Minimizing Micro-Cracks in Solar Cell Interconnections

Paul Wood

Advanced Product Applications Manager

Soldering Problems

Solder quality and panel-to- panel consistency

Micro-cracks on photovoltaic solar cells

Higher temperatures can crack the wafer substrate

Pressure can crack wafer substrate usually caused by heater in solder tool cooling

Buss bar tagging or repair is difficult if one panel needs to be replaced

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Solar factories still use soldering irons for touch up and rework

The flatness and pressure of soldering is still an issue for fully automated lines, so touch up and panel replacement is done by hand soldering.

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Manual Panel Assembly

Two Soldering

Processes:

Stringing or Tabbing

Bussing

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Solar cell manual assembly soldering video

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Soldering iron must not overshoot temperature

Solder tip temperature must not overshoot

Is display temperature the actual correct tip temperature?

Having an unstable temperature causes operator variations in pressure. 1. Temperature self-regulation

2. No temperature overshoot3. Fast thermal response + recovery4. High watt density direct power

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How to measure repeatability

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Tip temperature display issue

Display temperature may not be tip temperatures

Up to 60°C difference between display and actual temperature is common at the tips. This is a common fault in conventional solder iron stations and is dangerous to substrate.

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Micro-cracks are caused by high tip temperatures

Temperature stability and repeatability is critical, do not heat substrate connection temperature over 300°C

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How Micro-cracks occur

During the manual soldering operation, differential thermal expansion of the copper and the silicon elements can occur at temperatures greater than 300°C.

This differential can result in the formation of micro-cracks that may not be detected during the manufacturing process and result in a less than expected field lifespan.

The need for precise time and temperature control is additionally critical by the intermetallic layer requirement of 1-2µm that must be created during the solder joint formation.

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Buss bar soldering

4-5mm ribbon is connected at the end of the panel to connect power ribbon. This is a heavier load than stringing operation, which is generally 2-3mm ribbons.

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Using hoof tips instead of chisel tips is a great improvement

Originally 2mm chisel tips were used by many companies, but they cooled down too fast.

OKi developed 2mm hoof tips for ribbon soldering, making it faster and safer.

OKi developed 4mm hoof tips for soldering busses faster, with no cooling at tips.

Chisel 2 mm

2mm hoof4mm hoof

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Smartheat PS-900 soldering system

OKi manufactures a thermal technology that will produce and maintain a specific self-regulated temperature, with a heater that requires no calibration

The heater technology is known as “Curie point heating”

Dynamically responds to PV tabbing and bussing thermal loads with measurably improved results

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Tip life and stability through the life of the tip is critical

Tips are tested for life and repeatability automatically

Plating thickness is optimized for performance

Too thick iron plate means poor performance, but long life

Hoof tips have been developed and proven as ideal for the solar industry

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Conclusion

OKi can provide economic solutions with high degree of repeatability

Proven hand soldering application with happy customers within China’s solar industry

Can supply standard equipment or work with OEM manufacturers to give custom solutions