IPC-CM-770D-1996 - 第110页

January

1996

IPC-CM-770

as larger gull-wing types). Conversely, the circuit may be

heated by convection or direct transfer. Liquid thermoset

adhesive materials are processed differently. The material is

coated or printed onto the circuit. The entire board can be

covered. Components are conventionally placed and heat-

ing them cures the adhesive. Electrical contact is made

between the circuit and the components. The conductive

particles in the unpopulated areas remain dispersed within

the resin

so

that it serves as an insulative coating. Caution

-

use only with manufacturer’s approved component and

land surface finishes. Other finishes may lead to reliable

connections.

21

.O

THROUGH-HOLE MOUNTING TECHNIQUES

The most significant advantage of using the through-the-

board mounting method is its compatibility with conven-

tional mass soldering techniques, such as dip and wave

soldering. In addition, replacement of flatpacks mounted in

this manner is readily accomplished by melting and remov-

ing the solder from each land individually or in multiples.

Parts and components should be mounted on the side of the

printed board opposite that which would be in contact with

the solder if the board is machine soldered. Except when

mounted in cordwood modules or on nonrepairable printed

boards, parts and components should be

so

spaced and

so

located that any part can be removed from the printed

board without removing another part.

21.1 General Considerations

The selection of a particu-

lar method for mounting and connecting components in

equipment will depend on the type of component package

involved: on the equipment available for mounting and

interconnecting; on the connection method used (soldered,

welded, crimped, etc.); on the size, shape, and weight of

the equipment package; on the degree of reliability and

maintainability (ease of replacement) required; and, of

course, on cost considerations. See Figure

21-1

for compo-

nent mounting sequence.

The size and shapes of the solder-land terminations for

flatpack integrated circuits will depend on whether

“inline” configurations or “staggered” terminals configu-

rations are used. The sizes and shapes of lands for “TO”

type packaged circuits will depend on the effective lead-

circle diameter. For applications where speed and ease of

installation and removal are major considerations, devices

can be mounted in suitable sockets.

In any method which involves bending or forming of the

device leads, it is extremely important that the lead be sup-

ported and clamped between the bend and the seal, and that

bending be done with extreme care to avoid damage to lead

plating. Bending, forming and clinching of component

leads produce stresses in the leads and can cause stresses in

the seals if precautions are not taken. In no case should the

radius of the bend be less than the diameter of the lead, or

in the case of rectangular leads such as those used in dual

inline or flat-packaged integrated circuits, less than the lead

thickness. It is also extremely important that the lead exit-

ing the body of the component is parallel to the axis of the

component, and that the ends of the bent leads be perfectly

straight and parallel to assure proper insertion through the

holes in the printed board.

Tests performed on clinched cover leads show that the pull

and yield stresses produced by lead-clinching operations

are

far

below the allowable limits for these leads.

21.2 Component Preparation

Care needs to be exer-

cised to assure leads are prepared and insertion controlled

such that insulation on component leads does not extend

into the hole. This condition can have adverse impacts on

the resultant solder joint. This condition is often a problem

with two leaded disk type components not retained firmly

in the hole. Such components tend to tilt before soldering

introducing insulation in one hole and less than adequate

protrusion in the other.

21.3 LeadlHole Relationships

The lead to hole clear-

ance must be such as to provide for good soldering condi-

tions. Generally

0.25

mm to

0.5

mm clearance in diameter

is used.

If

the clearance is too small or too large, adequate

wicking of solder does not result.

A

minimum protrusion

through the substrate is often specified. The maximum is

dependent on specific later process equipment used and end

product design clearances.

For rectangular leads the dimension across the diagonal

should be considered as being the lead diameter.

21.3.1 Unsupported Holes

In determining the difference

between the diameter of an unsupported hole and that of

the lead to be placed in the hole, the hole should be from

0.25

mm to

0.5

mm larger than the lead diameter.

21.3.2 Supported Holes

In determining the difference

between the diameter of a supported hole and that of the

lead to be placed in the hole, the hole should be from

0.25

mm to

0.7

mm larger than the lead diameter.

21.4 Component Retention.

21.4.1 Assembly Process Methods

Two methods of

using solder as a means for component retention for lead

cutting are in general use. Both methods are practical for

inline applications whereby printed boards can be

assembled, lead cut, soldered and cleaned on the same con-

veyorized system.

A. Solder-Cut

The solder-cut method is commonly used

throughout the electronics industry. It is preceded by a

printed board soldering assembly procedure, followed by

the cutting operation.

5-7

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

lncommg

receiving

I

boards

S

I

inspaclion

commnents

L

m.

Board

S

preparation

T

-

Solder

components

Misc.

componenls

-

Radial

-

Dips

Component

.I

l

I

4

I

l

I

Preclean

prebake PWB

inspection

I

l

louc,h-up

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

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.

5-9

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services