Bonding and Sealing

Sealing structures to produce ftquid-tight enclosures (chambers and channels) is a key step in the fabrication of a microdevice. Several different processes are available depending on the materials employed. However, in nearly all cases some method of surface cleaning is necessary before the bonding process. 

One of the most common methods of cleaning surfaces for subsequent sealing is the use of the so-called piranha clean, which involves emersion in a mixture of H2SO4 and H2O2 (3 1) followed by washing with deionized water. When this method is misuitable (e.g., the device contams metal electrodes), then alternative methods—such as treatment of tlie surfaces with oxygen plasma or laser radiation —have been used. 

Pyrex glass has been bonded to silicon using an anodic bonding technique. With it, the thermal expansion coefficients of the two materials are similar thus residual stress after bonding is minimized. Operationally, a mirror-polished, clean flat piece of glass is placed on top of a clean 

This method is used for bonding two silicon surfaces. A thin layer of gold is coated onto the surface of one of the wafers, or a thin sheet of gold-siiicon or gold-tin mixture is placed between the two surfaces (a preform). The assembled device is then clamped together and heated in a vacuum chamber above the eutectic temperature for the particular materials (e.g., eutectic temperature for Si/Au is 36 °C). The preform melts and a diffusion process (gold into the silicon surface) leads to the formation of a silicon alloy between the contacted surfaces. As this cools, a bond is formed between the two surfaces.  

Other methods of bonding include direct bonding and clamping.However, most of these methods preclude the treatment of channels or chambers within the device before bonding. Several methods have been employed for bonding polymeric materials. These include pressure between aluminum plates and the use of adhesives or materials modified by UV light.  

---

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

UV curable adhesives have been used in electronics and electrical manufacturing operations for many years. There are many uses, such as bonding of coils, bonding of loudspeaker membranes, bonding and sealing of wires in ducfs, bonding of liquid crystal displays, and bonding of membrane swifches. ... 

Petrie, E. M., Chapter 16 Bonding and Sealing Specific Substrates, in Handbook of Adhesives and Sealants, McGraw-Hill, New York, 2000. 

The vacuum-tube diode, invented by Fleming24 in 1904 [2,3], works because of the relative geometrical shapes of the two concentric electrodes, the cathode and the anode. It consists of a cylindrical glass enclosure that is partially evacuated, bonded, and sealed to a metal base. It contains an inner metallic thin-wire "cathode" (negative electrode, consisting of W, oxide-covered W, or a Th-W alloy), placed along the cylinder axis. This cathode is electrically heated to 900 K or above, using an auxiliary filament circuit, typically driven by a 6.3-V power supply, to foster thermoionic emission of electrons from the cathode. This cathode is cylindrically surrounded by a metallic outer electrode, the anode or "positive electrode" or "plate," which is a hollow metallic cylinder, whose axis coincides with that of the cathode. The... 

RTV-2 adhesives are solidified with a curing agent like all two-component adhesives. This reaction is not diffusion-controlled, so that the adhesives are suitable for bonding (and sealing) large adhesion gaps. There are two different reaction types for initiation of crosslinking condensation reaction and addition reaction. The end products differ in their chemical structures. 

A variety of two component adhesives and sealants are currently available for applications requiring the ultimate in bonding and sealing efficiency and performance. Included among these conventional systems are epoxies, urethanes, polyesters, silicones, polysulfides and some modified phenolics and acrylics. Although their performance can be and usually is quite satisfactory, most are limited by pot life and fixturing requirements - and the need for thorough substrate treatment and cleanliness. 

Since the Flat Panel Display industry must confront the problems of bonding and sealing relatively large areas with special atmospheric requirements (vacuums and/or special process gases), the equipment suppliers in this industry also may serve as a good source of information. 

Silicone-based chemistries match well with applications that require a low modulus of elasticity or high-temperature resistance. They also correspond with applications that require a combination of bonding and sealing. Curing mechanisms can include heat, moisture, or solvent evaporation. Room temperature vulcanization (RTV) materials are designed to polymerize as a solvent evaporates. 

Polyester Resin Kit. The proper shipping name Polyester resin kit covers different kits such as filler, bonding and sealing compounds, chemical anchors and fibreglass repair kits. A polyester resin kit commonly consists of an unsaturated polyester resin mixed with styrene and a separate hardener (usually a phlegmatized organic peroxide) as a minor component. The main component (viscous liquid or paste) is inherently flammable due to the styrene content (flash point 29°C to 32°C). ICAO A2, lATA App. A... 

The more viscous, mastic-type cements include some of the epoxies, urethanes and sflicones. Epoxies adhere well to both thermosets and thermoplastics. But epoxies are not recommended for most polyolefin bonding. Urethane adhesives have made inroads into flexible packaging, the shoe industry, and vinyl bonding. Polyester-based polyurethanes are often preferred over polyether systems because of their higher cohesive and adhesive properties. Sflicones are especially recommended where both bonding and sealing are desired. 

Figure 3 Application of a sealant automatically extruded onto an oil pan flange. Table 2 Main Adhesive Bonding and Sealing Applications in the Body Shop...

Apart from underbody coating (usually PVC based), which is not explained further, the majority of bonding and sealing products in the paint shop are also PVC compounds. Acrylate plastisols are not often used and polyurethane-based sealants are rarely found. The main applications are in seam sealing and antiflutter bonding. On a small scale there are adhesive applications to bond and seal caps and to fix sound deadeners. In addition, foams and butyls as well as bituminous or acrylic-based sealants are used to fill car body holes. 

The polymerase chain reaction (PCR) is the most widely used technique for oligonucleotide synthesis. Microchips/microdevices for PCR are studied extensively, and thus great progress has been made in the development and scope of microchip components of microchip-based PCR analyzers such as on-chip micromachining (fabrication, bonding, and sealing), choice of... 

Anaerobic adhesives and sealants are based on acrylic (usually meth cryXic) functional monomers and cure by a redox-initiated free radical polymerization (see Chain polymerization). They are so named because of the characteristic of requiring a relatively air-free condition to allow curing. Hence, anaerobic adhesives are very suitable for the bonding and sealing of close-fitting metal components. 

M. R. Bowditch and J. M. Lane, The development of resin systems and processes for nnderwater bonding and sealing. Procedures of the 24th Annual Meeting of the Adhesion Society, l fllliamsburg, 2001. 

Manufacturing of microreactors from metals, ceramics and plastic was described briefly. The material for the microreactor is chosen in consistence to the process demands as well as with regard to the number of microreactors to be produced. The most common techniques for different material classes—metals, ceramic and glass as well as polymers - have been described or referenced. Aspects like assembling, bonding and sealing have also been integrated into this chapter. However, it is not exhaustive. 

Externally Bonded and Sealed Insulation for Liquid-Hydrogen-Fueled Rocket Tanks (5) 131... 

---