IPC-CM-770D-1996 - 第96页

January

1996

IPC-CM-770

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IPC-1-00278

Figure 18-3 Typical Spacers

lead can spreaders, Figure

18-4,

serve as a similar function

for offset can mounting as do spacers for straight-thru can

mounting. Leads may be terminated in the clinched or

unclenched lead method.

cally secured to the mounting base prior to lead termination

and during assembly usage.

I

18.1.4 Thermally Conductive Insulators

The most com-

mon method of obtaining thermal conductive insulators has

been by the use of a silicone grease and mica or plastic

film on printed board assemblies which must be cleaned

and conformably coated. The use of silicone grease can

cause cleaning and coating adhesive problems. Thermally

conductive insulators made of silicone rubber and fiber

glass cloth are also available in standard package configu-

rations and custom forms. By using these cured silicone

insulators, many of the cleaning and coating problems

related to silicone grease smear can be avoided. (See

Figure

18

-5

.)

Figure 18-5 Thermally Conductive Insulator

SPREADER

IPC-1-00164

I

L

Figure 18-4 Can Mounting Spreader

18.1.5 Component Securing Devices

The shock and

vibration to which printed board components are subjected

during normal handling and environmental testing can

damage the lead terminations and lead-to- component body

seals. For this reason, many components, especially those

weighing more than

7.09

g per lead, should be mechani-

The more commonly used component securing methods

are: clips, clamps, and brackets; wire and elastic straps;

adhesives; and integral mounting provision.

18.1 5.1 Clips, Clamps and Brackets

The following are

the basic requirements which should be adhered to when

components are mechanically secured by clips, clamps and

brackets:

A.

All clips, clamps or brackets should be secured to pre-

vent their rotation, such as by using two fasteners or one

fastener and a non-turn device.

B.

Clamps and brackets which require their removal in

order to replace the component should be secured with a

threaded fastener or other non-permanent fastener, unless

the subassembly in which they are used is considered to be

disposable or nonrepairable.

C.

Spring type clips which need not be removed during

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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

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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