INTERCONNECTMATERIALS
applications specified low cost, non noble metals such as tin and tin alloys, the adhesive metal interface was susceptible to humidity induced corrosion resulting in unstable, increasing contact resistance [5].
ECA as Thin Film Interconnect Historically in the c Si solar industry, the cell interconnect was formed with high temperature solders. The bus bar on the top side of the cell is soldered to a metal alloy ribbon. The ribbon is then soldered to the back side of the adjacent c Si cell. Depending on the design, many cells are strung together by soldered ribbons.
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ECA has become a popular choice for thin film applications for various reasons. As mentioned because of material temperature limitations, high temperature solder is precluded from use as the interconnect. Stresses created by high temperature solders can lead stress cracking of rigid substrates. ECAs create less stress and provide improved survival of thermal cycling and high temperature conditioning. In addition, many thin film technologies enable flexible designs that could only be made possible with a flexible interconnect such as a flexible, rubber like ECA.
Reliability concerns
Low stress polymers, that are essential to minimizing stress, intrinsically possess a low glass transition temperature, Tg, and a high polymeric free volume at ambient temperature. Free volume in a polymer is an indicator of the space between molecules and is related to vapor permeability [6]. During damp heat conditioning, the high free volume passes relatively high levels of moisture vapor into the bulk of the adhesive that diffuses to the adhesive metal interface.
With water, electrolyte (ionic species typically in the adhesive), and an electrochemical potential difference between the ECA and the substrate, a galvanic cell is formed. The tin or tin alloy ribbon is the anode with a relatively high oxidation potential or likelihood to liberate electrons. The general reaction is shown below:
Stresses created by high temperature solders can lead stress cracking of rigid substrates. ECAs create less stress and provide improved survival of thermal cycling and high temperature conditioning
increase in the interfacial resistance of the ECA ribbon joint [7, 8].
Sn2+ 2OH– →Sn(OH)2 →TinOxide (3)
The table above shows the oxidation potential of several materials. The larger potential difference between the dissimilar metals results in a faster rate of corrosion.
Interfacial Stresses With any joining technique, whether solder or adhesive, an expansion coefficient mismatch exists between adhesive and substrate and stress at some level is unavoidable. Interfacial stress is a result of a summation of strains that occur at the interface of adhesive and substrate during the bonding process. The total strain along with an effective modulus of the entire assembly produces a resultant interfacial stress.
When the ECA passes through the gel point, the summation of strains begins as the adhesive takes on more elastic than liquid characteristics. Initial strains from the crosslink reaction are followed by cooling strain from the cure temperature since the adhesive will shrink with a different contraction coefficient than the substrates that are bonded.
Sn→Sn2+
+ 2e–
(1)
The silver filler in the ECA typically acts as a cathode and the reduction of water occurs at the cathode forming hyrdroxide ions:
2H2 O+O2 + 4e– →4OH– (2)
Finally, the hydroxide ions react with tin, and oxidation of tin to tin oxide occurs resulting in an
Table 1. Standard oxidation potentials of common materials in electronics and solar industry
www.solar-pv-management.com Issue III 2011
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