A photodefinable polyimide based on 3,3',4,4'-benzophenone tetracarboxylic dianhydride, oxydianiline, and metaphenylene-diamine (BTDA-ODA-MPD) was studied for its application as a stress buffer film over the passivation in a plastic-encapsulated device. The test vehicle was a bipolar-interface device containing piezoresistive circuits which, upon plastic encapsulation, would typically have 10% yield loss due to threshold voltage failure. Polyimide coating eliminates this loss by providing a buffer against stress imposed by the plastic encapsulant. Accelerated testing with unbiased and biased pressure cooker storage under 15 psig steam for over 1300 hr and 1000 temperature cycles between -65 degrees C and 150 degrees C confirmed the superior performance of the polyimide coated devices as compared with the uncoated control. The polyimide used, besides having the obvious advantage of requiring fewer process steps, was found to have higher film density and modulus of elasticity than to a standard polyimide of similar chemistry and is thus likely to have superior electrical and mechanical properties and higher resistance to moisture.
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