Tabbing with Soldering vs Tabbing with Melt-Bonding Conductive Adhesive

Traditional tabbing using pre-tinned copper ribbon of typically 2mm width and 0.2 mm thick that link to 4mm busbar of the same 0.2 mm thickness. The temperature of tabbing for the lead-free solder is now well over 250-300°C that induces high enough internal bonding stress as to warp the solar cells because of the different coefficient of expansion (CTE) between silicon (3 ppm/°C) and copper (17 ppm/°C). This embedded stresses cause yield loss during manufacturing and long-term reliability when not properly controlled.

Besides the fact that soldering process typically involve application of flux that must be clean off before encapsulation, another not often mentioned effect is the thickness of the solderable tab is 0.2mm or 8 mil that automatically dictate the thickness of the encapsulant such as EVA to be in the range of 12-15 mil or 0.3mm or more to ensure adequate insulation is provided for both the front and back side of the solar panel.

The obvious side effect of the thicker tabbing requirement and the resulting overall thickness of the encapsulant also increase the exposed edges that is susceptible to the moisture ingress and thus potential reliability in combination of heat for the EVA encapsulant that are known to degrade with acetic acid generation.

Improved tabbing and encapsulation solutions for the solar cells in the solar panel may cover the following:

• Lowering the tabbing temperature substantially: The stress arises from the CTE differential that is inherent with the use of tinned copper tab is directly proportional to the temperature rise above the ambient needed to reflow the solder for tabbing. Thus, lowering the temperature from 275°C to 150°C will reduces the stress by half.
• Lowering the thickness of the tabbing material: While low series electrical resistivity for the panel is important to maintaining the efficiency of the solar panel, the thickness of the tabbing copper can be reduced to 2-3 mil from 8 mil without observable effect.
• Using Chemically inert transparent encapsulant that are UV resistant: While EVA has great advantage of being low cost and low melt-encapsulation temperature, it also has the disadvantage of having to cure under pressue and heat. It also have the high susceptibility to moisture ingression and having the reactivity under heat to generate acetic acid that can cause electrical resistance issue and potential loss of continuity.

AIT SOLAR-TAB™ solution is engineered to reduce three known issues and reduces the costs of manufacturing:

• SOLAR-TAB™ provides the first instant melt-bonding tabbing solution that dramatically increases productivity, improved reliability and yields with low temperature processing of 150°C.
• The operation of the melt-bondable (similar to soldering) but at much lower temperature of 150°C. The exception is that there is no need for fluxes or cleaning after soldering.
• The internal stress is relaxed to "zero stress" with the flexible conductive adhesive that had been proven by AIT in the semiconductor and military industry for over 25 years.
• The same ultra-low electrical resistance between the solar cells charge collection circuit is achieved similar to the soldered tabbing.
• Fluorinated polymer is used that have more than 40 years of proven solar and UV stability.
• RoHS, REACH and WEEE compliant that meets UL94V-0 rating.

Tabbing Solutions That May Also Work On Ewt As Embedded Circuit

AIT has some patented technologies (US patent# 7,154,046; 6,581,267; 6,580,0356,297,564; 4,695,404) and pending patents that may be applied to the use of hyper-conductive UV-resistant melt-bonding application processes for solder-tabbing replacement with stress free solar cell interconnections:

• SOLAR-TAB™ is a solar cell interconnection adhesive designed for cost-effective melt-tabbing to replace traditional pre-tin tab soldering process for solar cell interconnection at substantially lower temperature of 150°C.
• Melt-tabbing at less than 150°C dramatically reduces solar cell stress induced by traditional solder tabbing.
• SOLAR-TAB™ melt-tabbing provides instant bonding and enables the use of direct lamination process for solar cells interconnections in solar panel manufacturing.
• SOLAR-TAB™ conductive tab is made for ambient storage.
• Besides automated melt-bonding lamination, traditional manual soldering irons can be used to melt-bond the SOLAR-TAB™ conductive tab directly onto solar cells and their interconnections.
• SOLAR-TAB™ is available pre-applied on passivated copper for melt-bonding interconnection to maintain long-term low contact resistance.
• SOLAR-TAB™ ST8150-DS and ST8150-SS melt-tabbing film adhesive on both sides of metal tab and film on single side of metal tab respectively.
• The SOLAR-TAB™ film adhesive application uses proven fluorinated polymers and patented process to ensure long-term contact resistance as low as traditional solder-tabbing

For more information and recommendation assistance, please contact AIT sales and engineering:

AIT technical sales and service department can also be reached at: 1-609-799-9388 or 1-800-735-5040 (EST) and Fax: 609-799-9308