Contact resistance and adhesion performance of ACF interconnections to aluminum metallization

Flip chip joining technology using anisotropically conductive films (ACFs) has become an attractive technique for electronic packaging. However, several factors have hindered the wide spread use of this technology. Along with the reliability issue, these factors also include the low availability and high cost of the bumped wafers. This paper introduces the feasibilities of using unbumped die with respect to ACF joints for flip-chip-on-flex (FCOF) assemblies. The unbumped dies contain only bare aluminum pads. Untill now the performance of ACF to Al metallization is a controversial issue from the published reports. In this study, two different test vehicles were used to study contact resistance and adhesion performance. Reliability of contact resistance for ACF joints with the unbumped dies was investigated in terms of varying the thickness of the Al pads. Adhesion performance of ACF to the Al metallization was compared with the adhesion performance of ACF to a glass substrate using the same ACF and the same bonding parameters. FCOF assemblies containing dies with thinner aluminum pads showed lower initial contact resistance and a lower rate of increment during accelerated aging tests. Three factors were considered as the potential causes for the above results: (1) lower concentration of aluminum oxide on the thin Al pad, (2) larger contact area per deformed particle with Au/Ni/Cu electrode for the interconnection of thin Al pad and (3) lower concentration of the defects in the thin Al pad. Contact resistance was found to increase during accelerated testing because of aluminum oxide formation on top of the pads. Contrary to the usual expectation, adhesion strength of ACF with the Al metallization was increased during 60 °C/95% RH testing. After 500 h of such moisture-soak testing, the adhesion strength becomes 3 times the initial value. The change in chemical state on the aluminum surface is considered to be responsible for higher adhesion strength. It is proposed that oxidation of Al surface due to diffused moisture and the new chemical bond formation at the adhesives/aluminum interface are the key reasons for good adhesion reliability. © 2003 Elsevier Ltd. All rights reserved.