IPC-CM-770D-1996 - 第98页

January

1996

IPC-CM-770

and part are assembled automatically prior to placing on

the printed board.

18.7 Handling and Storage

The handling and storage of

mechanical components should be in accordance with the

guidelines of Section 26.

18.8 Soldering

Mechanical components or devices

described in this section are not normally soldered. How-

ever, soldering should be in accordance with

J-STD-001.

General soldering guidelines are discussed in Section 27.

18.9 Cleaning

The cleaning of assemblies utilizing items

such as spacers, spreaders and some insulators must be

performed with great care to insure that the fluxes used are

removed in accordance with the end item equipment class

requirements and have no deleterious effect on the

assembly.

Devices utilizing thermally conductive insulators which

have had silicone grease applied can create cleaning and

conformal coating problems. Cleaning must be performed

using techniques that provide proper cleaning without caus-

ing the silicone grease to spread and contaminate other

areas of the assembly. Improper cleaning may also wash

the silicone grease away. General cleaning guidelines are

discussed in Section 28.

18.1

O

Conformal Coating

See Section 29 conformal

coating. Any special requirements must be considered dur-

ing the design of a particular assembly and included in the

design documentation.

19.0 PACKAGING AND INTERCONNECTING

STRUCTURES

Many different board types can be used to mount inter-

mixed assemblies. As stated earlier boards may be single,

double or multilayered. The material used to manufacture

the printed board, or printed board structure many times

plays a large role in the component mounting techniques

that may be used to place parts on the printed board struc-

ture or board. Rigid printed board design is detailed in

IPC-D-275 for single- and double-sided boards and for

multilayer printed boards.

If

flexible printed boards are

used for mounting components, the design aspects of

single- and double-sided boards are detailed in IPC-D-249.

19.1 Printed Boards

There are three basic types of

printed circuit (printed wiring) boards, all of which can be

manufactured in both “rigid” and flexible materials. They

are listed below in ascending order of interconnection wir-

ing and component density:

Single-Sided-With conductors on only one surface of a

dielectric (insulating) base.

Double-Sided-With conductors on both sides of a

dielectric base that are usually interconnected by plated-

through or otherwise reinforced holes.

Multilayer-Boards with three or more conductor layers

separated by dielectric material and usually intercon-

nected by plated-through interlayer holes.

The least expensive type of board, single sided, is used for

relatively unsophisticated circuitry, and they are applicable

when circuit types and circuit speeds do not demand

unusual electrical characteristics. The more expensive,

double sided type of board is required for more complex,

more dense circuit types requiring interconnecting layers.

The requirements for high speed circuits in computer and

space industries, with a requirement for a still further

increase in package density, has lead to the demand for the

most expensive type of board, multilayer.

All printed boards have an insulating base often referred to

as the dielectric or laminate. Laminate bases for single-

sided, double-sided or multilayer printed boards can be

either “rigid” or “flexible.”

The most common laminate bases for rigid printed boards

are:

Phenolic-resin-impregnated paper (commonly called

paper-phenolic)

Acrylic-polyester-impregnated random glass mat

Epoxy-impregnated paper

Epoxy-impregnated fiberglass cloth (commonly called

glass-epoxy)

19.1.1 Rigid Laminate Boards

“Rigid” laminate materi-

als are selected according to physical, thermal and electri-

cal requirements. The first three materials listed above are

punchable, enabling low-cost hole formation, making them

popular for single-sided board use. However, their dimen-

sional stability is unsuitable for making plated-through-

hole boards. The more expensive glass-epoxy laminates

have good dimensional stability, usually making them the

choice for plated-through- hole, double-sided and multi-

layer boards. Glass-epoxy is not as punchable as the other

laminate types

so

that holes are usually drilled. Other lami-

nate types are available for high temperature, high fre-

quency and other special requirements.

19.1.2 Flexible Laminate Boards

When flexible printed

board types are used for surface mounting or through-the-

board mounting or intermixed assemblies, the component

mounting task may become more difficult, depending on

how the detailed assembly is structured. Usually, stiffeners

or other rigid sections of the flexible board are provided to

insure proper surface for the component mounting. In addi-

tion flexible printed boards usually take many different

shapes and special fixturing may be required in order for

the component assembly equipment to adequately mount

and attach electronic parts.

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IPC-CM-770

Januaty

1996

19.1.3 Metal-Core Boards

Metal core boards or special

printed board constructions are detailed in IPC-D-275.

These types of products, due to their mechanical rigidity,

make the component mounting task somewhat easier. How-

ever, the thermal characteristics of the interconnection

structure require that the soldering or attachment technique

consider the impact that the thermal mass has on the solder

joint solidification.

19.2 Surface Mounting

Surface mounting techniques are

applicable to any type of board or substrate material. How-

ever, when using leadless components, the differing coeffi-

cients of expansion between the component and the sub-

strate must be considered.

Surface mounting is suitable for single sided, double sided

(with or without plated-through-holes), and multilayer

boards. Mounting techniques are independent of board

type.

Techniques which have been developed to minimize the

thermal expansion problem have included the development

of special board types with controlled expansion coeffi-

cients. These include ceramic boards, conventional epoxy-

glass and polyimide- glass boards, and special laminates

such as invar or copper substrate clad with epoxy-glass on

each side. Proper selection of material types and thick-

nesses permits tailoring the thermal expansion coefficients

to any desired value.

Table 19-1 provides a comparison of the advantages and

disadvantages of many of the available printed board

structures.

19.3 Supporting-Plane Printed Board Structures

sup-

porting metallic or non-metallic planes can be used with

conventional printed boards or with customer processing to

enhance printed board properties. Depending on the results

desired, the supporting plane can be electrically functional

or not and can also serve as a structure stiffener, heatsink

and/or CTE constraint.

I

n

I

Circuil

panern

Chip

carrier

I I

Figure 19-1 Printed Board Bonded to Supporting Plane

19.3.1 Printed Board Bonded to Support Plane (Metal or

Non-Metal)

A conventional thin printed board that has

been fabricated and bonded with a rigid adhesive insulation

to a supporting plane such as metal (Figure 9-1) or

graphite-fiber resin composite can create a printed board

structure with controlled thermal expansion in the

X

and

Y

axes, improved rigidity, improved thermal conductivity,

etc. depending on the properties of the supporting plane.

However, the printed board must be thin enough to pre-

clude warping of the assembly or else the board should be

bonded to both sides of the plane (see 19.4.2). The printed

board portion of the printed board structure can be either

unpopulated or completely assembled and tested prior to

being bonded. However, components can only be mounted

to one side of the printed board. Also, the support is not

normally electrically connected to the printed board.

19.3.2 Sequentially-Processed Structures with Metal

Support Plane

High-density, sequentially processed, mul-

tilayer printed board structures are available with organic

dielectrics of specific thickness, ultrafine conductors, and

solid plated vias for layer-to-layer interconnections with

thermal lands for heat transfer, all connected to a low-CTE

metal support heatsink. Thus, this technology combines

laminating materials, chemical processing, photolithogra-

phy, metallurgy, and unique thermal transfer innovations,

such that it is also appropriate for mounting and intercon-

necting bare integrated circuit chips, as shown in Figure

19-2.

The major advantage of this system is that the vias can be

as small as 0.20 mm square and conductor widths can

range from 0.12 to 0.20 mm for high interconnection den-

sity. Thus, most applications can be satisfied with two sig-

nal layers with additional layers for power and ground.

19.3.3 Discrete-Wire Structures with Metal Support

Plane.

Discrete-wire printed board structures have been

developed specifically for use with surface mounted com-

ponents, as shown in Figure 19-3. These structures are usu-

ally built with a low-expansion metal support plane that

also offers good heat dissipation.

The interconnections are made by discrete

0.06

mm diam-

eter insulated copper wires precisely placed on a 0.03 mm

grid by numerically-controlled machines. This geometry

results in a low-profile interconnection pattern with excel-

lent high-speed electrical characteristics and a density nor-

mally associated with thick-film technology.

The wiring is encapsulated in a compliant resin to absorb

local stresses and dampen vibration. Electrical access to the

conductors is by 0.25 mm diameter copper vias. The small

via size can be accommodated in the component-

attachment land, thus eliminating the need for fan-out pat-

terns when using components with terminals on centers

as close as

0.6

mm, and allowing very-high packaging

densities.

19.3.4 Flexible Printed Board with Metal Support

Plane

Another arrangement for a printed board structure

with leadless components involves conventional fine-line

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January

1996

IPC-CM-770

Table

19-1

Packaging and Interconnecting Structure Comparison

ORGANIC BASE SUBSTRATE

Epoxy Fiberglass

Polyimide Fiberglass

Epoxy Aramid Fiber

Polyimide Aramid Fiber

Polyimide Quartz (Fused Silica)

Fiberglass/Aramid Composite

Fiber

Fiberglass/Teflon @Laminates

Flexible Dielectric

Thermoplastic

NON-ORGANIC BASE

Alumina (Ceramic)

SUPPORTING PLANE

Printed Board Bonded to Plane

Support (Metal or Non-metal)

Sequential Processed Board with

Supporting Plane Core

Discrete Wire

CONSTRAINING CORE

Porcelainized Copper Clad Invar

Printed Board Bonded With

Constraining Metal Core

Printed Board Bonded to Low

Expansion Graphite Fiber Core

Compliant Layer Structures

Major Advantages

Substate size, weight,

reworkable, dielectric properties,

conventional board processing.

Same as Epoxy Fiberglass plus

high temperature

Z

axis CTE,

substrate size, weight,

reworkable, dielectric properties.

Same as Epoxy Fiberglass, X-Y

axis CTE, substrate size, lightest

weight, reworkable, dielectric

properties.

Same as Epoxy Aramid Fiber,

Z

axis CTE, substate size, weight,

reworkable, dielectric properties.

Same as Polyimide Aramid Fiber,

Z

axis CTE, substrate size,

weight, reworkable, dielectric

properties.

Same as Polyimide Aramid Fiber,

no surface microcracks,

Z

axis

CTE, substrate size, weight,

reworkable, dielectric properties.

Dielectric constant, high

temperature.

Light weight, minimal concern to

CTE, configuration flexibility.

3-D configurations, low

high-volume cost.

CTE, thermal conductivity,

conventional thick film or thin film

processing, integrated resistors.

Substrate size, reworkability,

dielectric properties, conventional

board processing. X-Y axis CTE,

stiffness, shielding, cooling.

Same as board bonded to

supporting plane.

High-speed interconnections.

Good thermal and electrical

features.

Same as Alumina.

Same as board bonded to

supporting plane.

Same as board bonded to low

expansion metal cores, stiffness,

thermal conductivity, low weight.

Substrate size, dielectric

properties, X-Y axis, CTE.

Major Disadvantages

Because of its high X-Y plane Thermal conductivity, X,Y and

Z

Comments

axis CTE. CTE. It should be limited to

environments and applications

with small changes in

temperature and/or small

packages.

Thermal conductivity, X and Y

axis CTE, moisture absorption.

Same as Epoxy Fiberglass.

Thermal conductivity, X and Y

Resin selection critical to water absorption.

controlled to tailor X-Y CTE. axis CTE, resin microcracking,

Volume fraction of fiber can be

reducing resin microcracks.

Thermal conductivity, X and Y Same as Epoxy Aramid Fiber.

axis CTE, resin microcracking,

water absorption.

Thermal conductivity, X and Y Volume fraction of fiber can be

axis CTE,

Z

axis CTE, drilling controlled to tailor X-Y CTE. Drill

availability, cost, low resin wearout higher than with

content required.

fiberglass.

Thermal conductivity, X and Y Resin microcracks are confined

axis CTE, water absorption, to internal layers and cannot

process solution entrapment.

damage external circuitry.

Same as Epoxy Fiberglass, low Suitable for high speed logic

temperature stability, thermal applications. Same as Epoxy

conductivity. X and Y axis CTE.

Fiberglass.

Size. Rigid-flexible boards offer

High injection-molding setup Relatively new for these

trade-off compromises.

costs.

applications.

Substrate size, rework limitations,

circuit technology. weight, cost, brittle, dielectric

Most widely used for hybrid

constant.

Weight.

The thickness/CTE of the metal

core can be varied along with the

board thickness, to tailor the

overall CTE of the composite.

supporting plane.

Weight.

expansion metal support plane. special equipment.

Same as board bonded to two

Licensed process. Requires

Same as board bonded to

Reworkability, compatible thick

Same as board bonded to Weight, internal layer registration.

under development. film materials.

Thick film materials are still

supporting plane.

board can be varied to tailor the

overall CTE of the composite.

difference in CTE between

ceramic package and substrate.

Cost. The thickness of the graphite and

Z

axis CTE, thermal conductivity. Compliant layer absorbs

4-3

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