IPC-CM-770D-1996 - 第28页

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

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770 Januaty

1996

R1

CR1

JI

9

R5

R4 R3

a

+Cl

Not

Recommended

Sleeved component bridges more than one conductor.

I

IPC-1-00177

Figure

5-1

3

Horizontally Mounted Components

prevent damage to seals. Lead spacing should have

1.5

mm

minimum forming allowances.

5.2.4.6 Rectangular Bodied Components

A component

with a rectangular body should be mounted

0.4

mm mini-

mum from the mounting base to enhance cleaning. When

required, assemblies with stringent conformal coating

requirements may need some type of board coating prior to

1

component mounting.

Recommended

Bridged conductors protected from moisture traps

by compatible insulating material.

IPC-1-00372

Figure

5-11

Components Mounted Over Conductors

Mounting

Hole

C

""""_

/

""_

I

""""_

""

""""_

""

¿

R7

4

Cl

06

I

R101

I

CR101

-+IlEl-

C"""""""1 C"""""""1 C"""""""1

Not recommended

f

Acceptable

1.5

mm

Preferred

IPc-I-00020

Figure

5-1

2 Alignmentlboundaries

minimum of

0.4

mm clearance between the end of the

component body, which includes any packaging meniscus

and the surface of the board, to prevent potential heat dam-

age and entrapment problems. (See Figure

5-15.)

The maximum vertical misalignment shall be

15"

between

the body axis and a line perpendicular to the mounting sur-

face. Any bend in the lead should not fracture the case.

5.2.4.7 Mechanical Securing

Section

20

provides tech-

niques for component mounting that will prevent compo-

nent damage due to shock and vibration characteristics.

5.3

Surface Mounting

Axial-leaded components are pri-

marily suited for use in through-hole mounting technology.

However, when it is necessary to surface mount such com-

ponents, the guidelines of Section

22.1.2

should be fol-

lowed.

5.2.4.5 Hermetically Sealed Components

Hermetically

sealed components, such as reed relays, glass sealed

capacitors, and diodes, must be handled with caution to

5.4 Mixed Technology

Cases where both axial leaded

components with leads, through-hole and surface mount,

2-4

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

are used in the same assembly is rare to non-existent. Refer

to general information related to class B/1 or

B/2

assem-

blies for consideration when leaded axial surface mount

components are used with chip components. When

through-hole axial components and chip components are

used in the same assembly, care must be taken to establish

proper assembly sequence, design for tool clearances and

shock resistance before soldering, cleaning, inspection/

testing, and rework.

5.5

Manual Assembly

Manual techniques for axial com-

ponents mounting are covered in Section 20.5 of this

guideline.

5.6 Automated Assembly

Automated techniques for

axial component mounting are covered in Section 20.6 of

this guideline. In addition, the following paragraphs repre-

sent additional considerations.

Multiple station sequencers and automatic variable center

distance (VCD) axial component insertion machines can be

utilized to perform high labor content kitting and compo-

nent preparation automatically for manual or operator

assisted component assembly. For this application, the cut

and clinch head of the VCD machine is removed and a

multiple cavity tray is placed in a modified board holder in

place of a printed board.

5.7

Handling and Storage

The handling and storage of

axial-leaded components should be in accordance with the

guidelines of Section 25.

5.10 Conformal Coating

General techniques for confor-

mal coating for all types of assemblies are described in

Section 28.

5.11 Quality Assurance

General quality assurance con-

siderations are described in Section 30.

6.0 RADIAL-LEADED DISCRETE COMPONENTS

Radial-leaded components come in a variety of shapes

from cylindrical, square, rectangular, wafer and kidney.

Leads exit from a common side of the component. The lead

is either ribbon or cylindrical. Selected devices can be

automatically inserted during assembly and also can be

modified with coined leads for surface mounting.

6.1 Part Type Description

Typical types of radial-lead

components are electrolytic, plastic, dipped, molded,

encapsulated capacitors, Figures 6-1, 6-2, 6-3, and transis-

tors, Figure 6-4.

6.1.1 Hermetically Sealed Components

Hermetically

sealed components such as electrolytic capacitors shall be

handled with caution to prevent damage to seals.

Bi-metallic welds should not be stressed.

6.1.2 Plastic Package Components

The integrity of the

interfacial adhesion between the plastic dip coating or

encapsulants to packaging or leads shall be maintained dur-

ing handling.

Washer or other mechanical hardware

1.5rnm

or more

Spacing less

(0.06

inches)

than

electrical

clearance

requirements

Not

Recommended

Spacing less than electrical

clearance requirements

Recommended

Spacing between printed wiring and mechancal hardware is 1.5rnm

(0.06")

or

more

IPC-I-O0030

Figure 5-1

4

Clearance

5.8 Soldering

General soldering techniques applying to

6.2 Through-Hole Mounting

all types of components are described in Section 26.

5.9 Cleaning

General techniques for cleaning applying

to all types of components are described in Section 27.

6.2.1 Component Preparation

Under certain circum-

stances, it may be advisable to provide stress relief for the

2-5

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services