IPC-CM-770D-1996.pdf - 第107页

IPC-CM-770

Januaty

1996

The extent to which the user wishes to implement these

guidelines may ultimately be validated by actual tests of

the assembled printed board in its intended shock and

vibration environment.

The ultimate ability of components to survive in shock and

vibration environments will depend upon the degree of

consideration given to the following factors:

The worst case levels of shock and vibration environment

for the entire structure in which the printed board assem-

bly resides and the ultimate level of this environment that

is actually transmitted to the components mounted on the

board. Particular attention should be given to equipment

which will be subjected to random vibration.

The method of mounting the board in the equipment to

reduce the effects of this environment, specifically the

number of board mounting supports and their interval and

complexity.

The attention given to the mechanical design of the

board; specifically its size, shape, type of material, mate-

rial thickness and degree of resistance to bowing and

flexing that the design provides.

The shape, mass, and location of the components

mounted on the board.

The component lead wire strain relief design as provided

by its package, lead spacing, lead bending, or a combina-

tion of these plus the addition of restraining devices.

The attention paid to workmanship during board assem-

bly

so

as to insure that component leads are properly

bent, not nicked and that components are installed in a

manner which minimizes component movement.

20.4.2.3 Clips, Clamps and Brackets

The following are

the basic requirements which should be adhered to when

components are mechanically secured by clips (Figure

20-l),

clamps or brackets:

All clips, clamps or brackets should be secured to prevent

their rotation, such as by using two fasteners or one fas-

tener and a non-turn device.

Clamps and brackets which require their removal in order

to replace the component should be secured with a

threaded fastener or other non-permanent fastener, unless

the subassembly in which they are used is considered to

be disposable or nonrepairable.

Spring type clips which need not be removed during com-

ponent replacement may be secured with permanent type

fasteners such as rivets or eyelets.

The use of twist type lugs, tabs, or ears, and clipping of

glass envelope components, should be avoided.

20.4.2.4 Strapping

When using wires and elastic straps

for mechanical securing, the strap is wrapped over the

component body and passed through holes in the mounting

Positive

re Clamp

to Board

IPC-I-O0032

Figure 20-1 Clip-mounted Component

base (see Figure

20-2).

When wire is used it is clinched

and soldered in the same manner as component leads to

lands. When wire is used with heat sensitive or fragile

components the part of the wire on the component should

be covered with a suitable sleeving.

Figure 20-2 Strap Securing

The elastic strap is secured by being stretched to reduce its

cross-section below that of the hole and then returned to its

larger than hole size by relieving the tension after it has

been passed through the hole. The resiliency of the strap

holds the component in place.

20.4.2.5 Adhesives

The advantages of using adhesives

are as follows:

A. Selective Deposition Not Required

Selective applica-

tion is not required and the entire circuit can be covered.

Conductivity only occurs where component leads make

contact during the thermal stage.

B. Extreme Simplicity

This is basically a one-step pro-

cess. The bonding film can be handled by the placement

equipment in a manner analogous to hot stamp printing.

This means that placement concurrent with heating is the

only step.

C. No Bridging

No

bridging or other shorting phenom-

enon occur.

D. No Defluxing.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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

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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

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services