Adhesives, nonconductive

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. 

A press-in connection is a soideriess eiectricai connection estabiished by pressing a pin into a piated hoie in a printed-circuit board. 

The press-in pin (soiid orfiexibie) has a speciaiiy shaped zone (press-in zone) of a design suitabie for the estabiishment of a press-in connection. 

By means of a suitabie press-in tooi (machine), the press-in pin is pressed into the through-piated hoie in the printed-circuit board. 

A gas-tight connection must be made between the press-in zone of the press-in pin and the metaiiization of the hoie waii. 

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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. 

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). 

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... 

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. 

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. 

The AES technique provides rapid surface analysis, but is not widely used on fracture surfaces due to beam damage and charging of nonconductive adhesives. On the other hand, the use of AES in the analysis of adherend surfaces has been widespread. An advantage of AES is... 

XPS is used to evaluate surfaces prior to bonding yielding information which may relate to bond performance, as well as to study failure surfaces. The added advantage in XPS is that more sample types can be analyzed, such as nonconductive adhesive surfaces without the problems of charging found with other techniques such as AES, SIMS, and ISS. For this reason XPS is often the method of choice in post-fracture analysis. 

The casein wood glues have a long history of use as interior structural adhesives, assembly adhesives, and panel-to-frame adhesives. A limited but important use is as a nonconductive adhesive in the construction of spacers for the large transformer boards for the electrical industry. The most popular use of casein and protein blend glues is in the production of hollow and solid core flush doors, an operation for which these glues are ideally suited. Most of the doors in this country are made with this type of glue. 

Schematic descriptions of three ECA technoiogies (a) isotropic conductive adhesive (ICA), (b) anisotropic conductive adhesive (ACA), (c) nonconductive adhesive (NCA)...

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

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. 

These two aspects, bleeding and tailing, have also been considered for using adhesives in surface-mount technology. Various nonconductive adhesives are currently used to hold the components during the... 

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. 

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. 

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. 

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. 

In soldering and when an ICA is used, the electrical connection is first established by the solder or the conductive adhesive before the bond is mechanically stabilized by application of an underfill. Using ACA or NCA makes it possible to combine electrical connection and mechanical stabilization in one process step. Electrically conductive connections made with nonconductive adhesives are based on mutual contact between the joining partners. A constant contacting force therefore has to be applied until the adhesive has cured. Contacting flip chips with adhesive... 

The pressure sensor is a two-shot molding. Production volume is on the high side of 10 million units per year. The printed-circuit board is contacted on the MID by conductive-adhesive gluing (isotropic conductive adhesive). Nonconductive adhesive is also applied for mechanical location. The contact pins of the MID are tiny bumps to which the conductive adhesive is applied in the dispensing process. [143]... 

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