IPC-CM-770D-1996 - 第95页

IPC-CM-770

Januaty

1996

18.0 MECHANICAL COMPONENTS

contain provisions which permit the heat sink and compo-

This section provides information concerning Some of the nent leads to be soldered directly to the board. See Figure

many types of components that are used for the transfer-

18-2.

ence of heat, securing of parts to an assembly, maintaining

a prescribed space between part and surface, providing

electrical insulation between parts and guides used for

installing plug in assemblies. (Note: screws, rivets, wash-

ers, nuts, etc., are mentioned only in the application

required.)

18.1 Part Type Description

18.1.1 Heat Sinks

Heat sinks are devices used to absorb

and/or transfer heat away from heat sensitive parts. Heat

sinks come in many styles, shapes, sizes and may be

designed for mounting on printed boards, on a component

or a series of components. They can be mounted by

rivetingholting to the component or the circuit board or by

clipping to a mounted component. Some common types are

depicted in Figure

18-

1.

Figure 18-2 Solderable Heat Sink

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IPC-1-00277

Figure 18-1 Common Component Heat Sinks

To facilitate heat sinking of components which must be

electrically insulated from heat sinks, chassis, washers,

etc., thermally conductive epoxy compounds and adhe-

sives, silicone grease, silicone rubber and other materials

are available. The silicone rubber and other sheet material

is usually provided in the shape of the various devices. The

effectiveness of heat sinks can be improved by forced air

convection. Heat pipes are also used for heat sinking,

spreading hot spots or causing several components to oper-

ate at the same temperature. Recently, heat sinks have

become available which permit mounting of the active

componentlheat sink combination. These can then be

mounted to the printed board. Some of these heat sinks

18.1.2 Spacers

Spacers are provided to maintain a com-

ponent above the mounting surface for electrical clearance,

increase soldering capabilities, improve air flow on heat

dissipative parts, provide mechanical support, facilitate and

minimize thermally cleaning induced stresses.

Spacers are manufactured from non-conductive materials,

(plastic, nylon, teflon) and materials that will dissolve dur-

ing the cleaning process. Spacers are usually used on mul-

tileaded radial devices (three or more leads), such as tran-

sistors, or amps, potentiometers, etc. Some examples are

depicted in Figure

18-3.

18.1.3 Component-Lead Spreaders

The use of multiple

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

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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.

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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

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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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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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