Adhesives, electrically conductive reliability

Conductive Adhesives. Electrically conductive adhesives are used today for specialized applications such as connections to LCD displays and attachment of small resistors and capacitors. These materials consist of conductive particles, usually silver flakes or carbon, suspended in a polymer matrix, most commonly epoxy.The electrical resistance of the contact to the PCB tends to be unstable over time, so these materials are not suitable for applications requiring a constant, low-resistance contact. The primary failure mechanism is moisture migration through the epoxy to the interface, resulting in oxidation of the contact metal. Adhesion strength is also a reliability concern. New materials suitable for a broader range of applications are under development. Further information can be found in Ref 39. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Su B. Electrical, Thermomechanical, and Reliability Modeling of Electrically Conductive Adhesives. PhD Dissertation. Georgia Institute of Technology May 2006. 

Kim HK, Shi FG. Electrical reliability of electrically conductive adhesive joints dependence on curing condition and current density. Microelectron J. 2001 32. 

Volume resistivity is a measure of the electrical conductivity of adhesives or the insulation resistance of a nonconductive material. Volume resistivity is used to qualify adhesives for electronics and also to evaluate long-term reliability after aging. Volume resistivity for conductive adhesives is given by the equation ... 

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. 

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). 

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. 

The reliability of conductive adhesive electrical interconnections depends on the individual formulation and process employed [55]. In addition, the test vehicle configuration will strongly influence results. No comprehensive studies have been published, however, and no attempts to correlate chemical composition or a specific process variable to reliability performance have been reported. 

Conductive Columns. Nitto Denko Corp. developed an ACF for fine pitch flip chip applications (27). The features of this ACF were (1) connectability between bumpless chips and fine pitch PCB (2) high electrical conductivity (3) repairabil-ity (easy to peal off chips from a printed circuit board at elevated temperatures) (4) high reliability and (5) potential storage at room temperature. There are other notable features too (7) ACF is usable at pitches down to 25 fim, (2) the conductive elements are micrometallic columns as opposed to random-shaped particles, and (3) this adhesive matrix consists of a thermoplastic polymer resin, conductive columns coated with an insulator, and a high Tg polymer, which completely separates the columns from the adhesive (Fig. 6). 

H.K. Kim and EG. Shi, Electrical Reliability of Electrically Conductive Adhesive Joints Dependence on Curing Condition and Current Density, Microelectronics Journal, Vol 32, 2001, p 315-321... 

From the viewpoint of their conduction and mechanical joining, ACAs are similar to ICAs, except that they have lower concentrations of conductive particles. This lower concentration provides unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate), which is why they are called anisotropic conductive adhesives. In the same way, ACA materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. The concentration of particles is controlled, so that sufficient particles are present to ensure reliable electrical conductivity between the assembled parts in the z-direction, while insufficient particles are present to achieve percolation conduction in the x-y plane (Kim et al. 2008b). O Figure 50.6 shows a schematic description of an ACA interconnect, showing the electrical conductivity in the... 

There are at least three important requirements for the apphcation of conductive adhesives in the electrical industry. The first one is the electrical resistance. Because of the typical low electrical conductivity of the polymer materials, a high electrical conductivity or low resistivity is definitely the most important factor for the conductive adhesive. Second, the thermal properties of the adhesives should be carefully considered. The bonding process with the adhesives is optimized by their thermal properties, e.g., their Tg and curing behavior. Finally, thermo-mechanical properties of the adhesives would be discussed in this chapter. The thermo-mechanical properties determine the long-term reliability of the adhesive interconnection, so it is necessary to review how the thermo-mechanical properties can be measured and the results analyzed. 

The snap curable conductive adhesives provide excellent adhesion and reliability. For applications with large coefficient of thermal expansion (CTE) mismatches between substrates, or fine pitch flip chip interconnections where electrical conductivity is desired in only one direction, we have an electrically conductive adhesive product to meet the challenge. 

The substrate material is exposed to relatively high localized temperature loading, particularly when SnAgCu solder is used. An alternative connection medium would be a conductive or nonconductive adhesive, which cures at a much lower temperature and does not call for a matched temperature profile. An adhesive can be made conductive by the admixture of electrically conductive metallic or metallized particles. These adhesives can work at steady-state temperatures in the range from 150 to 170 °C. Another advantage is the availabihty of hard and soft adhesives, so the choice can be matched to connection technology to afford optimum long-term reliability. 

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