Factors Affecting Quality of Solder Joints in Multi-Busbar Interconnection

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Introduction

Tabbing and stringing is the dominant method in the market for making crystalline silicon (c-Si) solar cell modules. Soldering ribbons are used as an interconnection medium in this process on the four or five busbars of the front- and back-side metallization. The process is well established and proven reliable ^[1]. However, there are still challenges to obtain high efficiency, cost-effective solar electricity. These challenges include ^[1,2]:

1. Low-cost manufacture with higher power output.

2. Reduce the amount of silver (Ag) paste per cell.

3. Achieve current redundancy.

4. Introduction of fine gridline print in front metallization to reduce electrical and optical losses.

5. Implementation of simple and low-cost processing along with advanced cell architectures to achieve higher efficiencies (>20%) in the field.

Amongst several new interconnection technologies, multi busbar (MBB) interconnection promises to alleviate some of the above problems. The technology was first introduced in 2006 ^[3] and over a period of time several approaches to achieve MBB interconnection have been emerged. Soldering ^[2], low temperature alloy lamination (Smart wire) ^[3,4], woven fabrics assisted soldering ^[5], and the prefabricated interconnected grid method (Merlin) ^[6], etc. to name few. The MBB technology of soldering is the closest to the traditional cell interconnection technology. The current module equipment can be upgraded to assemble MBB modules.

Schmid company was the first one to develop a special stringer machine for MBB modules. Now, several Chinese equipment manufacturers are developing this kind of machine. Typically, MBB cells are interconnected by seven to 15 solder coated copper wires with circular cross-section (diameter between 200 and 450 µm on each side of the solar cell. The wires are soldered by infrared soldering on silver pads printed on the front and back of the cells.

This article originally appeared in the August 2021 issue of SMT Magazine. To read the entire article, click here.