IPC-CM-770D-1996.pdf - 第97页

IPC-CM-770

Januaty

1996

component replacement may be secured with permanent

type fasteners such as rivets or eyelets.

D.

The use of twist type lugs, ears, or clips with glass

envelope components, should be avoided.

18.1 5.2 Strapping Devices

When using wires and elas-

tic straps for mechanical securing, the strap is wrapped

over the component body and passed through holes in the

mounting base. When wire is used it is clinched and sol-

dered in the same manner as component leads to lands.

When wire is used with heat sensitive or fragile compo-

nents the part of the wire on the component should be cov-

ered with a suitable sleeving.

The elastic strap is secured by being stretched, to reduce its

cross-section below that of the hole, and then returned to

its larger-than-hole size by relieving the tension after it has

been passed through the hole. The resiliency of the strap

holds the component in place.

18.1 5.3 Adhesives

Whenever possible, components

should be secured by conventional means; when this is not

possible, such as in the case of oddly shaped components,

or where special support is required, or where there are

special design requirements (limited space, heat transfer,

limited access, etc.), a suitable adhesive may be used.

18.1.5.4 Integral Mounting Provisions

Components

with integral mounting provisions should be considered as

parts with permanently fastened clamps or brackets and

should conform to the requirements mentioned above.

18.1.6 Card Guides

Card guides are often used in elec-

tronic equipment to facilitate installation of a large number

of cards in a relatively compact area. They also are used to

relieve stresses on the connector contacts. Guides allow the

boards to be installed and extracted easily.

The use of guides, such as in card cages, allows for better

cooling of heat producing devices. See Section on connec-

tions single and multiple socket.

Mounting of card guides is widely varied. They can be

mounted using rivets, screws, snap in buttons or clipped on

to a molded mating connector. Still others are part of a total

card cage assembly.

Card guides are constructed of molded plastic dielectric

material such as nylon or polycarbonate. They are provided

with slots or grooves to receive and guide the board to the

connector and maintain it in position.

There are some card guides made of metal such as beryl-

lium copper which are spring tempered. These are one

piece or provided with spring fingers. This type offers

guide plus holding features plus a method of shock damp-

ing. When using metal card guides care must be taken to

avoid any circuitry contact between the guide and the card.

18.2 Through-Hole Mounting

Hardware such as screws,

rivets, terminals, etc., that are used to mount the items

herein must be stipulated as part of the design of a particu-

lar assembly.

18.2.1 Component Preparation

The majority of the

components mentioned in this chapter require no special

preparation prior to their use. However, care must be exer-

cised in the installation of any specialized item such as

these.

18.2.2 Land Patterns

Considerations for the configura-

tion of land patterns will be included in the design criteria

of the board.

18.2.3 Lead Configuration After Assembly

Not appli-

cable.

18.2.4 Mounted Component Configuration

Not appli-

cable except as defined in figures.

18.3 Surface Mounting

Mechanical components are

usually never surface mounted; however spacers, insula-

tors, spreaders, and heat sinks must be designed in such a

manner that they sit on the surface of the printed board,

and facilitate cleaning.

Mounting hardware, or leads passing through the mechani-

cal parts, are usually used to secure the mechanical compo-

nent to the surface of the printed board.

18.4 Mixed Technology

Mechanical parts are usually

added separately, or if part of a component configuration

such as a spacer, may be added at the time the component

is positioned to the printed board or interconnection sub-

strate. Because of the need to have mechanical parts inter-

mix with both through-the-board and surface mount parts,

these parts are usually handled separately, and require

appropriate attention to allow clearances for their addition

before or after other parts are added to the assembly.

18.5 Manual Assembly

Manual techniques are usually

employed to add mechanical components. Adequate clear-

ances must be provided for tools to be able to secure

mechanical components to the surface of the board or to

mount an electronic component to the part.

Spacers and spreaders are mounted by inserting the compo-

nent leads through the holes provided or simply laying the

component on the spacers.

18.6 Automated Assembly

Automated techniques are

rarely used for mounting mechanical parts unless these

parts are a part of the component such that the component

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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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