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temperatures. Epoxies are one group of thermosetting adhesives, which are used widely in electronics assembly because they do not weaken at the high temperatures of wave and reflow soldering environments. Also, epoxies are resistant to attack by solvents and aqueousbased cleaning solutions. Epoxies can be one-part, in which the curing agent and resin are already mixed together, or two-part, in which the two chemistries are combined prior to application. Although the one-part epoxies are convenient from the assembly standpoint, their storage and handling must be rigidly controlled to prevent curing prior to use. Both epoxies are cured by exposure to elevated temperatures, which can range from less than 100 C to as high as 125 to 150 C, for time periods of 1 to 4 hours, depending on the specific product recommendations. There is very little outgassing associated with the curing of thermosetting adhesives. Higher residual stresses can be generated by these materials due to their rigidity at elevated temperatures when there is a significant thermal expansion mismatch among the package, the epoxy, and the circuit board substrate. The permanency of thermosetting adhesives complicates repair or rework functions. The removal of these adhesives usually requires mechanical scraping and abrasion that can damage the component and circuit board. Thermoplastic adhesives soften when exposed to elevated temperatures. These adhesives are weaker than thermosetting epoxies. However, when tolerances are less restrictive, these materials may be preferred for assembly applications, particularly when excessive residual stresses are of concern during the soldering process temperature cycle. Thermoplastic materials are less resistant to solvents and aqueous-based materials. These adhesives tend to absorb these liquids more readily, resulting in dimensional changes (swelling) and more outgassing than is observed with thermosetting adhesives. Thermoplastic materials have curing temperatures that are lower and time durations that are shorter than thermosetting materials. Some compositions will cure at room temperature, making them suitable for temperature-sensitive components or when thermal expansion mismatch, residual stresses are of concern. Another advantage of thermoplastic adhesives is that because they readily soften at elevated temperatures, they can be easily removed to allow for the rework of components. Elastomeric adhesives are a subset of thermoplastic adhesives.These materials tend to be very tough, yet have a higher degree of elasticity. The silicone (rubber) adhesives are examples of this category. The lack of rigidity limits the application of these adhesives in the soldering assembly process. Curing temperatures are relatively low, as some compositions cure at room temperature. However, the curing cycle of some silicone adhesives include considerable outgassing and, moreover, the outgassing of vapors that can be corrosive to metal surfaces (e.g., acetic acid). Toughened alloy adhesives are blends (or alloys) of elastomeric materials and epoxy resins that together form this special class of thermosetting-like adhesives. These adhesives are engineered to provide both high structural strength and sufficient toughness (ductility) to resist damage due to either thermal or mechanical shock. Examples of this type of material include epoxy-nylon adhesives. All of these materials have been engineered to have properties that can accommodate one or more of the various dispensing techniques used in printed circuit board assembly (which are discussed later in this chapter). However, these properties do not remain optimal indefinitely. There are two stages of degradation. The first stage is the shelf-life of the material, which is the time frame during which the adhesive keeps its properties while the container remains unopened. Manufacturers date-code specify shelf-life based on changes to the mechanical properties (strength, ductility, etc.) and physical properties (glass transition, density, liquid viscosity, etc.) that occur to the adhesive. Density and viscosity directly impact dispensability. The second stage of degradation occurs when the adhesive is removed from the container, mixed if required, and is loaded into the dispensing equipment. Exposure to air, even under room temperature conditions, causes the adhesive to begin curing on the assembly floor.The curing can alter density and viscosity and thus the dispensing properties of the adhesive. Indications of a significantly cured adhesive include a clogged dispensing machine, run-out or bleeding of the deposit, and stringers of material created as the dispensing tool moves from one location to another spot.
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