As we discussed in a recent feature article on adhesives, the use of multiple substrate materials is expanding beyond aerospace and automotive assemblies to consumer electronics and medical devices. All of these require adhesives that will stick reliably to metal, glass, and many varieties of plastics and elastomers. Some of the biggest challenges for adhesives are coming from consumer electronics, especially mobile and wearable devices.

Mobile devices like cell phones, tablet computers, and virtual reality devices contain component modules for various functions, such as touch panels, speakers, cameras, and flashlights. Within each of these modules are subcomponents constructed of different materials that must be attached to each other, Torsten Uske, North America president of DELO Industrial Adhesives, told Design News.

Many smartphones, for example, contain more than a dozen MEMS sensors, each with a typical size of 1.5 x 2 mm, said Uske. They function as accelerometers, gyroscopes, or barometric pressure sensors. Some of these sensors are microphones used for voice recognition and noise suppression, consisting of a microprocessor and a tiny microphone. Because of their extremely small dimensions, fasteners cannot be used. "The only joining technology capable of MEMS packaging is adhesive bonding," said Uske.

Typically, adhesives are used in four key areas in these modules: ASIC die attach, MEMS die attach, glob top, and cap bonding. In a MEMS transducer microphone in a mobile phone, for example, an ASIC and a MEMS die must be attached to a ceramic board. The adhesive must also compensate for different coefficients of thermal expansion of these varied materials: around 8 ppm for ceramic and 2 ppm for silicon, said Uske. "Other considerations include the fact that the ASIC needs rigid die-attach adhesives, but the MEMS must be attached with a super flexible die-attach adhesive to avoid sound distortion," he said. "Plus, the ASIC device needs protection like a glob-top material. Then you have to bond the entire cap onto the frame to seal it, and that requires EMI shielding."

Another example is a compact camera module assembly for smartphones. In these, there are more than a dozen bonding areas, from bonding the housing together to attaching the lens barrel, said Uske. A fully assembled, compact camera module consists of multiple widely divergent materials such as polycarbonate, polyamide, liquid crystal polymer, FR4 (aka fiberglass-reinforced epoxy laminates), glass, aluminum, and rare earths.

Wearable electronics, in particular, pose challenges, said Kent Larson, application scientist for Dow Corning Electronics Solutions, in an interview. The combination of aesthetics, reliability, and safety for skin contact with advanced electronics functions demands the construction of multilayer surfaces, which can only be done with adhesives. Dow Corning is developing silicones that can form strong and durable bonds to a wide variety of substrates, especially to more plastics and elastomers, in several electronics applications.

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Silicones are especially suited to multi-material electronics bonding needs, whether in automotive, aerospace, or consumer applications. "The main job to be done for silicones is to provide very durable environmental sealing," said Larson. "When used with other fastening mechanisms, such as screws, the silicone essentially acts as a gasket, and pre-fabricated silicone gaskets are commonly used in device assembly for just this purpose. But pre-formed gaskets typically only work well for flat surfaces with relatively tight gap tolerances and enough mechanical fasteners to ensure very steady, uniform pressure on the seal over its intended service life." Designing sealants to be dispensable can help solve most of those problems.

As sealants are formulated with greater strength, less mechanical fastening is required to ensure the joint can overcome expected stresses over its lifetime. In joint constructions where stable sealing performance is more important than super-high strength, the very low modulus and very highly stress relieving seals that silicone provides can deliver needed durability and reliability, said Larson. This is due to their stable properties in a wide range of operating temperatures and harsh environments, and their ability to provide stress relief.

For example, although cell phones are expected to work at temperatures below zero degrees C, some adhesives and sealants become very rigid or even fragile when they get that cold. "If you drop some phones from your cold-numbed hands at such temperatures, this may crack the assembly adhesives and environmental seals, causing the phone to stop working," said Larson. This is not true of more advanced silicone adhesives.

Wearable electronics are expected to hold up to very harsh chemical environments. That includes anything skin may be exposed to, such as sunscreen and other lotions, cleaning products, perspiration, foods and drinks of all kinds, and even an occasional dip in the ocean, said Larson. The continual miniaturization of electronics, especially in wearables, has also posed challenges for stress relief solutions. "There's less room, less ability to cushion components against the shock of dropping or impact," he said. "This is driving the need for even higher stress-absorbing adhesives and sealants, which are used in ever-thinner layers." Overheating from more powerful processors is another increasingly important design parameter: sealants and adhesives must not only be tolerant of such heat, but they may also be expected to help dissipate it or help transport it away. Often, they must also help relieve some of the stresses caused by thermal expansion and contraction of complex component constructions.

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Ann R. Thryft is senior technology editor, materials & assembly, for Design News. She's been writing about manufacturing- and electronics-related technologies for 27 years, covering manufacturing materials & processes, alternative energy, and robotics. In the past, she's also written about machine vision and all kinds of communications.