Conductive and Nonconductive Adhesives

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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The main electrical properties related to organic materials are volume resistivity (for both conductive and nonconductive adhesives) and dielectric constant and dissipation factor (for insulative materials). Other electrical tests for conductive materials that are more application specific are electrical stability, current-carrying capacity, and interconnect resistance. 

Both conductive paste adhesives and pressure-sensitive tapes are widely used for EMI protection. Two-part epoxies or one-part silicones filled with silver or silver-plated copper particles are widely used as paste adhesives while acrylic conductive and nonconductive adhesives on aluminum or copper foil are used as pressure-sensitive tapes. EMI shielding tapes are suitable fortemperatures ranging from -40°Cto 121°C and are available in thicknesses from 1.5 mil to 5 miLl" Shielding effectiveness is measured as attenuation in dB at several frequencies, for example, from 1 MHz to 20 GHz. Values for several ARclad shielding tapes are given in Table 5.13. 

There are three types of electrically conductive adhesives typically used in the electrical industry, e.g., isotropic conductive, anisotropic conductive, and nonconductive adhesives. 

Amkon . [Emerson Cuming] Conductive and nonconductive epoxy, polyindde, or silicone adhesive for IC assembly die coating enctqrsulant... 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photoetching of metals or by screen-printing and firing of thick-film pastes. With recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

Colored pigments or fluorescent UV indicators may be used to inspect for bleedout from adhesives during and after cure. They may also be used to inspect for particles of cured adhesives that may have detached, migrated and contaminated other parts of an electronic assembly. Particles of electrically conductive or nonconductive adhesives can cause failures in microelectronic assembhes, gyros, accelerometers, or other electromechanical systems and are especially serious in space electronics. Rhodamine B which is pink to red in both the visible and UV regions and 2,6-distyrylpyridine which is visually colorless but intensely blue in the UV region are effective indicators in some adhesives formulations. 

Copper has good electrical conductivity and high adhesion property in conductive adhesive systems. However, copper tends to form a nonconductive oxide surface layer (Zhao et al. 2007). 

When devices are glued with electrically conductive or nonconductive adhesive, the mechanical strength of the bond is the result of thermal curing of the adhesive. Considerably lower than process temperatures for soldering, adhesive curing temperatures have to be chosen to suit the adhesive system, the substrate material, and required process time. 

Nonconductive or electrically insulative adhesives are used where electrical shorting may be a problem or where electrical isolation is required. Nonconductive adhesives are available in either paste or film form and are generally filled with silica or alumina to provide optimum flow properties for dispensing and to improve their thermal conductivities. 

Epoxies became popular for the first generation of electrically conductive adhesives introduced in the mid-to-late 1960s. The early adhesive formulations contained organic solvents, which often became trapped in the bondhne during cure and resulted in voids, especially under large chips and substrates. These early adhesives also contained large amounts of ionic impurities, notably chloride ions, which, in the case of nonconductive adhesives, resulted in reduced electrical resistivity, ion migration, and corrosion, especially in humid environments. Early epoxies were also... 

Nonconductive adhesives have no conductive fillers but rather maintain a pure mechanical contact between the bumps and pads, due to the application of a compressive force, and it is this that ensures the electrical connectivity between them. 

Suitable connection mediums matched to the process and to the substrate material are essential for the dependability of mechanical and electrical connection. Solder alloys are the most widely used in standard electronics production. Viable alternatives for MID technology along with solder pastes are conductive adhesives for low thermal loading during the connection process, for example. Nonconductive adhesives can also be a good choice in certain circumstances. 

If the adhesive contains no conductive fiiier, eiectrical conductivity relies on contact between the joining partners with adhesive-fiim thicknesses of the same order of magnitude as the surface roughnesses. Poiyimides and epoxy resins are good materiais for nonconductive adhesives, on account of their property profiies. Compared to adhesives admixed with conductive fiiiers, materiai costs are much iower on account of the iower price for the adhesive and the reduction in consumption. 

Connections made with insulating adhesives free of fillers are electrically conductive if the partners are in contact with each other in adhesive no thicker than the surface roughness (< 10 pm). In principle the process is very similar to that of anisotropic adhesive gluing. The glue is applied by a dispenser or by print application of a paste. The electrically nonconductive adhesive can be applied allover across multiple connections. This implies low requirements for the process as such and therefore good affinity for fine-pitch applications. Once the partners to be joined have been positioned, the adhesive cures under pressure and temperature within a matter of seconds. 

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