Adhesives play a major role in assembling portable electronic devices. In a cell phone, for example, adhesives bond the glass display window and camera lens; the keypad and membrane switch components; flex circuits and cables; microphones and speakers; nameplates and decorative parts; electromagnetic interference (EMI) shielding; and the battery compartment and exterior case. That's in addition to the adhesives used to assemble the circuit board itself, including underfill materials, potting compounds, conformal coatings and conductive epoxies.
The mix of materials in these devices-metal, glass, rubber, FR4, Mylar, Kapton and assorted engineering plastics-make adhesive bonding an ideal assembly method, while they limit opportunities for welding and other thermal assembly methods. At the same time, the small form factor of portable electronics may preclude the use of mechanical assembly methods, such as threaded fasteners.
This is not to imply that using adhesives to assemble mobile electronics is without challenges of its own. Because the parts are so small, there's not much surface area to which an adhesive can stick. And because the devices are portable, adhesive bonds must stay together despite extreme heat, cold, moisture and shock loads. Finally, mobile electronics require low-outgassing formulations to prevent fogging of glass displays and camera lenses.
New Hot-Melt Makes a Case
Adhesive suppliers are finding innovative ways to meet these challenges. For example, 3M (St. Paul, MN) and Graco Liquid Control (North Canton, OH) have developed a new hot-melt adhesive and automatic dispenser for bonding parts in portable electronics. The system combines Liquid Control's C-500 Micro Melt automatic dispenser for hot-melt materials and 3M's Plastic Bonding Adhesive 2665.The one-component, moisture-curing urethane adhesive is applied at a temperature of 230 F. The high-tack, 100 percent solids adhesive produces strong bonds with glass, metal and plastic, including hard-to-bond materials such as polycarbonate. With a viscosity of 11,000 centipoise, or slightly thicker than chocolate syrup, 2665 was formulated specifically for applications requiring thin bond lines. It has a 4-minute open time, enabling assemblers to reposition parts even when the bond line is particularly thin.
"As parts get smaller and smaller, the adhesive needs to be stronger and stronger," says Hann Pang, product manager with 3M's Electronics Materials Div. "It also has to bond relatively quickly, to increase efficiency on the assembly line. Plastic Bonding Adhesive 2665 behaves very much like a double-sided tape. It has good initial ‘grab' to hold parts together until it fully cures."
Unlike tape, however, the liquid adhesive is easy to apply to curved surfaces. And, unlike tape, which must be precisely die-cut in the required shape, 2665 can be applied in any pattern. If the product design changes, the assembler can simply reprogram the dispensing path of the robot.
Plastic Bonding Adhesive 2665 is packaged in 30-milliliter syringes for use with handheld time-pressure dispensers, and 295-milliliter cartridges for use with Liquid Control's robotic dispenser.
In portable electronics, 2665 is most often used to bond the display window to the case, and the case components to each other. The minimum bond width is 1.5 millimeters. A tongue-and-groove joint design will maximize the surface area for bonding without unduly increasing the size of the parts. Moreover, the recess will keep excess adhesive from flowing out of the bond area.
The parts to be bonded with 2665 should be clean and dry. If necessary, plastic parts can be cleaned with isopropanol or isopropyl alcohol. Metal and glass parts can be cleaned with methyl ethyl ketone.
Bonding Film
Many of today's portable electronic devices, especially cell phones, have cases consisting of both metal and plastic parts. The combination has aesthetic appeal and feels good in the hands of the consumer. However, it presents a challenge for adhesive suppliers that have to find a way to keep the parts together. That's because metal and plastic parts have vastly different coefficients of thermal expansion. In extreme temperatures, that difference puts tremendous stress on a bonded joint."Pressure-sensitive adhesives generally don't have enough strength to hold the parts together," says Pang. "Thermobonding films provide that strength, while meeting the speed requirements for high-volume manufacturing."
Part hot-melt adhesive, part double-sided tape, a thermobonding film is a thin sheet of thermoplastic adhesive. Usually attached to a paper release liner, the base resin can be polyester, polyolefin, epoxy, nitrile phenolic, ethylene vinyl acetate or ethylene acrylic acid. Standard film thickness ranges from 3 to 10 mils, and the material is typically die-cut in the shape of the part.
To make a bond, the adhesive is tacked into place on one of the substrates using low heat. The liner is removed, the second part is installed, and the parts are held together with heat and pressure until the adhesive melts and bonds the joint. Heat and pressure can be applied with a heated press or a hot-roll laminator.
Heat, pressure and dwell time depend on the type and thicknesses of the resins and substrates. For example, as a starting point with polyester bonding film, 3M recommends holding the parts together for 2 to 5 seconds at a temperature of 270 to 280 F and a pressure of 10 to 20 psi.
Pressure-Sensitive Adhesive
Pressure-sensitive adhesive tapes remain the bonding technology of choice for a wide variety of applications in portable electronic devices."Pressure-sensitive adhesive tapes are much easier to use than liquid adhesives," says William Stratton, vice president and general manager of the ARclad Div. of Adhesives Research Inc. (Glen Rock, PA). "A liquid adhesive has to be dispensed and cured, and that requires ovens and sophisticated application equipment."
In contrast, says Stratton, pressure-sensitive adhesive tape arrives at the assembly plant cut in the precise shape of the parts to be joined. In many cases, the adhesive arrives at the plant already in place on one of the parts to be assembled.
"We precisely control the thickness of the tape and the adhesive on each side of the tape, so the assembler doesn't have to worry about dispensing the right amount of adhesive," he adds.
Adhesives Research makes a number of tapes specifically for assembling portable devices. For example, ARclad 8314 is used for bonding battery packs, while ARclad 7992 bonds lens assemblies, microphones and speakers. ARclad 9032 and 8001 are conductive tapes for bonding ground planes and making electrical connections. ARclad 6262 is a foam tape that bonds parts and dampens sound and vibration.
"These tapes have excellent adhesion to a wide variety of plastic surfaces," says Stratton. "They also have good cold temperature shock resistance, which is especially important for cell phones. Many cell phone manufacturers require the phone to withstand a 6-foot drop at -40 F without any parts popping off."
When using tapes for assembly, engineers should provide enough surface area for the tape to bond to. How much area varies with the tape, the materials, the strength requirements, and the shape of the parts, so engineers may need to do some testing. The bonding surfaces should be clean and dry, and they should have good coplanarity.
"If you don't have good coplanarity, you're going to put undue stress on the bond line," says Stratton. A
