Adhesion Characterization and Reliability Issues of Anisotropic Conductive Adhesives for Microsystem Applications

Doctoral thesis, 2005

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. Simple processing, low bonding temperature and fine-pitch capability are the major advantages of conducting adhesive technology. Tin/lead soldering technology is widely used in today's electronics manufacturing. Lead is, however, a toxic element, and is a well-known hazard to human health. Lead-free interconnecting materials are urgently needed and polymeric conductive adhesives are likely to be one of the best alternatives for tin/lead solders. Anisotropic Conductive Adhesives (ACAs) provide both electrical as well as mechanical interconnections. This type of conductive adhesives conducts only in one direction, namely in the vertical direction. The conductive particles used include pure metals such as gold, silver or nickel, or metal-coated particles with plastic or glass cores. The fine pitch and low processing temperature are the primary reason for the growing use of anisotropic conductive adhesives. Flip-chip bonding technology with ACAs has attracted significant interest in the area of microelectronics packaging. It has potential possibilities to offer low processing temperature, low cost, and capability of dealing with fine pitch. However, there is still a large concern about the reliability issues of these materials. One major concern is the poor adhesion strength of the adhesive joint; the other is contact resistance shift. The function of the ACA is to give the mechanical strength and electrical path of the joint. In recent years, extensive studies have been conducted for conductive adhesives in various areas such as joint reliability, electrical properties and microstructure development, and joint residual stress analysis. It is well known that the use of adhesives can increase fatigue life in flip-chip assemblies, hence improve reliability. Unfortunately, the use of adhesives results also in the formation of weak interfaces that tend to delaminate during thermal cycling. Delamination is therefore a major concern in microelectronic package (flip-chip assemblies) when using conductive adhesives. The main objective of this work is to study and characterize adhesion and reliability for flip-chip bonding using flexible substrates. Different factors, related to reliabilities, have been evaluated. Special attention was focused on the adhesion between the chip and the flexible substrate. Another important task of this work was to study the electrical and mechanical properties of anisotropic conductive adhesive pastes filled with conductive particles. In addition to this, the FEM simulation and a 4 parameters Weibull reliability model are presented for the reliability of ACA flip chip interconnect. The primary results show that the ACA flip chip interconnection is mechanically reliable and the contact resistance shift is small during elevated temperature and humidity aging.