Reliability of interconnects with conductive adhesives

Johan Liu, Chalmers University of Technology, Gothenburg, Sweden and Shanghai University, China 

After the adhesive is cured, the fillers are randomly distributed and form a network within the polymer matrix. By this network, electrons can flow from one adherent to the other across the filler contact points. The overall result is to create numerous electron pathways, but with each path made up of a large number of mechanical contacts. Any factors affecting intimate contacts among fillers will surely influence the performance and reliability of ICA interconnects. 

Besides die attachment, ICAs are utilized in surface mount and flip-chip packages as alternatives to traditional solders. Due to their low surface tensions, ICAs are not suitable for wave soldering (Ref 6). Despite the advantages of ICA interconnection, the wide use of this technology has not been adopted by the electronics industry. The main concern is long-term reliability. 

Since the conduction of ACAs is based on mechanical particle-electrode contacts, pressure is a requisite to form qualified joints. A typical ACA assembly is shown in Fig. 2. After alignment, pressure is applied on the backside of the chip. The adhesive resin is squeezed out and conductive particles are trapped and deformed between opposing electrodes. Once electrical continuity is generated, the adhesive resin is cured with heat or UV. The intimate particle-electrode contacts are maintained by the cured matrix, and the elastic deformation of particles and electrodes exerts a continuous contact pressure. 

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Chapter 11 Reliability of Interconnects with Conductive Adhesives / 251... 

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. 

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. 

Thermosetting matrices, such as epoxies and thermosetting hot-melt adhesives, are used where increased reliability is required. Repair of anisotropically conductive interconnections assembled with thermoset adhesives is problematic, however, as the adhesive matrix must be removed completely from the substrate and device prior to reassembly. 

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. 

Moisture absorption can influence the reliability of conductive adhesive interconnection joints. Moisture in polymer composites is known to have an adverse effect on both the mechanical and electrical properties of epoxy laminates [54,55]. Studies relating to the reliability and moisture sensitivity of electronic packages indicate similar degrading effects. It was determined that moisture absorption can cause an increase in contact resistance, especially if the metallization on the bond pads and components is not a noble metal [56]. Effects of moisture absorption on conductive adhesive joints are summarized in Table 2. In order to achieve high reliability, conductive adhesives with low moisture absorption are required. High adhesion strength to pad... 

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