Conductive adhesives thermally

A typical thermally conductive epoxy system used as an adhesive, as weU as for other purposes, has a thermal conductivity of 0.0026 cal/cm/sec/°C and a volume resistivity of 1.5 X 10 ohm.cm (1.5 x 10 ohm.m). Fillers include alumina (aluminum oxide), beryUia (beryllium oxide), other unspecified inorganic oxides, boron nitride, and silica. Boron nitride is an excellent choice as a thermally conductive filler, except that its content reaches a maximum at about 40% by weight in epoxy resins. The resultant products are always thixotropic pastes. Beryllia powder has excellent thermal conductivity by itself, but when mixed with a resin binder, its conductivity drops drastically. It is also highly toxic and high in cost. Alumina is a commonly used filler to impart thermal conductivity in resins.  

These so-called wonder adhesives are marginally thermosetting materials and were first introduced commercially by Eastman Chemicals. They form strong thermosetting bonds between many materials without heat or an added catalyst. They are particularly useful in bonding metal to nonmetal. Lap-shear strengths of 13.7 MPa have been reported. However, the resistance of these adhesives to moisture is still somewhat low. These materials set very quickly when squeezed out to thin film between many types of adherends. 

A cyanoacrylate adhesive is a very rapid curing adhesive, also from the acrylic family tree but having a completely different cure system. Cyanoacrylate monomer is made from a eomplex ehemical process. The monomer produces a very reaetive polymerization. The reaction or polymerization process is stabilized and the monomer kept in the liquid state by the addition of a small amount of an acid stabilizer material.  

When a drop of cyanoaerylate adhesive is put on the surface of a part, the aeid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaetion of the water and acid causes the aeid stabilizer to be neutralized. The cyanoacrylate moleeules then reaet with eaeh other and form polymer ehains without eross-linking.  

The amount of stabilizer molecules in cyanoaerylate is very small—measured in parts per million—and very little moisture moleeules are required to eause rapid polymerization. Cyanoacrylates begin to form polymer ehains immediately on contact with the water vapor on the surface of the part. If parts are moved during initial eontaet, the polymerization proeess and polymer ehains are stopped. The process must start again at a new eatalyzed site.  

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Thermally conductive adhesives for bonding heat sinks... 

Thermal Conductivity. Many of today s electronic products feature miniaturization. In these applications, higher thermal conductivity and better dimensional stability are required from adhesive systems. Thermal management has become a significant area of development, and thermally conductive adhesives provide a way to transfer heat away from sensitive electronic components. 

Theoretically, boron nitride is an optimum filler for thermally conductive adhesives. However, it is difficult to fill systems greater than 40 percent by weight with boron nitride. Beryllium oxide is high in cost, and its thermal conductivity drops drastically when it is mixed with organic resins. Therefore, aluminum, aluminum oxide, and copper fillers are commonly used in thermally conductive adhesive systems. 

Thermal Ctmductive Adhesive 991. [Dymax] Filled thermally conductive adhesive fix mounting heat-sensitive electronic ccxnpotwnts. 

Thermally conductive adhesives may be filled with metal, ceramic, or inorganic particles. Silver-filled epoxies have high thermal conductivities, but may not be used where there is a risk of electrical shorting. In such cases, epoxies or other polymers filled with electrically resistive, but thermally conductive materials such as aluminum nitride, boron nitride, alumina, or beryllia must be used. Some applications for thermally conductive adhesives include attachment of power devices, heat sinks, large components such as capacitors and transformers, large ceramic substrates, and edge connectors. 

Properties of nanofillers recently developed nano materials are reported to display greater mechanical strength, greater thermal conductivity and improved electrical performance when compared to materials of normal particle sizes. Nano dimensional materials are being studied as fillers in polymer matrices in a variety of formulations for electrically conductive adhesives, thermally conductive adhesives, encapsulants, printed circuit boards, coatings, catalysts, underfills for flip-chip-attached devices and wafer-level connections. ... 

DM 3030, Silver/glass die attach paste. Thermally Conductive Adhesives Selection Guide. Diemat, a Namics Co, www.diemat.com 2003. 

Self heating of highly-filled, thermally-conductive adhesives at high shear rates affects the amount of material dispensed. 

CSP Ultralow MOE, stress and thermoset (MCOT), silicone materials, highly filled thermally conductive adhesives Epoxy blends, cyanate-ester... 

Besides high thermal conductivities, adhesives must combine low cost and relieve stresses from mismatches in the coefficients of expansion of the LED material and the substrates or packages to which they are attached. 

Lastly, a technique that has been used for many years involves first eutectic or solder attaching high-power devices to small heat spreaders such as molytabs (gold-plated molybdenum tabs that are the same size or slightly larger than the die). Thermally conductive adhesive is then used to attached the die-tab parts to a ceramic or laminate interconnect substrate. 

Abletherm 2600 Series Adhesives, Processing Tip Sheet, Ablestik Labs Nov. 1999. Self Shimming Thermally Conductive Adhesive Multi-Cure 9-20699, Preliminary Product Data Sheet, Dymax Corp., (Apr. 2004)... 

IPC CA-821, General Requirements for Thermally Conductive Adhesives Jan. 1995. Adhesives for SMD Technology, Siemens Standard SN 59651 (Ed. 1997-09). 

For high thermal-conductivity adhesives, such as the silver-glass compositions whose thermal conductivities are greater than 20 W/m K, the indirect laser-flash method is used. Unlike the steady-state methods, the flash method does not measure thermal conductivity directly, but measures thermal diffusivity, from which thermal conductivity is calculated as follows ... 

Electrical-andjor thermal-conducting adhesives [1,9]. The epoxies and acrylates described above are filled with metal powders to get electrical-conducting adhesives. For special applications polyimide and silicone adhesives are used also. Since the metallic particles must touch each other inside the resins to reach a sufficient level of conductivity, a metal content of 70 to 80 wt % is necessary. Silver is the metal generally used, since specific resistances of the filled adhesives down to about 10 " S2cm can be achieved (metallic silver has a specific resistance of 1.6 x 10 S2cm). Using other metals, such as copper or nickel, the accessible electrical conductivity is too small. On the other side, copper-filled resins show good thermal conductivity and are therefore used for such... 

Thermal dissipation. Thermally conductive adhesives may be filled with metal, ceramie, or inorganie partieles. Silver-filled epoxies have high... 

Conductive adhesives Electrically conductive adhesives Thermally conductive adhesives... 

Landrock, A.H., Effects of Varying Processing Parameters on the Fabrication of Adhesive-Bonded Structures. Part VII Electrically and Thermally-Conductive Adhesives—Literature Search and Discussion, Picatinny Arsenal Technical Report 4179, March 1971. 

Output, Thermally conductive adhesive, Loctite Corp., North American Group... 

If type II or type III assemblies are used and thermal transfer between components and the substrate is a concern, the design team must consider thermally conductive adhesives. 

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