IPC-D-859.pdf - 第25页

F . Dif ferences in thermal coef f icients of expansion G. V ariations in electrical characteristics of screened com- ponents 3.8.1 Substrates The substrate serves as the supporting structure for the circuitry . It acts …

and electrical connections. This support should also be suf-

ﬁcient to help prevent cracking or loosening of conductors,

breaking of part leads or wire bonds, or cracking or chip-

ping of the substrate resulting from ﬂexing stress on the

assembly.

3.6.3 Shock and Vibration The ultimate ability of com-

ponents to survive shock and vibration environments will

depend upon the degree of consideration given to the fol-

lowing factors

A. The worst-case levels of shock and vibration environ-

ment for the entire structure in which the hybrid circuit

assembly resides, and the ultimate level of this environ-

ment that is actually transmitted to the components.

(Particular attention should be given to equipment that

will be subjected to random vibration.)

B. The method of mounting the assembly in the equipment

to reduce the effects of the shock and vibration environ-

ment, speciﬁcally the number of mounting supports,

their interval, and their complexity.

C. The attention given to the mechanical design of the

assembly, speciﬁcally its size, shape, type of material,

material thickness, and the degree of resistance to bow-

ing and ﬂexing that the design provides.

D. The shape, mass and location of the components

mounted on the assembly.

E. The component lead wire stress relief design as pro-

vided by its package, lead spacing, lead bending, or a

combination of these, plus the addition of restraining

devices.

F. The attention paid to workmanship during assembly, so

as to insure that component leads are properly bent, not

nicked, and that the components are installed in a man-

ner that tends to minimize component movements.

G. Conformal coating may also be used to reduce the

effect of shock and vibration on the assembly.

The selection of components to be mounted on assemblies

subjected to severe shock and vibration should favor,

where circuit design permits, the use of components that

have lightweight, low-proﬁle packages and that have inher-

ent strain relief provisions. Where discrete components

must be used, preference should be given to axially-leaded

types that present a relatively-low proﬁle, and that can be

mounted and easily clamped in intimate contact with the

board surface.

The use of irregularly-shaped components, especially those

having a large mass and a high center of gravity, should be

avoided where practical. If their use cannot be avoided,

they should be located toward the outer perimeter of the

assembly where the guides and/or connectors can help to

provide resistance to ﬂexing. Depending on the severity of

this problem, the use of mechanical clamping, adhesive

bonding, or embedding may be required.

3.7 Test Requirement Considerations The circuit lay-

out conﬁguration, uniformity of the hybrid circuit size,

shape and functional connector interface will have an effect

on the manufacturability, testability, and cost of the ﬁn-

ished assembly. These aspects must be considered early in

the design stage of product development. Some important

test considerations are speciﬁed in the following para-

graphs.

3.7.1 Hybrid Circuit Layout Design The design layout

from one design to another should be such that designated

areas are identiﬁed by function, i.e., power supply section

conﬁned to one area, analog circuits to another section, and

logic circuits to another, etc. This will help to minimize

cross-talk, simplify test ﬁxture design, and facilitate

trouble-shooting diagnostics. In addition, the design

should:

• Have circuit elements placed away from the assembly

edges to allow adequate test ﬁxture clearance.

• Have design grid layout compatible with testing plan.

• Allow provision for isolating parts of the circuit to

facilitate testing and diagnostics.

• Where practical, group test points and jumper points in

the same physical location on the circuit.

• Consider Large Scale Integrated (LSI) circuit socketing

for components so that parts can be easily replaced for

further testing.

• Consider surface mounted components and their patterns

require special consideration for test probe access.

• Allow adequate space between devices for rework

access.

• Orient devices in same direction if possible for ease of

trouble- shooting and rework.

3.8 Materials The effect that one material may have

upon another material within a hybrid is of extreme con-

cern. Therefore, contemplated material substitutions should

be reviewed to determine whether the proposed design is

susceptible to failure mechanisms.

Nonconforming materials can result in failure of the hybrid

circuit caused by one or more of the following conditions:

A. Metal migration

B. Corrosion or oxidation

C. Ionic contamination

D. Improper adhesion

E. Intermetallic formation

IPC-D-859 December 1989

F. Differences in thermal coefficients of expansion

G. Variations in electrical characteristics of screened com-

ponents

3.8.1 Substrates The substrate serves as the supporting

structure for the circuitry. It acts as a surface for depositing

the conductive, dielectric, and resistive materials that form

the passive circuit elements. Also, it is a base for mechani-

cal support of all active and passive chip components. It

must be an electrical insulator to isolate the various con-

ductive paths of the circuit, and it must have sufficient

thermal conductivity to remove heat generated by the cir-

cuit components. See Table 3–4 and 3–5 for substrate

selection criteria.

The material most suitable for the particular application

should be determined by examining the following proper-

ties of substrate materials:

A. Thermal conductivity—The ability of the material to

conduct heat away from critical circuit components.

Normally measured as cal–cm/sec–cm

–°C. High ther-

mal conductivity is desired.

B. Electrical insulation—The ability of the material to

insulate various circuit components from one another.

Normally measured in ohm/cm. High resistivity is

desired.

C. Mechanical strength—The ability to withstand

mechanical shock. Young’s modulus, ﬂexular strength,

tensile strength, and compressive strength are consid-

ered measures of mechanical strength. High strength is

desired.

D. Refractory property—The ability of the material to

withstand high temperatures. Melting point is an indi-

cator of this property. This property is usually desir-

able, and absolutely mandatory for processes requiring

heat treatment.

E. Thermal expansion—This property is an important

consideration, especially when the structure is placed

into a metal case or mounted to an organic interconnec-

tion substrate.

F. Chemical susceptibility—The ability to withstand

exposure to chemicals. It is very desirable for sub-

strates to be inert to processing chemicals.

G. Weight—Weight may or may not be a consideration

based on the application.

H. Metallizability—The ability of the material to be suc-

cessfully metallized.

I. Cost—There are wide variations in the cost of substrate

materials.

Table 3–4 Nonphysical Substrate Selection Criteria

Material

Cost per 25 x 25 mm [1 x 1-inch] Substrate for

Applications Remarks100 5000 100K

Alumina Medium Medium Very low General Use Very popular

Beryllia High Medium Low Heat dissipating Low thermal resistance

Porcelainized

steel

Medium Low Low Special shapes Becomes cost effective

for large substrates

Coﬁred multilayer

ceramic

Very high High Medium Package May be cost effective

in large quantities

Table 3–5 Important Characteristics of Substrates

Material

Tensile

Modulus

MPa

Coefficient

of Thermal

Expansion

(ppm/°C)

Tensile

Strength

MPa

Thermal

Conductivity

(W/°C—mm)

Dielectric

Constant

(1 MHz)

Dissipation

Factor

Alumina (90%) 324.07 6.70 317.17 16.74 9.40 *

Alumina (96%) 324.07 7.11 317.17 25.10 9.90 0.0001–0.0002

Alumina (99.6%) 344.75 6.30 448.18 37.40 10.00 0.0001–0.0002

Aluminum

Nitride

291.66 4.30 367.50 170.00 8.80 *

Beryllia 319.93 8.00 * 250.00 7.00 0.0001

Quartz 72.40 0.72 * 0.75 3.80 0.0002

Sapphire 344.75 7.74 0.00 25.00 0.00 0.002

Porcelainized

steel

* 4.40 89.64 1.67 5.50 *

* Indicates data not available

December 1989 IPC-D-859

3.8.1.1 Alumina Substrates Alumina substrates are

made of polycrystalline A1

with small amounts of

metal oxide glasses to achieve certain physical properties.

Alumina is the most popular substrate material since it is

readily available in sizes ranging from tiny chips to large

area substrates, in thicknesses ranging from 0.25 mm

[0.010 inch] to 1.25 mm [0.050 inch] or greater, and in a

variety of shapes and designs. It is refractory and the ﬁn-

ished substrate can be drilled or cut with diamond tools or

lasers.

3.8.1.2 Beryllia Substrates Beryllia (polycrystalline

BeO), with impurities for physical properties, is primarily

used in applications requiring rapid heat removal from the

circuit. Like alumina, Beryllia substrates are available in a

large variety of sizes, thicknesses, shapes and designs.

Beryllia is less-widely used than alumina due to its toxic

nature in the powder form and its higher cost.

3.8.1.3 Coated Metal-Core Relatively new as a sub-

strate, porcelainized steel substrates provide strength,

shock resistance, a built-in ground plane, and relatively low

cost for larger sized substrates. Prior to coating, the metal

can be inexpensively shaped and punched to allow it to be

used as a mounting bracket.

As a combination hybrid and discrete component mounting

surface, porcelainized steel substrates can withstand such

harsh environments as automobile engine compartments.

Processing of these substrates requires low temperature ﬁr-

ing and pastes compatible with that condition.

3.8.1.4 Quartz Substrates Quartz is available in two

forms: (1) fused quartz or silica, and (2) single-crystal

quartz.

A. Fused Quartz—Fused quartz is similar to glass and is

used in microwave applications because of its consis-

tent dielectric constant and low RMS surface ﬁnish. As

with glass, it is a poor thermal conductor and cannot be

used in high heat dissipation applications. It is very

fragile and extreme care must be exercised when it is

handled. Its cost is moderate and the material is readily

available from many sources.

B. Single-crystal quartz—Single-crystal quartz is used in

applications where an ordered crystal structure is nec-

essary, e.g., in acoustic wave devices. The tensile

strength of quartz varies according to the axis in which

the crystal has been cut. The material is brittle and must

be handled carefully. Its cost is generally considered to

be exorbitant and, as with most crystals, its thermal

conductivity is poor. However, single-crystal quartz

does offer a very low RMS surface ﬁnish and it exhib-

its little or no camber.

3.8.1.5 Sapphire Substrates Sapphire is used in micro-

wave applications as a replacement for alumina substrates

when a superior surface ﬁnish and low camber are

required. Sapphire is an expensive single-crystal material

(Al

) and is limited in size to the diameter of the boule

from which it is cut.

The material is relatively strong but is a poor conductor of

heat. The dielectric constant, as well as other mechanical

and physical properties, will vary depending upon the crys-

tal orientation. Therefore, care must be exercised to specify

the proper crystal orientation.

3.8.1.6 Ferrite Substrates Ferrites are typically used in

microwave applications where a magnetic material is

required. Circulators are one example of this application.

The material maintains a uniformity of substrate properties,

e.g., dielectric constant and dissipation factor. Ferrites can

be obtained with good surface ﬁnish qualities and low cam-

ber. However, ferrites are brittle and highly susceptible to

impact shock breakage.

3.8.2 Conductive Materials The conductor, analogous to

the wire in older technologies, is fabricated from materials

of very low resistivity that is used to advance the signal

(information to be processed or used) to the desired loca-

tion with minimal losses.

3.8.2.1 Thick-Film Metallizations Thick-ﬁlm conductors

are typically silver, gold and alloys of these materials with

either platinum or palladium. Glass frits and other additives

are used which will affect the electrical characteristic of

these materials. Copper-and nickel-based thick-ﬁlm pastes

are emerging as a new low-cost thick ﬁlm conductor.

(These materials are, however, in the development stage.)

3.8.2.1.1 Sheet Resistivity and TCR Table 3–6 lists

nominal sheet resistivities and temperature coefficient of

resistance (TCR’s) of various thick-ﬁlm conductor metalli-

zations.

3.8.2.1.2 Thick-Film Conductors

A. Gold (Au) conductors are used where high conductiv-

ity combined with long-term reliability are required.

B. Platinum gold (PtAu) conductors are used where sol-

dering is the only viable means of component attach-

ment.

C. Palladium gold (PdAu) is limited to use in those areas

discussed under PtAu and where cost is to be kept as

low as possible. PdAu is a relatively low cost alterna-

tive to PtAu.

D. Silver (Ag) is used primarily in commercial applica-

tions where low cost is essential. Typical applications

are in microcircuit displays and polymeric composites,

adhesives, and sealants.

IPC-D-859 December 1989