IPC-CM-770D-1996 - 第26页

January

1996

IPC-CM-770

Part Two

Discrete (Two- and Three-Lead) Components

5.0 AXIAL-LEADED DISCRETE COMPONENTS

Axial leaded components with two leads are perhaps the

most common electrical components used in printed wiring

assemblies. The component body is usually cylindrical in

shape with two leads exiting from the opposite ends of the

component along its longitudinal axis. The lead is usually

round in cross section. Component identification as well as

polarity, when necessary, are generally marked on the body

of the component. Many resistors, capacitors and diodes

are supplied in this configuration (see Figure

5-1).

IPC-1-00173

I

L

Figure 5-1 Axial-leaded Component

Automatic processing technology and equipment handle

this type of com- ponent very effectively when the compo-

nents are provided in tape reels (see Figure 5-2).

I

3

\

L

U

U U

v

IPC-1-00136

I

L

Figure 5-2 Taped Axial-leaded Components

5.1 Part Type Description

In addition to Figure

5-1,

axial-leaded components (2 leads) can also come in polar-

ized component body styles (see Figure 5-3), and where the

center of gravity of the component is not CO- linear with the

leads. Size and material of component bodies and leads

range and vary widely, based on device characteristics,

electronic rating, and component package style techniques.

Figure 5-3 Polarized Axial Lead Component

5.2 Through-Hole Mounting

5.2.1 Component Preparation

Component preparation

is the processing step which generally includes forming

and cutting of component leads to facilitate subsequent

component assembly and/or minimize component damage

due to stress.

5.2.1.1 Lead Forming

The lead should extend approxi-

mately one forming allowance straight out from the body

of the component. This forming allowance is usually

expressed as “2 lead diameters” or a minimum of

1.5

mm

[0.06

inch] prior to the start of the bend except that when

space is limited by high density packaging the minimum

may be

0.75

mm [0.03 inch]. The end of the body in this

application is defined to include any coating meniscus, sol-

der seal, solder or weld bead, or any other extension.

The minimum component center-to-center board lead spac-

ing can be represented by the equation: (see Figure 5-4)

L=Bmax+3D*+2FA

where:

L

=

Center to center lead spacing

B

=

Body length

D

=

Nominal lead diameter

FA

=

Forming allowance (lead should not be disturbed

within this distance from the body)

*For lead diameters up to

0.7

mm, 4D for lead diameters

between

0.7

mm and 12 mm and SD for lead diameters

over 1.2 mm.

2-

1

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Figure 5-4

The value of

"L"

is usually adjusted upward to coincide

with the grid used. The total length of both leads should

not exceed

25

mm in length unless this component is

mechanically supported to the mounting base.

5.2.1.2 Minimum Inside Bend Radius

The minimum

inside bend radius of a single

90

bend of a component lead

should be in accordance with Figure

5-5.

-

Straightfor2

11

[O.(u,hch]

,

diameters, but not

le88

than

0.76mm

A.

Standard bend

I-Dia.

B.

Welded bend

1.22mm

[0.048

inches] and larger

2

diameters

IPC-I-O0025

L

Figure

5-5

Lead Diameter Versus Bend Radius

5.2.1.3 Stress Relief

Properly formed leads on axial-

leaded components normally afford adequate stress relief

when formed as shown in Figures

5-4

and

5-5.

Mechani-

cally sensitive components, such as glass diodes, may

require additional stress relief and have one or more leads

formed with a stress relief loop. Whenever the possibility

exists of solder wicking into the stress relief bend usually,

as a result of a small diameter component, stress relief

loops as shown in Figure

5-6

or spacers should be provided

as the stiffened lead defeats the purpose of the stress relief.

5.2.1.4 Lead Forming for Component Retention

The

purpose of preforming component leads for retention to the

board is to combine the advantages of labor reduction

achieved in straight through leads and the mechanical reli-

ability of the clinched lead. Simple forms are used only to

hold parts to the board prior to soldering; complex forms

can be used for mechanical strength.

The forming of leads can be from a minimum of a simple

offset to a complex compound form. The major factors

affecting the forming of leads are board thickness, lead

m

IPC-1-00138

Figure

5-6

Lead Stress Relief Examples

diameter, lead material, hole size, and tooling required.

A.

Simple Offset Method

(Figure

5-7)

This gives the

appearance of a straight through lead. The advantages are

ease of forming and ease of insertion. The disadvantage is

a minimum of retention force, and the resiliency of small

or soft component leads.

IPC-I-O0140

Figure

5-7

Simple-offset Preformed Leads

B. The Dimple

(Figure

5-8)

This method increases the

retention to the board and gives better contact to the board

circuitry. The main disadvan- tages are the die sets required

to form the leads and the hole size to dimple height

requirements. With the variation of lead sizes on the board

this becomes the main concern.

IPC-1-00141

Figure 5-8 Dimple Preformed Leads

2-2

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

C. Compound Forms

(Figure

5-9)

This method gives the

best retention and has the advantage of near clinched lead

reliability. Disadvantages in- clude complex tooling

required to form leads and the need for tighter control of

hole sizes.

B

5

IPC-I-O0142

L

Figure 5-9 Compound Preformed Leads

D.

Combinations

(Figure

5-10)

These are used to gain

the advantages of the above methods and overcome some

of the disadvantages.

mm

IPC-1-00143

Figure 5-10 Combination Preformed Leads

5.2.2 Land Patterns

Land patterns must consider

minimudmaximum lead spacing requirements. Standard

land spacing patterns should be established for the purpose

of uniformity of assemblies and the practical use of assem-

bly tools and equipment. Spacing(s) should be located to

accommodate automatic assembly-and “bed-of-nails’’ type

of testing equipment, (usually

2.5

mm increments-i.e.,

7.5

mm,

10

mm,

12.5

mm, etc.).

Land patterns for unsupported holes should have more sol-

derable area than that for supported holes for a stronger

joint after soldering. The optimum dimension is dependent

on the device and its mounting characteristics.

The most common geometry is the round land with a cen-

tered hole. Square lands with centered holes are sometimes

used to indicate polarity for polarized components.

Additional information for holeAand requirements can be

found in Section

21

(Through-Hole Mounting Techniques).

5.2.3 Lead Configuration After Assembly

5.2.3.1 Straight Through Leads

Partially clinched leads,

where the components are secured with only limited move-

ment and the leads are in contact with the land patterns,

shall be considered as being straight through leads.

Lead projection from an unsupported hole should be

0.5

mm minimum. Lead bend at the solder side is allowed to

the extent that minimum electrical clearances are main-

tained from all conductive surfaces with different electrical

potential.

Lead projection from a supported hole should extend at

least beyond the bottom surface of the board to the extent

that, as a minimum, the lead contour is discernible after

soldering.

Lead projection of specially preformed leads for compo-

nent retention without clinching can be used to eliminate

the need for post-soldering lead cutting.

Additional information for lead projections can be found in

Section

21.

5.2.3.2 Clinched Leads

Clinched leads are leads that are

formed to the land within one lead diameter of the land for

both supported or unsupported holes. Leads should not

extend beyond the land to reduce, by more than half, the

electrical spacing to any adjacent conductive patterns.

5.2.4 Mounted Component Configuration

5.2.4.1 Lead Spacing

Where practical, components with

similar physical dimensions should have the same lead

spacing and be on the design grid.

5.2.4.2 Component Body

Through-hole components

should be mounted on one side of the board only. Com-

ponents should not be mounted across or on top of vias,

exposed conductive patterns and other components. (See

Figure

5

-

1 1

.)

Component bodies should not be closer than

1.5

mm from

the board edge. (See Figure

5-12.)

Resistors with power rating above

1

watt should be

mounted with a clearance of at least 0.5mm from the board

surface.

5.2.4.3 Component Orientation

Horizontally mounted

components should be oriented in either of the two board

axes preferably with component identification in the same

direction. (See Figure

5-13.)

The body of the component

should be approximately centered between the lead spac-

ing. In general, components with cylindrical bodies should

be in direct contact to the mounting base. Density of com-

ponent mount should consider clearance needed for compo-

nent assembly, electrical clearance, and ease of rework.

(See Figure

5-14.)

5.2.4.4 Vertical Mounting

Components mounted perpen-

dicular to printed wiring boards should be installed with a

2-3

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services