INTERCONNECTMATERIALS
The rubber like ECA will become glassy and more rigid when exposed to temperatures approaching 40°C. Some sensitive applications with complex geometries, large expansion mismatches with the adhesive, or fragile non compliant substrates may require a lower Tg ECA with a Tg
ECA. However, a rubber like near 0°C exhibits a very low
. Interfacial stresses are cumulative. Relative to the a rigid, high Tg
interfacial stress at ambient temperature after curing and cooling, and the only significant stress is during the cooling to lower temperatures below the Tg
ECA that is already highly
Figure 3. Shear modulus comparison of a rigid ECA with a rubber like ECA
Another more simplified model for adhesive interfacial stress is shown below [10]:
σ = 2.66Gg 22 (αs –αg )∆T2 (5)
Both models emphasize the same physical property data.
Measuring shear modulus
Shear modulus is easily measured by a parallel plate rheometer. In figure 3, an overlay of two significantly different ECAs is shown. The green line represents the typical rigid ECA with a high Tg
The material begins to soften (onset of Tg) at around 50°C. The Tg
would be identified as the
inflection of the transition near 85°C. Also shown is a low modulus, “rubber like” ECA with a Tg
near
0°C. From the graph it is apparent that the shear modulus of the rubber like ECA is an entire order of magnitude lower than the rigid ECA. Using interfacial stress models it becomes apparent that the 3rd
and most significant term in the model is reduced to nearly zero with the rubber like ECA.
Figure 5. Cohesive failure mode after peel testing
occur to create a strong bond [11]. Often components and additives in an adhesive are matched to the substrate based on the affinity for the substrate. In addition to the interfacial forces, adhesion strength is also highly dependent on the ability of the adhesive to plastically deform and dissipate stresses applied during the adhesion test [12].
Figure 4. Peel adhesion comparison between a rubberlike and rigid ECA
Because of the flexibility demands of the thin film industry and most likely because of the geometry of the ribbons employed, peel testing of the ECA has been adopted by many thin film manufacturers. The 180° peel test was conducted with tin silver coated ribbon on aluminium substrates comparing a rigid ECA and the ECA
.
stressed at ambient before cool temperature exposure, the rubberlike ECA will typically survive low temperature exposures without delamination or stress cracking.
Peel Adhesion With an ECA exhibiting rubber like elasticity as the starting point, adhesion to critical materials such as molybdenum, tin, tin silver, TCO, and conductive ink are essential. Fundamentally, adhesion is a function of interfacial interactions between the adhesive and the substrate. Molecular forces such as van der Waals forces, hydrogen bonding, electrostatic interactions, and covalent bonds
www.solar-pv-management.com Issue III 2011
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