Keywords BondingSolderingFluxless solderingDirect bondingAnodic bondingAdhesives Epoxies
environmental stability [5]. Thermally curable silicone resin was also used for high-speed optical coating with specific feature of coating thickness controll- ability [6].
2.1.1.3 Polyimides
Polyimide (PI) is a polymer of imide monomers. Polyimides have a glass transition temperature at least 2008F greater than epoxy resins [7]. As a result, polyimide adhesives can operate at higher temperature than epoxies or pheno- lics, and semiconductor industry uses polyimides as a high-temperature adhe- sive. They are often used in the electronic industry in the form of films for flexible circuits and cables, deposited films for interlayer dielectrics, passivation and buffer coating, substrates for multichip modules, adhesive pastes or tapes with some unique features of low dielectric constant, high thermal stability, and excellent mechanical properties [8]. A good example is the cable in a laptop computer, which connects the main logic board to the display. It is often made of polyimide with copper conductors. Polyimides in use can either be cured by condensation reaction or addition reaction mechanism. Three most important categories are polyimide recursors, self-standing polyimide films, and polyi- mide adhesives [4].
2.1.1.4 Acrylics
Acrylic resins (polymethyl methacrylate) are known to have exceptional optical clarity and good weather resistance, strength, electrical properties, and chemi- cal resistance with low water absorption characteristics [7]. Acrylic resins as adhesives in electronic industry are formed through radical or anionic poly- merization [9]. Radical polymerization can be initiated by UV radiation as well as by heat. Cyanoacrylates are of special interest for systems with very high reaction rates. Their reaction follows an anionic polymerization mechanism [10]. Since the cyanoacrylates have very high polarity, water is able to act as an initiator.
2.1.2 Applications of Adhesives in Electronics
2.1.2.1 Integrated Circuits
Polyimides are commonly used in the fabrication of integrated circuits. Poly- imides, particularly as dielectric, passivation, and protective layers, have advan- tages over other types of inorganic materials used for the same process [11].
Photosensitive polyimides can produce direct patterns using common photo- lithography process without using any toxic chemicals. In typical process for three-level metal interconnect design, polyimide films are inserted (coated) between metal 1 & 2 and metal 2 & 3. After all integrated circuits are
implemented on the silicon wafer, a thicker outermost layer of polyimide is coated as final buffer coating to absorb the interfacial stresses and prevent passivation crack and electrode displacement during the pressure cooker test [7]. An efficient fabrication process was recently developed for superconducting integrated circuits using new fine resolution photosensitive polyimide. It is synthesized using aliphatic material as the KrF photoresist [12, 13]. This specific photosensitive polyimide is synthesized directly by block copolymerization using a catalyst in solvent at 1808C. The fabrication is simplified because the photosensitive polyimide insulation layer can be patterned by conventional photolithography process without etching.
2.1.2.2 Flexible Circuit
Flexible circuit is a technology for building electronic circuits by depositing electronic devices on flexible substrates. Flex circuits have traditionally been made with polyimide or polyester films. Many techniques have been made using flexible circuits to overcome the limitations of the rigid multilayer board tech- nology [14]. Basically, tapes intended to the flexible circuit market are fabri- cated by using two- or three-layer construction schemes. For two-layer tapes, they are usually made by coating solutions of polyimide precursors over copper foil or plating copper onto polyimide films. For three-layer tapes, they are formed of polyimide films coated with organic adhesive and laminated to 35mm copper foil [4]. The entire circuit board can be fabricated on a flexible substrate, and then folded and stacked in an organized pattern to achieve the desired compactness. Common applications of flex circuits are in cameras, cell phones, computer keyboard, printers, and medical applications.
2.1.2.3 Liquid Crystal Display
Liquid crystal displays (LCDs) are becoming ever more popular due to low power consumption, compactness, flatness, lightweight, and high compatibility with large scale integrated circuits [15]. Not only is it very important to have high resolution and large capacity but also critical to have cost reduction by choosing competitive materials in display applications. Many different types of adhesives are used to connect driver chips to LCDs, including thermoplastic adhesives and thermo-setting adhesives [4, 16]. There are two different mechan- isms used for curing the adhesives, which are heat curing and UV curing. Heat curing process has been more common. On the other hand, the use of the transparent glass substrate in the LCD makes UV curing an interesting alter- native because the bonding process can be done rapidly by UV irradiating at room temperature. The low temperature curing is critical for LCDs because liquid crystals are particularly heat sensitive and cannot withstand normal soldering temperatures [17, 18].
2.1.3 New Adhesives
2.1.3.1 Liquid Crystal Polymer (LCP)
Liquid crystal polymers (LCPs) are a unique class of wholly aromatic thermo- plastic polyester polymers that provide previously unavailable high perfor- mance properties. LCPs combine the properties of polymers with those of liquid crystals. While LCPs show the same mesophases characteristic of ordinary liquid crystals, they retain many of the useful properties of polymers. These polymers were synthesized by linkage of rod-like or disk-like mesogenic side groups with flexible spacers to the polymer main chain [19]. Mesogens must be incorporated into the chains for flexible polymers to display liquid crystal effect. LCPs are known to be inert to organic solvents and acids and mechani- cally flexible. It has several unique properties such as low dielectric constant, low loss tangent, low water absorption coefficient, and low cost. Due to their excellent properties, LCP has been frequently used in microwave application and become commercially available as high performance flexible circuit boards [20, 21]. Not only could LCP be used as the substrate for microwave frequency application but also RF MEMS packaging. The temperature to bond LCPs is around 28083108C, which is acceptable for most RF MEMS switches. LCPs can be patterned by microfabrication or laser cutting. They can be directly bonded to metal, silicon, and glass without using adhesive [22]. It can thus be called as adhesive-less bonding technique. The coefficient of thermal expansion (CTE) for LCP can be adjusted through thermal treatments. It can facilitate integration of integrated circuits in SOP modules. Being flexible, LCP can lead to deployment of antennas in space [23]. Due to its unique features, large sheets of LCP containing antenna arrays can be flexed, rolled up, and easily deployed.
2.1.3.2 SU 8 Adhesive Bonding
Su-8 is a high contrast, epoxy-based photoresist designed for micromachining and other microelectronic applications, where a thick chemically and thermally stable image is desired. It is a negative type photoresist, and thus the exposed portion is cross-linked while unexposed portion is soluble to liquid developer.
A normal process is listed in sequence.
Substrate
pretreat Spin coating Soft baking Exposure
Post Expose Bake Develop
Rinse & dry Hard bake
Process guidelines give us the duration of the soft bake as well as the post exposure bake depending on film thickness of the resist and different
kinds of SU-8 resist. The advantages of SU-8 are its flexibility of layer thickness up to several hundreds of micrometers, its high chemical and thermal stability as well as its good mechanical properties [24]. SU-8 is known to be well suited for permanent applications where it is imaged, cured, and left in place. An adhesive bonding method has been developed at wafer level using SU-8 photoresist as intermediate layer [25]. Adhesive bonding is good for joining silicon or glass wafers at lower temperature (usually below 2008C), and this technique is known to be less dependent on the substrate material, particles, and surface roughness of the bonding surfaces. In their development, the layer was selectively deposited on one of the bonding surface by contact imprinting method. It is a good approach for some applications where the adhesive layer cannot be applied directly using classical spinning method. A cover die for a pressure sensor was bonded using SU-8. Zero-level-packaging (ZLP) technology has been developed for high aspect ratio microstructures with selective adhesive bonding using SU-8 photoresist [26]. In the processing, they developed three partial steps as the basis of the ZLP technology: (1) coating and patterning of the SU-8 photoresist, (2) adhesive bonding of the MEMS and the cap wafer, and (3) etching of the wafer stack using deep reactive ion etching (DRIE) method. The techniques developed are very promising for low-cost wafer-level MEMS packaging for monolithic integration of microelectronics.