ACP Mfg. tech. for rigid & flex board interconnection

Abstract

Anisotropic conductive paste / anisotropic conductive adhesive (ACP/ACA) type of flex-on-board (FOB) bonding method ensures good electricity between the flex electrode and organic substrate electrode for fine pitch ?0.3 mm design layout. ACP is an alternative material for connecting two circuits except anisotropic conductive film (ACF), besides, it is industry acceptable lead free environment friendly material. The conductive particle passes electricity after heat press and the binder bonds the circuits and spacing to form solid interconnection. By conductive particle size selection, particle density and distribution control, a Z-axis electrically conductive pathway is formed. The adhesive does not conduct in the X-Y plane, thereby isolating the adjacent pathways. ACP have been used for Flip Chip gold bump bonding process in liquid crystal display(LCD) driver IC or Chip On Flex(COF) assembly in finger print sensor, while the application for rigid & flex board interconnection is relatively few. This paper describes the investigation of alternative low cost flex board bonding process which using Anisotropic Conductive Paste as an interconnection material on organic board such as PET. The driving force of ACP to substitute for ACF is lower cost, about 10% to 50% compared with ACF, and it can increase the product throughput by shorter bonding time, stability after thermal press bonding. The restrictions of ACP are that it can't be used for ultra fine pitch circuit due to big conductive particle, connection pitch must be greater or equals to 0.1 mm. For same conductive particle size, ACP have higher resistance compared with ACF due to fewer conductive particles connected by different process fabrication methodology. As ACP has the lower cost benefit than ACF, Universal Scientific Industrial Co., Ltd. (USI) investigated the application of ACP. Two manufacturing methods (screen printing and dispensing type ACP) are introduced for rigid & flex board interconnection. Screen printing type ACP was considered as preferred material for its lower material cost, easy to rework, easier ACP volume control and higher peeling strength than dispensing type ACP. But as the screen printing paste is a mixture of the thermosetting and thermoplastic type, the reliability is not good. If the thermoplastic resin removed, bonding temperature becomes higher. Finally, dispensing type of the thermosetting paste is decided to use for good thermal stability. ACP overflow to the solder mask area and lower peeling strength for narrow bonding area were the major two issues need to be resolved for dispensing type ACP bonding process development at initial stage. By ACP volume control, bonding head design and bonding parameters optimization, USI successfully developed the ACP manufacturing technology for rigid & flex board interconnection and get good results in peel strength>500 g/cm enhancement. The experimental results demonstrate the feasibility of using the conductive paste to achieve a stable connection resistance at various environmental temperature, passing 1,000 cycles -35~85?C reliability test. A serious and complete study will be explained in the following chapters.