IPC-CM-770D-1996 - 第108页

January 1996 IPC-CM-770

E. Dry Processing

Precoated boards can be component

populated with only the addition of heat.

F. Fine Line Capability

Lines and spaces well below

10

mils can be connected.

No

shorting occurs with even the

highest density flat packs.

G.

TAB Compatible

The outer lead bonding of TAB can

be easily accomplished with adhesive film.

Depending on the attachment method, an adhesive may be

necessary to attach the device to the board.

Thermoset adhesive and two-part reactive, conductive and

non-conductive adhesives have been used in the electronic

industry for several decades. Perhaps the most extensive

use of conductive adhesives is for die attachment, although

component attachment is not uncommon, especially tempo-

rary or initial attachment of surface mount components for

machine soldering.

Several types of adhesives are listed in Table

20-1.

ease of rework and repair. An electrically-conductive epoxy

must be used for bonding when electrical contact is

required to the back of the semiconductor or IC. In general,

metal backing on chips helps promote good ohmic contact.

Epoxy-bonded assemblies must be vacuum baked prior to

sealing to remove any entrapped moisture or volatile

residues.

Epoxy is available as a paste for screen printing or pneu-

matic dispensing and as an impregnated glass cloth or auto-

matic placement. Concerning costs, screen printing the

epoxy is cheapest followed by machine-mounted pneu-

matic dispensing and hand dispensing or film placement.

However, the manufacturing process, not cost, usually dic-

tates the epoxy application technique. For example, screen

printing can only be performed on flat surfaces (i.e., on

unpackaged substrates without components already in

place), whereas dispensing can be performed at various

stages of assembly.

Table 20-4 Various Bonding Adhesive Types

TY Pe

Limitations

Advantages

Phenolics

Depending on type of curing agent used and degrees Some are easy to rework by thermomechanical

Epoxies

Moderate-to-poor bond strength, high coefficient of High temperature stability, easy to rework, high purity, Silicones

High cure temperatures, require solvents as vehicles Very high temperature stability

Polyimides

Used mostly for structural applications, possibly Very high bond strength

corrosive, difficult to process at low temperatures.

low outgassing. thermal expansion

means, some are low outgassers, easy to process, of cure: outgassing, catalyst leaching, corrosivity.

can be filled to 60-70 percent with a variety of

conductive or nonconductive fillers.

The adhesive is applied directly to the circuit by three basic

methods:

-

Screen printing

-

Stencil printing

-

Pneumatic dispensing

Stencil printing provides the best deposition control and is

preferred.

Epoxy adhesives are widely used for circuit components

sensitive to the high temperatures of soldering. Both con-

ducting and insulating epoxy compounds are available, as

well as some that are thermally conductive but electrically

insulating. Some epoxies are available as one-part, cata-

lyzed, premixed adhesives, but these usually require stor-

age at very low temperatures and special care to prevent

moisture condensation when they are brought to room tem-

perature for use. Furthermore, some epoxies may be

incompatible with the other compounds, especially sili-

cones, used in subsequent packaging operations.

Often, low-power semiconductor chips and integrated cir-

cuits (ICs) can only be attached with epoxy because of its

relatively low processing temperature (usually less than

150°C), its superior temperature cycling results, and the

Some automatic die attachment stations include epoxy dis-

pensing by stamp transfer techniques, which reduces costs

advantages.

For highest assembly yields, take care to design adequate

space both around the chip perimeter and the nearest wire

bond site.

Adhesive or bonding agents used on printed board assem-

blies should be continuous, tack free and fully cured show-

ing no evidence of flaking, chipping, blistering, or peeling

from the base material. There should be no striations,

blemishes, or cracks in the adhesive. There should be no

void or group of voids greater than

5

percent of the visual

adhesive area. Bubbles are permissible provided there is no

loss of bond strength.

For components such as transistors, relays, or modules

which require the use of spacers between their base and the

board surface, special attention should be given to insuring

that under vibration there is no vertical movement of the

spacer which might cause damage to surface conductors.

20.4.2.5.1 Material Types.

20.4.2.5.1.1 Epoxy

Because of their superior properties,

conductive and non-conductive epoxy bonding methods are

5-5

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770 January 1996

Table 20-2 Epoxy device

-

Attachment Materials

Mechanical Ease

of

Type

Special Consideration

Application Reliability Strength

Material Cost

Non-conductive Available in 1-part (preforms with Excellent Good Good Low

glass mesh bonding) and 2-part

systems

Conductive

Available in 1-part and 2-part sys- Excellent Good Fa¡ r Moderate Silver-filled

Gold-filled

Excellent Good

Fair to poor

Very high

tems.

usually best for attaching components to boards. A sum-

mary of properties in shown in Table

20-2.

In conductive epoxy bonding, metal particles, usually sil-

ver or gold, are added to the basic epoxy resin. When the

resin cures, the metal particles remain contacted in suspen-

sion and thus provide electrical connectivity and thermal

conduction. Non-conductive epoxies, on the other hand,

contain a basic resin or may incorporate additives to pro-

mote heat dissipation. Epoxies designed for use with bare

chips do not contain volatile solvents. Also, because they

do not significantly outgas, even at high temperatures and

in a low vacuum, they can be used to bond chips to boards

without forming voids between them.

Epoxies have several advantages:

-Most epoxies cure near 150°C. This low temperature

does not grade the electrical properties or the reliability

of the chips, as do some other bonding methods.

-

Epoxy-bonding equipment is straightforward. In many

instances, epoxy can be deposited on all designated

board areas simultaneously with the same silkscreening

equipment used for processing the board itself.

-Boards containing wet epoxy can be stored for a long

time before mounting the chips.

-Devices bonded with epoxy exhibit yields approaching

100%.

-After the epoxy heats to its softening temperature it is

easy to remove a faulty chip and replace it with a good

one.

-An epoxy bonds mechanical strength is sufficient to

withstand most rough handling and most shock and

vibration environments.

-Components are easily attached to both sides since the

adhesive do not remelt after hardening.

-

Adhesives stay where they are placed; balls and runs are

rare and stay attached to substrate.

-

Silver-filled adhesives show good bonding to virtually

all PTF inks including carbon. Carbon ink is often

printed over silver ink to prevent migration.

-When used on conventional board circuits, conductive

adhesives provide more pliable joints than solder. This

prevents joint fractures during thermal cycling.

However, most conductive adhesives use silver as the con-

ductor. This means that silver migration can occur and sil-

ver bearing adhesive pastes cost more than solder.

As with other mounting processes, epoxy bonding also

involves the making of tradeoffs, in particular, its use dic-

tates that certain precautions be taken:

-When thermocompression wire bonding a chip device,

make sure that a bonder with a heated probe tip is used

since the heat associated with some types of thermo-

compression wire bonding will soften the epoxy.

-If a vacuum-hold check is used, the epoxy might be

drawn to the top of the chip.

-Sinter semiconductor chips that have gold backing in

order to alloy the gold with the silicon. If the gold is not

alloyed, the epoxy can pull it away from the chip and

form a high-resistance contact.

20.4.2.5.1.2 Polyimide

The use of polyimides and

silver-filled glass adhesives is increasing due to their lower

cost and ability to withstand higher temperatures than

epoxies. The polyimides are thixotropic pastes containing

approximately

70

percent silver powder in a polyimide

resin dissolved in a high-boiling-temperature solvent.

To minimize the quantities of solvents and other vapors

released during cure, the polyimides used in chip attach-

ment adhesives are low molecular weight resins that cure

by an addition reaction. They are supplied and used in

much the same way as epoxies.

The main objection to their use is that they are organic

polymers and it is difficult to remove all of the organics. If

all of the organics are not removed, they may outgas and

cause chip degradation.

20.4.2.5.2 Anisotropic Conductive Adhesive

A film of

anisotropic adhesive can be placed between the circuit and

the components. Anisotropic conductive adhesives conduct

electricity in one direction only, the Z-axis (between paral-

lel traces). Heat and pressure are simultaneously applied

so

that component terminations press down into conductive

particles and, in turn, press the particles against the lands.

Upon cooling, the component is held in place by the adhe-

sive. Heat may be applied to certain component leads (such

5-6

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

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