Mechanical Engineer

OptoSigma Corporation offers motor controllers, high precision low profile 5-phase stepper motors, servo motors, actuators and vacuum compatible motorized stages in X, Y, Z and rotational configuration. Linear encoder (Magnescale) models are also available.

Zaber Technologies Inc. was founded in 1997 when precision motion control was typically accomplished with DC motors and encoders. In 2000 Zaber introduced the first daisy-chainable miniature linear actuator based on stepper motor technology with integrated RS232 communications and control electronics in one small package. Since then Zaber has added over 120 products to its family of precision motion control and laboratory automation devices including linear actuators, linear slides, motorized x-y-z stages, rotation stages, motorized mirror mounts, and micro-stepping motor controllers.

Pacamor Kubar Bearings is a domestic manufacturer of high precision miniature and instrument bearings and offers over 40 years of expertise in radial ball bearings. Located in Troy, NY, Pacamor Kubar Bearings is one of the last American owned and operated.

The most common resin used in thermoset prepreg materials is epoxy. These prepregs are generally stored in a low-temperature environment and have a limited shelf life. Room-temperature prepregs are also becoming available. Usually, the resin is partially cured to a tack-free state called B-staging. Several additives (e.g., flame retardants, catalysts, and inhibitors) are added to meet various end-use properties and processing and handling needs. Thermoset prepregs require a longer process cycle time, typically in the range of 1 to 8 hr due to their slower kinetic reactions. Due to higher production needs, rapid-curing thermoset prepregs are being developed.
Thermoset prepregs are more common and more widely used than ther­moplastic prepregs. They are generally made by solvent impregnation and hot melt technology. In the solvent impregnation method, the resin is dis­solved by a chemical agent, creating a low-viscosity liquid into which fibers are dipped. Due to growing environmental awareness, disposal of the sol­vent resulting from this process is becoming a concern. The hot melt tech­nology eliminates the use of solvents. In this process, the matrix resin is applied in viscous form. The drawback of this process is that fiber wetting is not easily achievable due to the higher viscosity of the resin.
Prepregs are generally used for hand lay-up, roll wrapping, compression molding, and automatic lay-up processes. Once the prepregs are laid on a tool, it is cured in the presence of pressure and temperature to obtain the final product.


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BMI and polyimide are used for high-temperature applications in aircrafts, missiles, and circuit boards. The glass transition temperature (Tg) of BMIs is in the range of 550 to 600°F, whereas some polyimides offer Tg greater than 700°F. These values are much higher than for epoxies and polyesters. The lack of use of BMIs and polyimides is attributed to their processing difficulty. They emit volatiles and moisture during imidization and curing. Therefore, proper venting is necessary during the curing of these resins; otherwise, it may cause process-related defects such as voids and delaminations. Other drawbacks of these resins include the fact that their toughness values are lower than epoxies and cyanate esters, and they have a higher moisture absorption ability Polyurethane is widely used for structural reaction injection molding (SRIM) processes and reinforced reaction injection molding (RRIM) processes.


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Thermoset Resins

Thermoset materials once cured cannot be remelted or reformed. During
curing, they form three-dimensional molecular chains, called
cross-linking, as shown in Figure 2.4. Due to these cross-linkings, the
molecules are not flexible and cannot be remelted and reshaped. The
higher the number of cross-linkings, the more rigid and thermally
stable the material will be. In rubbers and other elastomers, the
densities of cross-links are much less and therefore they are flexible.
Thermosets may soften to some extent at elevated temperatures. This
characteristic is sometimes used to create a bend or curve in tubular
structures, such as filament-wound tubes. Thermosets are brittle in
nature and are generally used with some form of filler and
reinforcement. Thermoset resins provide easy processability and better
fiber impregnation because the liquid resin is used at room temperature
for various processes such as filament winding, pultrusion, and RTM.
Thermosets offer greater thermal and dimensional stability, better
rigidity, and higher electrical, chem­ical, and solvent resistance. The
most common resin materials used in ther-moset composites are epoxy,
polyester, vinylester, phenolics, cyanate esters, bismaleimides, and
polyimides. Some of the basic properties of selected thermoset resins
are shown in Table 2.2.
Continue reading "Thermoset Resins,Epoxy,Phenolics,Polyesters,Vinylesters,Cyanate Esters"

Aramid fibers provide the highest tensile strength-to-weight ratio among reinforcing fibers. They provide good impact strength. Like carbon fibers, they provide a negative coefficient of thermal expansion. The disadvantage of aramid fibers is that they are difficult to cut and machine. Aramid fibers are produced by extruding an acidic solution (a proprietary polycondensa-tion product of terephthaloyol chloride and p-phenylenediamine) through a spinneret.
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Carbon and graphite fibers are produced using PAN-based or pitch-based precursors. The precursor undergoes a series of operations. In the first step, the precursors are oxidized by exposing them to extremely high temperatures. Later, they go through carbonization and graphitization processes. During these processes, precursors go through chemical changes that yield high stiff­ness-to-weight and stength-to-weight properties. The successive surface treat­ment and sizing process improves its resin compatibility and handleability.


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The properties of fibers depend on how the fibers are manufactured. The raw materials used for making E-glass fibers are silica sand, limestone, fluorspar, boric acid, and clay. Silica accounts for more than 50% of the total ingredients. By varying the amounts of raw materials and the processing parameters, other glass types are produced. The raw materials are mixed thoroughly and melted in a furnace at 2,500 to 3,000°F. The melt flows into one or more bushings containing hundreds of small orifices. The glass filaments are formed as the molten glass passes through these orifices and successively goes through a quench area where water and/or air quickly cool the filaments below the glass transition temperature. The filaments are then pulled over a roller at a speed around 50 miles per hour. The roller coats them with sizing. The amount of sizing used ranges from 0.25 to 6% of the original fiber weight. All the filaments are then pulled into a single strand and wound onto a tube.


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Pressurized Liquid Spray

Most spray methods are based on the principle of atomizing the coating into a fine mist that is then directed to the part to be coated. Atomization is accomplished by using compressed air, pressurized volatile solvents, or a high velocity jetstream. With the proper technique, a thin continuous wet layer of coating may be deposited without drips, sags, “curtains,” “orange peel,” or other imperfections.

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Some coatings, such as solder or plating resists and photoresists, are used temporarily during processing. These coatings are usually used to shield critical portions of the circuitry during subsequent processing, such as soldering or laser ablation, or to selectively mask portions of a surface in the photofabrication processing of microcircuits and thin-film circuits.
4.18.1 Maskants Used During Soldering or Laser Processing
Permanent maskants, previously described, are generally used for PWBs as solder maskants but, in some cases, removable maskants are more appropriate. For example, in assembling hybrid microcircuits, in which both wire bonding and solder attachment processes are used, the soldering processes are performed first, but the remainder of the circuit must first be protected from solder splatter and flux residues. Polyvinyl alcohol and polyvinylacetate coatings provide this protection and are subsequently removed by dissolving in water or alcohol. Urethane/acrylate formulations that cure on exposure to 365 nm wavelength radiation may subsequently be removed either by peeling or dissolving in water.
In the formation of microvias in polyimide film or in other substrates by laser ablation, carbonaceous, non-volatile residues deposit on adjacent surfaces. Here too, water-soluble coatings such as PVA have been used to shield adjacent surfaces, for example, in the processing of Tape Automated Bonding (TAB) devices.[92]
4.18.2 Photoresists
Photoresists are a major class of temporary organic coatings essential in the fabrication of all microelectronic devices and thin-film circuits. Advances in photoresists together with optical tools have enabled the continued shrinking that has been occurring in device dimensions from
Chapter 4 - Applications
365
5 µm in the late 1960s to approximately 0.1 µm today. During this period, chemists have synthesized unique photosensitive polymers to satisfy the resolution, light sensitivity, and processing requirements for each successive generation of semiconductor and integrated circuit chips. Concurrently, physicists have optimized the optics and optical tools. The process by which photoresists are used to etch intricate and precise lines, spacings, and vias in metals and dielectrics has been a key factor in the rapid development of microelectronics from the early 1960s until today.


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A unique and specific application for coatings involves tamper-proofing of electronic circuit boards and assemblies. Secure systems for military and space electronics must prevent access to stored information and prevent reverse engineering. Even some commercial and consumer products such as toys and smart cards may require such protection, e.g., preventing competitors from determining a circuit design or the nature of an IC chip.


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Metal-filled electrically conductive organic paints and coatings are used on electronic chassis and enclosures to absorb and attenuate electromagnetic energy, thus, shielding the internal electronics. Electrically conductive coatings are also used to dissipate electrostatic charges (ESD) that build up on plastic or glass parts, to ground surfaces, and to prevent corona discharge. Conductive coatings allow static charges to be distributed evenly over the surface and allow charges to be bled off to ground. Generally, a surface resistance of <105 ohm/sq is required, but the most effective compositions have surface resistances of 0.03 to 1.0 ohm/sq/mil. Besides these electrical requirements, EMI shielding coatings must meet a combination of other requirements among which are:
• Ability to be sprayed or otherwise deposited in uniform thicknesses of 1 to 2 mils.
• Good adhesion to a variety of metal, plastic, and glass surfaces of which enclosures may consist. Examples of plastics used for enclosures include ABS, Noryl*, polystyrene, and polycarbonate.


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COATINGS FOR AUTOMOTIVE APPLICATIONS

As the usage of electronics in automobiles increases, the protection of these electronics to assure reliable performance under very harsh environments is critical. Three general environments are well known in the automotive industry:
• Passenger compartment
• Under the hood
• On or near the engine
Of these, the least severe ambient occurs in the passenger compartment while the most severe is on or near the engine. Temperatures inside the car can range from -40° to 85°C and relative humidities can vary from near 0 to 100%. Water can condense on electronic surfaces and, combined with water-soluble contaminants, can cause momentary malfunctions and ultimately catastrophic failures. Generally, electronics inside the vehicle are not coated because of the sparsity of proven failures due to unprotected circuits and the added cost of coating. However, this situation is changing with the increasing use of sensors, advanced devices, PCBs having finer conductor lines and closer spacings, and the use of thin-film metallization— all of which are more susceptible to corrosion and transient electrical effects. Where organic coatings are used, those that meet the industry conformal coating specifications, such as acrylics, polyurethanes, epoxies, and silicones are the favorites. Silicones, because of their high thermal stabilities are used in the near-engine circuits, while polyurethanes and epoxies are used for passenger compartment electronics. Applications include sensor circuits for aid in parking, belt-locks, side airbags, and the central locking mechanism. Schenectady-Beck’s Bectron PK43, a polyure-thane, is used to selectively coat a portion of an airbag electronic module installed in the sidedoor of an automobile.[89]
The under-the-hood environment is much more severe than the passenger environment. The maximum temperature there can reach 105°C and temperatures can rise quickly from outdoor freezing to super hot. Concurrently, the electronics may be exposed to corrosive materials including gasoline, motor oil, antifreeze, battery acid, brake fluid, and numerous road contaminants. For this environment, thin conformal coatings may not suffice as adequate protection. Thick coatings or coatings combined with encapsulants or potting compounds are generally required. Silicones and
epoxies are favored because of their high temperature and chemical resistances.

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Another major concern in using organic materials in space is outgassing and degradation in a thermal-vacuum environment. Outgassed products can condense on optical lenses and instruments or create a corrosive ambient for electronics. The main mode of outgassing consists of the volatilization of low-molecular-weight, high-vapor-pressure species. These may consist of resins and hardeners that have not completely reacted due to insufficient cure or use of nonstoichiometric amounts of reactants. Additives such as plasticizers, flame retardants, diluents, and solvents are also vulnerable to outgassing. In some cases, outgassing may be due to sublimation which is the vaporization of a solid without going through the liquid state, followed by redeposition from the gaseous state onto a cooler surface. At sufficiently high temperatures, all polymers decompose by the rupturing of atomic and molecular bonds and by the release of smaller more volatile fragments.


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Polyimide Wire Insulation. Polyimide coatings are extensively used for
high temperature wire insulation and for insulation that must withstand
high radiation fluxes. Pyre-ML (tradename of Summit Precision Polymers;
available from Rea Magnet Wire Co.) polyimide varnish has been in use
for many years as a magnet wire insulation for dry-type transformers,
dc field coils, and submersible pump motors. It is rated for continuous
use at 220°C. The measured thermal endurance for PD Wire & Cable’s
polyimide Imideze® is given in Fig. 4.33. As an overcoating, polyimide
provides an extremely tough abrasion-resistant film that also exhibits
high resistance to radiation and heat. Polyimide-coated round wire is
specified in NEMA MW-16. Polyimide coatings are also used as insulation
and impregnants for motor, coils, and relays. Because of their low
weight loss at elevated temperatures and their high chemical
resistance, polyimide varnishes are useful in encapsulated windings and
hermetically-sealed components. Polyimide wire coatings are compatible
with numerous phenolic, polyester, and epoxy varnishes and encapsulants
and are resistant to most solvents and chemicals. The exceptional
toughness, thermal stability, and chemical resistance of cured
polyimide, however, becomes a drawback in its ability to be easily
stripped from wire in order to perform rework or make electrical
connections. Other limitations include its hydrolytic instability at
temperatures above 105°C in a sealed system containing moisture.
Polyimides under the DuPont tradename of Kapton® may also be used as a
film to insulate wire.
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Polyester Varnishes and Enamels

Polyester varnishes are produced by the condensation of terephthalic acid, a glycol, and a polyol such as glycerine. When the proper ratios are mixed and heated, partial polymerization occurs, leaving the product soluble in solvents

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Polyurethane coatings that are used as wire enamels are generally the reaction products of hydroxylated polyesters or hydroxylated polyethers with tolylene diisocyanate. Admixing these components, however, results in very rapid polymerization—too rapid to be of any practical use. For this reason, polyesters containing excess isocyan-ate groups are first reacted with phenol to yield adducts that are relatively stable at room temperature. After coating the wire and baking at elevated temperatures, the phenol portion of the adduct is released and the resulting free isocyanate groups then react with the hydroxyl groups of the polyester to form the desired polyurethane coating.

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Coatings based on polyvinyl formal (Formvar) were first introduced in 1938 as magnet wire insulation, but remain in wide use today. The base polymer is synthesized by partially hydrolyzing polyvinyl acetate and then reacting the free hydroxyl groups of the resulting product with formaldehyde to form the six-membered formal structure. The polymer, therefore, contains three basic functional units: vinyl formal, vinyl alcohol, and vinyl acetate, as in Fig. 4.29.
Polyvinyl formal is thermally stable for extended periods of time at 110°C; its thermal endurance curve is shown in Fig. 4.30. Used alone, Formvar has marginal solvent and abrasion resistance, but its properties are greatly improved by modifying the basic resin through reaction with phenol formaldehyde or cresol formaldehyde resins. Both resins condense with the free hydroxyl groups of the polyvinyl formal to produce coatings that are extremely tough and flexible and that possess good abrasion and solvent resistance. Polyvinyl formal can also be modified with resins such as melamine, urea, epoxy, and urethanes.[59]


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Wire must be electrically insulated and environmentally and physically protected. A variety of polymer coatings can and are being used for this purpose. They are classified according to their thermal and physical endurance and manner of application. Wires may be dip-coated in a liquid bath of the resin solution, then cured or dried, wrapped with fibrous coverings or resin-impregnated coverings, wrapped with tape, or covered with extruded polymers. Large-diameter wire, as used in cables, is generally insulated by the extrusion of solid thermoplastic resins.[57] Thermoplastic resins soften on heating then resolidify on cooling. During this process they can be shaped around a conductor wire by forcing the plastic material under pressure and heat through an orifice concurrently with the conductor. Extruded insulation such as polyvinyl chloride, polyethylene, or polytetrafluoroethylene (Teflon) is relatively thick compared to the diameter of the wire and is, therefore, used mainly for interconnect lead wires and cables. Tape wrapping involves the wrapping of thin tape or plastic film around the wire with a minimum of overlap. Several layers of tape can be used to build up the desired insulation thickness.


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Chip-on-board (COB) is a fairly new packaging technology that combines the low-cost benefits of PWAs with the high densities of hybrid microcircuits. According to COB, bare chips are wire bonded or flip-chip interconnected to a PCB designed and fabricated with finer dimensions than normally used. The active chips requiring protection are then selectively encapsulated by dispensing a filled epoxy over the device, a process called glob-topping. In the board design, several ICs can be partitioned closely together and encapsulated as a unit. Although there is always a concern in using plastic encapsulants as the prime method for moisture protection, epoxy glob-top materials on the market have performed very well in many applications and environments. Reliability is enhanced by assuring a

good primary passivation at the die level and a low-stress, low-moisture-absorption encapsulant. Even so, overcoating the entire COB with a high-performance coating such as parylene produced improved results. In one report, DRAM devices glob-topped with either Hysol-Dexter FP-4402, FP-4450, or Dow Corning silicone HIPEC Q14939 and then overcoated with parylene, all passed 1,000 hours of 85/85 THB while control parts without encapsulant began to fail at 250 hours. Tests also carried out on epoxy-encapsulated ATC-01 triple track resistors (Sandia National Laboratory test chip) survived 1,000 temperature cycles from -55° to 125°C.[46]

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Solder resist coatings (maskants) are heat-resistant polymer coatings applied to selected areas of a PWB prior to solder coating and component assembly. Solder maskants protect portions of the board during subsequent soldering and reflowing of solder. They are useful in preventing solder from splattering onto conductor traces or bridging conductors. Other benefits of solder masking include: reducing solder-pot contamination from copper or gold, protecting the circuit from handling and environmental damage, and providing isolation between components and conductor lines or vias when components are mounted directly over the conductor lines. Solder maskants are generally based on epoxy, acrylic, or epoxy-acrylic resins formulated with fillers and additives to render them screen printable in selected areas.

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Polymer coatings are finding new applications in optoelectronics where many of their attributes important for semiconductor and MCM fine-line delineations are also applicable to the fabrication of waveguides. Photolithographic processes and photocurable materials such as polyimides, BCB, and epoxies are used to form precise grooves in the coating for the transmission of light. Most of these processes have already been described in connection with via formation except that, in the case of waveguides, long narrow grooves are formed in the dielectric coating. Polymer technology for waveguides is still in a state of flux because of the wide variety of materials and processes that can be used and the need to achieve low losses within the structure and at the connections. However, the rewards can be great because of the potential for cost reduction over current methods.


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Semiconductor Junction Coatings and Stress Buffers

Organic coatings are used directly on semiconductor surfaces prior to plastic molding or glob-top encapsulation to stabilize semiconductor junctions and dissipate stresses. High-purity silicones, polyimides, and BCBs are used because of their wide operating temperatures (-65° to 275°C for silicones), high breakdown voltages (500 to 1,000 V/mil), electrical stabilities minimizing leakage currents and surface effects, and compatibility with epoxy molding or encapsulation.

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Dielectric coatings may be used to form an additional layer of electrical interconnections beyond those already formed during wafer fabrication. Processes have been developed to alter the bonding pad topography both at the die (wafer) level and at the substrate level using polymer dielectrics. For example, a bonding pad can be reconfigured, changing its size and shape, by spin coating a photocurable BCB layer over the entire surface, exposing to uv through a mask having the image of the desired geometry, and developing it (Fig. 4.6). Dielectrics such as BCB or polyimides may also be used to redistribute peripheral bonding pads on a die to form an area array of pads to permit solder bumping for flip-chip bonding or ball grid array packages (Fig. 4.7). Likewise, internal pads on a die can be redistributed to one side to permit wire bonding of vertically stacked dice.

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Conformal Coating Performance

The performance and long-term reliability of conformal coated PWAs depend not only on the type of coating selected and its curing conditions, but also on the nature of the surfaces to which it is applied and the cleanliness and dryness of those surfaces. Circuit boards consisting of epoxy-glass and polyimide-glass laminates can absorb large amounts of moisture especially during aqueous cleaning. Flux residues are generally hygroscopic and augment the absorption and retention of moisture. If not cleaned and dried well, a subsequently applied conformal coating such as polyurethane will blister and lose adhesion. Adhesion of coatings to gold-plated surfaces or to Teflon-insulated wire is poor and, unless these surfaces are primed, physically roughened, or chemically etched, the coating will peel off.

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Organic polymer coatings have been used for over forty years as
moisture protective conformal coatings for printed circuit boards (PCB)
and for printed wiring assemblies (PWA) used in high-reliability
military and space applications and, to a lesser extent, in commercial
electronic hardware.[1][2] The main functions of organic conformal
coatings for PWAs include:
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As with other forms of radiation energy, gamma radiation can be used to polymerize or co-polymerize certain gaseous, liquid, or solid monomers. Gamma-radiation-induced reactions are similar in nature and mechanism to those induced by uv radiation. Consequently, the same classes of polymers used for uv curing are amenable to gamma curing. These include the general class of ethylenic compounds such as vinyls, acrylics, allylics, and polyesters. Epoxy resins, however, are difficult to polymerize by gamma irradiation. High dosages of 4 × 108rads and greater are required and, even so, only a small percentage increase in molecular weight occurs.[82] However, indirect techniques may be used to radiation cure epoxies. The epoxy resin may be modified to incorporate vinyl or ethylenic groups that are radiation sensitive. Gamma irradiation can then induce polymerization through these functional groups.


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