Isotropic conductive adhesive

The electrical conductivity of isotropic conductive adhesives is established in all directions by metal particles, generally silver flakes. These silver flakes are embedded in a matrix consisting of a base polymer, generally epoxy resin, and hardeners such as amines or carboxylic acids. It is the hardeners that ensure the mechanical strength of the joint. The choice of these two components can influence reaction speed and curing temperature. Broadly speaking, any chemical reaction will be fast at a high temperature and slow at a low temperature. 

Purpose Embedding filler particles Gluing Join partners 

A study of the single-component, temperature-cured adhesives and the two-component adhesives available on the market reveals that the single-component systems predominate. Most two-component adhesives cure at room temperature. No external energy is required and the curing process starts when the reaction partners are mixed together. 

Initial cost estimates of conductive adhesive/solder paste as alternative connection mediums indicate that the prices for conductive adhesives are considerably higher. A viable case-to-case estimate, however, has to take into account the joining material costs accruing for the production of a component joint  

So substituting conductive adhesive for solder paste is unlikely to contribute much to direct cost savings. Since, however, switching to conductive-adhesive gluing for an MID application can permit the use of more cost-effective base materials on account of the lower process temperatures, certain indirect cost benefits could accrue. Conductive-adhesive technology opens up considerable cost benefits for applications characterized by large base-material volume combined with few conductive joins. [48] 

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Lu D, Tong QK, Wong CP. Conductivity mechanisms of isotropic conductive adhesives. IEEE Trans Electron Package Mfg. 1999 22(3). 

Neither isotropic conductive adhesives nor conventional solder interconnections, can easily satisfy these requirements. 

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. 

IV. FORMULATION OF ISOTROPICALLY CONDUCTIVE ADHESIVES A. Requirements and Performance... 

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. 

When using isotropically conductive adhesives, placement of components is performed by the same equipment as used for nonadhesive attachment techniques. Die bonders are similar to those used for eutectic bonding except for the type of adhesive dispenser. Surface-mounted placement machines developed for solder paste assembly can also be... 

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. 

Isotropically conductive adhesive interconnections can be repaired using techniques similar to those used for solder rework. By application of heat locally at a temperature above the T value, a section of adhesive can be softened and the device can be removed... 

Conductive adhesives have a wide range of applications. Wu et al. [2006] worked on high-conductivity isotropic conductive adhesives (ICAs) filled with Ag NWs (p 50). The authors found that when the filler content reached 56 wt%, the bulk resistivity was seven times lower than that of traditional ICAs filled with microsized Ag fillers. 

Markley, P. L., Tong, Q. K., Magliacca, D. J., andHahn, T. P., Characterization of Silver Flakes Utilized for Isotropic Conductive Adhesives, Proc. IMAPS, Inti. Symp. On Advanced Packaging, Materials, Processes, Properties, and Interfaces,pp. 16-20(1999)... 

Isotropically Conductive Adhesives. There have also been a significant number of studies to improve ICA technology. However, several critical issues associated with this technology must also be addressed before it can be used as a replacement for soldering technology ... 

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... 

Durairaj, R., Mallik, S., Seman, A. Ekere, N.N. (2009a). Investigation of Wall-Slip Effect on Lead-Free Solder Paste and Isotropic Conductive Adhesives. Sadhana, Vol. 34, No.5, (October 2009), pp. 799-810. 

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... 

D. Lu, Q.K. Tong, and C.P. Wong, Conductivity Mechanisms of Isotropic Conductive Adhesives (ICAs), IEEE Transactions on Electronics Packaging Manufacturing, Vol 22, 1999,p 223-227... 

H. Better, R.B. Van Der Plas, and A. Arun-junai. Factors that Influence the Electrical Contact Resistance of Isotropic Conductive Adhesive Joints During Climate Chamber Testing, International Journal of Microelectronic Packaging Materials and Technologies, Vol 1, 1998, p 177-185... 

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... 

Z.M. Mo et al.. Electrical Characterization of Isotropic Conductive Adhesive under Mechanical Loading, J. Electron. Mater, Vol 31, 2002, p 916-920... 

J.H. Constable et al.. Continuous Electrical Resistance Monitoring, Pull Strength, and Eatigue Life of Isotropically Conductive Adhesive Joints, IEEE Transactions on Components and Packaging Technologies, Vol 22, 1999, p 191-199... 

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... 

ICA isotropic conductive adhesive TAL time above liquidus... 

The conventional method using screen printed solder pastes or isotropic conductive adhesives is drflicult to apply when the distance between adjacent connections is less than 0.5 mm. Thus, the main impetus for developing anisotropic conductive adhesives has been the fine pitch interconnection of electronic devices... 

Anisotropic adhesives are more sensitive than isotropic conductive pastes to the coplanarity of the connecting pads and leads of the substrate and the device because many of them are based on the single-particle bridging concept. The typical properties of a screen-printable isotropic conductive adhesive paste are a silver content of 70-75% by weight, a viscosity of about 80 Pa s, and a curing time of 15 s at 175°C. This fast cure cycle produces an adhesive layer with Tg... 

Irfen Mir, Kumar D (2008) Recent advances in isotropic conductive adhesives for electronics packaging applications. Int J Adhes Adhes 28 362-371... 

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