IPC-CM-770D-1996.pdf - 第111页

IPC-CM-770

Januaty

1996

lncommg

receiving

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boards

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inspaclion

commnents

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Board

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preparation

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Solder

components

Misc.

componenls

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Radial

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Dips

Component

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Preclean

prebake PWB

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IPC-1-00384

Figure 21-1 Component Mounting Sequence

Boards may be assembled in carriers, which then move

successively over fluxer, preheaters, and soldering unit.

After soldering, boards continue to move in same carriers

to the next inline station.

B.

Solder-Cut Solder

The solder-cut solder method is

used for requirements where base metal of component

leads, exposed by lead cutting, must be solder-coated. A

second inline soldering operation is added after the inline

lead cutter. All steps, except initial carrier loading and final

unloading are automatic.

21.4.3 Deep Solder

Another method of component

retention is to flux and solder long leads by the use of a

deep (up to

75

mm solder wave, or by dipping in a static

solder bath. (See Figure

21-2.)

After this first soldering the

leads are automatically trimmed to length by means of a

high speed cutter. The boards may be then fluxed and resol-

dered using a normal wave soldering system. This reduces

the possibility of damaged joints due to lead stressing and

solder coats the cut ends of the leads. Adequate fixturing

must be used to maintain board flatness through the cutting

system.

21.4.4 Conforming Material

Conforming sponge-like

material can be used to hold components in place during

wave soldering but has obvious disadvantages of fabrica-

tion, cleaning, removal and frequent replacement. “Bean

bags” or other weighty self- conforming substitutes can be

used.

Web-like materials are available for spraying on the top

surface of the printed board assembly. These set up on

contact and are removable with water or solvents. (See

Figure 21 -2 Component Retention-Deep Solder

Figure

21-3.)

Too heavy an application of the “web-like’’

material can prevent the flow of the solder along the com-

ponent lead and thus result in a lack of a solder fillet on the

component side of the board.

Blister pack, or skin pack, is sometimes used to hold com-

ponents in place during lead trimming and soldering. This

method employs a trans- parent (sometimes bubble) sheet

of plastic.

By means of a blister packing machine a sheet of plastic is

formed over the tops of the components and around edges

of the printed board assembly by the application of heat

and vacuum. Excess plastic is trimmed from the bottom of

the board before lead trimming wave soldering. Plastic is

removed while the board is still warm.

With the blister pack process it is sometimes found that the

plastic material softens and melts during the soldering

5-8

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

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

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b

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IPC-1-00364

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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.

5-9

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

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,

5-10

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services