IPC-D-859.pdf - 第24页

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 ﬂex…

where R = thermal resistance of the material layer

(°C/W)

t = thickness of the material layer (mm)

K = thermal conductivity of the material

in (W/°C–mm)

A = cross-sectional area of the material

layer (mm

)

Table 3–3 gives the thermal conductivity of typical materi-

als used in hybrids.

Heat spreading occurs when heat ﬂows into a material

layer of increased cross-sectional area. As illustrated in

Figure 3–14 it is adequate to assume that spreading occurs

at a 45 degree angle.

To calculate thermal resistance when heat spreading

occurs, the average cross-sectional area should be used

over the range of spreading. As shown in Figure 3–13 the

total thermal resistance (R

) will be:

= R

1/2t

Notes: 1. Use average area A over thickness 1/2t

. 2. Use area A

over thickness t

. 3. No spreading occurs over t

To the maximum extent possible, devices dissipating sig-

niﬁcant amounts of power should be spaced in the hybrid

in such a manner as to spread the heat ﬂow throughout the

case. When devices are close enough that their areas of

heat spreading overlap, based on the 45 degree assumption,

their thermal interaction should be taken into account. This

can be done by limiting the cross-sectional area used in the

thermal-resistance calculations to areas that do not overlap

the heat spreading of other devices.

When device power dissipation becomes signiﬁcant, steps

may be taken to minimize the temperature rise. Devices

may be eutectically mounted directly to the substrate or to

ceramic, molybdenum, or kovar ‘‘tabs’’ to increase heat

spreading and to reduce thermal resistance. In extreme

cases, but at considerable cost, devices can be mounted on

Beryllia substrates or metal heat-sinks.

Note that in the above discussion the temperature rise is

related to the hybrid case temperature, not to the ambient

air or to the hybrid mounting surface. The hybrid case tem-

perature will in turn be a function of the ambient air and

mounting surface temperature, the thermal properties of the

mounting surface, and the hybrid to mounting surface inter-

face.

3.5.5 Thermal Matching A primary thermal concern

with ceramic surface-mounted components is the thermal

expansion mismatch between the component and the sub-

strate. This mismatch may result in fractured solder joint

interconnections if the assembly is subjected to thermal

shock, thermal cycling, power cycling and high operating

temperatures. The number of fatigue cycles before solder

joint failure is dependent on, but not limited to, the thermal

expansion mismatch between the component and the sub-

strate, the delta temperature excursion over which the

assembly must operate, the solder joint size, the size of the

component, and the power cycling that may cause an unde-

sirable thermal expansion mismatch if a signiﬁcant tem-

perature different exists between the component and the

substrate.

With surface-mount components, the number of fatigue

cycles can be increased by reducing the thermal expansion

mismatch, reducing the temperature gradient, increasing

the height of the solder joint, using the smallest physical

size component wherever possible, and by optimizing the

thermal path between the component and the substrate.

3.6 Mechanical Requirement Considerations

3.6.1 Camber

All multilayer hybrids will warp to some

degree unless rigidly supported along all edges. The con-

struction of the board should be as symmetrical as possible

with respect to conductor area distribution and to a lesser

degree, the amount of dielectric applied in which there

exists a Coefficient of Thermal Expansion (CTE) mismatch

between the dielectric and the supporting substrate. Warp-

ing is more likely to occur on the boards having large

width-to-length ratios and in boards having large conductor

areas on one side.

3.6.2 Support All hybrid circuit assemblies should be

supported in order to minimize stress on the components

IPC-859-3-13

Figure 3–13 Example of total thermal resistance

calculation

Mounting

Material

Electrical Device

Substrate

Hybrid

Case

Mounting

Material

Mounting

Material

Electrical 

Device

Mounting

Material

Substrate

Hybrid Case

RRR

IPC-859-3-14

Figure 3–14 Thermal resistance during heat spreading

ELECTRICAL DEVICE

Heat

45°

1/2 t

December 1989 IPC-D-859

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