to clean them. Pay attention to the excessive cleaning force, which may cause damage to the battery cells.

5. Quality requirements

The quality IPQC personnel will conduct random inspection according to the sampling level stipulated by the national standard to determine whether the following quality objectives are achieved:

(1) The welding surface is bright and free of tin beads and burrs.

(2) Weld evenly and straightly in the back electrode.

(3) No de-soldering, virtual soldering and over-soldering to ensure good electrical performance.

(4) It has a certain mechanical strength, and the interconnection bar is gently pulled along the 45° direction without falling off, and the downward force can withstand a force of 50N.

(5) The welding surface of the negative electrode remains bright.

(6) The busbars of each single string of cells are on the same straight line, the misalignment is less than 0.5 mm, and the spacing between the cell strings is 2 mm ± 0.5 mm.

(7) Each single piece in the battery string is required to be complete, without fragmentation and cracking.

6. Precautions

(1) The hand holding the soldering iron should wear a sweat cloth over the cover, and the finger that takes the battery must be covered with a finger cover.

(2) The temperature of the soldering iron should be calibrated before work, and the calibration frequency is 6 h/time.

(3) Hidden fissures shall not be incorporated into the battery module, and the cracked battery shall be picked out and handed over to the process team leader for unified treatment.