Hygrothermal effects upon stress relaxation in a polyimide film

Abstract

Polyimide films are being used extensively in first and second level microelec-tronic packages. Despite their many advantages, the tendency of polyimide films to absorb moisture can pose serious reliability problems such as corrosion, internal shorts, delamination, loss in dimensional stability, and a reduction in mechanical performance. This paper presents the results of an investigation of the effects of moisture upon stress relaxation in Kapton#x00AE; polyimide films. Three relaxation frames were designed and built so that they could be placed side by side within an environmental chamber which controlled both temperature and relative humidity. Stress relaxation experiments were then conducted in triplicate at various combinations of temperature and relative humidity and the relaxation modulus was determined as a function of time, temperature, and moisture concentration. While the relaxation modulus was found to decrease with increasing temperature and moisture concentration, this plasticizing effect could not be described by the simplifying assumption of time-temperature-moisture superposition except for those results obtained at or below 50% relative humidity. Application of the time-temperature superposition principle to the results obtained at relative humidities higher than 50% produced several distinct master curves at constant moisture concentration. These curves had fundamentally different shapes which could not be superimposed by rigid horizontal shifting.