Kutz M. Handbook of materials selection

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3 DOMINANT CONSIDERATIONS 1229

The primary guide to the effectiveness of a material as a electrostatic shield

is the electrical conductivity of the material.

3.11 Magnetic Shielding

Magnetic shielding pertains to protecting a circuit from the presence of ﬂuctu-

ating magnetic ﬁeld energy that may induce currents in wires and metal struc-

tural elements and lead to circuit malfunction or failure. A simple Faraday shield

such as used for electromagnetic wave or static discharge protection will not

sufﬁce for protection from magnetic ﬁelds. It is usually necessary to restrict the

shielding materials to high-nickel alloy iron. Iron-bearing alloys are magnetic

and block penetration of magnetic ﬂux into an equipment, which may induce

noise within a circuit or which may cause malfunction of a circuit including

erasure of memory from several classes of memory storage devices such as

integrated circuits.

3.12 Fatigue Resistance

Fatigue is the loss of physical strength and eventual failure of a material as a

result of the material being subjected to repeated loads. Such loads may be the

result of thermal expansion stresses that occur during temperature cycles, me-

chanical loads induced during the operation of the device, shipping and handling

stresses, exposure to vibration, and repeated mechanical impact shock loads.

Repetitive loads are know to cause damage to accumulate until the material

fails.

Fatigue resistance may be judged on a relative basis by referring to tables

of fatigue resistance, which relate stress levels to time-to-failure for a speciﬁc

material or alloy under ideal conditions such as rotating beam test data.

The actual ability of a component to withstand fatigue damage is highly

sensitive to the loading history, the precise shape of the component (i.e., stress

risers), equipment construction details, and variations in temperature. As a result,

proof testing is usually required to verify the ability of the selected material and

design details to avoid fatigue damage.

3.13 Hardness

Hardness is taken as the ability of a material to resist denting as caused by a

load bearing on the material using a dimensionally standardized probe. There

are different scales by which hardness is measured, the most common are the

Brinell hardness and the Rockwell hardness number for a given Rockwell scale.

Unless otherwise speciﬁed the hardness in a chart refers to the intrinsic hardness

of the base material.

Hard surface materials are used to reduce wear in mechanical latches, sliding

surfaces, and electrical contacts. The lubricity of polymers such as nylon and

Teﬂon is used to reduce friction in applications such as drawer slides where a

plastic material is used to rub against a hard metal surface.

The hardness of a material is often established by the surface treatment of

the material and is not always a bulk property of the base material. That is, steel

and aluminum and many other materials may be chemically or physically surface

hardened to improve the wear of the part under design. The ability of a material

to be surface hardened varies widely among materials, with many materials

lacking the ability to be surface hardened.

1230 MATERIALS IN ELECTRONIC PACKAGING

3.14 Ductility

The ductility of a material refers to the ability of the material to be deformed

without breaking. Extreme examples would include tin (very ductile) and glass

(brittle and thus not ductile). This is an important property for materials that are

to be mechanically formed, are subjected to bending stresses during use, and

must withstand impact shock loading without cracking or shattering.

A guide to ductility of a material is its elongation, which refers to the ability

of the material to deform under load before failing. Elongation is seen to vary

with the degree to which a material is heat treated (in the case of copper and

iron alloys). Ductility in plastic materials is strongly affected by temperature.

For example, a plastic gear may work well at room temperature; however, at

low temperatures it may become brittle and fracture under load. Material duc-

tility varies widely among even a given general class of materials and each

individual material must be reviewed for ductility.

Closely related to ductility is the material’s malleability, which is the ability

of the material (usually metals) to be shaped by stamping, spinning, cold head-

ing, upsetting, rolling, or other physical processes involving deformation of the

basic material in solid form. A malleable material is one that may be physically

formed without tearing, cracking, fracturing, or developing surface defects. Ta-

bles of materials properties often provide guidance relative to malleability, and

subcontractors who provide metal shaping services are a good source of infor-

mation on malleability.

3.15 Wear Resistance

Wear resistance is a property of materials that does not often appear in tables

of materials properties. A guide to wear is hardness—the harder and smoother

the material the greater it’s resistance to wear. Also consider ductility for the

reason that a material may become brittle at low temperatures and exhibit surface

fractures and fretting under load. Wear life is also a function of the magnitude

of load-induced localized compressive stresses and the methods used to reduce

friction such as lubrication. Some nonmetallic materials, such as nylon, are self-

lubricating and useful as bearing materials.

3.16 Sublimation

Sublimation is the loss of material when the material is exposed to low pressures

(vacuum) and or elevated temperatures. It involves the conversion from solid

form immediately to gaseous form apparently without passing through a liquid

phase. Sublimation conversion may occur uniformly to the surface of a uniform

and pure material, or if the material is a more complex matrix one or more

constituent materials may sublimate leaving a residual material possessing poor

mechanical or electrical properties.

Sublimation may also result in the redeposition of material onto another sur-

face at a lower temperature where the presence of the sublimated material is not

desired (i.e., perhaps forming a conductive ﬁlm causing circuit elements to short

or reducing visibility through a window). Information on sublimation is limited

and may require inquiry to the manufacturer of a material or to experimental

test data.

3 DOMINANT CONSIDERATIONS 1231

Sublimation is of critical importance when the electronic packaging engineer

selects materials that will be subjected to low-pressure and vacuum environments

such as on electronic equipment for use in space vehicles.

3.17 Combustibility

Combustibility is the property of a material, under a given set of conditions, to

rapidly oxidize or burn, which may range from the ability to burn but not sustain

ﬂame when the ignition source is removed, or to burn and sustain ﬂame after

the ignition source is removed. Combustion may lead to the generation of nox-

ious gasses and destructive deterioration of the circuit elements in the vicinity

of the combustion.

This property is most often associated with plastic and organic materials;

however, some metals will burn in the presence of oxidizing gasses or chemicals.

Many tables of plastic material properties do not include combustibility infor-

mation; however, this information is usually available from the manufacturer of

the material. If combustibility is to be prevented in a design, it is sometimes

necessary to conduct tests to determine if a potential material is suitable for the

intended application.

3.18 Creep

Creep is the ongoing deformation (ﬂow) of a material under mechanical load.

Failure due to creep is often very slow, and it may occur only during certain

conditions (such as a solder-sealed case that is sealed at sea level but operated

in an aircraft, or the same case operating at seal level but within which the

internal pressure is increased due to heating during normal operation of the

equipment). It is good design practice to never place such a soldered joint in

sustained mechanical stress for the reason that the solder will creep until the

stress is relieved or the joint fails. Thus the design of a mechanical joint where

solder is employed (e.g., to achieve an enduring electrical connection or to seal

a joint) must have the mechanical load taken by structural members without the

solder present. Mechanical joints or seals that utilize adhesives, elastomers, and

other nonmetallic materials must be treated in the same way to avoid eventual

failure.

A creep failure may not become evident for days, weeks, or even years; but

it will happen if materials with the known ability to creep are improperly in-

corporated into a design.

Creep is not only a characteristic of some metals, it also is present in several

inorganic or organic materials as well. An example would the loosening of a

mechanical joint where the joint consists of metallic elements bolted together

and includes a nylon washer. This joint eventually loosens when the nylon

washer creeps to relieve the initial clamping stresses. Another example would

be when a compressible gasket, made of a material that will creep, is used in a

ﬂanged joint where the joint does not have mechanical stops to control the

separation of the ﬂanges when the bolts are tightened. In this case the joint will

eventually suffer loosening and possible leakage when the gasket creeps to re-

lieve the joint compressive forces. A classic example of a creep-induced failure

occurs when an insulated wire presses against a hard metallic object such as the

1232 MATERIALS IN ELECTRONIC PACKAGING

edge of a bracket or panel. In this case, the insulation will creep until the inner

conductor of the wire eventually makes contact (shorts to) the metal object.

3.19 Moisture Absorption

In many instances when a material is selected for use as an insulator, or the

material must have dimensional stability in humid environments, the moisture

absorption characteristics of the material becomes important. For metals, mois-

ture absorption is not a signiﬁcant consideration; however, the presence of mois-

ture will hasten galvanic corrosion when different metals are in intimate contact.

For nonmetals, porous sintered metal structures, and ceramics, moisture ab-

sorption is a signiﬁcant consideration because for an electrical insulator, the

insulation capability is reduced due to the absorption of moisture, and the in-

sulator may become electrically conductive to various degrees when moist. Hy-

drophobic (water-resisting materials, the surface tension of which causes water

to bead and not wet the surface of the material) materials are preferred for

insulator applications.

Most nonmetallic materials are tested for moisture absorption, and the results

of this data are found in many listings of material properties. The majority of

moisture absorption data relates the absorption of water over a speciﬁed period

of time.

In some cases, particularly for organic materials, moisture absorption leads

to a loss of strength and physical deterioration. If some materials become wet

and are subsequently allowed to dry, they may exhibit permanent shrinkage and

the inability to redevelop their original physical characteristics.

Should a material absorb moisture and be subjected to freezing temperatures,

the material may be permanently damaged due to expansion that occurs upon

freezing.

The chemical absorption properties of seals, paints, tubing, gasket materials,

and plastic materials expected to be used in applications involving chemicals

(oil, gasoline, fuels, alcohol, cleaning solutions, etc.) most likely have been in-

vestigated, and the materials supplier is usually able to provide test data and

guidance in the selection of an appropriate material.

Also, in conditions of high humidity and moisture in the presence of inorganic

salts, conditions are created that encourage the growth of fungus. Fungus growth

can lead to material degradation in appearance and loss of strength and the

development of undesirable electrical shorts. Materials are generally categorized

as fungus nutrient or fungus resistant. Materials such as metals, glass, ceramics,

mica, silicone resins, asbestos, acrylics, diallyl phthalate, and a limited list of

plastic resins are fungus resistant. Materials containing organic constituents,

rubber, cellulose acetates, epoxy resins, many lubricants, medium- and low-

density polyethylene, polyvinyl compounds, formaldehyde compounds, and oth-

ers are fungus nutrient. The above listing is far from complete, and the designer

is cautioned to verify the fungus nutrient characteristics of any materials used

in a design.

4 OVERRIDING CONSIDERATIONS

It is not unusual for an electronics packaging engineer to ﬁnd that there are

overriding considerations that impact the materials selection decision. Often

5 TYPICAL APPLICATIONS 1233

there is little if any opportunity to change the overriding considerations, thus the

materials selection must be made in a manner to best achieve the dominant

technical performance requirements within a framework of overriding consid-

erations.

Overriding considerations may include:

●

Customer preference for a given shape, material, or ﬁnish

●

Ergonometric (human factors) considerations

●

Product is part of a family of products that must have identical materials

●

Product manufacturing technique limited to available factory machinery

●

Industrial design constraints dictated by the application

●

Part to be designed must interface with a component or material for which

there is no alternate

●

Service temperature range

●

Shock or vibration environment

●

Physical abuse in service

●

Exposure to chemicals

●

Mechanical strength

●

Exposure to electromagnetic, electrostatic, ionizing, ultraviolet, or infrared

radiation

●

Corrosive environments

●

Availability of the material

5 TYPICAL APPLICATIONS

An important guide to materials selection is found by considering how materials

have been used for like or near-like applications. Electronic packaging appli-

cations often fall into one or more of the following applications:

●

Equipment attachment

●

Equipment racks, frames, and mounting structures

●

Equipment and module enclosures

●

Temperature control

●

Mechanical joints

●

Finishes

●

Position-sensitive assemblies

●

Electrical contacts

●

Harsh environment endurance

5.1 Equipment Attachment

Other than desktop and portable equipment, electronic equipment is usually me-

chanically fastened to an equipment rack, frame, shelf, or mounting structure.

These devices may sit on the ﬂoor in an equipment room or are bolted [using

high-strength Society of Automotive Engineers (SAE) rated fasteners) or welded

to the structure of the containing building or vehicle. The attachment may consist

1234 MATERIALS IN ELECTRONIC PACKAGING

of, for relay rack-mounted devices, passing two or more bolts through holes in

the equipment front panel into threaded fasteners in the rack frame. For equip-

ment used in aircraft, ships, and vehicles, ARINC standard modular ATR cases

are placed on an equipment self and secured thereto. The ATR module secure-

ment usually consists of an engagement device in the rear of the module and

brackets with retained thumbscrews or latching hook arrangements at the lower

edge at the front of the module, including a device fastened to the shelf into

which the screws or latches engage. Larger equipment used in moving vehicles,

such as tanks, battleships, and trucks, where the equipment must be protected

from higher levels of vibration and shock loading, often involves bolting the

equipment to a frame, which incorporates energy-absorbing dampers, and bolting

the frame to the structure of the vehicle. For consumer goods applications (car

radios and CD players, motorcycle and bicycle electronics, etc.), a variety of

mounting conﬁgurations are employed; some feature quick release of the at-

tached equipment.

Each of the applications place similar requirements on the materials used for

attachment. The dominant considerations are strength, wear resistance, corrosion

avoidance, temperature range, and fatigue resistance. The materials are usually

selected from the steel family to achieve a favorable strength-to-weight ratio.

For mounting bolts used to fasten commercial goods and other equipment used

in relatively benign environments, cadmium- or nickel-plated steel hardware is

used. If the mechanical loads are severe, higher strength SAE-grade steels must

be used. For more corrosive environments, stainless steel (usually passivated) is

employed. For aggressive high-stress applications where corrosion and wear are

a concern, fastening devices and brackets are made from the precipitation-

hardened stainless steels that are passivated.

5.2 Equipment Racks, Frames, and Mounting Structures

Equipment racks, frames, and mounting structures such as an equipment shelf

are designed to bear the weight of electronic equipment mounted thereto. The

dominant considerations are strength, wear resistance, corrosion, electrical con-

ductivity, and corrosion avoidance. The materials of preference include heat-

treated high-strength aluminum (such as 6061-T6) and mild steel materials. The

aluminum materials are almost always either anodized or treated (chromate dip

or similar) and painted for appearance and protection from the elements. Steel

materials are protected from rust by nickel, zinc, or other electroplated ﬁnish

and often by a rust-resistant paint. Stainless steel parts are passivated to improve

resistance to corrosion.

5.3 Equipment and Module Enclosures

Enclosures are the cases or boxes that house electronic equipment. Dominant

considerations include operating temperature range, electromagnetic and electro-

static shielding, magnetic shielding, strength, wear resistance, thermal conduc-

tivity, appearance, corrosion resistance, and perhaps chemical inertness.

Materials used for enclosures vary with the application and end-user market.

They include both metal and plastic materials. Cost containment, ease of fabri-

cation, and appearance are often the overriding considerations.

When electromagnetic, electrostatic, and magnetic shielding protection is not

required, plastics materials are often used to fabricate enclosures. Aluminum and

5 TYPICAL APPLICATIONS 1235

steel materials may be utilized to achieve electrostatic and magnetic shielding

protection. If magnetic shielding protection is required, iron-based materials are

required, and the material of choice is usually steel. Metal cases are almost

always protected against corrosion and other environmental damage by a ﬁnish

treatment, sometimes followed by paint to achieve a suitable appearance.

Plastics materials may be used for cases for electromagnetic and electrostatic

protecting enclosures if they are coated internally with a conductive paint that

may bear either silver or nickel powder. Enclosures fabricated from plastic usu-

ally do not require application of paint, chemical treatment, or other surface

protection. Another dominant consideration for plastic cases is combustibility

for the reason that many plastics will burn and emit noxious or toxic fumes if

exposed to ﬂame or high temperatures.

5.4 Temperature Control

Temperature control is directed at maintaining electronic component tempera-

tures within prescribed limits. This is accomplished by the proper selection and

application of materials used to conduct heat from or to the components.

The primary modes of heat transfer are conduction, convection, and radiation.

The dominant considerations for heat transfer material selection include thermal

conductivity, chemical inertness, resistance to corrosion, and sometimes wear

resistance, sublimation, thermal expansion, and the ability to resist ionizing and

ultraviolet radiation.

Conduction heat transfer involves the ﬂow of thermal energy through a ma-

terial from a heat source (component) to a heat sink. The design goal is to reduce

the resistance to heat ﬂow between the heat source and the heat sink. The ef-

fectiveness of heat transfer within a material is measured by its thermal con-

ductivity. Metals such as silver, copper, and aluminum are good heat conductors,

however, aluminum is most often used because it tends not to tarnish, is easy

to form, and costs less. Copper or copper alloys are used when large amounts

of heat are to be transferred and any small improvement in thermal conductivity

is meaningful in the design.

When the design requires electrical insulation and good thermal conduction

across a joint between two materials, thin layers of mica, silicon-impregnated

cloth, or ceramic materials are often used. These joints may be enhanced by

applying a thermal grease to the joining parts to prevent the entrapment of air

or other gas (a thermal insulator) between the mating surfaces.

Convection heat transfer involves the movement of heat between one material

(usually metal) and a surrounding material such as a gas (air) or a liquid (water,

freon, etc.). The usual design seeks to conduct heat from a heat source into a

physical conﬁguration, such as ﬁns, which present a relatively large area of

contact between the metal object and the gas or liquid with which the heat is to

be exchanged. Good thermal conductivity is very important, however, so is

strength and resistance to corrosion. Convection heat sinks are often made from

the high-strength aluminum alloys to meet these requirements. For resistance to

corrosion, which may result in the formation of a thermally insulating ﬁlm be-

tween the heat ﬁn surface and the heat transfer media, the aluminum heat sink

is sulfuric acid anodized black in color to also enhance heat transfer by radiation.

Painted surfaces are avoided because paint acts as an insulator and reduces the

efﬁciency of the heat transfer surface.

1236 MATERIALS IN ELECTRONIC PACKAGING

Radiation heat transfer is the primary mode of heat transfer when considering

space applications. Radiation at sea level can contribute 10% or more of the

total heat transfer process depending on the temperature gradient and distance

between the radiating and receiving objects. Radiation heat transfer applications

feature use of thermally conductive materials to transfer heat to radiating sur-

faces. The effectiveness of the radiation surfaces is dependent on the ﬁnish

applied to those surfaces. For materials that are to reduce the reception or reduce

the ability to emit heat, the surfaces are smooth and shiny. Such surfaces are

often electroplated with a material such as gold, chromium, or nickel, which are

chemically inert and may be buffed to a very high gloss. For materials for which

the design requires effective heat absorption or emission, the material surface is

not smooth (thus to increase surface area) and is usually black or another dark

color. This is achieved on aluminum surfaces by application of a black sulfuric

anodize ﬁnish such as used on the internal elements of solar collector devices.

Paint on a surface adds a layer of insulation and thus is an impediment to

radiation heat transfer. Due to the surface roughness of painted surfaces, the

color of the paint usually makes only small differences in surface radiation

effectiveness; however, a light-colored (white) paint will be somewhat less ef-

ﬁcient at radiation heat transfer than a dark (black) paint under extreme thermal

loads.

Plastic and ceramic materials are not effective radiation heat transfer materials

and may be employed as surface insulators to reduce the effectiveness of radi-

ation heat transfer.

5.5 Mechanical Joints

Mechanical joints are structural attachments within an electronic rack, enclosure,

case, shelf, or housing and are materials sensitive. Mechanical joints may be

either permanent or semipermanent. A permanent mechanical joint could be

brazed, welded, adhesively bonded, chemically joined, riveted, or mechanically

upset. Semipermanent mechanical joints involve the application of mechanical

fasteners and permit the mechanical joint to be attached and detached during

service.

Mechanical joints may involve two or more metal elements, two or more

nonmetallic elements, or a mixture of both metallic and nonmetallic elements.

For metallic-only fused joints the metals must be suitable to the joining

method and carefully selected ﬁller materials achieve the joint, such as welding

rod alloy materials or brazing materials. The dominant considerations include

temperature range (will the material properties be altered by the temperatures

required by the fusing process?), relative thermal expansion to avoid residual

stresses, and avoidance of materials that in combination will lead to galvanic

corrosion. If the metals gain their strength properties by being heat treated after

forming, the joint materials may require annealing prior to fusing and heat treat-

ing after fusing.

Soldering metal parts together using conventional electronic lead–tin solders

is not a valid method by which to form a mechanical joint. Solder will creep

under mechanical load and if the mechanical load is continuously applied, the

soldered joint will eventually creep to failure. Solder is a valid method for elec-

trically bonding and environmentally sealing mechanical joints, such as riveted

5 TYPICAL APPLICATIONS 1237

joints, which are designed to carry mechanical stress without the presence of

the solder and where the solder is used for bonding or sealing and is not used

to carry mechanical loads.

Nonmetallic (plastic) material joints may be formed by thermal bonding or

ultrasonically welding identical formulations together (thermoplastics only) or

by chemical fusing agents as recommended by the plastics manufacturer.

Mechanical joints involving a variety of materials (i.e., both metals and plas-

tics) require careful materials selection, and the dominant characteristics of in-

terest include thermal expansion, corrosion, creep, temperature range, and

electrical conductivity.

5.6 Finishes

The majority of metallic materials and many plastic materials require the appli-

cation of a surface ﬁnish to enhance resistance to corrosion and to provide an

attractive appearance. Dominant considerations regarding application of material

ﬁnishes include electrical conductivity, thermal conductivity, thermal expansion,

chemical inertness, temperature range, wear resistance, sublimation, and mois-

ture absorption.

Aluminum materials used inside of an equipment are usually chemically

cleaned and chromate conversion coated. Chromate conversion coating is also

used as a surface treatment that promotes improved adhesion of paint to the

aluminum. A chemical-ﬁlm-coated aluminum passes electrical signals and may

have an electrical conductivity greater that the base aluminum. These coatings

are easily scratched and have poor abrasion resistance.

Anodizing of aluminum creates a wear-resistant and sometimes attractive ﬁn-

ish that consists of a thin layer of aluminum oxide. This oxide ﬁlm is an elec-

trical insulator and may cause a problem if the part is to pass electricity from

to another component. Sulfuric acid anodize is used to provide protection from

corrosion, causes very little thickness growth to the base metal, works well for

dyed applications, and is avoided if the design has overlapping surfaces where

the acid may be trapped during the anodize process. Chromic acid anodize is

preferred when small dimensional change is necessary for highly stressed parts

or on assemblies that could entrap processing solution. Hard anodize provides

and extremely hard ﬁnish with excellent abrasion resistance and poor electrical

conductivity. Hard anodize coating is brittle and will crack should the anodized

part be ﬂexed. In addition, hard anodize causes a loss of strength because for

0.04 mm of ﬁlm thickness, 0.02 mm of the underlying material is lost. In ad-

dition, the anodize thickness increases the part dimensions by about one-half the

thickness of the ﬁlm.

Aluminum may be electroplated to achieve a variety of properties, such as

plating with a thin layer of gold to achieve a low thermal emissivity, or selective

plating with tin over electroless copper to permit the solder attachment of an

electrical conductor to the aluminum part. The entire part should be plated for

maximum protection, and voids in the plating may establish conditions for gal-

vanic corrosion.

Steel parts must be protected from oxidizing (rusting) in the presence of even

small amounts of moisture or humidity. Steel is usually protected by plating with

nickel or zinc for fasteners, and with tin when used as an electronic equipment

1238 MATERIALS IN ELECTRONIC PACKAGING

chassis to which electrical connections are required. Nickel and chrome plating

are used on steel parts to improve resistance to wear and to provide an attractive

appearance. Cadmium plating is used extensively to protect steel against cor-

rosion; however, it is limited to use below about 230

⬚C and is known to subli-

mate under vacuum conditions. Other treatments such as black oxide coating

and phosphate coating offer alternative approaches to protection of steel surfaces.

Caution is required when a metal is electroplated with another metal. The

two materials differ in coefﬁcient of thermal expansion, which may promote

delamination of the plating from the base metal when the composite is subjected

to thermal cycles. Also, the further apart the materials are listed on the galvanic

series, the greater the opportunity for galvanic corrosion to occur. This latter

consideration will require the addition of protective coatings (paint, adhesives,

etc.) at any location where the plating is removed and both the plating and the

base material are exposed to the environment. Aggressive mechanical fastening

techniques, such as the use of star-type (shakeproof) lockwashers in a bolted

joint, may cause local failure of the plating and eventually lead to joint failure

(a loss of electrical continuity, mechanical strength, or both) by corrosion.

Stainless steel parts used in electronic applications are passivated to remove

surface contamination and to form a uniform protective oxide ﬁlm that assures

that the material delivers maximum resistance to environmental contaminates.

Other commonly used metals, such as beryllium copper, bronze, and brass

also must be protected and may be treated by processes similar to those used to

protect steel.

Plastic materials may require ﬁnish treatments such as electroplating to

achieve surface electrical conductivity or a metallic ﬁnish for appearance pur-

poses such as applying chrome on a knob or bezel or other like application. Not

all plastics may be painted due to the inability of commonly used paints to

adhere to the plastic surface. The surface texture of thermoplastic parts may be

given an attractive matte ﬁnish by vapor blasting.

5.7 Position-Sensitive Assemblies

Position-sensitive assemblies include devices the operation of which requires an

invariant physical relationship between components over the range of tempera-

tures to which the electronic product will be exposed. One example would be

an antenna coupler or ﬁlter wherein the resonance frequency of the antenna

coupler is determined by the length of protrusion of a cylinder (tuning element)

into the cavity of the coupler. The protrusion distance is varied by the equipment

operator to be proper for each speciﬁc operational frequency and once set must

be maintained within less than a millimeter over a signiﬁcant temperature range.

A second example would be a variable-frequency tank circuit that after being

adjusted must maintain precise values of inductance, capacitance, and resistance

over a range of temperature. A third example would be an electronic structure

featuring panels or structural elements fabricated from different materials but

attached together and must not be allowed to deform when subjected to a change

in temperature.

The dominant consideration for position-sensitive assemblies is the thermal

expansion of the materials involved. Some materials, such as Invar, a nickel–

steel alloy, and structural ceramics have a very low coefﬁcient of thermal ex-

pansion and are used to facilitate the design of critical positioning mechanisms.