INTERCONNECTMATERIALS


modulus below the glass transition temperature, (αs


- αr ) and (αs - αg


expansion differences between substrate and adhesive in the rubbery and the glassy phase respectively, and ∆T1


and ∆T2


differences between cure temperature to Tg to final ambient temperature.


) are the coefficient of thermal


are the temperature and Tg


. Fundamentally polymers undergo a significant increase in modulus when transitioned to temperatures below the Tg


.


Figure 1. Strain development or dimension change during the cure and cool of the ECA


Figure 1 shows the development of strain from the adhesive perspective.


Depending on the materials and geometries, the resultant interfacial stress can contribute to delamination, cracking in the substrate or adhesive bondline, and potentially curling or deflection of the substrates. These effects may not manifest at ambient temperature, but further cooling to lower temperatures will amplify the interfacial stresses present at ambient. As long as the expansion coefficients of adhesive and substrate are different, cooling will increase the net interfacial strain and resulting stress. However, a lower interfacial stress at ambient will certainly deliver lower stress when the assembly is cooled to sub ambient temperatures.


Defining what is meant by “low stress” is always going to be a difficult challenge. Complex stress models that utilize physical property data and geometries of the materials are good predictors of the stresses created by an adhesive. A comprehensive model was developed for a bi material strip where the adhesive and one interface are isolated and the stress created by the adhesive during cure is measured from the deflection of a bonded strip [9].


The equation is shown below: σ≡3Gr


sc +6Gr (αs –αr )∆T1 +4Gg


The shear elastic modulus can change by 1-3 orders of magnitude after passing through the Tg making Gg


One important aspect of the model that should be emphasized is the magnitude of the glassy shear modulus, Gg


1-3 orders of magnitude larger than Gr


.


Therefore, for a glassy polymer (an ECA having a Tg


.


sufficiently above ambient temperature), the 3rd term will have the largest impact on the overall stress due to the relatively large magnitude of Gg


21


Figure 2. Stress development from an ECA during cure and cooling


and most significant term in the equation can be minimized by reducing the ∆T2 accomplished by simply reducing the Tg


The 3rd


which is of the


adhesive. Figure 2 depicts how the stresses begin to increase rapidly once the material passes through the Tg


during cooling. (αs –αg )∆T2 (4)


where σ is the interfacial stress between substrate and adhesive, sc Gr


is cure shrinkage after gelation,


is the rubbery modulus above the glass transition temperature (Tg


), Gg is the glassy


Furthermore, when the Tg ambient (~0°C), the 3rd


is reduced to below term disappears because


the adhesive is in the “rubbery” phase at ambient; it does not approach the glassy modulus unless the temperature is further reduced to below the glass transition temperature.


www.solar-pv-management.com Issue III 2011


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