Martha K. Raymond
Sticking to the same adhesive for specific design applications seems like the right thing to do, especially when it works. However, advances in technology such as electronics miniaturization and new composite materials continue to drive adhesive development. When choosing a fastening method, engineers should consider the advantages of adhesive bonding, along with basic principles of what makes adhesives stick. By doing so, you’ll open up the options for designing a fastening system. The following samples of the latest in adhesives technology give designers a jump start for new applications.
One of the advantages of using adhesives is they bond the entire surface area of a joint which minimizes stress and adds strength. Because mechanical loads are distributed over a larger area, stress doesn’t concentrate in the same way as it does for rivets, bolts, or welds. Plus, another benefit is that bond strength isn’t sacrificed for thinner, lighter materials.
Adhesives are typically more flexible than welds and metal fasteners, and they can damp out vibration, preventing it from transmitting across bond lines. When joining dissimilar materials, adhesives produce strong bonds, and their inherent flexibility balances different coefficients of expansion.
Adhesives also provide better aesthetics for joint interfaces. Without holes for mechanical fasteners, surfaces maintain a smooth appearance. Engineers can cut the number of finishing steps when adhesively bonding joints which also reduces costs. Other benefits include weight savings because of less components and reduced labor costs due to ease of assembly. Furthermore, adhesives can electrically insulate joined materials and seal to protect against environmental conditions.
A Sticky Situation
Adhesives bond by attracting the molecules between unlike materials. Whether or not the bond or attraction is strong is a function of the material’s surface energy. A low surface energy results in weaker attraction, while a high surface energy yields greater attractive forces. The adhesives’ viscosity is also a factor in bond strength. Adhesives that flow across or wet out a high surface energy material produce stronger bonds. And to adequately bond materials, adhesives must have a surface energy the same or lower than the substrate.
Metals typically have the highest surface energy among engineering materials. For example, tungsten has a surface energy of 6,800 mN/m and stainless steel ranges between 700 and 1,100 mN/m. Plastics, on the other hand, are broken down into high and low surface energies. Plastics with high surface energies fall between 38 to 50 mN/m, such as nylon which has a value of 46 mN/m. Resins with low surface energies range from 18 to 37 mN/m. An example is polypropylene with a surface energy of 31 mN/m. Another consideration for adequate bonding is the surface finish of the substrate, including coatings and the substrate’s size and flexibility.
Other dynamics influencing adhesive bonds include the types of stress on a joint. Tensile stresses are one example and act perpendicular to and away from the bond plane. The stresses distribute over the whole plane, making adhesives play a major role in determining bond strength. Compressive forces, in contrast, act in the opposite direction, perpendicular to the bond plane but directed toward each other. Another force distributed over the bond plane is shear stress, directed parallel to the joint plane, across the adhesive bond.
Additional stresses act on adhesives at the edge of the bond. A cleavage force acts to pry on adhesive bonds because not all the bond area contributes to the overall strength at one time. Another force restricted to the edge is peel. Joints subjected to peel should be designed with at least one flexible surface.
Whether or not a bond is structural or nonstructural is a factor in adhesive selection. Structural bonds are necessary for loadbearing joints. These are common in aircraft, appliance, and vehicle headlight assembly, as well as in electronic controls and relays. The role of adhesives used in nonstructural bonds is primarily to cushion and insulate. They also have applications in gasketing, packaging, general assembly, and sealing.
Adhesives are being used in more demanding applications, particularly in electronics where engineers are designing components smaller and smaller in order to shrink the size of printed-circuit boards (PCBs). In such applications, adhesives take on a variety of duties including bonding surface- mount components, potting and encapsulating components, protection as conformal coatings, and tacking wires.
For bonding surface-mount devices, adhesives need to withstand the effects of surface mount technology (SMT), where soldering components directly to pads on the PCBs eliminates drilled holes and throughboard connections. The two bonding methods are solder paste or cream.
Many PCBs are built by a combination of SMT and through-hole components. The challenge for adhesives used with through hole components is to withstand wave soldering, which deposits hot molten solder between the lead connector and the hole to attach components to the board. Cured adhesive needs enough strength to hold the device to the board during the wave solder process. Choosing the adhesive with the proper wet or green strength helps hold the component in position until cured. Applied by pin transfer, screen or stencil printing, and syringe dispensing, the adhesives are placed between solder pads on the board. In order to be effective, manufacturers must be able to use high-speed dispensing equipment to apply adhesives in very small dots, maintaining a constant dot profile and size. When cured, these adhesives combine flexibility, shock and solder wave resistance, and good electrical properties. Other uses for adhesives on PCBs include securing loose wires, replacing tie-wraps, forming coil terminals, mounting LEDs, and relaminating boards.
Besides being used to attach components, adhesives also function as conformal coatings to protect boards from moisture and mildew, hydraulic fluids, and process solvents that can cause problems such as solder joint corrosion. Coatings also protect electronics from extreme temperature swings, dirt, short circuits, rough handling and electrical and mechanical interference.
Other protective uses for adhesives are as potting or encapsulating compounds. These protect the electronics by filling small surfaces or spaces and provide added insulation.