Process development and reliability study with anisotropic conductive film bonding on multiple types of PCB surface finishes

ACF (Anisotropic Conductive Film) is a widely-used technology for flex-on-board (FOB) and flex-on-flex (FOF) assembly. Flexible Printed Circuit (FPC) boards enable the use of three dimensional space within the envelope of products such as sliding cell phones, mp3 players, etc. In the past few years, the usage of FPC´s made of polyimide, polyester or Teflon has continued to grow substantially and have become a key assembly technology in PCBA´s. ACF is made up of two main components: the adhesive itself, typically a formulation of epoxy resins, acrylic resins, or mixture thereof; and conductive particles used to connect both sides of the interconnection. The conductive particles can be made from nickel-gold plated polymer spheres, gold plated nickel particles, or recently from lead-free solder materials. This paper will report the development of 0.2mm fine pitch ACF bonding with different combination of PCB surface finishes (OSP, ENIG and I-Ag), including the mechanical ACF bonding strength, and thermo-mechanical reliability of the ACF interconnection. ACF material characteristics that contribute to the differences in yield will be discussed as well.