IPC-CM-770D-1996.pdf - 第25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 wir…

IPC-CM-770

Januaty

1996

1-10

Licensed by Information Handling Services

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

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).

U U

IPC-1-00136

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

[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:

Center to center lead spacing

Body length

Nominal lead diameter

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.

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

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

bend of a component lead

should be in accordance with Figure

5-5.

Straightfor2

[O.(u,hch]

diameters, but not

le88

than

0.76mm

Standard bend

I-Dia.

Welded bend

1.22mm

[0.048

inches] and larger

diameters

IPC-I-O0025

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

IPC-1-00138

Figure

5-6

Lead Stress Relief Examples

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

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

Licensed by Information Handling Services