Joining of displays using thermosetting anisotropically conductive adhesives

Abstract

This paper describes the reliability of using thermosetting anisotropically conductive adhesives for two applications: (1) joining LCD modules to rigid boards via flexible circuitry in a large panel video screen application; and (2) joining of LCD modules directly to flexible circuitry in a camera application. Three different types of thermosetting adhesive, one mixed type of thermosetting and thermoplastic anisotropically conductive adhesive and two heatseal connectors were used in the present investigation. The heatseal connectors are currently used in production and were used in the present investigation as reference materials. In the present study, ITO glass was used instead of real LCD devices. The interconnections were made using conventional heatseal press equipment for the two applications. The reliability of the adhesives was investigated in terms of a temperature-cycling test from −40 to 85oC for 1000 cycles using a temperature change rate of approximately 20oC/min, a constant-humidity test at 60oC, 95% RH for 1000 h, and a humidity-cycling test from 5 to 75oC at 95% RH for 1000 h. Both mechanical and electrical properties were measured before and after the environmental tests. The results indicate that control of process parameters such as bonding temperature, bonding pressure and time is critical for good bonding quality. Under the right process conditions, thermosetting anisotropically conductive adhesives are much better in environmental performance than heatseal connectors. The resistance change with time in the humidity test has been explained by oxidation of metal fillers or by oxidation of metallizations on interconnecting materials. It is shown that the oxide layer thickness and type control the rate of resistance change in a humid environment.