IPC-CM-770D-1996.pdf - 第94页

January 1996 IPC-CM-770

may be surface mounted (lap soldered) to a printed board

trace, conductor, or land under the following conditions:

The attachment area meets the requirements defined in

Section 22. The land width should be 1-1/2 times the wire

diameter, and the length of the connection should be five

times the wire diameter.

The connection must be protected from peeling stresses,

either by external clamps, or by adhesives to attach the

wire to the board.

17.4 Mixed Technology

Mixed assembly technology has

no unique requirements for these components.

17.5 Manual Assembly

A. Terminals

Terminals are often installed manually. Ter-

minal swaging tools are available from each terminal

manufacturer to permit funnel or roll swages as required.

Elliptical swages, after suitable tool modification, can also

be performed manually.

B. Pins

Pins may be installed manually; however, it

should be noted that pressing the pins into the printed

board and achieving a uniform insertion depth manually is

difficult.

Tooling should be provided to control the depth of inser-

tion. This is a minor problem when using insertion tooling

similar to that shown in Figure 17-26. Another method of

controlling the insertion depth of the pins is to use pins

with a “board stop” which will contact the board or other-

wise indicate proper insertion.

C. Wire and Cable Preparation

Sufficient insulation shall

be stripped from the wire or leads to provide for insulation

clearances as specified. Chemical stripping agents shall be

used for solid wire only and shall be neutralized or

removed prior to soldering. After insulation removal, defor-

mation of remaining insulation shall not exceed 20% of the

insulation thickness. In stripping insulation, care should be

taken to avoid nicking or otherwise damaging the wire or

the remaining insulation. For Class

1

or 2 assemblies, the

number of damaged or severed strands in a single wire

shall not exceed the limits given in Table 17-5. For wires

used a potential of

6kV,

or greater, or for Class

3

assem-

blies, there shall be no broken strands; nicked strands shall

be per Table 17-5. Insulation discoloration resulting from

thermal stripping is permissible.

Table 17-5 Nicked or Broken Strand Limits

Maximum allowable nicked or

Number

of

Strands broken strands

Fewer than 7

O

7-1 5

37-40

4 26-36

3 19-25

2 16-1

8

1

5

41 or more 6

D.

Bus Bars

Because of the size of the bus bars, these

devices are usually installed manually. The bars are

inserted in the appropriate holes, and pins are clinched to

retain the bus bar.

E.

Test Points

Test points can be installed manually.

17.6 Automated Assembly

A. Terminals

Tooling is available to install and flare most

terminals automatically.

B. Pins

Pins are most often installed automatically. Pins

are available with carriers which feed the pins to the tool-

ing, which inserts the pins to a uniform depth.

C. Wire

Wire is usually installed manually with the

exception of jumpers and wire wrap. Wire wrap connec-

tions can be installed automatically by wire wrap machines

which can follow preplanned wire lists, install the wire

wraps, and route the wires between the connections.

D.

Bus Bars

Bus bars are not normally installed auto-

matically. The size of the bars and the tolerances involved

normally precludes automating installation of these

devices.

E.

Test Points

Test points may be installed automatically.

17.7 Handling and Storage

The handling and storage of

interconnect components should be in accordance with the

guidelines of Section 26.

17.8 Soldering

Soldering of devices in this Section fol-

low the guidelines outlined in Section 27.

17.9 Cleaning

Cleaning techniques are covered in Sec-

tion 28.

17.1

O

Conformal Coating

Conformal coating techniques

are covered in Section 29. In addition, the devices in this

Section may require masking. Care must be taken to apply

and remove the masking without contaminating the device.

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Licensed by Information Handling Services

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

I

1

n

o

*

n

U

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9

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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Licensed by Information Handling Services

January

1996

IPC-CM-770

@@O00

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O0

o

O0

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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

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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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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services