Thermal Effects on Lifetime Evaluation of Adhesiveless Copper–Polyimide

Abstract

Adhesiveless materials are commonly used for electric or portable products that require high flexibility during operation. This structure incorporates a copper substrate and polyimide film. For life-time studies, the key aspect to evaluate is the effect of the thermal factors. In general, techniques to measure the life-time of the materials involve storing samples over a long term period in order to monitor changes; however, this is time-consuming. The aging test is an alternative method to predict and evaluate the changes in the mechanic properties over a short term period; yet the test conditions influence the accuracy of the results. The temperature and relative humidity (RH) are the major parameters for the test, and the test conditions of 85°C/85% RH and 150°C will be discussed in this paper. It is observed that the lifetime of rolled-annealed copper foil with polyimide film is significantly higher than metal films produced by other common methods, and the results are explained using the analysis of the fracture mechanics. From comparing the test data with samples stored for three years, the best prediction was achieved with conditions of 85°C/85% RH; with errors of less than 5%. The best flexibility was achieved when the optimum thickness of the copper and polyimide was determined.

Flash‐Induced Stretchable Cu Conductor via Multiscale‐Interfacial Couplings

“…On the other hand, no fracture damage was observed for the flash‐activated Cu conductor, stemming from the excellent adhesion properties of the dense Cu on the polymer (Figure d‐iv) . The significant enhancement of the Cu adhesion energy could also improve the life spans of electrodes in humid condition. Figure e shows the results of an accelerated humidity‐resistance test conducted with the theoretical Arrhenius approach to calculate the lifetime of flash‐induced Cu on a polymer (PI).…”

“…If humid moisture (85%) at a high temperature (from 75 to 95 °C) infiltrates into the samples through the vulnerable Cu and PI interface, the Cu metal line would be destroyed by corrosion, which would then induce delamination, breakage, and oxidation of the Cu. The time‐to‐failure ( t f ) was defined as the length of time until the resistance became 10% higher than the initial status after putting the fabricated Cu into the humid chamber . The t f of the flash‐induced Cu conductors in humid chamber (85%) at different temperatures of 75, 85, and 95 °C were 1, 3, and 9 h, respectively.…”

“…The X‐ray photoelectron spectroscopy (XPS) depth profile result of the sputtered Cu in Figure S4a (Supporting Information) shows that severe oxidation damage occurs not only on the Cu surface but also at the Cu and polymer interface. This indicates that the adhesion of weakly bonded Cu onto the PI could be easily degraded by the environmental stress (e.g., polymer dehydration), resulting in Cu delamination from PI and therefore creating additional oxidation and crack sites . As illustrated in Figure f‐iii, the sputtered Cu with low adhesion on a polymer could easily peel due to environmental fatigue (85% humidity at 85 °C for 96 h), allowing the permeation of moisture through the Cu interface and thus causing islands to form of the Cu material.…”