IPC-CM-770D-1996 - 第118页

January

1996

IPC-CM-770

both electrical and thermal conduction. Non-conductive

epoxies contain a basic resin and can contain additives to

promote thermal conduction.

Epoxies designed for die attach usually limit content of

volatile solvents. This reduces the amount of out gassing

and formation of voids. Inspection for voids is simply done

by using a clear glass die and viewing through the die after

adhesive cure. Advantages of using epoxies include:

Low temperature cure, however:

Cure temperature should not reach Tg of the board

Cure temperatures may age solder joints

Ease of processing with silk-screening or dispensing

equipment:

Appropriate adhesive viscosity is necessary for each

process

High Yield: Dependent on die yield and handling

Ease of removal for repair: Before encapsulation

Mechanical strength for ruggedness:

Can be verified with a die shear test

Epoxy bonding also dictates certain precautions, including:

A.

Heat control:

When thermocompression bonding is

used, a heated probe should be used, as the heat associated

with some types of thermocompression bonding will soften

the epoxy. (Does not apply when thermosonic bonding is

used.)

B. Precision tools:

If a vacuum-hold chuck is used to

mount the chips, check that each component forms a posi-

tive seal against the probe tip. Otherwise, epoxy might be

drawn to the top of the chip.

C. Chip processing:

If the gold is evaporated from sin-

tered chips that have gold backing in order to alloy the gold

with the silicon, the epoxy can pull away from the chip and

form a high resistance contact.

23.2.2 Eutectic Die Bonding

Eutectic die bonding

requires the back of the silicon die to be gold plated. At

sufficiently high temperatures the gold plating on the back

of the silicon chip melts with the gold plating on the die

site and forms a eutectic bond with the silicon. The silicon

device can withstand these high temperatures without

endangering performance or life expectancy. The die-attach

station should have the die attach area flooded with an inert

(forming gas or nitrogen) atmosphere.

23.2.3 Polyimide

Polyimides and silver-filled glass

adhesives have the ability to withstand higher temperatures

than epoxies. The polyimides used are thixotropic pastes

containing approximately

70%

silver powder in a polyim-

ide resin that has been dissolved in a high boiling tempera-

ture solvent. The main objection to their use is that they are

organic polymers and it is difficult to remove all of the

organics, which may outgas and cause chip degradation.

23.3 Wire Bonding Materials and Techniques

23.3.1 Bonding Wire

Both gold and aluminum wire are

used for chip on board (COB) applications (Figure 23-1).

Gold wires are typically alloyed with small amounts of

beryllium copper to control grain growth during bonding.

Aluminum wires are typically alloyed with

1%

silicon or

magnesium. For reference of standard wire see ASTM-F-

1.07.

Wire selection is dependent on the bonding techniques

(Table 23-1). The bonding technique determines the degree

of heat at the chip and board sites during bonding. The

amount of heat allowed and the wire termination selected

will limit the material choice and diameter of the wire.

Table 23-2 lists wire sizes and typical resistance values for

aluminum and gold wire. The current should be limited to

less than

0.5

amps for most applications.

A good gold wire bond should have a pull strength from

8

to

10

grams for 0.025 mm diameter wire. Aluminum wire

bonds usually achieve slightly less pull strength than gold.

Table 23-3, Minimum Bond Strength, lists the wire diam-

eter and minimum acceptable pull strengths. Failure occur-

ring in the wire and not at the bond sites means the bonds

are stronger than the wire. This failure is preferred because

wire strength is usually under more control than the bond

sites.

23.3.2 Metallization

Metallization of the die and board

bond pads are very important to successful bonding and

reliability of the bonds. Verification of metallization purity

and thickness are critical. Limiting intermetallics between

wire and bond pad is important. The purity, additives, and

age of the bond wire are also important. The vendor can

supply all of this information. Bond integrity can be veri-

fied by bond pull and shear tests. Aging the wire bonds and

repeating pull and shear tests will give additional informa-

tion on reliability of the bond. Typical board plating for

gold wire bonding should be a minimum of 0.001mm of

99.99%

pure soft gold over

0.004

to 0.005mm of conven-

tional nickel. Aluminum wire bonding can be done reliably

with board plating that is similar to that for gold wire

bonding.

23.3.3 Printed Boards

Printed boards produced with

polymer conductive materials on low-temperature sub-

strates are now being made that are wire bondable. The

application of a plated-metal surface over the printed thick-

film polymer conductor pattern makes this possible. The

printed polymers currently being used for this type of

board construction are thermal-set, epoxy-silver conductors

and non-noble nickel polymers.

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IPC-CM-770 Januaty 1996

1

O0

70

30

20

::

10

I

O

J

v:

I

1980

1985

19

'NON-IBM YEAR

OTHER

GOLD

BALL

BONDING

73%

AI

BAL

L

3'1"

ALUMINUM

WEbGE

BONDING

22%

i%

t

2

2%

I

IPC-1-00338

Figure 23-1 Chip Termination Techniques

Table 23-1 Bonding Wire for COB Applications

Wire Diameter Substrate

(inch)

Comments

Temperature("C)

Attachment Method

Thermocompression (hot substrate)

I

>300

I

Limited repair ability

0.0007-0.001

(Gold)

0.0007-0.002

Thermosonic ball or wedge 125°C Quality of bonds improved if the

substrate is raised to 125°C. Can bond

chips without raising substrate

temperature.

(Aluminum)

I

Ultrasonic Wedge

0.0007-0.010

I

25 Low-temperature process, restricted due

to tool size

Table 23-2 Aluminum and Gold Wire Sizes and Ratings

Wire

Minimum

Standard Wires

Aluminum

1%

Si Aluminum

1%

Si Diameter

Resistance(Rm.)

(inch)

Gold

(Annealed) (Hard)

Gold Aluminum

I

0.0007

I I

X

I

36.06-39.86

I

34.62-38.273

I

27.71-29.43

I

0.001

I

X

I

X

I

17.67-19.53

I

16.0-18.75

I

13.58-14.42

I

0.002

I I

X

I

4.41-4.88

I

4.23-4.68

I

3.40-3.61

I

23.3.4 Wire Bonding Techniques

300"

to

400"

C. The majority of T/C bonding uses a ball

bond on the first bond site and a wedge bond on the sec-

23.3.4.1 Thermocompression (T/C) Bonding (Ball Bond-

ond bond site.

ing)

Thermocompression bonding utilizes heat and pres- When bonding an open flame (or spark discharge) melts

sure to bond wire to a metallized pad site.

A

typical system the end of the wire, molten metal wicks up the end of the

uses a capillary tool of refractory material, such as alumina wire forming the ball. Wire termination after the second

or tungsten carbide, gold wire, and a surface temperature of bond is achieved by flame

off

or as in wedge bonding cut

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January 1996 IPC-CM-770

Table 23-3 Minimum Bond Strength

Wire Composition rocessing and screening when

AU 0.0007 in

AU 0.0013 in

F

5

grams-force

x

number of bonds (bumps)

Flip-clip

Any

G

or

H

30 grams force in accordance with linear millimeter of Beam Lead

Any

nominal undeformed (before bonding) beam width.3

'For wire diameters not specified, use the curve of figure 2011-1 to determine the bond pull limit.

2For ribbon wire, use the equivalent round wire diameter which gives the same cross-sectional area as the ribbon wire

3For condition

G

or H, the bond strength shall be determined by dividing the breaking force by the total of the nominal

being tested.

beam widths before bonding.

or tear off. Flame off produces another ball for the next

bond, cut off used cutters, and tear off pulls the wire until

it breaks. See Figure 23-2 for T/C ball bonding and Figure

23-5 for wedge bonding.

23.3.4.2 Ultrasonic

(UIS)

Bonding (Wedge Bonding)

An

ultrasonic bond is accomplished by pressing and holding

the wire firmly to the metallized pad site. Ultrasonics are

transferred through a wedge tool, creating a scrubbing

action, producing a metallurgical cold weld. The scrubbing

action may remove thin oxide films which form in standard

atmosphere. Use caution with the amount of power sup-

plied to produce the ultrasonics. Using too much power

may damage the chip during bonding. The most common

failure associated with excessive power is cratering. The

majority of U/S bonding applications use aluminum-alloy

wire, although gold, copper, and other metals have been

used. See Figure 23-4. Bonding dissimilar metals at low

temperatures eliminates, or significantly decreases, the for-

mation of intermetallic compounds and allows bonds to be

made in the immediate vicinity of temperature-sensitive

components without adverse effects.

23.3.4.3 Thermosonic (TIS) Bonding (Ball or Wedge

Bonding)

Thermosonic bonding incorporates T/C and U/S

bonding. Using thermal compression in unison with ultra-

sonic to produce a bond, the amount of force, heat, and

ultrasonics are less than for T/C and U/S. The substrate

temperature for T/S is lower than T/C, around 120" to

150°C. Thermosonic bonding can use either ball or wedge

bonding methods with gold wire. See Figure 23-5.

23.4 Assembly

23.4.1 Equipment Assisted Hand Assembly

Systems

are available for limited hands off use. Stations for place-

ment of die on boards under a microscope include a

vacuum tip and a motion reduction system. The system

allows increased precision placement of one of a kind part.

Some systems also integrate adhesive dispensing options

which can easily toggle between dispensing adhesive and

die placement.

23.4.2 Low-Volume Equipment

The low-volume surface

mount assembly equipment shown in Figure 23-6 is a com-

puter controlled pick-and-place machine. The machine can

select surface mount devices from tray carriers, vibratory

feeders and tape input. The system orients and aligns sur-

face mount devices on an automatic centering nest, then

accurately assembles them on the boards at preprogrammed

locations. Such a system can be equipped with provisions

for chip on board (COB). The additional equipment may

include a die attachment epoxy dispensing system, wafer

frame or die tray loader elevators, and pattern recognition

for orienting the die.

The basic system, Figure 23-7, consists of a transfer mod-

ule and a controller. The system controller provides the

interface and control electronics; the transfer module is

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