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The determination of swelling stresses in polyimide films
Abstract
Polyimides have been the material of choice for use as an insulating layer in electronic circuit boards. The solvent resistance and temperature stability as well as the insulating properties that polyimides possess make them ideally suited for this application. Polyimides are applied to circuit boards by spin-coating or sputter-coating a poly(amic-acid) precursor solution. Curing, solvent removal and imidization of the poly(amic-acid) to the polyimide, causes shrinkage stress. This shrinkage stress can cause failure of the film by cracking or delamination. An understanding of the stress in these systems would allow for better design of the circuit boards thereby avoiding these modes of failure. The swelling stresses which occur in fully cured polyimide films exposed to solvents/permeants is the focus of this dissertation. A small strain linear elastic analysis has been performed and has shown the applicability of the theory to transient swelling. Techniques have been developed to measure the swelling stresses and swelling strains of these films. The swelling behavior of poly(N,N$\sp\prime$-bis(phenoxyphenyl)-pyromellitimide) (PMDA-ODA) has been fully characterized in water and in methylene chloride. It was found that the swelling stress of uniaxially constrained PMDA-ODA in water is approximately seven MPa and in methylene chloride is greater than twenty MPa. The actual swelling stress on PMDA-ODA in methylene chloride is difficult to measure since the swelling stress is so large that the uniaxially constrained film buckles when exposed. The corresponding strains are approximately 0.3% for water exposure and 3.8% for methylene chloride exposure. The swelling coefficient of a material is the amount of length change experienced upon exposure to a permeant normalized by the mass uptake of the permeant. These experiments showed that the swelling coefficient of PMDA-ODA in water is 0.17 (m/m)/(g/g) and in methylene chloride/nitrogen vapor is 0.025 (m/m)/(g/g). The swelling coefficient of PMDA-ODA in methylene chloride is much lower than the swelling coefficient in water despite PMDA-ODA's greater strain in methylene chloride. This would indicate a much larger mass uptake of methylene chloride by the PMDA-ODA. Coupling between stress, temperature and solvent content was also observed. These observations indicate that a PMDA-ODA film under stress will not completely dry after solvent exposure unless the stress is lowered or the temperature is raised. This effect was most noticeable in the films exposed to methylene chloride, chloroform and acetone. PMDA-ODA films pretensioned to 50 MPa, held at constant strain and allowed to stress relax to an apparent equilibrium of 35 MPa, experienced a stress of zero when exposed to methylene chloride, chloroform or acetone. The large change in stress is the result of swelling and not the result of viscoelastic time effects.
Subject Area
Chemical engineering|Materials science|Plastics
Recommended Citation
Sackinger, Scott Thomas, "The determination of swelling stresses in polyimide films" (1990). Doctoral Dissertations Available from Proquest. AAI9100535.
https://scholarworks.umass.edu/dissertations/AAI9100535