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The electronics assembly industry has fortunately rediscovered conductive adhesives as the search for lead‐free joining materials and improved performance intensifies. Although these intrinsically clean bonding agents are often first sought for their favourable environmental attributes, many are surprised to find that conductive adhesives can solve old and new problems. Today, new polymer solders for SMT allow low temperature processing, finer pitch assembly and wider processing latitude while providing compatibility with a very much larger range of materials than solder. State‐of‐the‐art adhesives are oxide‐tolerant and absolutely no fluxing or cleaning is required. Adhesives work where solder cannot be used. What's more, polymer‐based solder alternatives can run on existing SMT lines — no new equipment is needed. Z‐axis, or anisotropic, bonding agents are uni‐directional conductive materials that solve fine pitch interconnect problems in several areas. The anisotropics now dominate the flat panel interconnect field. Nearly every LCD and other flat panel display is connected with a polymer adhesive. The Z‐axis adhesives are also beginning to enable high density multilayer circuits and MCMs to be built more effectively. Finally, Z‐axis appears to offer the simplest and most cost‐effective means for flip chip bonding. However, special equipment is required. The paper compares the metallurgical solder joint, the present de facto standard, with the polymer composite bond to highlight similarities and important differences. All types of conductive adhesives are discussed including the latest — Area Array Z‐axis types. Bonding materials, assembly processes and performance are also covered.

This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the significant research and development work for the mechanical properties of ACF material and joints, which helps to the development and application of ACF joints with better reliability in microelectronic packaging systems.

The ACF material was cured at high temperature of 190°C, and the cured ACF was tested by conducting the tensile experiments with uniaxial and cyclic loads. The ACF joint was obtained with process of pre-bonding and final bonding. The impact tests and shear tests of ACF joints were completed with different aging conditions such as high temperature, thermal cycling and hygrothermal aging.

The cured ACF exhibited unique time-, temperature- and loading rate-dependent behaviors and a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrained the ratcheting strain in subsequent cycling with lower mean stress or stress amplitude. The impact strength and adhesive strength of ACF joints increased with increase of bonding temperature, but they decreased with increase of environment temperature. The adhesive strength and life of ACF joints decreased with hygrothermal aging, whereas increased firstly and then decreased with thermal cycling.

This study is to review the recent investigations on the mechanical properties of ACF material and joints in microelectronic packaging applications.

Tape automated bonding (TAB) circuits were joined by hot compression bonding to copper or nickel conductors on glass with two anisotropic electrically conductive adhesives. One of the adhesives had a thermoplastic polystyrene‐polyester matrix which contained easily deforming metal‐coated polymer particles, while the other was a thermosetting bisphenol (A) based epoxy resin filled with nickel particles. The resistance values and the mechanical strengths of the joints were measured before and after the ageing treatments. The thermoplastic adhesive had the lowest resistance values with copper conductors and the joints produced with this adhesive showed increasing strength values during the ageing tests. The joints between the Ni conductors had smaller values of electrical conductivity irrespective of the adhesive used. The SEM/EPMA technique revealed that particles of the thermoplastic adhesive tended to agglomerate. This may cause problems when components with very fine lead pitch are joined, either by short circuiting or leaving some contacts without particles.

Flip chip assembly using anisotropic conductive adhesives offers an interesting alternative for making high‐density interconnections. The use of conventional organic printed circuit boards makes this technique even more attractive. However, a low‐cost adhesive flip chip bonding process will require a reduced bonding cycle time or the use of multi‐head joining equipment. Adhesive flip chip bonding is characterized by a long bonding cycle time due to the relatively long curing time of adhesives and the need for simultaneous application of pressure during the curing process. In soldered flip chip techniques, the bonding time per assembly is shorter, because all the chips on the substrate can be soldered in a reflow oven at the same time. In this study, the minimum pre‐curing time needed to make a reliable adhesive joint was determined using one commercial anisotropic conductive adhesive film used on FR‐4 substrates. The results are promising, since bonding time reduction from 40 s to 10 s does not reduce the joint reliability.

Polymer thick film (PTF) technology provides the lowest cost, cleanest and most efficient manufacturing method for producing flexible circuits. Non‐contact radio frequency (RF) smart cards and related information transaction devices, such as RFID tags, appear to be a good fit for PTF‐flex. Flip chip also seems well suited for these “contactless” RF transceiver products. Flip chip and PTF adhesive technologies are highly compatible and synergistic. All PTF SMT adhesives assembly methods are viable for flip chip. However, the merging of flip chip with PTF‐flex presents major challenges in design, materials and processing. This paper will compare assembly methods and discuss obstacles and solutions for state‐of‐the‐art flip chip on flex within the RFID product environment.

Polyester connector strips were joined to polyimide substrates with anisotropic electrically conductive adhesives. Copper conductors as well as Au/Ni‐coated copper conductors were used on flexible circuits. The adhesives were composite materials consisting of heat curing, one‐component epoxy resin and powdered ternary solder alloys: tin‐bismuth‐zinc, tin‐indium‐zinc and tin‐zinc‐aluminium. An adhesive filled with eutectic tin‐bismuth alloy powder was used as reference. The effect of bonding parameters (e.g., temperature, dwell time and pressure) on contact resistance values was evaluated. The contact resistance values were measured for evaluating the reliability of adhesive joints during a 60°C/95%RH test. Furthermore, the joint microstructures were examined with optical and scanning electron microscopy. The results showed that with the copper conductors the initial contact resistance values were lower than with the Au/Ni‐coated copper conductors. The most reliable joints were produced with low melting filler alloys (with respect to bonding temperature) on bare copper metallisation. The most likely reason for failure of the Au/Ni‐coated circuits was strong oxidation of locally exposed nickel in the presence of moisture.

This paper discusses flip chip on FR‐4 and ceramics using non‐conductive adhesive (NCA), anisotropic conductive film (ACF), or anisotropic conductive paste (ACP). Several ACF and ACP materials with different types of adhesive resin and conductive particles and one NCA material were evaluated. Flip chips were assembled on test vehicles for temperature cycling and high‐temperature high‐humidity tests. The reliability performance of the processes was compared. Flip chip processes using NCA, ACF, or ACP could give satisfactory reliability and high assembly yield for some applications, when the bonding parameters were optimised.

The purpose of the work is to investigate the feasibility of using anisotropically conductive adhesives to join surface‐mount devices as solder replacement. The results from a literature and market survey are reported. Based on industrial demands, two anisotropically conductive adhesives were chosen for the experimental work. During the experimental work, the conductive adhesive joints were produced at various curing conditions. The joints were characterised by shear testing and electrical resistance measurement after ageing at 20, 70 and 120°C to 1000 hours. Optical and scanning electron microscopy were used to characterise the adhesive joints. In addition to this, temperature cycling tests, humidity test and pull tensile tests were used to qualify the adhesive joint reliability and quality. From the results of the present work, it can be concluded that the anisotropically conductive adhesive A joints are stable in the 85°C/85% RH environment and therefore have better corrosion resistance than adhesive B joints. Neither of the adhesives can pass temperature cycling from −55 to 125°C for 1000 cycles according to military standard 883C.

Presents some experimental and theoretical results from research exploring the design rules and relevant process parameters in the assembly of electronic components using anisotropic conductive adhesive materials. The experimental configurations studies have geometries representative of flip‐chip and micro ball grid array chip scale packaging. Evaluates a range of materials combinations, including (random filled) adhesive materials based on both thermoplastic and thermo‐setting resin systems, combined with both glass reinforced polymer printed circuit board and silver palladium thick film on ceramic substrate materials. Also presents a summary of assembly experiments which have been conducted using a specially developed instrumented assembly system. This test rig allows the measurement of the process temperatures and pressures and their relationship with the consequent bondline thickness reduction and conductivity development. Finally summarizes the capabilities of models which have been developed of the assembly process and of the final joint properties.