Kutz M. Handbook of materials selection

Подождите немного. Документ загружается.

5 TYPICAL APPLICATIONS 1239

5.8 Electrical Contacts

Electrical contacts in electronic applications usually consist of switch contacts

and connector contacts. The requirement for each are similar; however, the usage

varies in that the switch contacts may be frequently operated and the connector

contacts are rarely engaged or disengaged.

Materials selected to make electrical contacts depend substantially upon the

amount of electrical energy that the contacts must carry. The dominant consid-

erations for selecting contact materials include electrical conductivity, chemical

inertness, resistance to corrosion, thermal conductivity, and wear resistance.

Electrical contacts are usually made from metal materials; however, in unusual

instances where a low electrical conductivity is not required, conductive plastics

(plastics loaded with carbon or metallic particles) are employed. Sometimes a

metal contact will ride upon a nonmetallic contact such as in a carbon compo-

sition potentiometer.

Connectors that must maintain reliable electrical contact over extended peri-

ods of time are often designed to produce high contact forces between the con-

tacting elements. High contact forces result in signiﬁcant mechanical erosion of

the contacting materials whenever the connector is engaged or disengaged and

can seriously shorten the number of uses of the connector. Connector engage-

ment is beneﬁcial because the mechanical action of the contacting materials

sliding over each other tends to remove surface ﬁlms and corrosion and thus

lead to a lower contact resistance. This lowered contact resistance will last until

the contact resistance is increased due to the formation of chemical ﬁlms or

corrosion products.

Contacts that are required to carry (make and break) very low electrical cur-

rent, such as CMOS level signals, are known as ‘‘dry contacts.’’ Dry contacts

are fabricated using inert metals or metals plated with an inert metal such as

gold or rhodium, upon which surface ﬁlm formation is inhibited. Silver contacts

and lead-based metal contacts are not used because they tarnish, and the tarnish

ﬁlm is a poor conductor of electricity and thus induces resistance in the switched

circuit. Long-term resistance to environmental chemicals is sometimes improved

by coating the contacts with a protective grease or other media.

Circuit contacts where the circuit carries higher levels of current provide

greater freedom in selection of contact materials because the current ﬂow is

adequate to dispel the tarnish and other ﬁlms that may form on the contacts.

These contacts are often made from beryllium copper, silver alloys, lead–tin

alloys, nickel, rhodium, and other materials.

Dominant considerations for connectors include electrical conductivity, chem-

ical inertness, prevention of galvanic corrosion, hardness, and wear resistance.

Many of the same materials used for switch contacts are used for connector

elements, however, with more emphasis on wear resistance and chemical resis-

tance. It is necessary to prove electrical contact performance for both switches

or connectors by extensive life testing with the primary variables being contact

force and material hardness.

5.9 Encapsulation

Encapsulation or potting of an electronic component or subassembly is utilized

to promote ruggedness, resist harsh environments, provide electrical isolation,

1240 MATERIALS IN ELECTRONIC PACKAGING

and to promote improved heat transfer for cooling of the encapsulated compo-

nents. Encapsulation is also used to seal an assembly, thus to protect proprietary

design details from the curious.

There are a variety of material properties that must be considered when se-

lecting an encapsulant. Sometimes the requirements are inconsistent with the

range of properties offered by any one material. Of interest are the ability of the

material to ﬂow freely, not capture or entrain air bubbles, be light weight, offer

acceptable electrical, thermal, and chemical properties, and resist cutting and

abrasion after cure. During cure the material should not produce excessive tem-

peratures nor produce excessive stresses on the encapsulated parts. After cure,

the material should not absorb moisture, not produce excessive forces during

temperature cycles, must be stiff enough to support components and connections,

and should be able to be removed for purposes of repair.

Some of the properties of the materials exist for a short period of time, such

as during cure and for some duration following cure. An example would be

short-term dielectric strength that may not stabilize for several days after cure.

There are at least three primary groups of material formulations used for

encapsulation:

●

Epoxy Resin–Based Materials. These materials may be formulated for

strength and rigidity. The addition of glass microspheres would reduce

density, and addition of metal or other powders will increase thermal and

electrical conductivity. Epoxy resins are usually two-part mixtures and

require strict control of the mixing, pouring, and curing processes.

●

Silicon-Based Materials. These materials are know for having an extended

temperature range over which they do not degrade. They tend to remain

ﬂexible and are resistant to water. They are either one-part or two-part

formulations. Some formulations generate acetic acid during cure and may

cause corrosion of parts and conductors. Cure time is a function of the

thickness and mass of the encapsulant and may require more than one day

to achieve full cure.

●

Polyurethane-Based Materials. These materials generally consist of a cat-

alyst and resin and require mixing prior to pouring. They tend to remain

ﬂexible over a useful temperature range and after cure are of a rubberlike

consistency; however, they have less tendency to adhere to component,

wires entering the potted volume, and potting box surfaces than epoxy or

silicone encapsulants.

Epoxy polymers are used to provide low-cost, high-performance reliability

without hermeticity to provide environmental protection for bare circuit chips

when used in chip-on-board (COB), ball grid array (BGA), multichip module

(MCM), and chip-scale packaging (CSP) assemblies.

Data provided on encapsulants is found on manufacturer’s data sheets; how-

ever, this information may be incomplete and sometimes does not relate physical

properties to the method of cure and the material thickness. It is often necessary

6 CANDIDATE MATERIALS 1241

to consult with the technical support personnel of the manufacturer and to per-

form testing to determine suitability for the intended design goals.

5.10 Harsh Environment Endurance

Environmental endurance is the ability of the selected material(s) to resist deg-

radation when subjected to the service conditions faced by the equipment or

device that is constructed from those materials. Unusually harsh operating con-

ditions may cause the materials selection process to focus on the ability of the

material to resist the service environment before the designer considers other

important desired material properties.

Applications involving high temperatures require the use of metals, ceramics,

and a few plastics.

A highly corrosive chemical environment possibly involving strong acids or

strong alkali vapors or solutions are resisted by some glasses, some ceramics,

stainless steels, and some plastics.

Solar radiation creates heating of materials and degrades some plastics, such

as nylon and PVC, which lose weight and strength when exposed to ultraviolet

radiation.

Glasses, ceramics, and many metals react well in the low pressures and the

vacuum conditions of outerspace without sublimating or losing strength due to

ultraviolet radiation exposure.

6 CANDIDATE MATERIALS

6.1 General

With the dominant and overriding design considerations deﬁned, the electronics

packaging engineer is ready to survey general classes of materials for the pur-

pose of identifying one or more candidate material for the design task under

consideration.

One primary materials selection consideration is often the conductivity of the

material; that is, whether the material is a conductor or an insulator of electrical

signals and currents. Conductivity is an important characteristic (printed circuit

traces, wires, etc.) and is often the reason that metals are the materials of choice.

Other important characteristics of metals include strength, durability, thermal

conductivity, and ability to work over a wide temperature range.

Nonmetallic elements may exhibit many of the characteristics of metals; how-

ever, they are used to provide electrical and thermal insulation. Other properties

such as inertness to chemicals, dielectric strength, appearance, low cost, and east

of fabrication may lead to the use of nonmetallic elements.

Semiconductor materials may be employed to achieve, when properly for-

mulated as in transistor junctions, the ability to become either an electrical con-

ductor or an electrical insulator depending on the conditions of use.

Several major categories of materials exist

from which the designer may

identify subsets and speciﬁc materials for detailed review and selection.

6.2 Metals

A guide to the electrical conductivity of a metal is its electrical resistance.

Table

2 gives volume resistivities of selected materials, where:

1242 MATERIALS IN ELECTRONIC PACKAGING

Table 2 Materials Categorized by Resistivity

Conductors Semiconductors Insulators

Basic metals Silicon Ceramics

Metal alloys Germanium Elastomers

Metal bearing compounds Gallium arsenide Gasses

Plasma Selenium Glass

Some liquids Solvents

Thermoplastics

Thermosets

●

Insulators are considered to have a volume resistivity of 10

⍀-cm or

greater;

●

Semiconductors in the range of 10

⍀-cm to about 10

⫺

⍀-cm

●

Conductors are materials with volume resistivity of about 10

⫺

or lower

Other selected properties of metals used in electronic packaging are provided

by Table 3.

Iron and Its Alloys

Iron-bearing materials used in electronic packaging include various alloys of

steel.

Steel materials

are strong and durable; however, they are subject to de-

terioration by oxidation (rust) and relative to other metals, such as copper and

aluminum, are less efﬁcient conductors of electricity and heat.

Steel alloys containing chromium, commonly referred to as ‘‘stainless steels’’

or CRES (corrosion-resistant steel),

resist oxidation and tarnishing and are often

used in electronic packaging as fastener devices such as screws, nuts, washers,

and other hardware. Some alloys of stainless steel are not magnetic, a property

that is sometimes useful when used in conjunction with circuits that are sensitive

to magnetic ﬁelds.

There are a wide variety of properties that may be developed for steel alloys

such as very high strength-to-weight ratios, hardness, wear, selective resistance

to chemicals, the ability to be formed by mechanical cutting and shaping, and

the ability to be joined by welding and brazing. Consumer goods electronic

equipment chassis are sometimes constructed from tin-plated steel, which is a

low-cost material that can be easily cut or formed and that may be soldered

using conventional lead-tin electronic solders.

Aluminum and Its Alloys

Aluminum alloys

are commonly used in electronic equipment for manufacturing

cases, chassis, covers, brackets, and other mechanical parts. Aluminum alloys

range from being ‘‘dead soft’’ through alloys for which substantial strength and

surface hardness improvements are achieved by heat treatment.

Aluminum alloys are subject to degradation by both acids and bases and are

subject to galvanic corrosion when in contact with other metals. Aluminum is a

good conductor of both electricity and heat. Common surface protection methods

include iridite or chromate dip coating, anodizing, and painting.

6 CANDIDATE MATERIALS 1243

Table 3 Typical Properties of Selected Metals

Metal

Melt Point

(⬚C)

Density

⫻ 10

⫺3

kg/m

(20⬚C)

Resistivity

⫻ 10

⍀-m

(20⬚C)

Thermal Cond.

W/ m-K

(0–100⬚C)

Thermal

Expansion

⫻ 10

1/⬚C

Aluminum 660 2.7 2.7 238 23.5

Antimony 630 6.7 42.0 17.6 8.11

Beryllium 1284 1.8 5.0 167 12.0

Bismuth 271 9.8 116 7.9 13.4

Cadmium 321 8.6 7.4 92.0 31.0

Chromium 1875 7.2 13.0 69.0 6.5

Copper 1491 8.9 1.7 393 17.0

Germanium 937 5.3

⬎10

59.0 5.8

Gold 1064 19.3 2.3 263 5.8

Indium 156 7.3 9.1 81.9 25.0

Iron 1525 7.9 9.7 71.0 6.8

Lead 328 11.7 21.0 34.3 20.0

Lithium 179 0.5 9.4 71.1 55.0

Magnesium 650 1.7 4.0 167 26.0

Molybdenum 2595 10.2 5.6 142 5.2

Nickel 1254 8.9 6.8 88.0 13.4

Palladium 1550 12.0 10.9 71.0 10.9

Platinum 1770 21.5 10.7 71.0 9.1

Rhodium 1959 12.4 4.7 84.0 8.4

Silicon 1414 2.3

⬎10

83.0 7.5

Silver 961 10.5 1.6 418 19.0

Tantalum 2985 17.0 14.0 54.0 6.5

Tin 232 7.3 12.9 65.0 12.0

Titanium 1670 4.5 55.0 17.0 8.9

Tungsten 3380 19.4 6.0 165 4.5

Uranium(a) 1130 19.0 29.0 29.3 23.0

Zinc 420 7.1 26.0 111 30.0

Zirconium 1860 6.5 45.0 20.0 5.9

Aluminum alloys are easily cut and formed by most mechanical methods. The

softer aluminum alloys are very ductile and may be extruded, impact formed,

rolled, or mechanically upset. Other alloys may be punched and formed to

achieve a deﬁned shape and then heat treated to achieve structural properties.

Aluminum has a good strength-to-weight ratio and is used to achieve lightweight

products. Commonly used aluminum alloy series include:

●

1100 Resists corrosion, good weldability, good conductivity, commonly

heat treated, used for sheet metal parts, rivets, etc. without high-strength

requirements

●

3003 Similar to 1100 except higher strength, more corrosion resistant,

used for heat exchanger ﬁns and like applications

●

5052 Excellent corrosion resistance especially in marine environments,

higher strength than 3003, used for chassis parts, fan blades, and the like

1244 MATERIALS IN ELECTRONIC PACKAGING

●

6061 A heat-treatable high-strength material used for structural parts

●

356 series alloy Used for cast aluminum parts; may be heat treated and

used for sand, permanent mold, and investment casting processes

Magnesium

Magnesium has similarities to aluminum. It is a good but slightly poorer elec-

trical and thermal conductor with a high strength-to-weight ratio and is sometime

used in place of aluminum to effect weight reduction. Care must be taken when

using magnesium because magnesium corrodes readily, is highly reactive to

oxidizing chemicals, and magnesium in shaving and powder form will oxidize

(burn).

Copper and Its Alloys

The primary attributes of copper are its low electrical resistance and low resis-

tance to the ﬂow of heat (high thermal conductivity). Copper is used for con-

ductive traces on printed circuit boards, in wiring, and in switches and relays.

Copper may be machined, formed, and cast. It is easily soldered by conventional

lead–tin electronic solders. Copper will tarnish and usually requires surface treat-

ment to avoid tarnishing in service.

Copper is heavy, soft, and ductile thus not a good structural material unless

used in an alloy with other materials such as beryllium. Beryllium–copper alloys

are used for electrical contacts (improved wear) and are good materials for

constructing springs and spring-type electrical contacts.

Cadmium, Chromium, Nickel, and Zinc

These metals are often used as an electroplated ﬁnish on iron alloy parts, such

as fasteners, to improve the appearance of the parts and to achieve protection

against galvanic corrosion.

Gold and Silver

Gold and silver are precious metals that exhibit very low electrical resistance

and very high thermal conductivity. Both are used to electroplate connector,

relay, and switch contacts to achieve low contact resistance, durability, and re-

sistance to corrosion. Gold is an inert metal and not given to forming noncon-

ductive surface ﬁlms over time and thus is a good candidate for ‘‘dry circuit’’

switching. (A dry circuit is one where very low currents are involved, such as

signals in a CMOS circuit. The low currents are insufﬁcient to penetrate and

dispel the effect of environmental-induced nonconductive ﬁlms on the contacts.

The result is a detrimental increase in contact resistance over time.)

Care must be used when silver is employed in a design because silver will

easily tarnish; and, when used as switch or other contacts, this tarnish will create

a nonconductive surface ﬁlm on the silver part. Sulfur, such as found in brown

wrapping paper, will quickly cause silver to tarnish. The application of antiox-

idant grease is used to protect electrical contacts fabricated from silver.

Both gold and silver are easily soldered using conventional electronic solders.

When soldering to contacts that are silver plated, the solder will leach the plated

silver from the wetted area resulting in degraded electrical performance. As a

6 CANDIDATE MATERIALS 1245

result, a ‘‘silver solder,’’ containing approximately 2% silver, is be used to pre-

vent leaching.

Lead and Tin

Lead is very heavy and may be used for shielding of electronic components that

are subjected to ionizing radiation.

Tin is a soft and ductile material, substantially inert to the environment that

has lubrication properties. Thus tin may be used to electroplate electrical contacts

in connectors to improve the mechanical mating of connector elements.

Used together, lead and tin are the primary constituents in the majority of

electronic solder alloys commonly used in the electronics industry. Lead–tin

solders vary in the ratio of lead to tin in the alloy, with 63% lead and 37% tin

being a eutectic mixture featuring the lowest melting point of solder and when

cooling the entire mixture ‘‘freezes’’ at the same temperature, which leads to

improved joint integrity particularly regarding the attachment of parts to a

printed circuit board. Another common solder mixture consists of 60% lead and

40% tin. This mixture melts at a slightly higher temperature; however, it exhibits

higher mechanical strength in the solidiﬁed form and, thus is used to attach

wires to connectors and in other general applications.

The presence of lead in solder is believed to cause health risks, and lead-free

solders

and lead-free conductive adhesives are being used in sensitive appli-

cations where lead must be avoided.

Rhodium

This metal has very good wear resistance and is used to coat electrical contacts

subjected to repetitive usage, such as high-performance relay contacts.

Titanium

Heavier than aluminum but lighter than steel, titanium has a melting point com-

parable to steel, offers a relatively inert structural material with a high strength-

to-weight ratio and is difﬁcult to weld. Its electrical resistance is about 20 times

that of aluminum, and its thermal conductivity is one-fourth that of aluminum.

Its coefﬁcient of thermal expansion is nearly the same as that of steel. Titanium

is used for weight reduction when high strength is required for equipment at-

tachments, mounting structures, and cases.

6.3 Plastics and Elastomers

Plastics or organic polymers ﬁnd extensive use in electronic applications.

There

are two basic types of plastics; thermoplastics and thermosetting plastics. The

difference between thermoplastics and thermosets is whether or not the plastic

is completely polymerized at the time of manufacture (i.e., a thermoplastic), or

whether the plastic is provided as a resin plus a hardener, which must be mixed

and polymerized when the plastic is formed into a component (i.e., a thermo-

setting plastic).

Plastics are commonly used in applications where electrical insulation is re-

quired or acceptable. Plastics are unable to operate over the temperature range

of metals and are subject to softening at warm temperatures and embrittlement

1246 MATERIALS IN ELECTRONIC PACKAGING

Table 4 Plastics Used in Electronic Packaging

Thermoplastics Thermosets Elastomers

ABS Allyl Natural rubber

Acrylic Epoxy Isoprene

Fluoropolymer Melamine Chloroprene, neoprene

Nylon Phenolic Ethylene–propylene

Polycarbonate Polyimide Fluorinated copolymers

Polyethermide Polyurethane Butyl

Polyethylene Silicone Nitrile, Buna N

Polyimide PVC

Polystyrene Silicone copolymers

PVC Polysulﬁde

Polyurethane

at cold temperatures. The physical properties vary widely, however, plastics are

used in the construction of many electronic components and may be used for

equipment housings and appearance hardware. Care must be taken when sol-

dering near plastic parts, as touching plastic by a soldering iron tip can often

cause local melting and deformation. Resistance to chemicals varies widely as

does moisture absorption and mechanical load-bearing capacity. Most plastics

are subject to creep and must be employed where permanent deformation due

to sustained loading is controlled or may be tolerated.

Plastic materials may be formed into component parts by a wide variety

of methods, and the cost per unit for large-volume production can be quite

low. Typical plastics used in electronic packaging applications are identiﬁed in

Table 4.

Thermoplastics

Thermoplastic materials are available in sheet, granules, and powder form; and

may be fabricated by transfer molding, injection molding, laminating, pultrusion,

compression molding, ﬁlament winding and pouring or casting. Thermoplastic

materials can be softened by heat and merged or shaped. The properties of

thermoplastics may be modiﬁed by the addition of ﬁllers or reinforcing ﬁbers.

ABS thermoplastics are tough and used to make enclosures, insulators, ap-

pearance sheet materials, and a variety of molded parts for electronic applica-

tions. They may be varied in color; however, the temperature range is limited.

Most applications of the ABS thermoplastics are in consumer goods and other

low-cost applications.

Acrylics may be used for conferral coating, and in solid form they are opti-

cally clear with very good weather resistance. They are temperature sensitive

and will readily burn when exposed to ﬂame.

Fluoropolymers are widely used in electronic applications due to the wide

range of properties for which they may be formulated. They are highly heat

resistant, are inert to most chemicals, have essentially zero water absorption,

have a low coefﬁcient of friction, have high arc resistance, resist creep, and have

low dielectric losses. Teﬂon, as manufactured by DuPont, is an example of a

ﬂuoropolymer used for wire insulation, electrical insulation, low load bearings,

6 CANDIDATE MATERIALS 1247

and dielectric purposes. Fluoropolymers are expensive, relatively soft, and hard

to process.

Nylon is a polyamide polymer that is tough and abrasion resistant with good

mechanical strength. It is used in ﬁber form to make fabrics and in solid form

to make gears, standoffs, low-pressure unlubricated bearings, wire ties, con-

nectors, wire jackets, and like items. Nylon absorbs water and its dielectric

strength and resistivity varies with the humidity of the environment to which it

is exposed. Nylon will degrade when subjected to sunlight and will exhibit creep

or cold ﬂow under sustained loads.

Polycarbonate materials exhibit high impact strength, are optically clear, ﬂame

resistant, and will perform in higher temperature applications. This material is

used to make windows and other functional and appearance-related components.

These materials absorb water and when under mechanical stress are degraded

by many chemicals and solvents.

Polyetherimide polymer ﬁnds usage as fuse blocks, circuit board insulators,

chip carriers, equipment housings, switch and circuit breaker housings, and like

applications. It exhibits low dielectric losses, functions over a wide temperature

range, has high radiation resistance, and withstands boiling water.

Polyethylene is an inert, abrasion-resistant oleﬁn, with low weight per unit

volume and has very low water absorption. This material is used in molded form

for discrete component enclosures, battery cases, and like applications. In sheet

or ﬁlm form it is used as a dielectric in the manufacture of capacitors. It also is

used as a vapor barrier and as insulation and sheath for wires and cables. When

exposed to ﬂame, oleﬁns will burn.

Polymide plastic materials are used for radomes, antenna housings, ﬁlms,

printer circuit boards, and connectors. They have high radiation resistance, low

coefﬁcient of thermal expansion, high electrical resistance, are wear resistant,

have a low dielectric loss factor and resist high temperatures. They are chemi-

cally resistant but are degraded by hot caustic solutions and highly polar sol-

vents. Hard to mold, polymide shapes are formed by powder metallurgy and

compression-molding techniques.

Polystyrene has a very low dissipation factor of about 0.0001 and about a 2.4

dielectric constant; thus, it is used in foam insulation materials and in radio

frequency (rf) striplines and similar applications. It is degraded by many solvents

and chemicals and by modestly elevated temperatures.

PVC, or polyvinyl chloride, materials are used to build wire sleeves and

jackets, tubing, and insulation due to ﬂexibility and resistance to most chemicals.

They are sensitive to ultraviolet (sunlight) radiation, which causes them to be-

come brittle and shrink. PVC use is restricted to temperatures below about

100

⬚C, and it stiffens at temperatures below 0⬚C.

Thermosets

Thermoset plastic materials are supplied in the form of a resin and hardener,

and the ﬁnal polymerization of the plastic occurs during the ﬁnal forming pro-

cess. This ﬁnal polymerization may be referred to as curing, hardening, vulca-

nizing, or cross-linking. Sometimes heat is required to complete or hasten the

polymerization. The fabricator must be aware that the chemical process of curing

involves the generation of heat by exothermic reaction within the material, and

1248 MATERIALS IN ELECTRONIC PACKAGING

this heat must be shunted away to prevent cracking and other deterioration. The

material also shrinks up to 10% by volume during cure, thus having an impact

on the conﬁguration of the mold. The addition of ﬁllers, pigments, and lubricants

may also be used to control shrinkage.

The allyl polymer family is characterized by good dimensional stability, hav-

ing electrical insulation properties, the ability to withstand elevated temperatures,

and good moisture resistance. A commonly used allyl includes glass-ﬁlled diallyl

phthalate, which is dimensionally stable and is used to make inserts in con-

nectors, electronic parts, housings, and to form insulating standoff posts.

Epoxy materials adhere well and are used as adhesives as well as for potting,

casting, and encapsulation purposes. The properties may be varied from ﬂexible

to rigid. The ﬁnal weight, strength, thermal conduction, and thermal expansion

may be modiﬁed by inclusion of ﬁller materials. Epoxies are used to encapsulate

circuits such as microcircuit chips in multichip modules (MCM), chips-on-board

(COB), and conventional components in cordwood array packaging. Epoxy res-

ins are used with glass fabrics and phenolic laminates to fabricate printed circuit

boards; they also are used for conformal coating, encapsulation, adhesives, and

in forming durable paints and ﬁnishes.

Phenolic materials are used for high-temperature, high mechanical strength

(relative to other plastics), and low-cost applications. Phenolics are used to fab-

ricate knobs, handles, connectors, chip carriers, and in laminate form are used

to build small enclosures. Phenolics are not elastic and may fracture upon bend-

ing or impact loading.

Polyimide materials exhibit low outgassing, good radiation resistance, and

have very good high- and low-temperature performance. They must be used with

care because they are sensitive to moisture and are degraded by organic acids

and alkalis. Applications include bearings, seals, insulators, ﬂexible cable, tapes,

sleeving, moldings, wire enamel, chip carriers, and laminates.

Polyurethane materials are often used as paints, wire enamels, conformal coat-

ings, embedding compounds and foam dielectric and thermal insulation. Some

polyurethane materials may be poured and used as encapsulants to protect cir-

cuits from the environment. They exhibit elastomeric properties and have tear

resistance, abrasion resistance, and unusual toughness. They can be degraded by

strong acids and bases. The operating temperature range is limited and should

not exceed 120

⬚C.

Silicone-based materials have a wide range of general-purpose applications

in electronic equipment. They may be formulated as sealants, elastomers, ad-

hesives, paints, conformal coatings, and encapsulants. They have a broad tem-

perature range (

⫺40 to ⫹250⬚C), have excellent arc resistance, are nonburning,

and are water resistant when fully cured.

A commonly used silicone family is known as the room temperature vulca-

nizing (RTV) adhesives and sealants, which are often used to waterproof a com-

ponent or electrical joint. These products polymerize with water when curing

and in the process release acetic acid in some formulations or ethyl alcohol in

other formulations. In applications where the presence of acetic acid vapor would

degrade circuit elements such as switch and relay contacts, the RTV should be

allowed to fully cure before sealing RTV within an electronic enclosure. Alter-