IPC-CM-770D-1996.pdf - 第127页

January 1996 IPC-CM-770 ........................................................................... ......... ”... .... ”.%.”% ....-.-..- . .” t IPC-I- I L Figure 24-6 TAB Mounting Options TAB is also used for small-size…

IPC-CM-770

Januaty

1996

Layer

0.061 mm [O.O024"l

Copper

PATTERM

COPPER

FORMED

"""

"-""""

Laver

0.036 mm [O.O014"l

Copper

-7

PATTERNS

CUPPER

FORMED

IMAGE

-IN0

0.051 mm

10.002

Polyimide

Layer

kmrd

pmcWon

punching

Rnm.

POlylmM~

and

0.036 mm

[0.00141

Copper

0.025

[0.001

Adhesive

"""""""_

""""""

0.127 mm

[0.0057

Polyimide~

IPC-I-

Figure 24-4 1-, 2-, and

layer TAB Contructions

The first step in the OLB process is to excise the leaded die

from the carrier tape (Figure 24-5). The leaded die is then

transferred to the board in a way that precisely maintains

the original relationship of the die with respect to its tape

sprocket holes or to a vision system that positions leads to

lands. Thus, the die will be accurately placed on the board.

Depending on the TAB mounting configuration (see Figure

24-6) a secondary lead-forming operation may be included.

When required, care must be taken to assure that the leads

are shaped down to the board level without shorting them

against the electrically-active edge of the die.

A bonding operation is then performed on all interconnec-

tions by either thermocompression bonding, thermosonic

bonding, ultrasonic bonding, reflow, or the use of conduc-

tive adhesives. Leads may be bonded one-at-a-time using a

single-point border, or mass bonded using thermocompres-

sion, reflow and conductive adhesive techniques.

24.6 TAB Applications

Applications for TAB vary

widely depending on overall end-product packaging

IPC-I-

requirements. TAB is commonly used to attach chips to

Figure

24-5

Excising

the

leaded die

from

the

tape

both organic (printed wiring board) and inorganic

carrier

(ceramic) substrates. It has also been used in large assem-

blies, as well as in single-chip and multidie packages.

5-24

Licensed by Information Handling Services

January

1996

IPC-CM-770

...........................................................................

.........

”...

....

”.%.”%

....-.-..-

.”

IPC-I-

Figure 24-6 TAB Mounting Options

TAB is also used for small-size and relatively-low cost

military personnel “dogtags.“ In addition, TAB dice are

replacing conventional through-hole and surface mounting

printed wiring board assemblies in several applications.

TAB is also being used to package gate array dice for mid-

range and high-end computers, because of its ability to

support high lead counts (300+) with good electrical per-

formance.

24.7 Pattern Standardization (TAB Tape)

This section

describes the preliminary outlines and features for JEDEC

registered TAB tape (Figure 24-7). Unlike other surface-

mount components, TAB tape is delivered to the user in a

format that is different from the package format after bond-

ing to the substrate. Test lands and supporting areas are cut

away or excised prior to packaging or board mounting.

Because each IC die may have different pad pitches, pad

dimensions, and pad

and

locations, only those features

outside the die area can be standardized. Features that can

be registered are:

Package size prior to the leads begin formed, in an “as-

shipped” condition.

Package body size after lead forming and excising

The outer lead window inside dimension

Test pad size and location

Outer lead dimension and pitch

Registration Features

The following are suggested guidelines for good lead

forming:

-Bend radii of 0.125mm

Lead forming angle of

degrees

Polyimide-up configuration

Foot length of 0.75mm

Inner clamping distance,

of 0.375mm

Height above board or substrate to be determined by

user (depending on die thickness, board clearance

desired, etc.).

TAB footprints are designed with the same principles used

in the design of fine pitch chip carrier footprints to provide

a robust assembly process and a reliable product.

24.8 Encapsulation

Encapsulation involves placing a

protective coating on the top and/or bottom surface of the

inner-lead bonded device; coating on the active side of the

chip, and optionally on all six sides. The reasons for encap-

sulation includes the following:

Protection from mechanical damage-scratches, bending,

etc.

-Protection from surface contaminants

solvents, par-

ticles, dust, etc.

-Protection from edge shorts

created when the lead

touches the scribed edge of the device.

Increased long-term reliability

by protecting the device

from moisture and the environment

24.8.1 Encapsulation Types

The following encapsulants

are used for protection of TAB devices.

24.8.1.1 Unfilled Epoxies

Epoxy types available include

anhydride and novotac epoxies. Ionic impurity levels with

chloride ion under

ppm are available. Typical coating

thickness for an unfilled epoxy is

0.05

to 0.25mm. Epoxies

are generally heat-cured.

24.8.1.2 Filled Epoxies

These are epoxies that have

been filled with quartz. The presence of the filler results in

a thicker coating on the chip, typically

9.5

to l.Omm. The

increased stress on the die caused by the thicker coating

may be partially or completed offset by the lower thermal

coefficient of expansion of the filled epoxy relative to the

unfilled epoxy. Filled epoxies used for encapsulation are

similar to the semi-conductor grade molding compounds

used for plastic integrated circuit packages. Ionic impurity

levels are similar to molding compound and chloride levels

less than 20 ppm are available. The majority of filled

epoxies are anhydride hardened. As a result, they break-

down prematurely during pressure cooker testing, some-

times after 48 hours of testing.

24.8.1.3 Silicones

Several silicones are suitable for

encapsulation. The ionic contamination levels in the mate-

rials have been steadily reduced. Currently, chloride levels

5-25

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Defines

the

tape

width

format

(i.

35mm,

48mm

70mm)

IID

Area

NOTES:

All dimensions and tolerances conform

ANSI

Y14.5-1982

Datums A-B and

are

be determined where the center leads exlt the

body

Contrdlmg dimensions are in inches.

4. D1

and

are the inner edges

the OLB

window

and are equlvaknt

the package

body

size: the area between

and

is user delInable.

Tie

bar

feature shown

opttonal.

IPC-I-

Figure

24-7

Registered Features for TAB Tape Outer-lead Bonding from

JEDEC

outline

UO-O17

less than

ppm are common. The applicability of silicones

is generally good. Heat curing is most common but several

UV curable silicones are available which may have advan-

tages in an in-line cure process. Typical cured thickness is

0.125 to 0.5mm. Alpha particle protection is good. The

UV-cured silicones are generally transparent. As a result

they may not be suitable for certain applications as many

companies are not interested in exposing the active side

the die to day or room light, or visual inspection (after pro-

cessing).

24.8.1.4 Polyimides

Ionically-clean polyimide resins

now boast chloride levels under

ppm. Polyimide mate-

rials are especially attractive when alpha-particle protection

is a priority. Polyimides are heat dried also. Pre-imidized

polyimides are preferred because the drying temperature is

lower than the monomeric materials. Typical coating thick-

ness is

0.05

to 0.25mm. Polyimides of interest to TAB are

siloxane modified. As such, they are delivered pre-cured,

5-26

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