IPC-CM-770D-1996 - 第112页

January

1996

-

IPC-CM-770

Figure 21-3 Conforming Web-like Material

operation and it then adheres to the components leads,

making it difficult to remove.

21.4.5 Wax Stabilizing

Stabilizing is a method by which

long and short component leads are held in position for

automatic lead cutting by a material, not solder, but simi-

larly applied, having adhesive-like holding power. The low

temperature plain wax is applied to the bottom surface by

a deep wave of wax after automatic wave (or spray) flux-

ing and preheating, then through a chiller which hardens

the wax, locking components securely in place for auto-

matic lead cutting which is followed by preheating and

wave soldering. The stabilizing process using flux mixed

with the wax eliminates the separate flux station. Most of

the flux and wax is removed during wave soldering and the

little remaining is removed by either solvent or water

cleaning as appropriate.

21.4.6 Heat Sinking

Excessive exposure time and tem-

perature can cause damage to heat sensitive parts. Thermal

shunts or heat sinks (Figure

21-4)

should be used, as

required, for the protection of heat sensitive parts. They

should be made of a material with good heat conductivity

(e.g. copper) and must be of a size and shape such that

adequate thermal protection and minimum mechanical

interference will be provided during the soldering

operation.

Thermal

Shunt

h(&

b

I

IPC-1-00364

I

Figure 21-4 Thermal Shunt

21.5 Lead Configuration After Assembly

The objectives

of lead termination are to form the lead and electrically

connect it to the conductors in such a manner that the

required circuit continuity is provided through the life of

the equipment regardless of the environments to which the

assembly may be subjected. See Figure

21-5

for the most

frequently used methods.

LEAD AT

i

CLINCHED

LEADS

I

PLATED THROUGH HOLE

I

1

NON-PLATED THROUGH HOLE1

I

CLINCHED

RIBBON LEAD

LAPPED

PLANAR MOUNTED LEADS

RIBBON AXIAL

LtAD LEAD

STRAIGHT THROUGH

PLATED THROUGH HOLE

I

NON~PLATED THROUGH HOLE

-

TERMINALS

IPC-1-00327

Figure 21

-5

Lead Terminations

Leads may be attached to unsupported printed conductor

lands by clinching or straight-through (unclinched) lead

attachment. The attachment should be completed by

soldering.

Component leads should not be used as interfacial

connections.

Component attachment to printed boards should have the

lead or terminal pass through the board and be soldered to

the conductor pattern on the opposite side of the board.

Lead attachment should normally be an option as to

whether clinched or straight-through attachment is used,

with the following restrictions:

Where flat swaged eyelets (unfused) are used, a clinched

lead attachment should be used.

For straight-through attachment the diameter of unsup-

ported holes should not exceed the diameter of the

inserted lead by more than

0.5

mm.

For straight-through lead attachment to supported holes

the inside diameter of the supported hole should not

exceed the diameter of the inserted lead by more than

0.7

mm.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Leads should be terminated in such a manner that they do

not exert a lifting force on the copper foil terminal area

or conductor.

Each functional lead should have an associated terminal

area.

There should be no more than one lead in any lead

mounting hole.

21.5.1 Unclinched Leads

The most direct method for

mounting components to the printed board is the straight

through method with unclinched leads.

The use of straight-through unclinched leads requires the

minimum of device handling; a straightening of the com-

ponent leads and cutting the leads to length before or after

insertion.

The disadvantages associated with this approach are:

The device is subject to movement both before and dur-

ing the soldering operation. This makes it difficult to con-

trol the component height off the mounting surface. This

movement can be a source of solder joint problems.

It is difficult to maintain a suitable clearance between the

body of the component and the printed board surface for

flux removal and, when applicable, conformal coating of

the assembly. This is greatly minimized when multiple

lead cans with integral standoffs are used creating a com-

ponent seating plane below the surface of the can from

which the leads emerge.

When the leads are rigid, precise drilling of the compo-

nent mounting hole pattern is required due to small lead

circle required and the inflexibility of the unformed leads.

Supported holes are preferred in the printed board for the

component lead holes to enhance the mechanical strength

of the solder joint. Otherwise the clearance between the

component lead, the hole, and the circumscribing land

must consider the lead-to-hole ratio and the hole-to-land

difference that would allow sufficient remaining conduc-

tor to promote solderability.

The automatic insertion of the device leads in limited

space can present problems.

The mounting of multilead component cans with plastic

spacers has been used to overcome some the disadvantages

for the more conventional straight-through lead mounting

techniques. Spacers with protrusions on one side should be

mounted with the protrusions against the board. In addition

to the considerations common to the straight-through

mounting techniques, the mounting of component with

spacers has the advantages

of

A suitable clearance between the component body and the

printed board can be maintained to facilitate soldering

flux removal and conformal coating.

A bearing surface for the body is provided if the compo-

nent leads are to be clinched.

The extension of unclinched leads beyond the printed

board surface can be more accurately controlled.

The height of the component body above the printed

board surface can be more accurately controlled; this is

especially important when the printed board assemblies

are closely spaced.

The spacer helps to reduce the magnitude of mechanical

stresses that are transmitted to the leadhody interface

seal.

The lead mounting hole pattern need not be held as accu-

rately as for unprepositioned component leads. The use of

spacers has the following additional disadvantages:

Additional cost.

Increases the amount of assembly labor.

21.5.1.1 Preformed Leads

In order to overcome the

restrictions of small component mounting hole circles and

their associated small hole and land diameters with little or

no space for conductors between the leads, which is com-

mon to the straight- through mounting technique, offset

multiple lead can mounting methods have been developed.

Lead forming requires cutting the leads and then straight-

ening them, usually by combing,

so

they can be inserted

into the lead forming and cutting die. When removed from

the die, the packages are hand assembled and clinched to

hold them in place. Spacers are available for units with

press-on heat sinks or formed leads that have to be kept off

the board. The considerations of using the offset multiple

lead can mounting method without clinched leads are:

Allows for larger lands and component lead holes.

Requires less stringent board fabrication tolerances, if

desired.

May be used with conductors which can be routed

between lands.

May reduce (circular pattern) or eliminate (rectangular

pattern) the number of off-grid mounting holes and lands.

The formed leads can provide the needed spacing func-

tion for flux removal and conformal coating.

This method requires considerably more mounting labor

and usually tooling or fixturing.

Care must be taken to assure that the lead forming pro-

cess does not unduly stress the leadhody interface seal.

More printed board area is taken up by the component

mounting pattern.

This method has the disadvantages common to all

unclinched lead mounting methods.

21 5.1.2 Lead Retention

Swaging or crimping of com-

ponent leads is done by either automatic tools or by hand

tools, some of which are power assisted. Leads are cut and

flattened close to the bottom surface of the printed board,

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January 1996

IPC-CM-770

leaving a spade-shaped area on the lead ends which, if the

diametrical clearance between lead and hole is not too

great, holds the component in the hole. Clinching, or bend-

over, of component leads is a method used by automatic

component insertion machines, and hand tools are also

available to perform the same function.

The method and means chosen for component retention

should take the following factors into consideration.

End use of assembly and possible need for repair; com-

ponent removal, replacement and resoldering without

damage to the printed boards, plated-through-holes and/or

lands. Straight through leads are the simplest in this

respect.

Stresses on leads at the junction of the component body,

especially for hermetically sealed or glass bodied compo-

nents.

Allowable distance from the bottom of the printed board

to the end of the component leads, and desirable solder

fillet.

Area and direction available for clinching or bend-over

without danger of proximity to other leads or conductors

which may result in shorts or solder bridges.

Possibility for internal voids and entrapments of flux

gases, etc., if leads are curved and bent inside the hole

contacting hole comers.

Hazards to personnel created by sharp or knife-like lead

ends.

Potential for fractured solder joints when leads are cut

after soldering.

Potential for lifted lands.

Requirement for lead ends to be covered with solder.

Applicable specification constraints on component reten-

tion techniques and results.

Lead forming is normally along the centerline of the com-

ponent and inward, back toward the body. The clinch is

either

90

or

45

.

(See Figure 21-6.) The length of the com-

ponent lead under the printed board is a function of the

lead diameter, as shown in Table 21-1. (Note: the clinched

lead length is measured parallel to the printed board, after

clinch.)

1

uu

Stand.off

from

Board

Optional

45'

Clinch

[Dimenslon

"C']

IPC-1-00326

Figure 21-6 Clinch Patterns

Table 21-1 Lead Clinch

Wire Diameter Minimum Clinch

[0.080 inch] [0.030 inch]

[0.20

inch]

2.0

mm 0.8mm 0.5 mm

Maximum Clinch

0.8 mm 1 .Omm

2.0

mm

[0.030 inch]

[0.040

inch] [0.080 inch]

1.0 mm

2.0

mm 1.3mm

[0.040

inch] [0.050 inch [0.080 inch]

1.3mm 1.5mm

2.0

mm

[0.050 inch] [0.060 inch] [0.80 inch]

The clinch length for the normal

90"

clinch pattern is mea-

sured from the centerline of the component lead as it

extends through the hole in the printed board. The mini-

mum clinch lengths for small diameter leads are somewhat

dependent on the hole diameter in the board. To maintain a

minimum clinch length, the hole size must not be more

than 0.36 mm larger than the component lead diameter.

When printed boards are drilled to close positional toler-

ances and with minimum recommended hole sizes, the

45

clinch length, dimension "C", may be as small as

0.8

times the lead diameter. This allows only 0.3 times the lead

diameter extending over the land. In this case two wire

diameters should be allowed for the distance below the

board.

Use of automatic insertion machines provides a built-in

means for holding of components. (For most reduced reli-

ability applications, the components need not be retained

rigidly in position during wave soldering.) Numerically

controlled insertion equipment provides for accurate place-

ment of components (if tape sequence and program are

correct). Inspection of automatically inserted components

is usually through a sampling plan and/or a first article

inspection to check out tape sequence and program.

21.5.1.3 Lead Cutting

Cutting of component leads may

be accomplished individually, by hand, or by automated

equipment. When automated lead cutting is used, it is usu-

ally necessary to retain the parts on the board during the

cutting process. Leads may be cut either before or after

soldering. However, cutting leads after soldering requires

that cutting methods, sharpness of cutting tools, lead mate-

rial, or procedures are such that they do not induce solder

fractures. Some government specifications require that the

leads clipped after soldering be reflowed to insure the

integrity of the solder joint. The retention method may also

serve to retain the parts during a second soldering

operation.

As with any cutting instrument, as blade usage on the auto-

matic lead cutter increases, cutting quality diminishes. In

some instances, the small lead section is not entirely

removed from the lead leaving a "flag" or "lid" attached

to the lead end.

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