IPC-CM-770D-1996.pdf - 第114页

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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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Surface mount components mounted to the bottom side of

the board may also interfere with the lead cutting

operation.

21.5.2 Clinched Leads

Clinching of leads prior to sol-

dering is commonplace, either as part of machine insertion

or following hand insertion. The substrate land configura-

tion and spacing to adjacent lands must be considered.

Clinching in line with traces is good practice and trimming

of leads before clinching is recommended where clinch

direction may cause shorting to adjacent lands. It is gener-

ally not felt to be necessary to clinch all leads of a multi-

leaded device unless required by the customer and equip-

ment class. The lead is passed through-the-board and is

clinched to make contact with the land or conductor and is

then soldered. The lead or terminal should make contact

with the conductor pattern before soldering. Leads should

not extend beyond the edge of their lands, however,

if

overlap does occur, the lead should never violate electrical

spacing requirements. The lead termination hole may be

supported by eyelets or plated-through- holes or it may be

unsupported.

The lead should be cut and clinched prior to the soldering

operation. The maximum height of lead spring back should

not exceed the maximum lead diameter plus 0.8 mm above

the plated surface or eyelet. See Figure 21-7.

As its name implies, the component leads for this method

are clinched to the printed board land after they have

passed through the lead hole.

1

IPC-1-00329

Figure 21-7 Clinched Leads

In addition to the considerations common to all straight-

through mounting methods, this method has the following

advantages:

A reinforced mounting hole is not required; tear drop and

offset lands can be used.

This method does afford some resistance to movement

during soldering.

This method has some of the disadvantages mentioned for

unclinched straight-through mounting, in addition:

Care must be taken when cutting the lead to length and

forming the clinch to assure that minimum conductor

clearances are provided when the clinched lead overhangs

the land.

The lead clinching operation, if not controlled properly,

can unduly stress the component lead-to-can body seal.

21 5.2.1 Preformed Leads

A slight modification of the

mounting method just described is the offset multiple lead

can mounting method with clinched leads. As shown in

Figure 21-8 this method adds the feature of having the

leads clinched to the conductor land after they pass through

their mounting holes.

m

IPC-1-00330

Figure 21-8 Offset Clinched Lead

21.5.2.2 Semi-Clinched Leads

(The lead is passed

through a hole in the board, cut to length, and soldered.)

When straight-through leads are used inconjunction with

unsupported holes, the leads should extend from

0.5

mm

minimum to

1.5

mm maximum from the surface of the foil.

When straight-through leads are used in conjunction with

plated- through holes or eyelets, the lead should extend at

least to the surface of the plating or rim of the eyelet and

no more than 2.3 mm from the plating surface eyelet.

Semi-clinched leads should be considered as straight-

through leads providing the degree of clinch meets the

requirements shown in Figure 21-9.

n

a!

ID

IPC-1-00328

Figure 21-9 Straight Through Leads (Semi-clinched

Leads)

21.6 Assembly

Some basic principles for assembly,

especially automated are:

Design to the minimum number of different components,

and purchase these to industry or prescribed standards for

automatic insertion.

Arrange all components on

X

and, if necessary,

Y

axes.

Also arrange components in columns andor rows, if pos-

sible.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Sequence all types of axial lead components prior to

insertion.

Minimize the distance between the components and fol-

low a grid pattern for component layout.

Minimize the number of different center spacings.

Make provisions for tooling holes at or near the edge of

the board in an area not occupied by components.

Minimize the number of different hole sizes to minimize

manufacturing time if boards are drilled or die costs if

punched.

Provide clearance areas as large as the tooling footprint

between components for the insertion tools both above

and below the board.

22.0 SURFACE MOUNTING

22.1 General Considerations

Surface mounting is a

technique which is applicable to most component types,

and is used for a variety of reasons. The technique was

developed and proven for stripline and other high fre-

quency applications where lead placement and discontinui-

ties had to be strictly controlled.

The technique was found to be a reliable and viable solu-

tion to a number of problems, and is now in wide use.

Surface mounting consists of placing the component on the

printed board or other suitable substrate, and making the

necessary electrical connection to the component on the

same side of the board. The leads of surface mounted com-

ponents do not pass through the board to make electrical

connections. The specific connection technique to be used

will depend on several factors.

In general, leaded components are lap-soldered to the ter-

minal areas, while leadless components are attached by

solder fillets between a solderable area on the component

and the land area.

Orientation and placement of components on the “solder

side” of the assembly should be such that bridging or

depleted solder on the following component does not

occur. (Figure

22-

1).

f-

Direction

of

travel

F

Solder

wave

IPC-1-00369

Figure 22-1 Depleted Solder

22.1.2 Lead Forming

General requirements for lead

extension and forming for leaded components are illus-

trated in Figure

22-2.

When formed the leads must be held

close to the body to prevent damage to seal.

IPC-1-00326

Figure 22-2 Lead Forming for Surface Mounting

Leaded components should be mounted with the body

0.25

mm minimum above the surface of the printed board

except when the part body is sealed to the surface of the

board with adhesive such that no flux, moisture, or other

contaminants can become trapped. Round axial leaded

parts need not be elevated, since the body configuration

permits thorough cleaning.

A major consideration in using leaded components is to

assure that all leads are coplanar. A tolerance of

+

0.05

mm

is recommended to assure adequate solder filleting to each

lead.

Alternative definition: use the three longest non-colinear

leads to define a plane (sit the part on a flat surface) then

all leads should be within

+0.05

mm of the surface.

It should be noted that the JEDEC standards do not contain

such a stringent coplanarity requirement. Therefore, lead

forming by the user is usually required.

Lead forming is not applicable to leadless components.

Solderability, however, is of prime importance. All surfaces

to be soldered should be pretinned and verified as very sol-

derable through one of the recognized solderability test

methods, such as

J-STD-002.

The formed leads should be parallel and in contact with the

lands on the mounting base without unplanned overhang.

Maximum angle allowed between the lead pad and parallel

mounting base is

15.

22.1.2.1 Lead Support

Leads should be supported dur-

ing forming to protect lead to body seal.

22.1.2.2 Coined Leads

Round cross section leads usu-

ally are coined to enhance mounting stability.

Components with axial leads of round cross section should

be utilized for planar mounting only if the leads are coined

or flattened for positive seating.

22.1.2.3 Component Support

Depending upon weight,

components may be secured for soldering in the following

ways:

For light components, solder paste with relative high

solid content can be used to hold pretinned lead and land

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services