IPC-CM-770D-1996 - 第106页

January

1996

IPC-CM-770

Table 20-1 Through Hole and Surface Mount Assembly Process

Flow

Comparison

~~

Surface Mount

Single Sided

A

Surface

Mount

Single Sided B

Surface Mount

through Board

Mi

Surface Mount

Double Sided

PRECLEAN

Through

Board

n

PRECLEAN

W

PRECLEAN

u

PRECLEAN

APPLY SOLDER

I

CREAM

APPLY SOLDER

U

APPLY SOLDER

CREAM

APPLY COMP.

ADHESIVE

PICWPLACE

COMPONENTS

SIDE

1

ADHESIVE

SIDE

1

PlC"WUPTLOACE

I

COMPONENTS

I

PlC%$CE

I

COMPONENTS

AUTO INSERT

COMPONENTS

MANUAL INSERT

I

MANUAL

COMPONENTS

PICWPLACE

I

MANUAL

COMPONENTS

PICWPLACE

[Tl

CURE SOLDER

I

CRYM

I

CURE SOLDER

CREAM

SOLDERCREAM

CLEAN

U

FLOW MELT

SOLDER CREAM SOLDERCREAM

FLOW MELT

I

AUTO PICK

6

SIDE

2

AUTO INSERT

COMPONENTS

ANUAL INSERT

COMPONENTS

PUCL1

MANUAL PICK

6

I

PREHEAT THEN

MACHINE

SOLDER

I

PREHEAT THEN

MACHINE

SOLDER

I

FLOW MELT

SOLDER CREAM

CLEAN

MACHINE

SOLDER

I

CLEAN

I

REPAIR

.

RETEST

REPAIR

RETEST

REPAIR

RETEST

REPAIR

RETEST

I

RETEST

5-3

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

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.

5-4

COPYRIGHT Association Connecting Electronics Industries

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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

5-5

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services