Isotropic conductive adhesives reliability

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. 

Table 2 lists the requirements for a type I (electrically conductive) adhesive and test results of a typical current generation isotropically conductive adhesive as reported by Estes [44]. These requirements specify test ranges for characteristics that will establish processing, performance, and reliability. 

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. 

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. 

D. Klosterman, L. Li, and J.E. Morris, Materials Characterization, Conduction Development, and Curing Effects on Reliability of Isotropically Conductive Adhesives, IEEE Transactions on Components Packaging and Manufacturing Technology Part A, Vol 21, 1998, p 23-31... 

L. Li et al.. Reliability and Failure Mechanism of Isotropically Conductive Adhesives Joints, 45th Electronic Components and Technology Conference (ECTC), May 1995, Proceedings, Las Vegas, NV, IEEE CPMT Society, 1995, p 114-120... 

R.S. Rorgren and J. Liu, Reliability Assessment of Isotropically Conductive Adhesive Joints in Surface-Mount Applications, IEEE Transactions on Components Packaging and Manufacturing Technology, Part B, Vol 18, 1995,p 305-312... 

L. Chen et al.. Reliability Investigation for Encapsulated Isotropic Conductive Adhesives Rip Chip Interconnection, Sixth IEEE CPMT Conference on High Density Microsystem Design and Packaging and Component Eailure Analysis (HDP 04), June/July... 

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. 

The long-term reliability of isotropic and anisotropic conductive adhesives for surface-mount applications has been extensively studied [144-148]. The data accumulated with die attached chips cannot be transposed to the hybrids or surface-mounted wiring boards because of the dissimilar requirements in mechanical and electrical properties [149]. As shown in Figure 41(a) and (b) of Section 6.5.7, the active and passive devices can be attached to thick film... 

---