Sealing silicon devices

Hermetically sealed electrical devices must be verified by a testing laboratory to meet mechanical abuse and to withstand aging and exposure to expected chemicals. Devices potted with common silicones and similar materials by an end user or even a manufacturer, without testing, and devices merely provided with O-rings seldom meet acceptable criteria. Normally, hermetically sealed devices must be sealed through metal-to-metal or glass-to-metal fusion. Many electrical relays, switches, and sensors are available as hermetically sealed devices for common oil and gas producing facility applications. Hermetically sealed devices are often desirable to protect electrical contacts from exposure to salt air and other contaminants. 

For sealing ra-dioelectronic equipment and screw joints for repairing articles sealed with other silicone sealants the temperature range is from -60 to+250°C For surface sealing of electronic and radio devices which operate in air and increased humidity at -60 -+250°C For sealing electronic devices to protect them from humidity and air the temperature range is from -60 to+300°C. 

Polymeric encapsulation has largely displaced techniques of hermetically sealing electronic devices, due to the low cost and ease of production. However, since all polymers are permeable to some small extent to water vapour, problems of adequate protection do still occur. Interestingly, it is not the inherent permeation of water into the polymer which alone causes these problems. Rather, they arise from the presence of trace amounts of catalyst residues in the polymer, which are transported to the surface of the device by the permeating water and which there result in the formation of an aggressive electrolyte solution. This problem is of particular concern with epoxide resins and of lesser concern with silicone encapsulants. 

Fig. 14-8 Circuit diagram for a dc decoupling device with silicon diodes. (KE) insulated cable end sealing, (E) grounding installation, (1) silicon power diodes.

Silicone elastomers find use as gaskets and seals, wire and cable insulation and hot gas and liquid conduits. They also find use in surgical and prosthetic devices. The RTV elastomers are used for sealing and encapsulating. 

Microelectronic devices on silicon chips are typically made from layers of n-type and p-type silicon. Films of silica act like the plastic sheath on copper cable, since silica is insulating. A layer of p-type silicon back to back with a layer of n-type, called a p-n junction, allows a current moving across the junction to flow in one direction but not the reverse. This one-way behaviour is the fundamental characteristic of a device called a diode. Early diodes in electronics were made from metal plates sealed inside evacuated glass tubes, which could be seen glowing in the innards of old radio sets. Diodes made from doped silicon can be much smaller and more robust since they are made from solid materials, they are components of solid-state electronics. 

Because of the extremely wide variety of reactions, educts, products, and process conditions, a sufficiently broad spectrum of materials is required to realize suitable microdevices for chemical processes. Metals and metal alloys, plastics, glass, ceramic materials, semiconductor materials like silicon, and various auxiliary materials for sealing, surface treatment, etc. have been successfully applied for realizing microreaction devices. 

FIGURE 2.12 Fabrication procedure of a PDMS chip (a) silicon master wafer with positive surface relief, (b) pre-mixed solution of Sylgard 184 and its curing agent poured over the master, (c) cured PDMS slab peeled from the master wafer, (d) PDMS slab punched with reservoir holes, and (e) ready-to-use device sealed with another slab of PDMS [159]. Reprinted with permission from the American Chemical Society. 

Low-molecular silicone elastomers are viscous liquids which do not contain solvents and solidify at room temperature. The specific properties of SKTN allow one to use them as insulation against heat, moisture and electricity in various miniature and large units of machines, mechanisms and devices, as well as for thermal, electrical and vibration sealing of various devices. The physiological inertness of elastomers accounts for their wide applications in medicine. 

As shown in Table 23, silicone sealants are used to seal various constructions and devices that operates in a wide temperature range (from -60 to 250-300 °C). Seals made from them are resistant to vibration, alternate and shock loads and atmosphere. 

Their high resistance to aging, moisture and ozone allows silicone rubbers to be used in lighting and signalling facilities, as well as in special-purpose electrical installations, as seals in meteorological devices and lights for airports, as shock absorbers, etc. 

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