Micro cracks in solar cell interconnection during manual soldering

(December 13, 2010) -- Solar cell substrates are delicate. The manual soldering process applies stress to the material, which can cause micro-cracks to form in the substrate. Hoa Nguyen, OK International, identifies the requirements and equipment needed to reduce micro-cracking during solar cell assembly.

Photovoltaic cell (PV) module assembly

There are two soldering process steps used to assemble a PV module: photovoltaic cell interconnection, called stringing or tabbing; and PV module assembly, called bussing.

Initially, the cells are electrically connected using tinned copper ribbon that is typically 2mm wide. The solder-coated ribbon is dipped into, or sprayed with, flux. Flux promotes wetting and removes oxides from the ribbon surface. A point of contact between the collector grid (substrate) and the ribbon is formed by screen printing a metallization paste to the substrate. The tabbing ribbon provides all the solder required to form a bond between the ribbon and the substrate.

Figure 1. Conventional solder joint after peel-test using optical and SEM micrographs.

Next, individual PV cells are attached in series by reflow soldering, forming a column precisely placed onto a glass substrate. Six to ten rows of cells are then "bussed" together using 5mm-wide bus ribbon to create the collector grid. Bussing solar cell columns is performed by hand, using a soldering iron. The need to reduce PV manufacturing costs is driving a steady reduction in wafer and cell thicknesses. As a result, the soldering process has become more challenging because the thinner cells are even more sensitive to thermally induced micro-cracking.

During the manual soldering operation, different thermal expansion of the copper and the silicon elements can occur at temperatures greater than 300°C. This temperature differential can result in the formation of micro-cracks, which might not be detected during the manufacturing process. Undetected microcracks could result in a less than expected field lifespan. In addition to the need for precise time and temperature control, it is also critical to adhere to the intermetallic layer requirement (within 0.5-1.5µm) during the solder joint formation. To meet this criteria, a process window to reduce cell damage and consistently produce good inter-metallic bonds between the ribbon and the solar cells must be identified and maintained. Precise control of time and temperature reduce the possibility of forming micro-cracks in the substrate.

Figure 2. Peel test diagram; Pb-free solder interconnection on standard metallization. The variation in force results from the manual soldering process.

Defect detection

Damage to the crystalline silicon can be detected by conducting a peel test according to conventional soldering interconnect joint specifications. Peel force values up to 6N (1.1lbs of force), which is within IPC/EIA J-STD-001 requirements for a conventional solder interconnection joint, should not damage the interconnection. After peeling off the ribbons, the tabbing ribbon remains in partial contact with the metalized substrate of the solar cell (Figures 1 and 2).

Figure 3. Sn96Ag4

Suitable solder types

There are two suitable solder alloys utilized for solar cell soldering application: Sn96Ag4 with a melting point of 221°C, or bismuth containing Bi58Sn42 with a melting point of 138°C. Proper soldering will result in an intermetallic layer that is within 0.5- 1.5μm (Figures. 3 and 4). The soldering temperature must be reduced to a minimum so that the solar cells will not be subjected to mechanical or thermal stresses from the hand soldering process.

Figure 4. Bi58n42

Self-regulated soldering equipment

A thermal technology exists that will produce and maintain a specific, self-regulated temperature with a heater that requires no calibration and responds directly to thermal loads. The proprietary SmartHeat Technology consists of a high-frequency alternating current (AC) power supply and a self-regulating heating element. The heater utilizes the electrical and metallurgical characteristics of two different metals: copper, which has high electrical conductivity, and a magnetic material with high resistivity.

When the heating element is energized by the high frequency alternating current (AC) power source, the current will automatically begin to flow thru the conductive copper core of the heater. However, as the AC current continues to flow, a physical phenomenon known as the "skin effect," occurs and the current flow is directed to the skin of the heater assembly, driving the majority of the current through the high resistance magnetic layer and causing rapid heating.

As the outer layer reaches a particular temperature (controlled by its heater alloy formula), it loses its magnetic properties. This "Curie point" temperature is when the skin effect begins to decrease, again permitting the current back into the conductive core of the heater and repeating the cycle.

Figure 5. Temperature of manual soldering iron tips.

SmartHeat technology is therefore an inherently temperature-stable form of resistance heating. Because of this inherent stability, it eliminates the need for external temperature control devices, and is characterized by:

• temperature self-regulation,
• no temperature overshoot,
• fast thermal response and recovery,
• and high watt density direct power.

Experiments were performed using the OK International SmartHeat PS-900 soldering system to evaluate its suitability for solar cell applications. Custom designed STV-DRH420A, STV-DRH430A, STV-DRH-440A and STV-DRH440R hoof tip geometries were chosen to optimize the power delivered to the solder joint, increasing efficiency and tip life. The temperature-sensitive "T" heater series was chosen to ensure low-temperature soldering, minimizing thermally induced stresses on the surface of the solar cells. To minimize thermally-induced damage of the solar cells due to micro-cracking, it is critical to maintain the solder joint below 300ºC at all times during the soldering operation. This was demonstrated by using the soldering system with the STV-DRH4XXA series tips to solder a 12″ strip of 0.4mm ribbon onto the surface of a solar cell.

The solder ribbon was placed on top of five thermocouples positioned three inches apart and the heated soldering tip was moved along the surface of the ribbon. Results show that by moving the heated soldering tip at an approximate rate of one inch per second, solder reflows efficiently at the ribbon and results in the formation of an inter-metallic bond of within 1-2µm. Moreover, since the heat is delivered directly to the solder joint, the temperature profile measured at the ribbon is observed to be uniform and follows a repeatable curve below 300°C with no thermal overshoot. The quick response of the soldering system combined with the lower "T" series tip temperatures helps minimize the thermal gradients and thus maintains high yields and module reliability for the solar cell application.

Conclusion

A variable power output power supply combined with a tip designed specifically for the application will measurably reduce the potential for cell damage. By applying only the thermal energy required to form a good intermetallic bond between the ribbon and the solar cell, the system described above will operate within a much smaller process window.

Acknowledgment
SmartHeat is a registered trademark of OK International.

Hoa Nguyen received his EE, CS at Columbia University and is VP of technology at OK International, 12151 Monarch St., Garden Grove, CA 92841; ph.: 714-230-2354; hnguyen@okinternational.com.