Anisotropically conductive adhesives

MD-200 Technical Data Sheet, Thermoset, Lord Chemical Products Dec. 18, 1997. Watanabe I, Takemura K. Anisotropic conductive adhesive films for flip-chip interconnection. In Liu J, ed. Conductive Adhesives for Electronic Packaging. Electrochemical Publications Ltd 1999 [chapter 10]. 

The traces are at densities of over 200-line pair per inch. The adhesive must survive temperatures from —40 °C to - -80 °C and be reparable. The resolution of the LCD is limited by the achievable line-pair density. If the lines are too closely spaced, the conductivity between the lines becomes too high. The LCD industry has been the prime motivator for improving anisotropic conductive adhesives with very low line-pair pitch. 

Rasul JS. Chip on paper technology utilizing anisotropically conductive adhesive for smart label applications. Microelectronics Reliability. Jan 2004 Vol. 44(Issue 1). Holmberg M, Lenkkeri J, Lahti M, Wiik B. Reliability of Adhesive Joints in Dual Interface Smart Cards. Proc. 2nd Inti. IEEE Conf. on Polymers and Adhesives in Microelectronics and Photonics, Zalaegerszeg, Hungary. Jun. 2002 23-26. 

Kwon W, Yim M, Paik K, Ham S, Lee S. Thermal cycling reliability and delamination of anisotropic conductive adhesives flip chip on organic substrates with emphasis on thermal deformation. Trans ASME (Jun) 2005 Vol. 127 86-90. 

There are two types of conductive adhesives conventional materials that conduct electricity equally in all directions (isotropic conductors) and those materials that conduct in only one direction (anisotropic conductors). Isotropically conductive materials are typically formulated by adding silver particles to an adhesive matrix such that the percolation threshold is exceeded. Electrical currents are conducted throughout the composite via an extensive network of particle-particle contacts. Anisotropically conductive adhesives are prepared by randomly dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. A schematic illustration of an anisotropically conductive adhesive interconnection is shown in Fig. 1. The concentration of particles is controlled such that enough particles are present to assure reliable electrical contacts between the substrate and the device (Z direction), while too few particles are present to achieve conduction in the X-Y plane. The materials become conductive in one direction only after they have been processed under pressure they do not inherently conduct in a preferred direction. Applications, electrical conduction mechanisms, and formulation of both isotropic and anisotropic conductive adhesives are discussed in detail in this chapter. 

In this section we discuss three applications of electrically conductive adhesives die attach adhesives, anisotropically conductive adhesives for liquid crystal display (LCD) assembly, and conductive adhesives for surface-mounted assembly of packaged components on printed wiring boards (PWBs). These applications were selected based on overall... 

Figure 1 Anisotropically conductive adhesive assembly process (a) bare substrate (b) apply anisotropically conductive adhesive (c) align device, cure under pressure.

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

Most commercially available anisotropically conductive adhesives are formulated on the bridging concept, as illustrated in Fig. 1. A concentration of conductive particles far below the percolation threshold is dispersed in an adhesive. The composite is applied to the surface either by screen printing a paste or laminating a film. When a device is attached to a PWB, the placement force displaces the adhesive composite such that a layer the thickness of a single particle remains. Individual particles span the gap between device and PWB and form an electrical interconnection. For successful implementation of anisotropically conductive adhesives, the concentration of metal particles must be carefully controlled such that a sufficient number of particles is present to assure reliable electrical conductivity between the PWB and the device (Z direction) while electrical isolation is maintained between adjacent pads (X,Y directions). 

V. FORMULATION OF ANISOTROPICALLY CONDUCTIVE ADHESIVES A. Requirements and Performance... 

Materials for use as anisotropically conductive adhesives must satisfy requirements even more stringent than those defined previously for isotropically conductive adhesives. No specifications, however, have been defined specifically for these materials. When used for flip-chip applications, the adhesive not only serves as a physical and electrical interconnection between the device and the substrate, but also serves as the environmental protection and passivation layer. This fact, combined with high adhesive concentrations, makes the ionic contamination levels of these materials more critical than for isotropic conductive adhesives. In addition, the processing of these materials has a greater influence on joint reliability as the anisotropic electrical properties develop only after heat and pressure are applied to the joint. 

Numerous geometrical factors of the specific interconnection will also influence anisotropic adhesive formulation and processing, including lead planarity, IC pad metallization, and IC test patterns. The planarity of the leads on the substrate and/or device and the compliance of the conductive particles will determine if anisotropically conductive adhesives can be used in a particular application. For systems with large disparities between lead height, no electrical interconnection will be formed, as shown in Fig. 5. Fine-pitch IC packages for surface-mounted applications, such as the plastic quad flat pack (PQFP), often use gullwing leads that olfer much compliance to the joint. Even if the... 

Figure 5 Effect of lead nonplanarity on anisotropically conductive adhesive interconnections.

Data describing the reliability of joints assembled with anisotropically conductive adhesives are incomplete. Several papers have been published, but usually the sample size investigated is small, the accelerated stress tests are not standardized, and the results are highly dependent on device type (e.g., flexible circuit to rigid PWB, surface-mounted components, and flip-chip assembles). Further work is required in this area. 

The goal in formulating anisotropically conductive adhesives is to maximize particle concentration without compromising electrical isolation in the X Y plane. Higher particle loadings increase the probability that an electrical interconnection will be made (especially for relatively small contact areas) and decrease contact resistance. Typical concentrations range from 5 to 15 vol % (30 to 60 wt % based on pure silver particles). The size of the particles usually ranges from 10 to 20 pm in diameter. Smaller particles offer the best results for very fine pitch applications. 

Table 5 Current-Carrying Capability of Some Anisotropically Conductive Adhesives... 

Assembly of silicon chips onto substrates with anisotropically conductive adhesives uses specialized equipment, initially developed for ffip-chip solder and TAB inner lead bonding. Heat and pressure are transmitted to the adhesive through a thermode attached to a robotic arm or a high-precision linear translator. Equipment requirements are more demanding than for solder assembly, as no self-alignment can occur. A minimum placement accuracy of 0.0005 in. is required. Coplanarity between the substrate and die is critical one study reports maintaining coplanarity to within 0.00004 in. [19]. The pressure required to achieve interconnection depends on the size of the die, the type of conductive particle used, and the viscosity of the adhesive at the bonding temperature. 

Most conductive adhesive failures are accelerated by elevated temperature and humidity. In a study of 12 commercially available isotropically conductive adhesives, joint resistance increased between 160 and 35,000% when exposed to 65°C and 85% relative humidity (65/85) [56]. However, some adhesive manufacturers claim resistance change of less than 10% after 1000 h at 60°C and 90% relative humidity [57] and less than 4% after 1000 h at 85/85 [58]. Anisotropically conductive adhesive joints are even more susceptible to early failures under accelerated test conditions due to process variations [16]. Reliability screening tests can be used effectively to iteratively optimize process parameters. 

Isotropic adhesives conduct current equally in all directions and are the most common and widely used in industry. The anisotropic types, also referred to as z-direction adhesives or anisotropic-conductive adhesives (ACA), although filled with metal particles, are filled at much lower levels (0.5%-5% by volume) than isotropic types (filled 25%-30% by volume). The volume Iraction of filler is well below the percolation threshold at which the adhesive becomes highly conductive in all directions. Because of the low volume Iraction of metal particles, there are no continuous electrically conductive paths in the x-y plane. During the connection process, the anisotropic adhesive, either as a film or paste, is positioned between a flip-chip bumped die or a tape-automated bonded (TAB) die and the corresponding pads of an interconnect substrate. Pressure and heat are... 

Watanabe, L, and Takemura, K., Anisotropic Conductive Adhesive Films For Flip-Chip Interconnection, Conductive Adhesives for Electronic Packaging, Ch. 10, (J. Liu, ed.), Electrochemical Publications Ltd. (1999)... 

Anisotropically conductive adhesives (AGAs) represent the first major division of polymer bonding agents. The anisotropic class of adhesives provides unidirectional electrical conductivity in the vertical or 2-axis. This directional conductivity is achieved by using a relatively low volume loading of conductive filler (5-20 vol%) (4-6). The low volume loading is insufficient for interparticle contact... 

AniSOtropically Conductive Adhesives. Significant research has been conducted on ACAs as a potential solder replacement for some electronics packaging appUcations. However, many aspects of this technology must be better... 

IPC-3406 Guidelines for Electrically Conductive Surface Mount Adhesive IPC-3407 General Requirements for Isotropically Conductive Adhesives lPC-3408 General Requirements for Anisotropically Conductive Adhesive Films lPC-4101 Specification for Base Materials for Rigid and Multilayer Boards lPC-4103 Specification for Plastic Substrates, Clad or Unclad, for High Speed/High Frequency Interconnection... 

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